WO2001048961A1 - Optical apparatus for dropping and/or adding optical signals - Google Patents

Optical apparatus for dropping and/or adding optical signals Download PDF

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Publication number
WO2001048961A1
WO2001048961A1 PCT/EP2000/012788 EP0012788W WO0148961A1 WO 2001048961 A1 WO2001048961 A1 WO 2001048961A1 EP 0012788 W EP0012788 W EP 0012788W WO 0148961 A1 WO0148961 A1 WO 0148961A1
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Prior art keywords
optical
signal
wavelength
output
input
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PCT/EP2000/012788
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French (fr)
Inventor
Simona Scotti
Aurelio Pianciola
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Optical Technologies U.S.A. Corp.
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Priority to AU20111/01A priority Critical patent/AU2011101A/en
Publication of WO2001048961A1 publication Critical patent/WO2001048961A1/en

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Classifications

    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
    • G02B6/24—Coupling light guides
    • G02B6/26—Optical coupling means
    • G02B6/28—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals
    • G02B6/293—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals with wavelength selective means
    • G02B6/29346—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals with wavelength selective means operating by wave or beam interference
    • G02B6/2935—Mach-Zehnder configuration, i.e. comprising separate splitting and combining means
    • G02B6/29352—Mach-Zehnder configuration, i.e. comprising separate splitting and combining means in a light guide
    • G02B6/29355—Cascade arrangement of interferometers
    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
    • G02B6/24—Coupling light guides
    • G02B6/26—Optical coupling means
    • G02B6/28—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals
    • G02B6/293—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals with wavelength selective means
    • G02B6/29379—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals with wavelength selective means characterised by the function or use of the complete device
    • G02B6/2938—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals with wavelength selective means characterised by the function or use of the complete device for multiplexing or demultiplexing, i.e. combining or separating wavelengths, e.g. 1xN, NxM
    • G02B6/29382—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals with wavelength selective means characterised by the function or use of the complete device for multiplexing or demultiplexing, i.e. combining or separating wavelengths, e.g. 1xN, NxM including at least adding or dropping a signal, i.e. passing the majority of signals
    • G02B6/29383—Adding and dropping
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04J—MULTIPLEX COMMUNICATION
    • H04J14/00—Optical multiplex systems
    • H04J14/02—Wavelength-division multiplex systems
    • H04J14/0201—Add-and-drop multiplexing
    • H04J14/0202—Arrangements therefor
    • H04J14/0209—Multi-stage arrangements, e.g. by cascading multiplexers or demultiplexers
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04J—MULTIPLEX COMMUNICATION
    • H04J14/00—Optical multiplex systems
    • H04J14/02—Wavelength-division multiplex systems
    • H04J14/0201—Add-and-drop multiplexing
    • H04J14/0202—Arrangements therefor
    • H04J14/021—Reconfigurable arrangements, e.g. reconfigurable optical add/drop multiplexers [ROADM] or tunable optical add/drop multiplexers [TOADM]
    • H04J14/0212—Reconfigurable arrangements, e.g. reconfigurable optical add/drop multiplexers [ROADM] or tunable optical add/drop multiplexers [TOADM] using optical switches or wavelength selective switches [WSS]

Definitions

  • the present invention relates to an optical apparatus for dropping and/or adding optical signals. More in particular, the present invention relates to an optical apparatus for dropping and/or adding optical signals from a wavelength division multiplexed optical signal (or WDM) , a WDM optical communication system comprising said apparatus and a method for dropping and/or adding optical signals.
  • WDM wavelength division multiplexed optical signal
  • a WDM optical signal is a signal comprising a plurality of N optical signals independent of one another, each at a predetermined central wavelength ⁇ l, ⁇ 2 ... ⁇ N different from that of the other signals.
  • the signals can be both digital and analogue, and they have a certain spectrum width around the value of the central wavelength.
  • the signals having different wavelengths are each assigned a specific band of wavelengths having predetermined width - hereinafter called “channel” .
  • Each of said channels is hereinafter characterised by a central wavelength value and by a range of wavelength, centred about said central wavelength, which is defined “channel band” or “channel width” .
  • optical signal having a wavelength ⁇ , optical channel at a wavelength ⁇ and residual component of a signal having a wavelength ⁇ respectively refer to an optical signal, an optical channel and a signal residual component having a predetermined spectrum width ⁇ centred about the wavelength ⁇ .
  • the spectrum width typically depends on the characteristics of the laser source generating it, and on the modulation given to it to associate the information to be transmitted thereto. Typical values of spectrum width of a signal emitted by a laser source, in absence of modulation, are of about 10 MHz, and, in the presence of an external modulation, (for example, at 2.5 Gbit/s) they are of about 5 GHz.
  • the spectrum width ⁇ depends, for example, on the band of reflection or of transmission of said filter.
  • spacing between channels refers to the separation in frequency (or wavelength) between the central wavelengths of two adjacent channels of a plurality of N channels at wavelengths ⁇ l , ⁇ 2 ... ⁇ N.
  • the wavelength separation between channels (or spacing) of a WDM signal is typically in the range of nanometres .
  • the various optical signals are generated by a plurality of optical sources, multiplexed so as to form a WDM signal, transmitted along the same line of optical transmission (for example, an optical fibre) and then, demodulated so as to be received each one by a respective receiver.
  • WDM optical systems such as, for example, Local Area Networks (or LAN)
  • LAN Local Area Networks
  • WDM optical systems such as, for example, Local Area Networks (or LAN)
  • LAN Local Area Networks
  • the following optical devices have been proposed.
  • US 4 900 119 discloses wavelength selective optical devices comprising a 3 dB optical directional coupler and wavelength selecting elements - such as Bragg reflection gratings or Fabry-Perot resonators - usually positioned in optical symmetry with respect to the coupler.
  • a tap that can be used as device for adding-dropping channels, comprising a Mach-Zehnder interferometer having two arms ' with two identical Bragg reflection gratings. A first and a second 3 dB directional coupler are connected with one another by the two arms of the interferometer.
  • US 5 636 309 discloses a planar waveguide Mach-Zehnder device.
  • the arms of the Mach-Zehnder interferometer are of essentially equal length, with a maximum spacing between the arms (that is, between the waveguide core centres) selected to make possible simultaneous exposure of both arms to refractive index-altering radiation.
  • US 5 657 406 discloses a multiplexer/demultiplexer comprising a plurality of 2x2 optical couplers each having two ports attached to a pair of gratings adapted to transmit a predetermined wavelength and reflect the others .
  • US 5 717 798 discloses an optical waveguide system comprising a mode coupling grating and a mode discriminating coupler that can be used as device for adding-dropping optical signals.
  • US 5 751 456 discloses a multi-wavelength add/drop multiplexer comprising an optical circulator, arranged to direct all of the light transmitted along a main optical fibre towards a branch optical fibre, a Fabry-Perot filter, comprised into said branch optical fibre, arranged to transmit light of a predetermined wavelength and reflect the non-transmitted light backwards to the main optical fibre, a receiver for receiving the transmitted light, and a transmitter arranged to transmit an optical signal, in succession, along said branch optical fibre, through the filter, the optical circulator and the main fibre.
  • US 5 778 118 discloses a device comprising a first optical coupler having an input port and a first and a second output port, and a second optical coupler having a first and a second input port and an output port.
  • An optical path optically connects the first output port of the first coupler to the first input port of the second coupler, and includes an optical filter for reflecting portions of a WDM optical signal in input from the first coupler. Said portions of the WDM signal that are not sent to an input port of the second coupler exit the device.
  • An optical path communicating with the second output port of the first coupler comprises a splitter and wavelength selectors configured to select a wavelength of the WDM signal in output of that port.
  • Optical channels to be added are first combined by an optical combiner and then, sent to the second input port of the second coupler to be combined with the portion of the WDM signal that has passed through the optical filter.
  • US 5 822 095 discloses a device wherein the components ⁇ l- ⁇ n of a multi-wavelength input signal are received by an input optical fibre and transmitted, through a first optical circulator and an optical fibre, to a pass-band filter which allows a specific wavelength ⁇ l to pass and reflects the other wavelengths ⁇ 2- ⁇ n.
  • the reflected wavelengths ⁇ 2- ⁇ n return backwards to the first circulator while the wavelength ⁇ l runs through another optical fibre and a second optical circulator, and is • dropped from an output optical fibre.
  • another signal component of the wavelength ⁇ l is added and introduced from another input optical fibre and sent through the second optical circulator and the second optical fibre to the optical pass-band filter.
  • the added wavelength ⁇ l signal component Once the added wavelength ⁇ l signal component has passed the pass-band filter, it is mixed with the reflected wavelengths ⁇ 2- ⁇ n so that a resultant signal sum of the full wavelengths ⁇ l- ⁇ n is passed through the first optical circulator and an output optical fibre.
  • F. Bilodeau et al . (“An all -fiber dense-wavelength-division mul tiplexer /demul tiplexer using photoimprinted Bragg gratings ", IEEE Photonics Technology Letters Vol. 7, no. 4, April 1995, pages 388-390) describe a device used to add and drop a single channel from/to a WDM connection with a spacing between channels of 100 GHz at 1550 nm, comprising a Mach-Zehnder interferometer made with an all-fibre technology with identical Bragg gratings in the arms of the interferometer.
  • the multiplexing (signal adding) and demultiplexing (signal dropping) operations can be carried out simultaneously within the same device.
  • the crosstalk can be minimised with gratings having a reflectivity higher than 99%, or by positioning in the arms of the interferometer an additional pair of gratings with a resonance frequency different from that of the other pair of gratings, so as to use a different wavelength for the dropped signal and for the added signal .
  • Turantechnik et al ( "Integrated-optical Mach-Zehnder add- drop fil ter fabri cated by a single UV-induced grating exposure", Applied Optics, Vol. 36, No. 30, October 1997, pages 7838-7845) propose a method for fabricating a Mach- Zehnder interferometer with Bragg gratings by si ultaneously exposing the arms of the interferometer to obtain identical gratings in aligned positions, thus minimising the asymmetries of the optical paths. Said technology can allow avoiding the arms to be subjected to a further UV trimming for balancing the interferometer.
  • the Authors propose two methods for solving the problem of crosstalk.
  • the first method consists of using a drop channel and an add channel having a spacing in frequency equal to twice the bit rate of transmission.
  • the second method consists of using two devices in cascade.
  • the first method requires a precise control of the channel spacing in frequency and a difficult realisation of a spectrum profile of the gratings.
  • said solution is complex, difficult and expensive to implement.
  • the second method implies a significant increase of insertion loss of the device since it requires the use of a number of devices equal ' to twice the number of channels with different wavelengths that have to be added and dropped.
  • the Applicant faced the problem of reducing the crosstalk of an optical device for adding-dropping channels from a
  • insertion losses refers to the difference between the optical power in output from an optical device and the optical power in input to the same, when the values of said optical powers are expressed in dB;
  • crosstalk refers to the difference between the power of a noise component and the power of an optical signal, when the values of said optical powers are expressed in dB (for example, on the output of an optical -signal add-drop device, the noise component can be the possible residual component of a not perfectly dropped or added signal) ;
  • in-band crosstalk refers to the optical signal and the noise component having the same wavelength
  • out-of-band crosstalk refers to the optical signal and the noise component having different wavelengths.
  • the present invention relates to an optical apparatus for adding-dropping signals from a WDM optical signal comprising a plurality of N signals having wavelengths ⁇ l , ⁇ 2 ... ⁇ N, said apparatus comprising
  • a first optical device comprising an input for said WDM optical signal, a first output and a second output, said first device being adapted to drop from said WDM optical signal at least one signal at a wavelength ⁇ x, preselected among said wavelengths ⁇ l, ⁇ 2 ... ⁇ N, to send said at least one signal dropped at wavelength ⁇ x to said first output and to send said WDM optical signal to said second output;
  • a second optical device comprising a first input in communication with said second output of said first optical device, a second input, a first output, a second output and a portion of waveguide, between said first input and said second input, housing a wavelength selective element, said wavelength selective element being adapted to
  • said wavelength selective element of said second optical device is also adapted to drop, from said WDM optical signal, at least one signal at a wavelength ⁇ y, preselected among said wavelengths ⁇ l, ⁇ 2 ... ⁇ N,
  • said second optical device is adapted to send the signal dropped at the wavelength ⁇ y to said first output, and said WDM optical signal to said second output;
  • a third optical device having a first input in communication with said second output of said second optical device, a second input and an output, said third device being adapted to drop from said WDM optical signal a possible residual component of said at least one signal at wavelength ⁇ y; to receive from said second input at least one add signal having a wavelength ⁇ y substantially equal to said wavelength ⁇ y and add it to said WDM optical signal; and to send said WDM optical signal to said output.
  • the optical apparatus of the invention advantageously allows reducing the crosstalk between two possible residual components of two signals dropped at wavelengths ⁇ x and ⁇ y and two signals added at substantially the same wavelength, reducing the insertion losses of the WDM signal passing through them.
  • the dropping of the possible residual component of the signal at wavelength ⁇ x, the dropping of the signal at wavelength ⁇ y and the adding of the signal at wavelength ⁇ x are carried out by the wavelength selective element into the same portion of waveguide of the second optical device, it allows using, for adding/dropping two optical signals, only three add/drop optical devices in series. More in particular, it allows using a limited number of passive optical components such as, for example, optical couplers and/optical circulators, which would otherwise introduce undesired additional losses to the WDM optical signal.
  • the optical apparatus of the invention allows reducing the crosstalk, at the same time limiting the insertion losses on the WDM optical signal, using only a number of add/drop optical devices equal to the number of signals at different wavelengths that have to be added and/or dropped, increased by one.
  • two optical signals typically have two central wavelengths slightly different from one another.
  • ⁇ 1 substantially equal to the wavelength ⁇ of another optical signal
  • ⁇ 1 and ⁇ are nominally equal according to the standard defined by the ITU-T Standards.
  • said first optical device comprises at least one optical filter FI.
  • Said at least one optical filter FI is advantageously adapted to reflect at least said wavelength ⁇ x and to let the other wavelengths of said WDM optical signal pass (or vice versa) so that at least the signal at wavelength ⁇ x of said WDM optical signal is sent to said first output while the other signals of said WDM optical signal are sent to said second output of said first optical device.
  • said at least one optical filter FI has a reflection (or transmission) spectrum comprised within a predetermined band of wavelengths ⁇ x centred about said ⁇ x.
  • said predetermined band of wavelengths ⁇ x has such a width as to allow isolating the signal at wavelength ⁇ x from the other signals of said WDM optical signal, according to the system requirements.
  • the width of said predetermined band of wavelengths ⁇ x is smaller than twice the spacing between the channels associated to said optical signals at wavelengths ⁇ l , ⁇ 2 .... ⁇ N.
  • said spacing between the channels is smaller than or equal to 200 GHz.
  • the spacing between the channels is smaller than or equal to 100 GHz. More preferably, smaller than or equal to 50 GHz. Even more preferably, smaller than or equal to 25 GHz.
  • said at least one optical filter FI is a conventional Bragg grating. According to a variant, it is a conventional interferential filter or a conventional Fabry- Perot resonator.
  • said first optical device also comprises a first portion of optical waveguide which is in communication with said input and said first and second output of said first optical device.
  • said first portion of optical waveguide is a portion of optical fibre. More preferably, it is a portion of a conventional single-mode optical fibre. Said first portion of optical waveguide is advantageously adapted to house said at least one optical filter FI .
  • said first optical device also comprises an input optical coupler having a first and a second port respectively corresponding to said input and to said first output of said first optical device, and a third port connected to a first end of said first portion of optical waveguide.
  • said first optical device also comprises an output optical coupler having a first port connected to a second end of said first portion of optical waveguide and a second port corresponding to said second output of said first optical device.
  • said optical device also comprises a second portion of optical waveguide connected, at a first end, to a fourth port of said input coupler, and at the opposed end, to a third port of said output coupler.
  • Said second portion of optical waveguide is advantageously adapted to house at least a second optical filter FI .
  • said first optical device comprising said input coupler, said first and second optical waveguides, said optical filters FI and said output coupler, is a conventional add/drop device made up of a Mach-Zehnder interferometer with an optical filter on each arm of the interferometer as described, for example, in the US Patent 4 900 119.
  • said first optical device also comprises an optical circulator having a first and a second port respectively corresponding to said input and to said first output of said first optical device and a third port connected to an end of said first portion of optical waveguide, the opposed end of said portion of optical waveguide being said second output port of said first optical device.
  • said wavelength selective element of said second optical device is an optical filter F2.
  • Said one optical filter F2 is advantageously adapted to reflect at least said wavelengths ⁇ x and ⁇ y and to let the other wavelengths of said WDM optical signal pass (or vice versa) .
  • said one optical filter F2 has a reflection
  • each of said predetermined bands of wavelengths ⁇ x and ⁇ y have such a width as to allow respectively isolating the signals at wavelength ⁇ x and ⁇ y from the other signals of said WDM optical signal, according to the system requirements.
  • the width of said predetermined bands of wavelengths ⁇ x and ⁇ y is smaller than twice the spacing between the channels associated to said optical signals at wavelengths ⁇ l , ⁇ 2 .... ⁇ N.
  • said optical filter F2 is made up of two conventional Bragg gratings. According to a variant, it is made up of two conventional interferential filters or two conventional Fabry-Perot resonators.
  • said wavelengths ⁇ x and ⁇ y are two adjoining wavelengths between said wavelengths ⁇ l, ⁇ 2 .... ⁇ N.
  • said band of wavelengths ⁇ xy has such a width as to allow isolating the signals at wavelengths ⁇ x and ⁇ y from the other signals of said WDM optical signal, according to the system requirements.
  • the width of said predetermined band of wavelengths ⁇ xy is smaller than twice the spacing between the channels associated to said optical signals at wavelengths ⁇ l , ⁇ 2 .... ⁇ N.
  • said optical filter F2 is preferably made up of a single Bragg grating. According to a variant, it is made up of a single interferential filter or a Fabry-Perot resonator.
  • said portion of optical waveguide of said second optical device is a portion of optical fibre. More preferably, it is a portion of a conventional single-mode optical fibre.
  • said second optical device also comprises an input optical coupler having a first and a second port respectively corresponding to said first input and to said first output of said second optical device, and a third port connected to a first end of said portion of optical waveguide.
  • said second optical device also comprises an output optical coupler having a first port connected to a second end of said portion of optical waveguide, a second port corresponding to said second output of said second optical device, and a third port corresponding to said second input of said second optical device.
  • said second optical device also comprises a second portion of optical waveguide connected, at a first end, to a fourth port of said input coupler, and at the opposed end, to a third port of said output coupler.
  • Said second portion of optical waveguide is advantageously adapted to house at least a second optical filter F2.
  • said second optical device comprising said input coupler, said first and second optical waveguides, said optical filters F2 and said output coupler, is an add/drop device made up of a Mach-Zehnder interferometer with a suitable optical filter on each arm of the interferometer .
  • said second optical device besides said at least one optical filter F2 and said first portion of optical waveguide, said second optical device also comprises an optical circulator having a first and a second port respectively corresponding to said first input and to said first output of said second optical device, and a third port connected to a first end of said portion of optical waveguide.
  • this embodiment of said second optical device also comprises an output optical circulator having a first port connected to a second end of said portion of optical waveguide, and a second and a third port respectively corresponding to said second input and to said second output of said second optical device.
  • said third optical device comprises at least one optical filter F3.
  • Said at least one optical filter is advantageously adapted to reflect said wavelength ⁇ x and to let the other wavelengths of said WDM optical signal pass (or vice versa) .
  • said at least one optical filter F3 has a reflection (or transmission) spectrum comprised within a predetermined band of wavelengths ⁇ y centred about said ⁇ y.
  • said predetermined band of wavelengths ⁇ y has such a width as to allow isolating the signal at wavelength ⁇ y from the other signals of said WDM optical signal, according to the system requirements.
  • the width of said predetermined band of wavelengths ⁇ y is smaller than twice the spacing between the channels associated to said optical signals at wavelengths ⁇ l, ⁇ 2 .... ⁇ N.
  • said optical filter F3 is a conventional Bragg grating. According to a variant, it is a conventional interferential filter or a conventional Fabry-Perot resonator.
  • said third optical device also comprises a first portion of optical waveguide which is in communication with said first and second inputs and with said output of said second optical device.
  • said first portion of optical waveguide is a portion of optical fibre. More preferably, it is a portion of a conventional single-mode optical fibre. Said first portion of optical waveguide is advantageously adapted to house said at least one optical filter F3.
  • said third optical device also comprises an output optical coupler having a first port connected to a first end of said first portion of optical waveguide, a second port corresponding to said second input of said third optical device, and a third port corresponding to said output of said third optical device.
  • said third optical device also comprises an input optical coupler having a first port corresponding to said first input of said third optical device, and a second port connected to a second end of said first portion of waveguide.
  • said third optical device also comprises a second portion of optical waveguide connected, at a first end, to a third port of said input coupler, and at the opposed end, to a fourth port of said output coupler.
  • Said second portion of optical waveguide is advantageously adapted to house at least a second optical filter F3.
  • said third optical device comprising said input coupler, said first and second optical waveguides, said optical filters F3 and said output coupler, is a conventional add/drop device made up of a Mach-Zehnder interferometer with an optical filter on each arm of the interferometer.
  • said third optical device also comprises an optical circulator having a first port connected to an end of said portion of waveguide and a second and third port respectively corresponding to said second input and to said output of said third optical device, the other end of said portion of waveguide being said first input of said third optical device.
  • the optical apparatus of the invention also comprises a first optical switch having a first input port connected to said first output of said first optical device, a second input port to receive said at least one add signal having said wavelength ⁇ , a first output port connected to said second input of said second optical device, and a second output port.
  • said first optical switch When it is in a "bar" status, said first optical switch is adapted to connect its first input port to its first output port, and its second input port to its second output port.
  • it is adapted to connect the first input port to the second output port and the second input port to the first output port .
  • said first optical switch is a conventional optomechanical or thermal-optical device.
  • the optical apparatus of the invention also comprises a second optical switch having a first input port connected to said first output of said second optical device, a second input port to receive said at least one add signal having said preselected wavelength ⁇ y, a first output port connected to said second input of said third optical device, and a second output port.
  • said second optical switch When it is in a "bar" status, said second optical switch is adapted to connect its first input port to its first output port, and its second input port to its second output port .
  • it is adapted to connect the first input port to the second output port and the second input port to the first output port .
  • the second optical switch when the second optical switch is in the bar status, the signal dropped at said wavelength ⁇ y is re- added to the WDM optical signal, while when it is in a cross status, the add signal having said preselected wavelength ⁇ x y is added to the WDM optical signal .
  • said second optical switch is a conventional optomechanical or thermal-optical device.
  • the present invention relates to a WDM optical communication system comprising:
  • a transmission station adapted to provide a WDM optical signal comprising a plurality of N signals having wavelengths ⁇ l , ⁇ 2 .... ⁇ N;
  • a first optical device comprising an input for said WDM optical signal, a first output and a second output, said first device being adapted to drop from said WDM optical signal at least one signal at wavelength ⁇ x, preselected among said wavelengths ⁇ l, ⁇ 2 .... ⁇ N, sending said at least one signal dropped at wavelength ⁇ x to said first output, and sending said WDM optical signal to said second output;
  • a second optical device comprising a first input in communication with said second output of said first optical device, a second input, a first output, a second output and a portion of waveguide, between said first input and said second input, housing a wavelength selective element, said wavelength selective element being adapted to
  • said wavelength selective element of said second optical device is also adapted to drop, from said WDM optical signal, at least one signal at a wavelength ⁇ y, preselected among said wavelengths ⁇ l , ⁇ 2 ... ⁇ N,
  • said second optical device is adapted to send the signal dropped at the wavelength ⁇ y to said first output, and said WDM optical signal to said second output;
  • optical apparatus also comprises
  • a third optical device having a first input in communication with said second output of said second optical device, a second input and an output, said third device being adapted to drop from said WDM optical signal a possible residual component of said at least one signal at wavelength ⁇ y; to receive from said second input at least one add signal having a wavelength ⁇ x y substantially equal to said wavelength ⁇ y and add it to said WDM optical signal; and to send said WDM optical signal to said output.
  • said optical transmission line comprises at least one intermediate station (or line site) .
  • said intermediate station comprises an amplification section. More advantageously, said amplification section comprises at least one optical amplifier.
  • said at least one optical apparatus is comprised into said intermediate station. According to a variant, it is comprised into said transmission station or into said reception station.
  • the present invention relates to a method for adding-dropping signals from a WDM optical signal comprising a plurality of N signals having wavelengths ⁇ l , ⁇ 2 .... ⁇ N, said method comprising the steps of
  • step a) adding, in said third optical device, at least one add signal having wavelength ⁇ y substantially equal to said wavelength ⁇ y, into said WDM optical signal .
  • the step a) is carried out through an optical filter FI adapted to reflect at least said wavelength ⁇ x and to let the other wavelengths of said WDM optical signal pass (or vice versa) .
  • the steps b) , c) and d) are carried out through an optical filter F2 adapted to reflect at least said wavelengths ⁇ x and ⁇ y and to let the other wavelengths of said WDM optical signal pass (or vice versa) .
  • the steps e) and f) are carried out through an optical filter F3 adapted to reflect at least said wavelength ⁇ y and to let the other wavelengths of said WDM optical signal pass (or vice versa) .
  • FIG. 1 shows a schematic view of an embodiment of an optical apparatus according to the invention
  • FIG. 2 shows a schematic view of a preferred embodiment of the optical apparatus of figure 1 ;
  • Figure 3 schematically shows three reflection spectrums of optical filters that can be used on the optical apparatus of figure 2 ;
  • FIG. 4 shows a schematic view of a second preferred embodiment of an optical apparatus of the invention
  • Figure 5 shows a schematic view of another embodiment of the optical apparatus of figure 1;
  • Figure 6 shows a schematic view of an optical communication system according to the present invention
  • FIG. 7 shows a schematic view of a graph of spectrum emission of optical amplifiers usable on the system of figure 6;
  • FIG. 8 shows a schematic view of part of a transmitting station of the system of figure 6;
  • FIG. 9 shows a measurement bench used in experimental tests carried out to check the performances of an optical apparatus of the invention
  • FIG. 10 shows the spectral responses obtained between the input and the drop port of each optical device of the apparatus of figure 2 ;
  • FIG. 11 shows the spectral responses obtained between the input and the output of each optical device of the apparatus of figure 2;
  • FIG. 12 shows the spectral response obtained between the input and the output of the apparatus of figure 2 , without adding channels, (curve A) and the spectrum of losses in function of the polarisation - PDL - (curve B) ;
  • FIG. 13 shows the spectral responses obtained between the input of the first optical device and the output of the second optical device by adding the signal at wavelength ⁇ x (curve A) and by adding the signal at wavelength ⁇ y (curve B) ;
  • FIG. 14 shows the spectral responses obtained between the input and the output of the first optical device (curve A) , between the input and the output of the second optical device (curve B) , and between the input of the first optical device and the output of the second optical device (curve C) ;
  • FIG. 15 shows the spectral responses obtained between the input and the output of the optical apparatus of figure 2 (curve A) , between the input and the output of the third optical device (curve B) , and between the input of the first optical device and the output of the second optical device (curve C) .
  • Figure 1 shows a schematic view of an embodiment of an optical apparatus 4 of the invention, comprising a first optical device 1, a second optical device 2 and a third optical device 3.
  • the first optical device 1 has an input 11, a first output 12 and a second output 13.
  • the second optical device 2 has a first input 21, a second input 22, a first output 23 and a second output 24.
  • the third optical device 3 has a first input 31, a second input 32 and an output 33.
  • the second output 13 of the first optical device 1 is connected to the first input 21 of the second optical device 2, and the second output 24 of the second optical device 2 is connected to the first input 31 of the third optical device 3.
  • the optical apparatus 4 of figure 1 is adapted to drop from a WDM optical signal 1000, comprising a plurality of N signals having wavelengths ⁇ l, ⁇ 2 .... ⁇ N, two signals having wavelengths ⁇ x and ⁇ y, preselected among ⁇ l, ⁇ 2 .... ⁇ N, and to add to it two add signals having wavelengths ⁇ x x and ⁇ y substantially equal to ⁇ x and ⁇ y.
  • the N optical signals are associated to N channels having a predetermined spacing or band ⁇ ch.
  • the first optical device 1 is adapted to drop from said optical signal 1000, which enters into apparatus 4 through input 11, a signal at wavelength ⁇ x, to send it to output
  • the optical device 2 is adapted to receive, at its first input 21, the WDM optical signal coming from the first device 1, to eliminate a possible residual component of the signal at wavelength ⁇ x, to drop from it a signal at wavelength ⁇ y, to send the possible residual component at wavelength ⁇ x and the signal dropped at wavelength ⁇ y to output 23, to receive at its second input 22 the add signal having wavelength ⁇ x, to add said add signal to the WDM optical signal 1000 and to send the WDM optical signal 1000, thus modified, to the second output 24.
  • the optical device 3 is adapted to receive, at its first input 31, the WDM optical signal coming from the second device 2, to eliminate a possible residual component of the optical signal at wavelength ⁇ y, to receive at its second input 32 the add signal having wavelength ⁇ y , to add said add signal to the WDM optical signal 1000, and to send the WDM optical signal 1000, from which the signals at wavelengths ⁇ x and ⁇ y have been dropped, and to which said add signals at wavelengths ⁇ x x ⁇ y have been added, to output 33.
  • Figure 2 shows a preferred embodiment of the apparatus of figure 1, wherein the optical devices 1, 2 and 3 comprise each a conventional Mach-Zehnder interferometer with Bragg gratings (the operation of which is described, for example, in the already mentioned US Patent 4 900 119 herein included as reference) .
  • said interferometers are made with an all- fibre technology, and they are each made up of only two portions of optical fibre reciprocally connected and treated so as to obtain the structure shown in figure 2.
  • the Bragg gratings are obtained in the portions of fibre by inserting both arms of the interferometer into the same phase mask adapted to write the gratings on the fibres, so as to obtain a Bragg grating on both arms .
  • the Mach-Zehnder interferometer of the first optical device 1 comprises an input coupler 17, an upper arm and a lower arm respectively comprising a- first and a second portion of optical fibre 15 and 16, and an output coupler 18.
  • the input coupler 17 is provided with a first and a second port respectively corresponding to input 11 and to the first output 12, and a third and a fourth port respectively connected to two ends of said portions of optical fibre 15 and 16.
  • the output coupler 18 is provided with four ports of which two are connected to two ends of said portions of optical fibre 15 and 16, one corresponds to the output port 21 and the other one is unused.
  • the portions of optical fibre 15 and 16 respectively house two optical filters FI 14 and 19.
  • the two optical filters FI 14 and 19 are two substantially equal Bragg gratings having a reflection spectrum comprised in a band of wavelength ⁇ x centred about ⁇ x (curve A of figure 3) .
  • said band of wavelengths ⁇ x has a width substantially equal to said spacing between channels ⁇ ch.
  • the Mach-Zehnder interferometer of the second optical device 2 comprises an input coupler 27, an upper arm and a lower arm respectively comprising a first and a second portion of optical fibre 25 and 26, and an output coupler 28.
  • the input coupler 27 is provided with four ports, two of which respectively correspond to input 21 and to the first output 23, and the other two are respectively connected to two ends of said portions of optical fibre 25 and 26.
  • the output coupler 28 is provided with four ports of which two are connected to two ends of said portions of optical fibre 25 and 26, and two respectively correspond to the first input 22 and to the second output 24.
  • the portions of optical fibre 25 and 26 respectively house two optical filters F2 29 and 30.
  • the two optical filters F2 29 and 30 are substantially equal, and each of them comprises two Bragg gratings having two reflection spectrums respectively comprised in a band of wavelength ⁇ x centred about ⁇ x and in a band of wavelength ⁇ y centred about ⁇ y (respectively, curves A and C of figure 3) .
  • said band of wavelengths ⁇ y has a width almost equal to said spacing between channels ⁇ ch.
  • the unused ports of the output coupler 18 of the first optical device 1 and of the input coupler 37 of the third optical device 3 are optically terminated so as to prevent back reflections (that is to say, they are adapted in a conventional way) .
  • the use of a single Bragg grating, with wider reflection band, in each arm of the interferometer advantageously allows making the optical device 2 easier and less expensive to be manufactured since said gratings can be obtained by means of a single step of writing on fibre.
  • the Mach-Zehnder interferometer of the third optical device 3 comprises an input coupler 37, an upper arm and a lower arm respectively comprising a first and a second portion of optical fibre 35 and 36, and an output coupler 38.
  • the input coupler 37 is provided with four ports, one of which corresponds to the first input 31, two are respectively connected to two ends of said portions of optical fibre 35 and 36, and the other one is unused.
  • the output coupler 38 is provided with four ports of which two are connected to two ends of said second input 32 and to the output port 33.
  • the portions of optical fibre 35 and 36 respectively house two optical filters F2 34 and 39.
  • the two optical filters F2 34 and 39 are substantially equal, and each of them comprises a Bragg grating having a reflection spectrum comprised in a band of wavelengths ⁇ y (curve C of figure 3) .
  • couplers 17, 18, 27, 28, 37 and 38 are 3 dB directional couplers.
  • the two arms of each Mach-Zehnder interferometer are preferably balanced. That is to say, the two arms substantially have the same optical path length.
  • the WDM optical signal 1000 When the WDM optical signal 1000 enters into the first optical device 1 through input 11, its optical power is divided into two substantially equal parts by the input coupler 17, and is sent to the two optical fibres 15, 16 where the two optical filters FI 14 and 19 reflect the signal at wavelength ⁇ x back. In this way, the non- reflected signals of the WDM optical signal 1000 travel towards the output coupler 18 and exit from the first optical device 1 through the second output 13, while the optical signal at wavelength ⁇ y, reflected backwards, returns towards coupler 17 and exits from the first device 1 through the first output 12.
  • the WDM optical signal 1000 enters from input 11 of the first optical device 1, the signal having wavelength ⁇ x is dropped from the first output 12 whereas the other signals exit from the second output 13 to enter into the second optical device 2 through its first input 21.
  • the power of the WDM optical signal 1000 is divided into two substantially equal parts by the input coupler 27, and is sent to the two optical fibres 25, 26 where the two optical filters F2 30 and 29 reflect the possible residual component of the optical signal at wavelength ⁇ x and the optical signal at wavelength ⁇ y back.
  • the non-reflected signals of the WDM optical signal 1000 travel towards the output coupler 28 and, in a similar way to what described above with reference to the first optical device 1, exit from the second optical device 2 through the second output 24.
  • the possible residual component of the optical signal at wavelength ⁇ x and the optical signal at wavelength ⁇ y return towards coupler 27 and exit from the second optical device 2 through the first output 23.
  • both the signal at wavelength ⁇ y and the possible residual component of the signal at wavelength ⁇ x, coming from the first optical device 1, are dropped from the first output 23.
  • the remaining signals of the WDM optical signal 1000 exit from the second output 24 to enter into the third optical device 3 through its first input 31.
  • the Applicant has noted that, although in output 23 both the signal at wavelength ⁇ y and the possible residual component of the signal at wavelength ⁇ x are dropped, the drop signal of interest at wavelength ⁇ y can be easily separated in reception from the undesired residual component by means, for example, of a suitable optical filtering to eliminate said noise component at wavelength ⁇ x.
  • the power of the WDM optical signal 1000 arriving to the first input 31 is divided into two substantially equal parts by the input coupler 37, and is sent to the two optical fibres 35, 36 where the two optical filters F3 35 and 39 reflect the possible residual component of the optical signal at wavelength ⁇ y back.
  • the non-reflected signals of the WDM optical signal 1000 travel towards the output coupler 38 and, in a similar way to what described above with reference to the first optical device 1, exit from the third optical device
  • the add signal at wavelength ⁇ x x is sent to the second input 22 of the second optical device 2, its optical power is divided into two substantially equal parts by the output coupler 28, and is sent to the two optical fibres 25, 26.
  • the two optical filters F2 30 and 29 reflect the two parts of optical power of said signal backwards to the output coupler 28, and said output coupler 28 recombines them into the second output 24, thus adding the signal at wavelength ⁇ x x to the WDM optical signal 1000.
  • the add signal at wavelength ⁇ x y is sent to the second input 32 of the third optical device 3
  • its optical power is divided into two substantially equal parts by the output coupler 38, and is sent to the two optical fibres 35, 36.
  • the two optical filters F3 34 and 39 reflect the two parts of optical power of said signal backwards to the output coupler 38, and said output coupler 38 recombines them into output 33, thus adding also the signal at wavelength ⁇ x y to the WDM optical signal 1000.
  • the dropping of the possible residual component at wavelength ⁇ x coming from the first optical device 1, the addition of the signal at wavelength ⁇ x x and the dropping of the signal at wavelength ⁇ y are carried out by the same add/drop optical device (second optical device 2) by using a suitable optical filter F2 on the two arms of the Mach-Zehnder interferometer.
  • the optical apparatus 4 allows reducing the crosstalk between possible residual components of the signals dropped at wavelengths ⁇ x and ⁇ y and the signals respectively added at wavelengths ⁇ x x and ⁇ , using only three optical devices in cascade and, thus, limiting the insertion losses of the WDM optical signal 1000 passing through them.
  • a limited number of optical devices in cascade causes a reduction in the number of passive optical elements such as for example, optical couplers and/or optical circulators, which would otherwise introduce undesired additional insertion losses on the WDM optical signal.
  • the optical apparatus 4 allows eliminating the crosstalk between a dropped signal and the possible residual component of an add signal at the same wavelength since the possible residual component exits from the optical apparatus 4 from another output than that from which the dropped signal exits.
  • the signal dropped at wavelength ⁇ x exits from output 12 whereas the possible residual component of the signal added at wavelength ⁇ x x (added from input 22) exits from output 23.
  • the optical apparatus 4 of the invention also comprises a first and a second switch 5 and 6. Typically, they are conventional optomechanical or thermal-optical devices.
  • the first optical switch 5 is provided with a first input port connected to the first output 12 of said first optical device 1, a second input port for receiving the add signal having said wavelength ⁇ x x, a first output port connected to the second input 22 of the second optical device 2, and a second output port .
  • the second optical switch 6 is provided with a first input port connected to the first output 23 of said second optical device 2, a second input port for receiving the add signal having said wavelength ⁇ y, a first output port connected to the second input 32 of the third optical device 3, and a second output port.
  • Said switches are adapted to operate according to one of two "bar” and "cross” status. When they are in a “bar” status, they connect their first input port to their first output port, and their second input port to their second output port. On the contrary, when they are in a "cross” status, they connect their first input port to their second output port, and their second input port to their first output port .
  • Said switches advantageously allow making the optical apparatus 4 of the invention re-configurable.
  • said switches when they are in the bar status, said switches allow re-adding the dropped signal to the WDM optical signal 1000, whereas when they are in the cross status, they allow adding the add signal, having a different information content from that of the dropped signal, to the WDM optical signal 1000.
  • the input and output ports of the first and/or second optical switch 5 and 6 are connected to the ports of the optical devices 1, 2, 3 in an opposed way with respect to that shown in figure 2.
  • the switches allow re-adding the dropped signal to the WDM optical signal 1000, whereas when they are in the bar status, they allow adding the add signal, having a different information content from that of the dropped signal, to the WDM optical signal 1000.
  • the optical apparatus 4 also comprises an insulator in correspondence with input 11. Said insulator allows suppressing possible undesired components of light that return back to said input due to reflections inside apparatus 4.
  • a suitable optical filter (not shown) can advantageously be provided at output 23 of the second optical device 2, said filter being adapted to eliminate the noise component at wavelength ⁇ x.
  • Figure 4 shows an embodiment of an optical apparatus 4 of the invention adapted to drop three signals at wavelengths ⁇ x, ⁇ y and ⁇ z from the WDM optical signal 1000, and to add three signals having wavelengths ⁇ x x, ⁇ x y and ⁇ x z substantially equal to ⁇ x, ⁇ y and ⁇ z and an information content different from that of the drop signals.
  • Said embodiment is completely similar to that of figure 2, with the exception that it also comprises an intermediate device 2' and a third optical switch 7, and that the filters F3 of the third optical device 3 (in figure 4 referred to with reference numeral 3') are adapted to reflect a band of wavelengths ⁇ z, centred about wavelength ⁇ z, instead of the above band of wavelengths ⁇ y.
  • the intermediate optical device 2' is completely similar to the optical device 2 with the exception that each optical filter F2 ' has a reflection spectrum comprised in two bands of wavelengths ⁇ y and ⁇ z, respectively centred about wavelengths ⁇ y and ⁇ z, or in a band of wavelengths ⁇ yz, centred about ( ⁇ y + ⁇ z) / 2 and having width equal to about ⁇ y + ⁇ z .
  • switch 7 its structural and functional characteristics are totally similar to those of the two switches 5 and 6; thus, reference shall be made to what has already been described.
  • the optical apparatus 4 of the invention comprises, in a way completely similar to the embodiment of figure 4, a first optical device 1, M-l intermediate optical devices 2 and 2', and a third optical device 3', connected in series to one another.
  • the optical apparatus 4 of the invention allows filtering each optical signal to be dropped from the WDM optical signal twice, and then adding to it a signal having substantially the same wavelength, using a limited number of optical devices arranged in series.
  • apparatus 4 only requires M+1 optical devices connected in series for adding and/or dropping M optical signals.
  • the optical apparatus of the invention allows reducing the crosstalk between an added signal and the possible residual component of a signal dropped at the same wavelength, limiting insertion losses of the WDM optical signal passing through it.
  • the optical apparatus of the invention has the advantage of making the WDM optical communication system on which it is used extremely flexible.
  • an apparatus of the invention (or a plurality of apparatuses arranged in series) can be easily added to the WDM system and, in case, removed, added to and/or replaced by another optical apparatus of the invention, adapted to add and/or drop different signals, in case it is not necessary anymore to add and/or drop some signals to said system, and/or it is necessary to add and/or drop new signals to it.
  • the ml and m2 dropped signals must then be conventionally demultiplexed in wavelength to be afterwards separately received by the respective photo-receivers, and the ml and m2 add signals must be conventionally multiplexed in wavelength, before being added to the apparatus of the invention.
  • Figure 5 shows another embodiment of the optical apparatus of figure 1. Since said embodiment has some elements in common with the embodiment of figure 2 , the same reference numerals as figure 2 have been used for these elements.
  • the first device 1 comprises a first three-port optical circulator 40, the portion of optical fibre 15 and the filter FI 14.
  • the second optical device 2 comprises a second three-port optical circulator 41, the portion of optical fibre 25, the filter F2 30 and a third three-port optical circulator 42.
  • the third optical device 3 comprises the portion of optical fibre 35, the optical filter F3 34 and a fourth three-port optical circulator 43.
  • the optical circulators 40-43 are conventional three-port optical devices wherein an optical signal can move from one port to the other only according to a predetermined direction of propagation.
  • the input 11 and the first output 12 of the first optical device 1 are made up of two ports of the first optical circulator 40, whereas the second output 13 is made up of an end of the portion of optical fibre 15.
  • the input 21 and the first output 23 of the second optical device 2 are made up of two ports of the second optical circulator 41, whereas the second input 22 and the second output 24 are made up of two ports of the third optical circulator 42.
  • the input 31 of the third optical device 3 is made up of an end of the portion of optical fibre 35, whereas the second input 32 and the output 33 are made up of two ports of the fourth optical circulator 43.
  • the optical signal 1000 when the optical signal 1000 enters into the first optical device 1 through input 11, it is sent along the portion of optical fibre 15 by the optical circulator 40; the optical signal at wavelength ⁇ x is reflected backwards by the optical filter FI 14, while the remaining signals of the WDM optical signal 1000 continue towards output 13 of the first optical device 1. The reflected signal returns backwards to the optical circulator 12, and exits from the first device 1 through the first output 12.
  • the WDM optical signal 1000 coming from output 13 of the first device enters from input 21, passing through the optical circulator 41; the possible residual component of the signal at wavelength ⁇ x and the signal at wavelength ⁇ y are reflected backwards, while the remaining signals propagate towards the third optical circulator 42.
  • the possible residual component of the signal at wavelength ⁇ x and the signal at wavelength ⁇ y return backwards to the optical circulator 41 and exit from the second optical device 2 through the first output 23.
  • the remaining signals of the WDM optical signal 1000 recombine into the optical circulator 42 with the add signal at wavelength ⁇ x (coming from input 22) and exit with it from the second optical device 2 through output 24.
  • the WDM optical signal 1000 coming from output 24 of the second device 2 enters from input 31, it continues towards the portion of optical fibre 25; the possible residual component of the signal at wavelength ⁇ y is reflected backwards by the optical filter F3 34, while the other signals of the WDM optical signal 1000 continue towards the optical circulator 43.
  • the reflected residual component returns backwards to the second optical device 2, where it is blocked by the optical circulator 42.
  • the non-reflected signals of the WDM optical signal 1000 recombine into the optical circulator 43 with the add signal at wavelength ⁇ x y (coming from input 32) and exit with it from the third optical device 3 through output 33.
  • the optical apparatus 4 of the invention can be used on any system for transmitting or distributing WDM optical signals from/to which it is necessary to drop/add at least one optical signal .
  • WDM optical transmission system comprising an optical apparatus 4 of the invention.
  • Said WDM system comprises a first terminal site 100, a second terminal site 200, an optical-fibre line 300a, 300b which connects the two terminal sites, and at least one line site 400 interposed along said optical-fibre line.
  • the transmission system described is monodirectional , that is to say, the optical signal travels from a terminal site to the other, but the following considerations can be applied also to bi- directional systems wherein the optical signal travels in both directions.
  • the WDM system is adapted to transmit on a maximum of 128 channels, but the maximum number of channels can differ according to the configurations of the WDM system.
  • the first terminal site 100 preferably comprises a multiplexing section 110 (MUX) for a plurality of input channels 160, and a power amplification section 120 (TPA) .
  • MUX multiplexing section 110
  • TPA power amplification section 120
  • the second terminal site preferably comprises a pre- amplification section 140 (RPA) and a demultiplexing section 150 (DMUX) for a plurality of output channels 170.
  • RPA pre- amplification section 140
  • DMUX demultiplexing section 150
  • Each input channel 160 is received by the multiplexing section 110 - hereafter described in detail with reference to figure 8 - which groups the channels preferably into three sub-bands, hereafter referred to as BB (blue band) , RBI (red band 1) and RB2 (red band 2) .
  • BB blue band
  • RBI red band 1
  • RB2 red band 2
  • the WDM optical transmission system can provide for the division of the above three into a greater or smaller number of sub- bands .
  • the three sub-bands are sent to the power amplification section 120 and then, to line 300.
  • the power amplification section 120 preferably receives the three sub-bands separated from one another by the multiplexing section, and separately amplifies them; afterwards, it combines them with one another so as to obtain a wide band WDM signal to be sent to the transmission line 300.
  • the line site 400 receives the wide band WDM signal, divides it again into the three sub-bands BB, RBI and RB2 , separately amplifies the signal of the three sub-bands, drops and/or adds some channels from/to said three sub- bands by means of an optical apparatus 4 according to the invention, and recombines the three sub-bands to reconstruct the wide band WDM signal .
  • Further line sites 400 can be distributed along line 300 in a suitable position according to the portion of line covered up to that point every time there is the need of amplifying the WDM optical signal or, more in general, of modifying its characteristics (for example, by adding and/or dropping channels by means of an apparatus 4 according to the invention) .
  • the second terminal site 200 receives the wide band signal and amplifies it through the pre-amplification section 140, which preferably divides the WDM signal again into the three sub-bands BB, RBI and RB2.
  • the demultiplexing section 150 receives the three sub-bands and divides then into single-wavelength signals 170.
  • the number of input channels 160 can differ from the number of output channels 170 due to the fact that in the line sites 400 some channels can be added or dropped from the optical apparatus 4 of the invention.
  • Figure 7 shows a graph of spectrum emission of the optical amplifiers of the system of figure 6, wherein the three sub-bands of the following example are highlighted.
  • the first sub-band BB preferably presents signals having wavelengths comprised between 1529 nm and 1535 nm;
  • the second sub-band RBI preferably presents signals having wavelengths comprised between 1541 nm and 1561 nm;
  • the third sub-band RB2 preferably presents signals having wavelengths comprised between 1575 nm and 1602 nm.
  • first sub-band 16 channels are preferably allocated; in the second sub-band, 48 channels are preferably allocated and in the third sub-band, 64 channels are preferably allocated.
  • adjacent channels in a 128-channel WDM system have a frequency spacing of 50 GHz.
  • sub-band BB the spectrum presents a central peak; thus, in said sub-band the amplifiers can advantageously present an equalisation device to make the curve of spectrum emission flat in said area.
  • Figure 8 shows in detail the input section of the first terminal site 100. Besides the multiplexing section 110 and the amplification section 120 (not shown in figure 8) , said site comprises a line terminal section 410 (OLTE) and a wavelength conversion section 420 (WCM) .
  • OLTE line terminal section 410
  • WCM wavelength conversion section 420
  • the line terminal section 410 corresponds to a terminal apparatus, for example according to one of the standard SONET, ATM, IP or SDH, and includes transmitters/receivers in a number corresponding to the channels to be transmitted along the line.
  • the OLTE is provided with 128 transmitters/receivers.
  • the OLTE transmits a plurality of channels, each one at its own wavelength.
  • Said wavelengths can be modified so as to make them compatible with the telecommunication system, through wavelength converters WCM1-WCM128 being part of the WCM section 420.
  • Converters WCM1-WCM128 are adapted to receive a signal at a generic wavelength and to convert it into a signal at a predetermined wavelength, according, for example, to what described in patent US 5 267 073 entitled to the Applicant itself.
  • Each wavelength converter WCM preferably comprises a photodiode that converts the optical signal into an electric signal; a laser source and an electro-optical modulator, for example of the Mach-Zehnder type, to modulate the optical signal generated by the laser source at the predetermined wavelength, with the electric signal converted by the photodiode .
  • said converter can comprise a photodiode and a laser diode directly modulated by the electric signal of the photodiode so as to convert the optical signal to the predetermined wavelength.
  • Devices such as amplifiers, signal re-timers and/or clippers can be inserted between the photodiode and the modulator or between the photodiode and the direct- modulation laser. Additionally, the connection of an FEC
  • (Forward Error Connection) module of transmission is provided to add information to the temporal frame of the signal, so as to allow the receiver to correct possible errors occurred along the line, thus improving the BER.
  • a further alternative provides for a receiver (for example, according to one of the previously mentioned standard types) to be comprised in said converter to receive an optical signal and convert it into a corresponding electric signal; and for a laser source and an electro-optical modulator to modulate the optical signal generated by the laser source at the predetermined wavelength, with the electric signal coming from the receiver.
  • a receiver for example, according to one of the previously mentioned standard types
  • a laser source and an electro-optical modulator to modulate the optical signal generated by the laser source at the predetermined wavelength, with the electric signal coming from the receiver.
  • Wavelength converters of the type mentioned are marketed by the Applicant with the name initials of WCM, RXT, LEM.
  • wavelength converters or optical signal generators present in the first terminal site 100 generate respective operating optical signals having wavelengths inside respective channels comprised in the effective operating band of the amplifiers arranged in succession into the WDM system.
  • the multiplexing section 110 preferably comprises three multiplexers 430, 440 and 450.
  • the first multiplexer 430 combines the signals coming from the first 16 WCM converters 1-16 so as to form the first sub-band BB;
  • the second multiplexer 440 combines the signals coming from the WCM converters 17-64 so as to form the second sub-band RBI, and
  • the third multiplexer 450 combines the signals coming from the WCM converters 65-128 so as to form the third sub-band RB2.
  • Multiplexers 430, 440 and 450 are passive optical devices, through which the optical signals transmitted on respective optical fibres are multiplexed in a single fibre; devices of this type are, for example, made up of fused fibre couplers or in planar optics, Mach-Zehnder devices, AWG, polarisation filters, interferential filters, micro-optical filters, or the like.
  • a suitable combiner is the 8-WM or the 24 -WM combiner, marketed by the Applicant itself.
  • the amplification section 120 is capable of amplifying the signals of the sub-bands so as to raise their level up to a sufficient value to cover the following portion of optical fibre 330a existing prior to new amplifying means (comprised in the first line site 400) , keeping at the end a sufficient power level to guarantee the required transmission quality.
  • the signals of the bands are combined with one another downstream of said power amplifier 120, through a pass-band combiner filter (not shown) , so as to be inserted in the first portion 300a of optical line, usually made up of a single-mode optical fibre inserted into a suitable optical cable which is some dozen- (or hundred-) kilometers long, for example about 100-kilometers long.
  • optical fibres used for links of the type described can be optical fibres of the dispersion-shifted type.
  • the type of fibre with step-index profile is preferable in those cases in which the prevention or reduction of non-liner effects of intermodulation between close channels is desired, as it could be important in dispersion-shifted fibres, especially if the distance between the channels themselves is very small .
  • Step-index fibres have a dispersion of about 17 ps/mm km at wavelengths of about 1550 nm. Smaller dispersion values, sufficient to make said intermodulation phenomena unimportant, for example from 1.5 to 6 ps/km, can be obtained with the fibres called NZD described, for example, by the ITU-T Standards G.655.
  • the first line site 400 adapted to receive the multi- wavelength (or WDM) signals, attenuated in the fibre path, and to amplify them up to a sufficient level to feed them to the second portion of optical fibre 300b, having similar characteristics to those of the previous one.
  • Successive line amplifiers and respective portions of optical fibre usually inserted into respective optical cables, cover the required overall transmission distance until they arrive to the second terminal station 200.
  • demultiplexing section 150 it is possible to use a component of the same type as that used in the multiplexing section 110, already described, mounted in an opposed configuration, in connection with respective pass-band filters arranged on the output fibres.
  • Pass-band filters of the above type are, for example, marketed by MICRON-OPTICS.
  • a demultiplexing section 150 suitable for the purpose comprises, for example, an AWG (Array Waveguide Grating) named 24WD or 8WD, marketed by the Applicant itself.
  • AWG Array Waveguide Grating
  • the configuration described is especially adapted for transmissions on distances of about 500 km, with a high transmission speed, for example 5 Gbit/s per channel, or higher .
  • the line amplifiers are provided for operation at an overall output optical power of about 22 dBm.
  • the power amplifier 120 can advantageously have the same configuration as the line amplifiers.
  • the configuration of WDM transmission system described above is especially adapted to provide the desired performances, in particular for transmissions of more channels by wavelength multiplexing, in the presence of a particular choice of the properties of the line amplifiers with which it is provided, in particular as regards the capacity of transmitting the selected wavelengths without affecting any of them with respect to the others.
  • the amplifier configuration changes according to the bands of wavelength it must amplify.
  • the first, the second and the third optical device 1, 2, 3 used were Mach-Zehnder fused fibre interferometers with Bragg gratings written in a single exposure on both arms of the interferometer.
  • Said fused fibre interferometers have the advantage of having low insertion loss, and do not introduce significant additional insertion losses in case of fused junction of the optical apparatus 4 with a standard single-mode fibre.
  • couplers, arms and Bragg gratings of each interferometer are made using only two optical fibres suitably treated and coupled to one another
  • the optical devices 1, 2, 3 used in the experimental tests are those produced by Amphenol Fiber Optic Products, and marketed with the name of DWDM ("Dense" WDM) Product .
  • the Bragg gratings FI and F3 had a reflection spectrum respectively comprised in the bands of wavelengths ⁇ x and ⁇ y, having width almost equal to the spacing between channels, and respectively centred about the wavelengths 1549.32 nm ( ⁇ x) and 1549.71 nm ( ⁇ y) .
  • the Bragg gratings F2 had a reflection spectrum comprised in a band of wavelengths ⁇ y, centred about the intermediate wavelength 1549.515 nm ( ⁇ m) between said ( ⁇ x) and said ( ⁇ y) , and having width almost equal to twice the spacing between channels.
  • Switches 5 and 6 were optomechanical switches available on the market as "SN Series Fiberoptic Latching Switches" , produced by JDS-Uniphase in a 2x2 configuration. They have the advantage of having low insertion loss (about -0.5 dB) and high isolation on the non-connected port (over 70 dB) . In addition, their commutation speed is equal to some milliseconds (4 ms) . Before assembly, the three devices 1, 2, 3, were characterised one by one for the purpose of obtaining the spectral responses in correspondence of the inputs and the outputs of each of them.
  • a measurement bench was used for the experimental measures, adapted to provide the spectrum PDL (Polarisation Dependent Loss) , that is to say, the spectral response in the function of the polarisation of the signal in input to the optical device from time to time under test (or DUT) .
  • PDL Polarisation Dependent Loss
  • Said bench shown in figure 9, comprised a laser source 44 tunable in wavelength (model 3642 CR00 by PHOTONETICS, 52 Avenue de 1 'Europe - BP 39, 78160 MARLY-LE-ROY, FRANCE), and a polarisation controller device 45 (model HP 11896A by HEWLETT PACKARD Company, P.O. Box 4026, Englewood, CO 80155-4026) to change the polarisation status of the signal from time to time emitted by the laser source 44.
  • a laser source 44 tunable in wavelength model 3642 CR00 by PHOTONETICS, 52 Avenue de 1 'Europe - BP 39, 78160 MARLY-LE-ROY, FRANCE
  • a polarisation controller device 45 model HP 11896A by HEWLETT PACKARD Company, P.O. Box 4026, Englewood, CO 80155-4026
  • the maximum, minimum and mean values on all the polarisation statuses were acquired for each DUT and in correspondence with each signal emitted by the laser source 44, using an optical power meter 46 (model HP 8163A together with HP 815333, and optical head HP 81524A by HEWLETT PACKARD Company) .
  • a wavelength meter 47 was used (model WA-1600 Wavemeter produced by Burleigh Instruments Inc. Burleigh Park, Fisher, NY 14453- 0755) to check from time to time the wavelength actually emitted by the laser source 44.
  • the measure was automated through a personal computer 48 - provided with boards for IEEE 488 communication (GPIB) and for acquiring analogue signals (for example, AT-GPIB / TNT and AT-MIO-16E-10 by NATIONAL INSTRUMENTS, 11500 N Mopac Expwy Austin, TX 78759-3504) - which controlled, with a special software, the entire measurement system.
  • GPSB IEEE 488 communication
  • analogue signals for example, AT-GPIB / TNT and AT-MIO-16E-10 by NATIONAL INSTRUMENTS, 11500 N Mopac Expwy Austin, TX 78759-3504
  • the spectral responses shown in the figures from 10 to 15 represent the mean values obtained from time to time as the polarisation of the signals in input to the devices under test (DUT) was changed.
  • Table 1 shows the nominal values of wavelengths ⁇ x, ⁇ m, ⁇ y, the values of said wavelengths measured by the wavelength meter 47, the deviation between nominal wavelengths and those measured, the insertion loss (IL) values, expressed in dB, obtained at the wavelengths ⁇ x, ⁇ m and ⁇ y between the IN port and the DROP port of each device.
  • IL insertion loss
  • figure 10 shows the spectral responses obtained between the IN input and the DROP port of each optical device 1, 2, 3 (respectively, curves A, B, and C)
  • table 2 shows the bandwidth (BW) values of the spectral responses obtained at -0.5 dB, in the worst case of polarisation, and at -3 dB, in the case of depolarised light.
  • the measurement of the spectral responses as the signal polarisation changed provided for a spectrum PDL value, around the wavelengths of interest ⁇ x, ⁇ m and ⁇ y, smaller than 0.1 dB .
  • the spectral responses from the IN ports to the OUT ports correspond to those from the ADD ports to the DROP ports .
  • the insertion loss values obtained for the three optical devices 1, 2, 3 - respectively represented by the curves A, B and C of figure 11 - include the optical connectors with which they were terminated.
  • Table 3 shows the insertion loss (IL) values, expressed in dB, obtained between the IN port and the OUT port of each device, at the wavelengths transmitted by the respective filters and at the nominal wavelengths ⁇ x and ⁇ y.
  • Figure 12 shows (curve A) the spectral response obtained by dropping the signals at wavelength ⁇ l and ⁇ 2 without re- adding any other signal (operation as fixed DROP of apparatus 4) .
  • the insertion losses (IL) obtained for the signals at the wavelengths passing through the apparatus were comprised between -1.8 and -2 dB (that is to say, they were substantially equal to the sum of the insertion losses of the single devices) .
  • this figure shows (curve B) the spectrum of losses depending on the polarisation (PDL) , expressed in dB .
  • Figure 13 shows the spectral responses obtained by re- adding the signals at wavelengths ⁇ x (curve A) and ⁇ y (curve B) through switches 5 and 6.
  • the insertion losses (IL) obtained for said re-added signals are respectively equal to -2.7 dB and -2.5 dB .
  • Figure 14 shows the spectral responses obtained from the IN input to the OUT output of the first optical device 1 (curve A) , from the IN input to the OUT output of the second optical device 2 (curve B) , and from the IN input of the first optical device 1 to the OUT output of the second optical device (curve C) .
  • the crosstalk value was equal to about 35 dB.
  • the crosstalk was smaller than -26 dB (value which experimentally caused a loss on the BER ⁇ 0.5 dB) .
  • Figure 15 shows the spectral responses obtained from the IN input to the OUT output of apparatus 4 (curve A) , from the IN input to the OUT output of the third optical device 3 (curve B) , and from the IN input of the first optical device 1 to the OUT output of the second optical device 2 (curve C) .
  • the crosstalk values obtained for the signal at wavelength ⁇ y were smaller than -40 dB . This value was better than that previously obtained because the components used for the third optical device 3 had better performances than those used for the first optical device 1.

Abstract

An optical apparatus (4) for dropping and/or adding signals from an optical signal WDM (1000) comprising a plurality of N signals having a wavelength μ1, μ2 ... μN, said apparatus (4) comprising a first optical device (1) adapted to drop from said WDM optical signal (1000) at least one signal at a wavelength μx, preselected among said wavelengths μ1, μ2 ... μN, a second optical device (2) adapted to drop from said WDM optical signal (1000) a possible residual component of said at least one signal at a wavelength μx, to drop at least one signal at a wavelength μy , preselected among said wavelengths μ1, μ2 ... μN, to receive from said second input (22) at least one add signal having a wavelength μix substantially equal to said wavelength μx and to add it to said WDM optical signal (1000), and a third optical device (3) adapted to drop from said WDM optical signal (1000) a possible residual component of said at least one signal at a wavelength μy; to receive from said second input (32) at least one add signal having a wavelength μix substantially equal to said wavelength μy and add it to said WDM optical signal (1000).

Description

Optical apparatus for dropping and/or adding optical signals
DESCRIPTION
The present invention relates to an optical apparatus for dropping and/or adding optical signals. More in particular, the present invention relates to an optical apparatus for dropping and/or adding optical signals from a wavelength division multiplexed optical signal (or WDM) , a WDM optical communication system comprising said apparatus and a method for dropping and/or adding optical signals.
A WDM optical signal is a signal comprising a plurality of N optical signals independent of one another, each at a predetermined central wavelength λl, λ2 ... λN different from that of the other signals. The signals can be both digital and analogue, and they have a certain spectrum width around the value of the central wavelength.
In a WDM optical communication system, the signals having different wavelengths are each assigned a specific band of wavelengths having predetermined width - hereinafter called "channel" . Each of said channels is hereinafter characterised by a central wavelength value and by a range of wavelength, centred about said central wavelength, which is defined "channel band" or "channel width" .
In the present description and following claims, the expressions "optical signal having a wavelength λ, optical channel at a wavelength λ and residual component of a signal having a wavelength λ" respectively refer to an optical signal, an optical channel and a signal residual component having a predetermined spectrum width Δλ centred about the wavelength λ. For example, in the case of an optical signal, the spectrum width typically depends on the characteristics of the laser source generating it, and on the modulation given to it to associate the information to be transmitted thereto. Typical values of spectrum width of a signal emitted by a laser source, in absence of modulation, are of about 10 MHz, and, in the presence of an external modulation, (for example, at 2.5 Gbit/s) they are of about 5 GHz. In addition, in case of an optical signal or of a residual signal component being reflected or made pass by an optical filter, the spectrum width Δλ depends, for example, on the band of reflection or of transmission of said filter.
Additionally, the expression "spacing between channels" refers to the separation in frequency (or wavelength) between the central wavelengths of two adjacent channels of a plurality of N channels at wavelengths λl , λ2 ... λN.
For the purpose of transmitting a high number of channels, making use of one of the so-called transmission windows of optical fibres, and of the effective bandwidth of the optical amplifiers, the wavelength separation between channels (or spacing) of a WDM signal is typically in the range of nanometres .
Typically, in a WDM system the various optical signals are generated by a plurality of optical sources, multiplexed so as to form a WDM signal, transmitted along the same line of optical transmission (for example, an optical fibre) and then, demodulated so as to be received each one by a respective receiver.
In many applications of WDM optical systems such as, for example, Local Area Networks (or LAN) , distribution networks for television signals or telecommunications in general, it is increasingly felt the need of dropping one or more channels of a WDM signal from the transmission line to route them towards different directions and, at the same time, to add along the line one or more channels having the same wavelengths as the dropped channel, but with a different information content. In the prior art, the following optical devices have been proposed.
US 4 900 119 discloses wavelength selective optical devices comprising a 3 dB optical directional coupler and wavelength selecting elements - such as Bragg reflection gratings or Fabry-Perot resonators - usually positioned in optical symmetry with respect to the coupler. In an embodiment, it describes a tap that can be used as device for adding-dropping channels, comprising a Mach-Zehnder interferometer having two arms' with two identical Bragg reflection gratings. A first and a second 3 dB directional coupler are connected with one another by the two arms of the interferometer.
US 5 636 309 discloses a planar waveguide Mach-Zehnder device. The arms of the Mach-Zehnder interferometer are of essentially equal length, with a maximum spacing between the arms (that is, between the waveguide core centres) selected to make possible simultaneous exposure of both arms to refractive index-altering radiation.
US 5 657 406 discloses a multiplexer/demultiplexer comprising a plurality of 2x2 optical couplers each having two ports attached to a pair of gratings adapted to transmit a predetermined wavelength and reflect the others .
US 5 717 798 discloses an optical waveguide system comprising a mode coupling grating and a mode discriminating coupler that can be used as device for adding-dropping optical signals.
US 5 751 456 discloses a multi-wavelength add/drop multiplexer comprising an optical circulator, arranged to direct all of the light transmitted along a main optical fibre towards a branch optical fibre, a Fabry-Perot filter, comprised into said branch optical fibre, arranged to transmit light of a predetermined wavelength and reflect the non-transmitted light backwards to the main optical fibre, a receiver for receiving the transmitted light, and a transmitter arranged to transmit an optical signal, in succession, along said branch optical fibre, through the filter, the optical circulator and the main fibre.
US 5 778 118 discloses a device comprising a first optical coupler having an input port and a first and a second output port, and a second optical coupler having a first and a second input port and an output port. An optical path optically connects the first output port of the first coupler to the first input port of the second coupler, and includes an optical filter for reflecting portions of a WDM optical signal in input from the first coupler. Said portions of the WDM signal that are not sent to an input port of the second coupler exit the device. An optical path communicating with the second output port of the first coupler comprises a splitter and wavelength selectors configured to select a wavelength of the WDM signal in output of that port. Optical channels to be added are first combined by an optical combiner and then, sent to the second input port of the second coupler to be combined with the portion of the WDM signal that has passed through the optical filter.
US 5 822 095 discloses a device wherein the components λl- λn of a multi-wavelength input signal are received by an input optical fibre and transmitted, through a first optical circulator and an optical fibre, to a pass-band filter which allows a specific wavelength λl to pass and reflects the other wavelengths λ2-λn. The reflected wavelengths λ2-λn return backwards to the first circulator while the wavelength λl runs through another optical fibre and a second optical circulator, and is • dropped from an output optical fibre. Meanwhile, another signal component of the wavelength λl is added and introduced from another input optical fibre and sent through the second optical circulator and the second optical fibre to the optical pass-band filter. Once the added wavelength λl signal component has passed the pass-band filter, it is mixed with the reflected wavelengths λ2-λn so that a resultant signal sum of the full wavelengths λl-λn is passed through the first optical circulator and an output optical fibre.
D.C. Johnson et al . ( "New design concept for a narrowband wavelength- selective optical tap and combiner", Electronics. Letters, June, 1987, vol. 23, no. 13, pages 668-669) describe a wavelength selective device comprising a Mach- Zehnder interferometer with identical Bragg reflection filters in each arm of the coupler.
F. Bilodeau et al . ("An all -fiber dense-wavelength-division mul tiplexer /demul tiplexer using photoimprinted Bragg gratings ", IEEE Photonics Technology Letters Vol. 7, no. 4, April 1995, pages 388-390) describe a device used to add and drop a single channel from/to a WDM connection with a spacing between channels of 100 GHz at 1550 nm, comprising a Mach-Zehnder interferometer made with an all-fibre technology with identical Bragg gratings in the arms of the interferometer. Thanks to the symmetry of the device, the multiplexing (signal adding) and demultiplexing (signal dropping) operations can be carried out simultaneously within the same device. However, in this case there is the problem of crosstalk between the multiplexed signal and the demultiplexed signal, unless the reflectivity of the Bragg gratings is very high. The Authors say that, in practice, the crosstalk can be minimised with gratings having a reflectivity higher than 99%, or by positioning in the arms of the interferometer an additional pair of gratings with a resonance frequency different from that of the other pair of gratings, so as to use a different wavelength for the dropped signal and for the added signal .
Turan Erdogan et al . ( "Integrated-optical Mach-Zehnder add- drop fil ter fabri cated by a single UV-induced grating exposure", Applied Optics, Vol. 36, No. 30, October 1997, pages 7838-7845) propose a method for fabricating a Mach- Zehnder interferometer with Bragg gratings by si ultaneously exposing the arms of the interferometer to obtain identical gratings in aligned positions, thus minimising the asymmetries of the optical paths. Said technology can allow avoiding the arms to be subjected to a further UV trimming for balancing the interferometer.
M. Jouanno et al . ( "Low crosstalk planar optical add-drop mul tiplexer fabricated wi th UV-induced Bragg gratings ", Electronics Letters, Vol. 33, No. 25, December 1997, pages 2120-2121) describe the performances of an add-drop device made with a silica-on-silicon technology by writing Bragg gratings on the arms of a Mach-Zehnder interferometer.
R.J.S. Pedersen et al . { "Impact of coherent crosstalk on usable bandwidth of a grating-MZI based OADM", IEEE Photonics Technology Letters, Vol. 10, No. 4, April 1998, pages 558-560) examine the influence of the "in-band crosstalk" (falling within the band of the receiver) on the performances of add-drop optical devices made of a Mach- Zehnder interferometer and Bragg gratings on the arms of the interferometer.
Takashi Mizuochi et al . ( "Interferometri c cross talk-free optical add/drop mul tiplexer using cascaded Mach Zehnder fiber gratings" , OFC 1997 Technical Digest, Wednesday Afternoon - 177, WL14) deal with the problem of crosstalk between the residual part of a signal dropped at a wavelength and a signal added at the same wavelength into a Mach Zehnder filter with Bragg gratings (or MZ-FG) on the two arms of the interferometer. The Authors say that, for obtaining a power loss smaller than 0.1 dB the crosstalk must be kept below -40 dB. Since it is difficult to obtain an MZ-FG with such a low level of crosstalk, a device is proposed comprising two MZ-FG devices in cascade. However, the Applicant has noted that this solution implies a significant increase of insertion loss of the device since it requires the use of a number of devices in cascade equal to twice the number of channels with different wavelengths that have to be added and dropped. T. Mizuochi et al . ( "Jnt erf erome tri Crosstalk- free optical add/drop mul tiplexer using Mach-Zehnder -Based fiber gratings ", Journal of Lightwave technology, vol. 16, no. 2, February 1998, pages 265-276) propose two methods for reducing the problem of crosstalk present on the device of the Mach Zehnder interferometer type with Bragg gratings. The Authors describe that, when the dropped channel and the added channel have the same wavelength ( "wavelength reuse scheme") , an interferometric beat noise is generated in the photo-receivers of these channels respectively due to the crosstalk between the dropped channel and the leakage light of the added channel, and to the crosstalk between the added channel and the leakage light of the dropped signal, since the reflectivity of actual (not ideal) Bragg gratings is lower than 100%. In addition, they state that, for guaranteeing a power loss of 0.1 dB the crosstalk must be smaller than -35 dB and the reflectivity of the Bragg gratings must be greater than 99, 97%. Since it is very difficult to obtain a grating with such high values of reflectivity, the Authors propose two methods for solving the problem of crosstalk. The first method consists of using a drop channel and an add channel having a spacing in frequency equal to twice the bit rate of transmission. The second method consists of using two devices in cascade.
The Applicant has noted that the first method requires a precise control of the channel spacing in frequency and a difficult realisation of a spectrum profile of the gratings. Thus, said solution is complex, difficult and expensive to implement. On the contrary, the second method implies a significant increase of insertion loss of the device since it requires the use of a number of devices equal' to twice the number of channels with different wavelengths that have to be added and dropped.
The Applicant faced the problem of reducing the crosstalk of an optical device for adding-dropping channels from a
WDM optical signal while reducing at the same time the insertion losses.
In the present description and following claims, the expression
- "insertion losses" refers to the difference between the optical power in output from an optical device and the optical power in input to the same, when the values of said optical powers are expressed in dB;
- "crosstalk" refers to the difference between the power of a noise component and the power of an optical signal, when the values of said optical powers are expressed in dB (for example, on the output of an optical -signal add-drop device, the noise component can be the possible residual component of a not perfectly dropped or added signal) ;
- "in-band crosstalk" refers to the optical signal and the noise component having the same wavelength;
- "out-of-band crosstalk" refers to the optical signal and the noise component having different wavelengths.
In a first aspect thereof, the present invention relates to an optical apparatus for adding-dropping signals from a WDM optical signal comprising a plurality of N signals having wavelengths λl , λ2 ... λN, said apparatus comprising
- a first optical device comprising an input for said WDM optical signal, a first output and a second output, said first device being adapted to drop from said WDM optical signal at least one signal at a wavelength λx, preselected among said wavelengths λl, λ2 ... λN, to send said at least one signal dropped at wavelength λx to said first output and to send said WDM optical signal to said second output;
a second optical device comprising a first input in communication with said second output of said first optical device, a second input, a first output, a second output and a portion of waveguide, between said first input and said second input, housing a wavelength selective element, said wavelength selective element being adapted to
• drop, from said WDM optical signal, a possible residual component of said at least one signal at wavelength λx, and
* add to said WDM optical signal at least one add signal having a wavelength λ^x substantially equal to said wavelength λx, coming from said second input;
characterised in that
• said wavelength selective element of said second optical device is also adapted to drop, from said WDM optical signal, at least one signal at a wavelength λy, preselected among said wavelengths λl, λ2 ... λN,
• said second optical device is adapted to send the signal dropped at the wavelength λy to said first output, and said WDM optical signal to said second output;
and in that it also comprises
• a third optical device having a first input in communication with said second output of said second optical device, a second input and an output, said third device being adapted to drop from said WDM optical signal a possible residual component of said at least one signal at wavelength λy; to receive from said second input at least one add signal having a wavelength λ y substantially equal to said wavelength λy and add it to said WDM optical signal; and to send said WDM optical signal to said output.
The optical apparatus of the invention advantageously allows reducing the crosstalk between two possible residual components of two signals dropped at wavelengths λx and λy and two signals added at substantially the same wavelength, reducing the insertion losses of the WDM signal passing through them.
In fact, since the dropping of the possible residual component of the signal at wavelength λx, the dropping of the signal at wavelength λy and the adding of the signal at wavelength λx are carried out by the wavelength selective element into the same portion of waveguide of the second optical device, it allows using, for adding/dropping two optical signals, only three add/drop optical devices in series. More in particular, it allows using a limited number of passive optical components such as, for example, optical couplers and/optical circulators, which would otherwise introduce undesired additional losses to the WDM optical signal.
In general, the optical apparatus of the invention allows reducing the crosstalk, at the same time limiting the insertion losses on the WDM optical signal, using only a number of add/drop optical devices equal to the number of signals at different wavelengths that have to be added and/or dropped, increased by one.
Typically, although generated by two light sources having the same nominal emission wavelength, two optical signals have two central wavelengths slightly different from one another. In the present description and following claims, when reference is made to an optical signal having a wavelength λ1 substantially equal to the wavelength λ of another optical signal, it means that λ1 and λ are nominally equal according to the standard defined by the ITU-T Standards.
Advantageously, said first optical device comprises at least one optical filter FI. Said at least one optical filter FI is advantageously adapted to reflect at least said wavelength λx and to let the other wavelengths of said WDM optical signal pass (or vice versa) so that at least the signal at wavelength λx of said WDM optical signal is sent to said first output while the other signals of said WDM optical signal are sent to said second output of said first optical device. Typically, said at least one optical filter FI has a reflection (or transmission) spectrum comprised within a predetermined band of wavelengths Δλx centred about said λx. Advantageously, said predetermined band of wavelengths Δλx has such a width as to allow isolating the signal at wavelength λx from the other signals of said WDM optical signal, according to the system requirements. Preferably, the width of said predetermined band of wavelengths Δλx is smaller than twice the spacing between the channels associated to said optical signals at wavelengths λl , λ2 .... λN.
Typically, said spacing between the channels is smaller than or equal to 200 GHz. Preferably, the spacing between the channels is smaller than or equal to 100 GHz. More preferably, smaller than or equal to 50 GHz. Even more preferably, smaller than or equal to 25 GHz.
Preferably, said at least one optical filter FI is a conventional Bragg grating. According to a variant, it is a conventional interferential filter or a conventional Fabry- Perot resonator.
Advantageously, said first optical device also comprises a first portion of optical waveguide which is in communication with said input and said first and second output of said first optical device. Preferably, said first portion of optical waveguide is a portion of optical fibre. More preferably, it is a portion of a conventional single- mode optical fibre. Said first portion of optical waveguide is advantageously adapted to house said at least one optical filter FI .
According to an embodiment, said first optical device also comprises an input optical coupler having a first and a second port respectively corresponding to said input and to said first output of said first optical device, and a third port connected to a first end of said first portion of optical waveguide. Preferably, said first optical device also comprises an output optical coupler having a first port connected to a second end of said first portion of optical waveguide and a second port corresponding to said second output of said first optical device. Advantageously, said optical device also comprises a second portion of optical waveguide connected, at a first end, to a fourth port of said input coupler, and at the opposed end, to a third port of said output coupler. Said second portion of optical waveguide is advantageously adapted to house at least a second optical filter FI . Advantageously, said first optical device, comprising said input coupler, said first and second optical waveguides, said optical filters FI and said output coupler, is a conventional add/drop device made up of a Mach-Zehnder interferometer with an optical filter on each arm of the interferometer as described, for example, in the US Patent 4 900 119.
As regards the characteristics of said at least one second optical filter FI and of said second portion of optical waveguide, reference shall be made to what previously described with reference to said at least one optical filter FI and to said first portion of optical waveguide, respectively .
According to an alternative embodiment, besides said at least one optical filter FI and said first portion of optical waveguide, said first optical device also comprises an optical circulator having a first and a second port respectively corresponding to said input and to said first output of said first optical device and a third port connected to an end of said first portion of optical waveguide, the opposed end of said portion of optical waveguide being said second output port of said first optical device.
Typically, said wavelength selective element of said second optical device is an optical filter F2. Said one optical filter F2 is advantageously adapted to reflect at least said wavelengths λx and λy and to let the other wavelengths of said WDM optical signal pass (or vice versa) .
Advantageously, said one optical filter F2 has a reflection
(or transmission) spectrum comprised within two predetermined bands of wavelengths Δλx and Δλy respectively centred about said λx and λy. Advantageously, each of said predetermined bands of wavelengths Δλx and Δλy have such a width as to allow respectively isolating the signals at wavelength λx and λy from the other signals of said WDM optical signal, according to the system requirements. Preferably, the width of said predetermined bands of wavelengths Δλx and Δλy is smaller than twice the spacing between the channels associated to said optical signals at wavelengths λl , λ2 .... λN.
Preferably, said optical filter F2 is made up of two conventional Bragg gratings. According to a variant, it is made up of two conventional interferential filters or two conventional Fabry-Perot resonators.
Advantageously, said wavelengths λx and λy are two adjoining wavelengths between said wavelengths λl, λ2 .... λN. In this case, said at least one optical filter F2 advantageously has a reflection (or transmission) spectrum comprised within a predetermined band of wavelengths Δλxy centred about the mean wavelength λm between said λx and said λy [λm= (λx+λy) / 2] . Advantageously, said band of wavelengths Δλxy has such a width as to allow isolating the signals at wavelengths λx and λy from the other signals of said WDM optical signal, according to the system requirements. Preferably, the width of said predetermined band of wavelengths Δλxy is smaller than twice the spacing between the channels associated to said optical signals at wavelengths λl , λ2 .... λN.
In a case as the latter, said optical filter F2 is preferably made up of a single Bragg grating. According to a variant, it is made up of a single interferential filter or a Fabry-Perot resonator. Preferably, said portion of optical waveguide of said second optical device is a portion of optical fibre. More preferably, it is a portion of a conventional single-mode optical fibre.
According to an embodiment, said second optical device also comprises an input optical coupler having a first and a second port respectively corresponding to said first input and to said first output of said second optical device, and a third port connected to a first end of said portion of optical waveguide. Preferably, said second optical device also comprises an output optical coupler having a first port connected to a second end of said portion of optical waveguide, a second port corresponding to said second output of said second optical device, and a third port corresponding to said second input of said second optical device. Advantageously, said second optical device also comprises a second portion of optical waveguide connected, at a first end, to a fourth port of said input coupler, and at the opposed end, to a third port of said output coupler. Said second portion of optical waveguide is advantageously adapted to house at least a second optical filter F2. Advantageously, said second optical device, comprising said input coupler, said first and second optical waveguides, said optical filters F2 and said output coupler, is an add/drop device made up of a Mach-Zehnder interferometer with a suitable optical filter on each arm of the interferometer .
As regards the characteristics of said at least one second optical filter F2 and of said second portion of optical waveguide, reference shall be made to what previously described with reference to said at least one optical filter F2 and to said first portion of waveguide of said second optical device.
According to an alternative embodiment, besides said at least one optical filter F2 and said first portion of optical waveguide, said second optical device also comprises an optical circulator having a first and a second port respectively corresponding to said first input and to said first output of said second optical device, and a third port connected to a first end of said portion of optical waveguide. Preferably, this embodiment of said second optical device also comprises an output optical circulator having a first port connected to a second end of said portion of optical waveguide, and a second and a third port respectively corresponding to said second input and to said second output of said second optical device.
Typically, said third optical device comprises at least one optical filter F3. Said at least one optical filter is advantageously adapted to reflect said wavelength λx and to let the other wavelengths of said WDM optical signal pass (or vice versa) .
Advantageously, said at least one optical filter F3 has a reflection (or transmission) spectrum comprised within a predetermined band of wavelengths Δλy centred about said λy. Advantageously, said predetermined band of wavelengths Δλy has such a width as to allow isolating the signal at wavelength λy from the other signals of said WDM optical signal, according to the system requirements. Preferably, the width of said predetermined band of wavelengths Δλy is smaller than twice the spacing between the channels associated to said optical signals at wavelengths λl, λ2 .... λN.
Preferably, said optical filter F3 is a conventional Bragg grating. According to a variant, it is a conventional interferential filter or a conventional Fabry-Perot resonator.
Advantageously, said third optical device also comprises a first portion of optical waveguide which is in communication with said first and second inputs and with said output of said second optical device. Preferably, said first portion of optical waveguide is a portion of optical fibre. More preferably, it is a portion of a conventional single-mode optical fibre. Said first portion of optical waveguide is advantageously adapted to house said at least one optical filter F3.
According to an embodiment, said third optical device also comprises an output optical coupler having a first port connected to a first end of said first portion of optical waveguide, a second port corresponding to said second input of said third optical device, and a third port corresponding to said output of said third optical device. Preferably, said third optical device also comprises an input optical coupler having a first port corresponding to said first input of said third optical device, and a second port connected to a second end of said first portion of waveguide. More preferably, said third optical device also comprises a second portion of optical waveguide connected, at a first end, to a third port of said input coupler, and at the opposed end, to a fourth port of said output coupler. Said second portion of optical waveguide is advantageously adapted to house at least a second optical filter F3. Advantageously, said third optical device, comprising said input coupler, said first and second optical waveguides, said optical filters F3 and said output coupler, is a conventional add/drop device made up of a Mach-Zehnder interferometer with an optical filter on each arm of the interferometer.
As regards the characteristics of said at least one second optical filter F3 and of said second portion of optical waveguide, reference shall be made to what previously described with reference to said at least one optical filter F3 and to said first portion of waveguide.
According to an alternative embodiment, besides said at least one optical filter F3 and said first portion of optical waveguide, said third optical device also comprises an optical circulator having a first port connected to an end of said portion of waveguide and a second and third port respectively corresponding to said second input and to said output of said third optical device, the other end of said portion of waveguide being said first input of said third optical device.
Advantageously, the optical apparatus of the invention also comprises a first optical switch having a first input port connected to said first output of said first optical device, a second input port to receive said at least one add signal having said wavelength λ^, a first output port connected to said second input of said second optical device, and a second output port. When it is in a "bar" status, said first optical switch is adapted to connect its first input port to its first output port, and its second input port to its second output port. On the contrary, when it is in a "cross" status, it is adapted to connect the first input port to the second output port and the second input port to the first output port .
This advantageously allows re-adding the signal dropped at the wavelength λx into the WDM optical signal when the first optical switch is in the bar status, and adding the add signal having said wavelength λ^ into the WDM signal when the said switch is in the cross status .
Typically, said first optical switch is a conventional optomechanical or thermal-optical device.
Advantageously, the optical apparatus of the invention also comprises a second optical switch having a first input port connected to said first output of said second optical device, a second input port to receive said at least one add signal having said preselected wavelength λ y, a first output port connected to said second input of said third optical device, and a second output port. When it is in a "bar" status, said second optical switch is adapted to connect its first input port to its first output port, and its second input port to its second output port . On the contrary, when it is in the "cross" status, it is adapted to connect the first input port to the second output port and the second input port to the first output port .
In this way, when the second optical switch is in the bar status, the signal dropped at said wavelength λy is re- added to the WDM optical signal, while when it is in a cross status, the add signal having said preselected wavelength λxy is added to the WDM optical signal .
Typically, also said second optical switch is a conventional optomechanical or thermal-optical device.
In a second aspect thereof, the present invention relates to a WDM optical communication system comprising:
* a transmission station adapted to provide a WDM optical signal comprising a plurality of N signals having wavelengths λl , λ2 .... λN;
* an optical transmission line, optically connected to said transmitting station, to transmit said WDM optical signal;
* a reception station, optically connected to said optical transmission line, to receive said WDM optical signal;
* at least one optical apparatus for adding/dropping signals from said WDM optical signal, comprising in turn
- a first optical device comprising an input for said WDM optical signal, a first output and a second output, said first device being adapted to drop from said WDM optical signal at least one signal at wavelength λx, preselected among said wavelengths λl, λ2 .... λN, sending said at least one signal dropped at wavelength λx to said first output, and sending said WDM optical signal to said second output;
- a second optical device comprising a first input in communication with said second output of said first optical device, a second input, a first output, a second output and a portion of waveguide, between said first input and said second input, housing a wavelength selective element, said wavelength selective element being adapted to
• drop, from said WDM optical signal, a possible residual component of said at least one signal at wavelength λx, and
• add to said WDM optical signal at least one add signal having a wavelength λ^ substantially equal to said wavelength λx, coming from said second input;
characterised in that
• said wavelength selective element of said second optical device is also adapted to drop, from said WDM optical signal, at least one signal at a wavelength λy, preselected among said wavelengths λl , λ2 ... λN,
• said second optical device is adapted to send the signal dropped at the wavelength λy to said first output, and said WDM optical signal to said second output;
and in that said optical apparatus also comprises
• a third optical device having a first input in communication with said second output of said second optical device, a second input and an output, said third device being adapted to drop from said WDM optical signal a possible residual component of said at least one signal at wavelength λy; to receive from said second input at least one add signal having a wavelength λxy substantially equal to said wavelength λy and add it to said WDM optical signal; and to send said WDM optical signal to said output.
Typically, said optical transmission line comprises at least one intermediate station (or line site) . Advantageously, said intermediate station comprises an amplification section. More advantageously, said amplification section comprises at least one optical amplifier.
Preferably, said at least one optical apparatus is comprised into said intermediate station. According to a variant, it is comprised into said transmission station or into said reception station.
As regards the functional and structural characteristics of said optical apparatus, reference shall be made to what has already been said with reference to the apparatus of the invention.
In a third aspect thereof, the present invention relates to a method for adding-dropping signals from a WDM optical signal comprising a plurality of N signals having wavelengths λl , λ2 .... λN, said method comprising the steps of
a) dropping, in a first optical device, at least one signal at wavelength λx, preselected among said wavelengths λl , λ2 .... λN, from said WDM optical signal;
b) dropping, in a portion of waveguide of a second optical device, a possible residual component of said at least one signal at wavelength λx from said WDM optical signal;
c) adding, in said portion of waveguide, at least one add signal having wavelength λ^x substantially equal to said wavelength λx, into said WDM optical signal,
characterised in that it further comprises the following steps
d) dropping, in said portion of waveguide of said second optical device, at least one signal at wavelength λy, preselected among said wavelengths λl, λ2 .... λN, from said WDM optical signal;
e) dropping, in a third optical device, a possible residual component of said at least one signal at wavelength λy from said WDM optical signal;
f) adding, in said third optical device, at least one add signal having wavelength λ^y substantially equal to said wavelength λy, into said WDM optical signal . Preferably, the step a) is carried out through an optical filter FI adapted to reflect at least said wavelength λx and to let the other wavelengths of said WDM optical signal pass (or vice versa) .
Advantageously, the steps b) , c) and d) are carried out through an optical filter F2 adapted to reflect at least said wavelengths λx and λy and to let the other wavelengths of said WDM optical signal pass (or vice versa) .
Preferably, the steps e) and f) are carried out through an optical filter F3 adapted to reflect at least said wavelength λy and to let the other wavelengths of said WDM optical signal pass (or vice versa) .
As regards the functional and structural characteristics of said first, second and third optical devices, and of said optical filters FI , F2 and F3 , reference shall be made to what described above .
Further features and advantages of the present invention will appear more clearly from the following detailed description of some exemplifying and not limitative embodiments, made with reference to the attached drawings. In such drawings :
- Figure 1 shows a schematic view of an embodiment of an optical apparatus according to the invention;
- Figure 2 shows a schematic view of a preferred embodiment of the optical apparatus of figure 1 ;
- Figure 3 schematically shows three reflection spectrums of optical filters that can be used on the optical apparatus of figure 2 ;
- Figure 4 shows a schematic view of a second preferred embodiment of an optical apparatus of the invention;
- Figure 5 shows a schematic view of another embodiment of the optical apparatus of figure 1; Figure 6 shows a schematic view of an optical communication system according to the present invention;
- Figure 7 shows a schematic view of a graph of spectrum emission of optical amplifiers usable on the system of figure 6;
- Figure 8 shows a schematic view of part of a transmitting station of the system of figure 6;
- Figure 9 shows a measurement bench used in experimental tests carried out to check the performances of an optical apparatus of the invention;
- Figure 10 shows the spectral responses obtained between the input and the drop port of each optical device of the apparatus of figure 2 ;
- Figure 11 shows the spectral responses obtained between the input and the output of each optical device of the apparatus of figure 2;
- Figure 12 shows the spectral response obtained between the input and the output of the apparatus of figure 2 , without adding channels, (curve A) and the spectrum of losses in function of the polarisation - PDL - (curve B) ;
- Figure 13 shows the spectral responses obtained between the input of the first optical device and the output of the second optical device by adding the signal at wavelength λx (curve A) and by adding the signal at wavelength λy (curve B) ;
- Figure 14 shows the spectral responses obtained between the input and the output of the first optical device (curve A) , between the input and the output of the second optical device (curve B) , and between the input of the first optical device and the output of the second optical device (curve C) ;
- Figure 15 shows the spectral responses obtained between the input and the output of the optical apparatus of figure 2 (curve A) , between the input and the output of the third optical device (curve B) , and between the input of the first optical device and the output of the second optical device (curve C) .
Figure 1 shows a schematic view of an embodiment of an optical apparatus 4 of the invention, comprising a first optical device 1, a second optical device 2 and a third optical device 3.
The first optical device 1 has an input 11, a first output 12 and a second output 13.
The second optical device 2 has a first input 21, a second input 22, a first output 23 and a second output 24.
The third optical device 3 has a first input 31, a second input 32 and an output 33.
The second output 13 of the first optical device 1 is connected to the first input 21 of the second optical device 2, and the second output 24 of the second optical device 2 is connected to the first input 31 of the third optical device 3.
The optical apparatus 4 of figure 1 is adapted to drop from a WDM optical signal 1000, comprising a plurality of N signals having wavelengths λl, λ2 .... λN, two signals having wavelengths λx and λy, preselected among λl, λ2 .... λN, and to add to it two add signals having wavelengths λxx and λ y substantially equal to λx and λy.
The N optical signals are associated to N channels having a predetermined spacing or band Δλch.
The first optical device 1 is adapted to drop from said optical signal 1000, which enters into apparatus 4 through input 11, a signal at wavelength λx, to send it to output
12 and to send the remaining signals of the WDM optical signal 1000 to output 13.
In turn, the optical device 2 is adapted to receive, at its first input 21, the WDM optical signal coming from the first device 1, to eliminate a possible residual component of the signal at wavelength λx, to drop from it a signal at wavelength λy, to send the possible residual component at wavelength λx and the signal dropped at wavelength λy to output 23, to receive at its second input 22 the add signal having wavelength λ^x, to add said add signal to the WDM optical signal 1000 and to send the WDM optical signal 1000, thus modified, to the second output 24.
Finally, the optical device 3 is adapted to receive, at its first input 31, the WDM optical signal coming from the second device 2, to eliminate a possible residual component of the optical signal at wavelength λy, to receive at its second input 32 the add signal having wavelength λ^y , to add said add signal to the WDM optical signal 1000, and to send the WDM optical signal 1000, from which the signals at wavelengths λx and λy have been dropped, and to which said add signals at wavelengths λxx λ y have been added, to output 33.
Figure 2 shows a preferred embodiment of the apparatus of figure 1, wherein the optical devices 1, 2 and 3 comprise each a conventional Mach-Zehnder interferometer with Bragg gratings (the operation of which is described, for example, in the already mentioned US Patent 4 900 119 herein included as reference) .
Advantageously, said interferometers are made with an all- fibre technology, and they are each made up of only two portions of optical fibre reciprocally connected and treated so as to obtain the structure shown in figure 2. For example, the Bragg gratings are obtained in the portions of fibre by inserting both arms of the interferometer into the same phase mask adapted to write the gratings on the fibres, so as to obtain a Bragg grating on both arms .
More in particular, the Mach-Zehnder interferometer of the first optical device 1 comprises an input coupler 17, an upper arm and a lower arm respectively comprising a- first and a second portion of optical fibre 15 and 16, and an output coupler 18. The input coupler 17 is provided with a first and a second port respectively corresponding to input 11 and to the first output 12, and a third and a fourth port respectively connected to two ends of said portions of optical fibre 15 and 16. In turn, the output coupler 18 is provided with four ports of which two are connected to two ends of said portions of optical fibre 15 and 16, one corresponds to the output port 21 and the other one is unused. Additionally, the portions of optical fibre 15 and 16 respectively house two optical filters FI 14 and 19. Advantageously, the two optical filters FI 14 and 19 are two substantially equal Bragg gratings having a reflection spectrum comprised in a band of wavelength Δλx centred about λx (curve A of figure 3) .
In the embodiment shown, said band of wavelengths Δλx has a width substantially equal to said spacing between channels Δλch.
In turn, the Mach-Zehnder interferometer of the second optical device 2 comprises an input coupler 27, an upper arm and a lower arm respectively comprising a first and a second portion of optical fibre 25 and 26, and an output coupler 28. The input coupler 27 is provided with four ports, two of which respectively correspond to input 21 and to the first output 23, and the other two are respectively connected to two ends of said portions of optical fibre 25 and 26. In turn, the output coupler 28 is provided with four ports of which two are connected to two ends of said portions of optical fibre 25 and 26, and two respectively correspond to the first input 22 and to the second output 24. Additionally, the portions of optical fibre 25 and 26 respectively house two optical filters F2 29 and 30. Advantageously, the two optical filters F2 29 and 30 are substantially equal, and each of them comprises two Bragg gratings having two reflection spectrums respectively comprised in a band of wavelength Δλx centred about λx and in a band of wavelength Δλy centred about λy (respectively, curves A and C of figure 3) .
In the embodiment shown, also said band of wavelengths Δλy has a width almost equal to said spacing between channels Δλch.
The unused ports of the output coupler 18 of the first optical device 1 and of the input coupler 37 of the third optical device 3 are optically terminated so as to prevent back reflections (that is to say, they are adapted in a conventional way) .
According to a preferred embodiment, when λx and λy are two adjacent wavelengths among λl, λ2 .... λN, the two optical filters F2 29 and 30 comprise a single Bragg grating each, having a reflection spectrum comprised in a band Δλxy centred about the mean wavelength λm between λx and λy [λm= (λx+λy) / 2] , and having a width almost equal to the sum of Δλx and Δλy (curve B of figure 3) . The use of a single Bragg grating, with wider reflection band, in each arm of the interferometer advantageously allows making the optical device 2 easier and less expensive to be manufactured since said gratings can be obtained by means of a single step of writing on fibre.
Finally, the Mach-Zehnder interferometer of the third optical device 3 comprises an input coupler 37, an upper arm and a lower arm respectively comprising a first and a second portion of optical fibre 35 and 36, and an output coupler 38. The input coupler 37 is provided with four ports, one of which corresponds to the first input 31, two are respectively connected to two ends of said portions of optical fibre 35 and 36, and the other one is unused. In turn, the output coupler 38 is provided with four ports of which two are connected to two ends of said second input 32 and to the output port 33. Additionally, the portions of optical fibre 35 and 36 respectively house two optical filters F2 34 and 39. Advantageously, the two optical filters F2 34 and 39 are substantially equal, and each of them comprises a Bragg grating having a reflection spectrum comprised in a band of wavelengths Δλy (curve C of figure 3) .
Advantageously, couplers 17, 18, 27, 28, 37 and 38 are 3 dB directional couplers. In addition, the two arms of each Mach-Zehnder interferometer are preferably balanced. That is to say, the two arms substantially have the same optical path length.
When the WDM optical signal 1000 enters into the first optical device 1 through input 11, its optical power is divided into two substantially equal parts by the input coupler 17, and is sent to the two optical fibres 15, 16 where the two optical filters FI 14 and 19 reflect the signal at wavelength λx back. In this way, the non- reflected signals of the WDM optical signal 1000 travel towards the output coupler 18 and exit from the first optical device 1 through the second output 13, while the optical signal at wavelength λy, reflected backwards, returns towards coupler 17 and exits from the first device 1 through the first output 12. In fact, as known, when the optical signal 1000 enters into the Mach-Zehnder interferometer through input 11, its two power components that propagate through the optical fibres 15 and 16 interfere constructively into output 13 and destructively into the other unused port of the output coupler 18. In addition, the power components of the signal at wavelength λx, that are reflected backwards by the optical filters FI 14 and 19, interfere constructively into output 12 and destructively into input 11 of the optical device 1. This is due to the fact that, as known, an optical signal which goes through a 3 dB optical directional coupler passing from a portion of fibre to the other of the coupler is subject to a 90° phase shift with respect to the optical signal that goes through it but remains into the same portion of fibre.
Thus, when the WDM optical signal 1000 enters from input 11 of the first optical device 1, the signal having wavelength λx is dropped from the first output 12 whereas the other signals exit from the second output 13 to enter into the second optical device 2 through its first input 21.
In the second optical device 2, the power of the WDM optical signal 1000 is divided into two substantially equal parts by the input coupler 27, and is sent to the two optical fibres 25, 26 where the two optical filters F2 30 and 29 reflect the possible residual component of the optical signal at wavelength λx and the optical signal at wavelength λy back. In this way, the non-reflected signals of the WDM optical signal 1000 travel towards the output coupler 28 and, in a similar way to what described above with reference to the first optical device 1, exit from the second optical device 2 through the second output 24. In turn, the possible residual component of the optical signal at wavelength λx and the optical signal at wavelength λy return towards coupler 27 and exit from the second optical device 2 through the first output 23.
Thus, in the second optical device 2 both the signal at wavelength λy and the possible residual component of the signal at wavelength λx, coming from the first optical device 1, are dropped from the first output 23. On the contrary, the remaining signals of the WDM optical signal 1000 exit from the second output 24 to enter into the third optical device 3 through its first input 31.
The Applicant has noted that, although in output 23 both the signal at wavelength λy and the possible residual component of the signal at wavelength λx are dropped, the drop signal of interest at wavelength λy can be easily separated in reception from the undesired residual component by means, for example, of a suitable optical filtering to eliminate said noise component at wavelength λx.
In addition, should the signal at wavelength λy coming from output 23 be re-added to the WDM optical signal 1000 through input 32 (as it will be described hereafter) , it would be reflected backwards by the optical filters F3 34 and 39 and, then, re-added to the WDM optical signal 1000, whereas the possible residual component at wavelength λx would be let to pass by said filters F3 34 and 39 so as to arrive to the unused port of the input coupler 37, where it would be eliminated.
In the third optical device 3, the power of the WDM optical signal 1000 arriving to the first input 31 is divided into two substantially equal parts by the input coupler 37, and is sent to the two optical fibres 35, 36 where the two optical filters F3 35 and 39 reflect the possible residual component of the optical signal at wavelength λy back. In this way, the non-reflected signals of the WDM optical signal 1000 travel towards the output coupler 38 and, in a similar way to what described above with reference to the first optical device 1, exit from the third optical device
3 through output 33. In turn, the reflected residual component returns backwards coupler 37, where it is eliminated through the unused port of the input coupler 37.
In addition, when the add signal at wavelength λxx is sent to the second input 22 of the second optical device 2, its optical power is divided into two substantially equal parts by the output coupler 28, and is sent to the two optical fibres 25, 26. The two optical filters F2 30 and 29 reflect the two parts of optical power of said signal backwards to the output coupler 28, and said output coupler 28 recombines them into the second output 24, thus adding the signal at wavelength λxx to the WDM optical signal 1000. In turn, when the add signal at wavelength λxy is sent to the second input 32 of the third optical device 3, its optical power is divided into two substantially equal parts by the output coupler 38, and is sent to the two optical fibres 35, 36. The two optical filters F3 34 and 39 reflect the two parts of optical power of said signal backwards to the output coupler 38, and said output coupler 38 recombines them into output 33, thus adding also the signal at wavelength λxy to the WDM optical signal 1000.
In the optical apparatus 4, the dropping of the possible residual component at wavelength λx coming from the first optical device 1, the addition of the signal at wavelength λxx and the dropping of the signal at wavelength λy are carried out by the same add/drop optical device (second optical device 2) by using a suitable optical filter F2 on the two arms of the Mach-Zehnder interferometer.
Thus, the optical apparatus 4 allows reducing the crosstalk between possible residual components of the signals dropped at wavelengths λx and λy and the signals respectively added at wavelengths λxx and λ^, using only three optical devices in cascade and, thus, limiting the insertion losses of the WDM optical signal 1000 passing through them. In fact, a limited number of optical devices in cascade causes a reduction in the number of passive optical elements such as for example, optical couplers and/or optical circulators, which would otherwise introduce undesired additional insertion losses on the WDM optical signal.
In addition, the optical apparatus 4 allows eliminating the crosstalk between a dropped signal and the possible residual component of an add signal at the same wavelength since the possible residual component exits from the optical apparatus 4 from another output than that from which the dropped signal exits. For example, in the embodiment of figure 2, the signal dropped at wavelength λx exits from output 12 whereas the possible residual component of the signal added at wavelength λxx (added from input 22) exits from output 23.
Resuming the description of the embodiment of figure 2, the optical apparatus 4 of the invention also comprises a first and a second switch 5 and 6. Typically, they are conventional optomechanical or thermal-optical devices.
The first optical switch 5 is provided with a first input port connected to the first output 12 of said first optical device 1, a second input port for receiving the add signal having said wavelength λxx, a first output port connected to the second input 22 of the second optical device 2, and a second output port .
In turn, the second optical switch 6 is provided with a first input port connected to the first output 23 of said second optical device 2, a second input port for receiving the add signal having said wavelength λ^y, a first output port connected to the second input 32 of the third optical device 3, and a second output port.
Said switches are adapted to operate according to one of two "bar" and "cross" status. When they are in a "bar" status, they connect their first input port to their first output port, and their second input port to their second output port. On the contrary, when they are in a "cross" status, they connect their first input port to their second output port, and their second input port to their first output port .
Said switches advantageously allow making the optical apparatus 4 of the invention re-configurable. In fact, when they are in the bar status, said switches allow re-adding the dropped signal to the WDM optical signal 1000, whereas when they are in the cross status, they allow adding the add signal, having a different information content from that of the dropped signal, to the WDM optical signal 1000.
According to another embodiment (not shown) of the optical apparatus 4 of the invention, the input and output ports of the first and/or second optical switch 5 and 6 are connected to the ports of the optical devices 1, 2, 3 in an opposed way with respect to that shown in figure 2. In this case, when they are in a cross status, the switches allow re-adding the dropped signal to the WDM optical signal 1000, whereas when they are in the bar status, they allow adding the add signal, having a different information content from that of the dropped signal, to the WDM optical signal 1000.
In a variant (not shown) , the optical apparatus 4 also comprises an insulator in correspondence with input 11. Said insulator allows suppressing possible undesired components of light that return back to said input due to reflections inside apparatus 4.
In addition, a suitable optical filter (not shown) can advantageously be provided at output 23 of the second optical device 2, said filter being adapted to eliminate the noise component at wavelength λx.
Figure 4 shows an embodiment of an optical apparatus 4 of the invention adapted to drop three signals at wavelengths λx, λy and λz from the WDM optical signal 1000, and to add three signals having wavelengths λxx, λxy and λxz substantially equal to λx, λy and λz and an information content different from that of the drop signals.
Said embodiment is completely similar to that of figure 2, with the exception that it also comprises an intermediate device 2' and a third optical switch 7, and that the filters F3 of the third optical device 3 (in figure 4 referred to with reference numeral 3') are adapted to reflect a band of wavelengths Δλz, centred about wavelength λz, instead of the above band of wavelengths Δλy.
The intermediate optical device 2' is completely similar to the optical device 2 with the exception that each optical filter F2 ' has a reflection spectrum comprised in two bands of wavelengths Δλy and Δλz, respectively centred about wavelengths λy and λz, or in a band of wavelengths Δλyz, centred about (λy + λz) / 2 and having width equal to about Δλy + Δλz .
Thus, as regards to the structural and functional characteristics of said intermediate optical device 2', reference shall be made to what has previously been said with reference to the second optical device 2.
Also as regards switch 7, its structural and functional characteristics are totally similar to those of the two switches 5 and 6; thus, reference shall be made to what has already been described.
In the case of adding and dropping of M signals (with M < N) to/from the WDM optical signal 1000, the optical apparatus 4 of the invention comprises, in a way completely similar to the embodiment of figure 4, a first optical device 1, M-l intermediate optical devices 2 and 2', and a third optical device 3', connected in series to one another.
Thus, the optical apparatus 4 of the invention allows filtering each optical signal to be dropped from the WDM optical signal twice, and then adding to it a signal having substantially the same wavelength, using a limited number of optical devices arranged in series. In fact, apparatus 4 only requires M+1 optical devices connected in series for adding and/or dropping M optical signals.
In other words, in the photo-receivers used for receiving the signals of the WDM optical signal, the optical apparatus of the invention allows reducing the crosstalk between an added signal and the possible residual component of a signal dropped at the same wavelength, limiting insertion losses of the WDM optical signal passing through it.
In addition, the optical apparatus of the invention has the advantage of making the WDM optical communication system on which it is used extremely flexible. In fact, an apparatus of the invention (or a plurality of apparatuses arranged in series) can be easily added to the WDM system and, in case, removed, added to and/or replaced by another optical apparatus of the invention, adapted to add and/or drop different signals, in case it is not necessary anymore to add and/or drop some signals to said system, and/or it is necessary to add and/or drop new signals to it.
In the optical apparatus 4 of the invention, if there is the need of adding and/or dropping M signals from the WDM optical signal 1000, it is also possible to use a first optical device 1 with an optical filter FI adapted to reflect ml wavelengths, a third optical device 3 with an optical filter F3 adapted to reflect m2 wavelengths (with ml + m2= M) , and a second optical device 2 with an optical filter F2 adapted to reflect the M wavelengths.
In this last embodiment (not shown) , the ml and m2 dropped signals must then be conventionally demultiplexed in wavelength to be afterwards separately received by the respective photo-receivers, and the ml and m2 add signals must be conventionally multiplexed in wavelength, before being added to the apparatus of the invention.
Figure 5 shows another embodiment of the optical apparatus of figure 1. Since said embodiment has some elements in common with the embodiment of figure 2 , the same reference numerals as figure 2 have been used for these elements.
In the embodiment of figure 5, the first device 1 comprises a first three-port optical circulator 40, the portion of optical fibre 15 and the filter FI 14.
The second optical device 2 comprises a second three-port optical circulator 41, the portion of optical fibre 25, the filter F2 30 and a third three-port optical circulator 42.
Finally, the third optical device 3 comprises the portion of optical fibre 35, the optical filter F3 34 and a fourth three-port optical circulator 43.
The optical circulators 40-43 are conventional three-port optical devices wherein an optical signal can move from one port to the other only according to a predetermined direction of propagation.
The input 11 and the first output 12 of the first optical device 1 are made up of two ports of the first optical circulator 40, whereas the second output 13 is made up of an end of the portion of optical fibre 15.
The input 21 and the first output 23 of the second optical device 2 are made up of two ports of the second optical circulator 41, whereas the second input 22 and the second output 24 are made up of two ports of the third optical circulator 42.
Finally, the input 31 of the third optical device 3 is made up of an end of the portion of optical fibre 35, whereas the second input 32 and the output 33 are made up of two ports of the fourth optical circulator 43.
In this embodiment, when the optical signal 1000 enters into the first optical device 1 through input 11, it is sent along the portion of optical fibre 15 by the optical circulator 40; the optical signal at wavelength λx is reflected backwards by the optical filter FI 14, while the remaining signals of the WDM optical signal 1000 continue towards output 13 of the first optical device 1. The reflected signal returns backwards to the optical circulator 12, and exits from the first device 1 through the first output 12.
In turn, in the second optical device 2, the WDM optical signal 1000 coming from output 13 of the first device enters from input 21, passing through the optical circulator 41; the possible residual component of the signal at wavelength λx and the signal at wavelength λy are reflected backwards, while the remaining signals propagate towards the third optical circulator 42. The possible residual component of the signal at wavelength λx and the signal at wavelength λy return backwards to the optical circulator 41 and exit from the second optical device 2 through the first output 23. On the contrary, the remaining signals of the WDM optical signal 1000 recombine into the optical circulator 42 with the add signal at wavelength λ x (coming from input 22) and exit with it from the second optical device 2 through output 24.
Finally, in the third optical device 3, the WDM optical signal 1000 coming from output 24 of the second device 2 enters from input 31, it continues towards the portion of optical fibre 25; the possible residual component of the signal at wavelength λy is reflected backwards by the optical filter F3 34, while the other signals of the WDM optical signal 1000 continue towards the optical circulator 43. The reflected residual component returns backwards to the second optical device 2, where it is blocked by the optical circulator 42. On the contrary, the non-reflected signals of the WDM optical signal 1000 recombine into the optical circulator 43 with the add signal at wavelength λxy (coming from input 32) and exit with it from the third optical device 3 through output 33.
The optical apparatus 4 of the invention can be used on any system for transmitting or distributing WDM optical signals from/to which it is necessary to drop/add at least one optical signal .
An example of WDM optical transmission system according to the invention, comprising an optical apparatus 4 of the invention, is shown in figure 6.
Said WDM system comprises a first terminal site 100, a second terminal site 200, an optical-fibre line 300a, 300b which connects the two terminal sites, and at least one line site 400 interposed along said optical-fibre line. For sake of simplicity, the transmission system described is monodirectional , that is to say, the optical signal travels from a terminal site to the other, but the following considerations can be applied also to bi- directional systems wherein the optical signal travels in both directions.
In said example, the WDM system is adapted to transmit on a maximum of 128 channels, but the maximum number of channels can differ according to the configurations of the WDM system.
The first terminal site 100 preferably comprises a multiplexing section 110 (MUX) for a plurality of input channels 160, and a power amplification section 120 (TPA) .
The second terminal site preferably comprises a pre- amplification section 140 (RPA) and a demultiplexing section 150 (DMUX) for a plurality of output channels 170.
Each input channel 160 is received by the multiplexing section 110 - hereafter described in detail with reference to figure 8 - which groups the channels preferably into three sub-bands, hereafter referred to as BB (blue band) , RBI (red band 1) and RB2 (red band 2) . Similarly, the WDM optical transmission system can provide for the division of the above three into a greater or smaller number of sub- bands . The three sub-bands are sent to the power amplification section 120 and then, to line 300.
The power amplification section 120 preferably receives the three sub-bands separated from one another by the multiplexing section, and separately amplifies them; afterwards, it combines them with one another so as to obtain a wide band WDM signal to be sent to the transmission line 300.
The line site 400 receives the wide band WDM signal, divides it again into the three sub-bands BB, RBI and RB2 , separately amplifies the signal of the three sub-bands, drops and/or adds some channels from/to said three sub- bands by means of an optical apparatus 4 according to the invention, and recombines the three sub-bands to reconstruct the wide band WDM signal .
Further line sites 400 can be distributed along line 300 in a suitable position according to the portion of line covered up to that point every time there is the need of amplifying the WDM optical signal or, more in general, of modifying its characteristics (for example, by adding and/or dropping channels by means of an apparatus 4 according to the invention) .
The second terminal site 200 receives the wide band signal and amplifies it through the pre-amplification section 140, which preferably divides the WDM signal again into the three sub-bands BB, RBI and RB2. The demultiplexing section 150 receives the three sub-bands and divides then into single-wavelength signals 170.
The number of input channels 160 can differ from the number of output channels 170 due to the fact that in the line sites 400 some channels can be added or dropped from the optical apparatus 4 of the invention.
Figure 7 shows a graph of spectrum emission of the optical amplifiers of the system of figure 6, wherein the three sub-bands of the following example are highlighted. In particular, the first sub-band BB preferably presents signals having wavelengths comprised between 1529 nm and 1535 nm; the second sub-band RBI preferably presents signals having wavelengths comprised between 1541 nm and 1561 nm; and the third sub-band RB2 preferably presents signals having wavelengths comprised between 1575 nm and 1602 nm.
In the first sub-band 16 channels are preferably allocated; in the second sub-band, 48 channels are preferably allocated and in the third sub-band, 64 channels are preferably allocated.
Advantageously, adjacent channels in a 128-channel WDM system have a frequency spacing of 50 GHz.
As it can be noted in the graph of figure 7, unlike sub- bands RBI and RB2 for which the curve is substantially flat, in sub-band BB the spectrum presents a central peak; thus, in said sub-band the amplifiers can advantageously present an equalisation device to make the curve of spectrum emission flat in said area.
Figure 8 shows in detail the input section of the first terminal site 100. Besides the multiplexing section 110 and the amplification section 120 (not shown in figure 8) , said site comprises a line terminal section 410 (OLTE) and a wavelength conversion section 420 (WCM) .
The line terminal section 410 corresponds to a terminal apparatus, for example according to one of the standard SONET, ATM, IP or SDH, and includes transmitters/receivers in a number corresponding to the channels to be transmitted along the line. In the example described, the OLTE is provided with 128 transmitters/receivers. The OLTE transmits a plurality of channels, each one at its own wavelength.
Said wavelengths can be modified so as to make them compatible with the telecommunication system, through wavelength converters WCM1-WCM128 being part of the WCM section 420. Converters WCM1-WCM128 are adapted to receive a signal at a generic wavelength and to convert it into a signal at a predetermined wavelength, according, for example, to what described in patent US 5 267 073 entitled to the Applicant itself.
Each wavelength converter WCM preferably comprises a photodiode that converts the optical signal into an electric signal; a laser source and an electro-optical modulator, for example of the Mach-Zehnder type, to modulate the optical signal generated by the laser source at the predetermined wavelength, with the electric signal converted by the photodiode .
Alternatively, said converter can comprise a photodiode and a laser diode directly modulated by the electric signal of the photodiode so as to convert the optical signal to the predetermined wavelength.
Devices such as amplifiers, signal re-timers and/or clippers can be inserted between the photodiode and the modulator or between the photodiode and the direct- modulation laser. Additionally, the connection of an FEC
(Forward Error Connection) module of transmission is provided to add information to the temporal frame of the signal, so as to allow the receiver to correct possible errors occurred along the line, thus improving the BER.
A further alternative provides for a receiver (for example, according to one of the previously mentioned standard types) to be comprised in said converter to receive an optical signal and convert it into a corresponding electric signal; and for a laser source and an electro-optical modulator to modulate the optical signal generated by the laser source at the predetermined wavelength, with the electric signal coming from the receiver.
Wavelength converters of the type mentioned are marketed by the Applicant with the name initials of WCM, RXT, LEM.
In any case, wavelength converters or optical signal generators present in the first terminal site 100 generate respective operating optical signals having wavelengths inside respective channels comprised in the effective operating band of the amplifiers arranged in succession into the WDM system.
The multiplexing section 110 preferably comprises three multiplexers 430, 440 and 450. Preferably, for a 128- channel WDM system the first multiplexer 430 combines the signals coming from the first 16 WCM converters 1-16 so as to form the first sub-band BB; the second multiplexer 440 combines the signals coming from the WCM converters 17-64 so as to form the second sub-band RBI, and the third multiplexer 450 combines the signals coming from the WCM converters 65-128 so as to form the third sub-band RB2.
Multiplexers 430, 440 and 450 are passive optical devices, through which the optical signals transmitted on respective optical fibres are multiplexed in a single fibre; devices of this type are, for example, made up of fused fibre couplers or in planar optics, Mach-Zehnder devices, AWG, polarisation filters, interferential filters, micro-optical filters, or the like.
By way of example, a suitable combiner is the 8-WM or the 24 -WM combiner, marketed by the Applicant itself.
The amplification section 120 is capable of amplifying the signals of the sub-bands so as to raise their level up to a sufficient value to cover the following portion of optical fibre 330a existing prior to new amplifying means (comprised in the first line site 400) , keeping at the end a sufficient power level to guarantee the required transmission quality. Thus, the signals of the bands are combined with one another downstream of said power amplifier 120, through a pass-band combiner filter (not shown) , so as to be inserted in the first portion 300a of optical line, usually made up of a single-mode optical fibre inserted into a suitable optical cable which is some dozen- (or hundred-) kilometers long, for example about 100-kilometers long.
The optical fibres used for links of the type described can be optical fibres of the dispersion-shifted type.
Nevertheless, the type of fibre with step-index profile is preferable in those cases in which the prevention or reduction of non-liner effects of intermodulation between close channels is desired, as it could be important in dispersion-shifted fibres, especially if the distance between the channels themselves is very small .
Step-index fibres have a dispersion of about 17 ps/mm km at wavelengths of about 1550 nm. Smaller dispersion values, sufficient to make said intermodulation phenomena unimportant, for example from 1.5 to 6 ps/km, can be obtained with the fibres called NZD described, for example, by the ITU-T Standards G.655.
At the end of said first portion of optical line 300a there is the first line site 400, adapted to receive the multi- wavelength (or WDM) signals, attenuated in the fibre path, and to amplify them up to a sufficient level to feed them to the second portion of optical fibre 300b, having similar characteristics to those of the previous one.
Successive line amplifiers and respective portions of optical fibre, usually inserted into respective optical cables, cover the required overall transmission distance until they arrive to the second terminal station 200.
For the demultiplexing section 150, it is possible to use a component of the same type as that used in the multiplexing section 110, already described, mounted in an opposed configuration, in connection with respective pass-band filters arranged on the output fibres.
Pass-band filters of the above type are, for example, marketed by MICRON-OPTICS.
Alternatively, a demultiplexing section 150 suitable for the purpose comprises, for example, an AWG (Array Waveguide Grating) named 24WD or 8WD, marketed by the Applicant itself.
The configuration described is especially adapted for transmissions on distances of about 500 km, with a high transmission speed, for example 5 Gbit/s per channel, or higher .
In the WDM system described, the line amplifiers, conveniently made in a multistage configuration, are provided for operation at an overall output optical power of about 22 dBm.
Additionally, the power amplifier 120 can advantageously have the same configuration as the line amplifiers.
The configuration of WDM transmission system described above is especially adapted to provide the desired performances, in particular for transmissions of more channels by wavelength multiplexing, in the presence of a particular choice of the properties of the line amplifiers with which it is provided, in particular as regards the capacity of transmitting the selected wavelengths without affecting any of them with respect to the others.
In particular, it is possible to guarantee an homogenous behaviour for all channels, in the band of wavelengths comprised between 1529 and 1602 nm, or 1529 - 1535 nm, or 1542 - 1561 nm, or 1575 - 1602 nm in the presence of amplifiers adapted to operate in cascade, making use of line amplifiers provided so as to have a substantially homogenous (or "flat") response at the different wavelengths, in the cascade operation.
Typically, the amplifier configuration changes according to the bands of wavelength it must amplify.
For the purpose of checking the performance of an optical apparatus of the invention, the Applicant has carried out some experimental tests.
Said tests have been carried out on an apparatus similar to that of figure 2, adapted to drop and add two signals at the nominal wavelengths of 1549.32 nm (λx) and 1549.71 nm (λy) , belonging to the grid defined for said spacing by the above ITU-T Standard, from a WDM optical signal, having a spacing of 50 GHz between the channels.
The first, the second and the third optical device 1, 2, 3 used were Mach-Zehnder fused fibre interferometers with Bragg gratings written in a single exposure on both arms of the interferometer. Said fused fibre interferometers have the advantage of having low insertion loss, and do not introduce significant additional insertion losses in case of fused junction of the optical apparatus 4 with a standard single-mode fibre. In fact, being made with an all-fibre technology (that is to say, couplers, arms and Bragg gratings of each interferometer are made using only two optical fibres suitably treated and coupled to one another) , inside said interferometers there is no junction between the elements. The optical devices 1, 2, 3 used in the experimental tests are those produced by Amphenol Fiber Optic Products, and marketed with the name of DWDM ("Dense" WDM) Product .
The Bragg gratings FI and F3 had a reflection spectrum respectively comprised in the bands of wavelengths Δλx and Δλy, having width almost equal to the spacing between channels, and respectively centred about the wavelengths 1549.32 nm (λx) and 1549.71 nm (λy) .
In their turn, the Bragg gratings F2 had a reflection spectrum comprised in a band of wavelengths Δλy, centred about the intermediate wavelength 1549.515 nm (λm) between said (λx) and said (λy) , and having width almost equal to twice the spacing between channels.
Switches 5 and 6 were optomechanical switches available on the market as "SN Series Fiberoptic Latching Switches" , produced by JDS-Uniphase in a 2x2 configuration. They have the advantage of having low insertion loss (about -0.5 dB) and high isolation on the non-connected port (over 70 dB) . In addition, their commutation speed is equal to some milliseconds (4 ms) . Before assembly, the three devices 1, 2, 3, were characterised one by one for the purpose of obtaining the spectral responses in correspondence of the inputs and the outputs of each of them.
A measurement bench was used for the experimental measures, adapted to provide the spectrum PDL (Polarisation Dependent Loss) , that is to say, the spectral response in the function of the polarisation of the signal in input to the optical device from time to time under test (or DUT) .
Said bench, shown in figure 9, comprised a laser source 44 tunable in wavelength (model 3642 CR00 by PHOTONETICS, 52 Avenue de 1 'Europe - BP 39, 78160 MARLY-LE-ROY, FRANCE), and a polarisation controller device 45 (model HP 11896A by HEWLETT PACKARD Company, P.O. Box 4026, Englewood, CO 80155-4026) to change the polarisation status of the signal from time to time emitted by the laser source 44.
The maximum, minimum and mean values on all the polarisation statuses were acquired for each DUT and in correspondence with each signal emitted by the laser source 44, using an optical power meter 46 (model HP 8163A together with HP 815333, and optical head HP 81524A by HEWLETT PACKARD Company) .
Since the components were for DWDM systems, a wavelength meter 47 was used (model WA-1600 Wavemeter produced by Burleigh Instruments Inc. Burleigh Park, Fisher, NY 14453- 0755) to check from time to time the wavelength actually emitted by the laser source 44.
The measure was automated through a personal computer 48 - provided with boards for IEEE 488 communication (GPIB) and for acquiring analogue signals (for example, AT-GPIB / TNT and AT-MIO-16E-10 by NATIONAL INSTRUMENTS, 11500 N Mopac Expwy Austin, TX 78759-3504) - which controlled, with a special software, the entire measurement system.
The spectral responses shown in the figures from 10 to 15 represent the mean values obtained from time to time as the polarisation of the signals in input to the devices under test (DUT) was changed.
In a first experimental test, for each optical device 1, 2, 3 the spectral responses between the input (respectively, inputs 11, 21 and 31 of figure 2, hereafter referred to as IN ports) and the drop output (respectively, outputs 12, 23 and that unused of the input coupler 37 of figure 2, hereafter referred to as DROP ports) were acquired.
Table 1 shows the nominal values of wavelengths λx, λm, λy, the values of said wavelengths measured by the wavelength meter 47, the deviation between nominal wavelengths and those measured, the insertion loss (IL) values, expressed in dB, obtained at the wavelengths λx, λm and λy between the IN port and the DROP port of each device.
In addition, figure 10 shows the spectral responses obtained between the IN input and the DROP port of each optical device 1, 2, 3 (respectively, curves A, B, and C) , and table 2 shows the bandwidth (BW) values of the spectral responses obtained at -0.5 dB, in the worst case of polarisation, and at -3 dB, in the case of depolarised light.
TABLE 1
Figure imgf000048_0001
TABLE 2
Figure imgf000049_0001
In addition, the measurement of the spectral responses as the signal polarisation changed provided for a spectrum PDL value, around the wavelengths of interest λx, λm and λy, smaller than 0.1 dB .
As regards the spectral responses of devices 1, 2, 3 from the inputs that add the signal (in figure 2, unused input of the output coupler 18 and inputs 22 and 32, hereafter referred to as ADD ports) and the outputs of the multi- wavelength signal (in figure 2, outputs 13, 24 and 33, hereafter referred to as OUT ports) , the results obtained were totally similar to those reported above. In fact, being said devices symmetrical, the spectral responses from the IN ports to the DROP ports coincide with those from the ADD ports to the OUT ports.
Similarly, the spectral responses from the IN ports to the OUT ports correspond to those from the ADD ports to the DROP ports .
Analogously to what reported with reference to the DROP ports, the spectral responses from the IN port to the OUT port of each optical device 1, 2, 3, were measured.
The insertion loss values obtained for the three optical devices 1, 2, 3 - respectively represented by the curves A, B and C of figure 11 - include the optical connectors with which they were terminated.
Table 3 shows the insertion loss (IL) values, expressed in dB, obtained between the IN port and the OUT port of each device, at the wavelengths transmitted by the respective filters and at the nominal wavelengths λx and λy.
TABLE 3
Figure imgf000050_0001
Figure imgf000050_0002
Figure imgf000050_0003
After assembling the single optical devices 1, 2, 3, another experimental test was carried out to obtain spectral responses from the input port of the optical apparatus 4 (input 11 of figure 2) to the output port
(output 33 of figure 2) .
Figure 12 shows (curve A) the spectral response obtained by dropping the signals at wavelength λl and λ2 without re- adding any other signal (operation as fixed DROP of apparatus 4) . The insertion losses (IL) obtained for the signals at the wavelengths passing through the apparatus were comprised between -1.8 and -2 dB (that is to say, they were substantially equal to the sum of the insertion losses of the single devices) . Additionally, this figure shows (curve B) the spectrum of losses depending on the polarisation (PDL) , expressed in dB .
Figure 13 shows the spectral responses obtained by re- adding the signals at wavelengths λx (curve A) and λy (curve B) through switches 5 and 6. The insertion losses (IL) obtained for said re-added signals are respectively equal to -2.7 dB and -2.5 dB .
On the contrary, by adding through switches 5 and 6 two signals at wavelengths λx and λy, with a different information content with respect to the dropped signals, insertion losses respectively equal to -2 dB and -1 dB were obtained, measured from the input port of the switches to output 33 of the optical apparatus 4.
Figure 14 shows the spectral responses obtained from the IN input to the OUT output of the first optical device 1 (curve A) , from the IN input to the OUT output of the second optical device 2 (curve B) , and from the IN input of the first optical device 1 to the OUT output of the second optical device (curve C) .
By comparing curve C with curve A, it is possible to notice that the signal at wavelength λx in input to the first device, when in output from the second device, was attenuated by at least 10 dB more with respect to that in output from the first device.
In addition, considering that for the purpose of determining the crosstalk at the output of the second device between the residual signal at wavelength λx and the channel added at the same wavelength it is necessary to compare the power levels of said signals in correspondence with said output, the crosstalk value was equal to about 35 dB.
Also considering a certain tolerance range (for example, ± 40 pm) within which the signal at wavelength λx can vary, the crosstalk was smaller than -26 dB (value which experimentally caused a loss on the BER < 0.5 dB) .
Figure 15 shows the spectral responses obtained from the IN input to the OUT output of apparatus 4 (curve A) , from the IN input to the OUT output of the third optical device 3 (curve B) , and from the IN input of the first optical device 1 to the OUT output of the second optical device 2 (curve C) .
With similar consideration to those made for the signal at wavelength λx, the crosstalk values obtained for the signal at wavelength λy were smaller than -40 dB . This value was better than that previously obtained because the components used for the third optical device 3 had better performances than those used for the first optical device 1.
The considerations expressed before with reference to the tolerance on the wavelength are applicable also in this case .
In addition, it must be noted that on the DROP port of the second optical device, in which the wavelength λy is dropped, there is a residual component of the signal at wavelength λx mainly coming from the signal added from the ADD port of said device. This residual signal is a contribution out of the band of the signal of interest at wavelength λy since it is located at a distance of 50 GHz from it. In addition, in the experiment carried out, it was at least 16 dB below the signal at wavelength λy. According to the system requirements, said residual signal can represent a critical noise contribution; thus, it can be filtered by connecting in series a filter to the output 23 of DROP (or to the output port of switch 6) , adapted to eliminate said undesired contribution.

Claims

1. An optical apparatus (4) for dropping and/or adding signals from an optical signal WDM (1000) comprising a plurality of N signals having wavelengths λl , λ2 ... λN, said apparatus (4) comprising
- a first optical device (1) comprising an input (11) for said WDM optical signal (1000) , a first output (12) and a second output (13), said first device (1) being adapted to drop from said WDM optical signal (1000) at least one signal at a wavelength λx, preselected among said wavelengths λl, λ2 ... λN, to send said at least one signal dropped at wavelength λx to said first output (12) and to send said WDM optical signal (1000) to said second output (13) ;
- a second optical device (2) comprising a first input (21) in communication with said second output (13) of said first optical device (1), a second input (22), a first output (23), a second output (24) and a portion of waveguide (25; 26) , between said first input (21) and said second input (22), housing a wavelength selective element (30; 29), said wavelength selective element (30; 29) being adapted to
* drop, from said WDM optical signal (1000) , a possible residual 'component of said at least one signal at wavelength λx, and
* add to said WDM optical signal at least one add signal, having a wavelength λxx substantially equal to said wavelength λx, (1000) coming from said second input (22) ;
characterised in that
• said wavelength selective element (30; 29) of said second optical device (2) is also adapted to drop, from said WDM optical signal (1000) , at least one signal at a wavelength λy, preselected among said wavelengths λl , λ2 ... λN, • said second optical device (2) is adapted to send the signal dropped at the wavelength λy to said first output (23) , and said WDM optical signal (1000) to said second output (24) ;
and in that it also comprises
• a third optical device (3) having a first input (31) in communication with said second output (24) of said second optical device (2), a second input (32) and an output (33), said third device (3) being adapted to drop from said WDM optical signal (1000) a possible residual component, of said at least one signal at wavelength λy; to receive from said second input (32) at least one add signal having a wavelength λxy substantially equal to said wavelength λy and add it to said WDM optical signal (1000) ; and to send said WDM optical signal (1000) to said output (33) .
2. An optical apparatus (4) according to claim 2, wherein said first optical device (1) comprises at least one optical filter FI (14; 19) .
3. An optical apparatus (4) according to claim 2, wherein said at least one optical filter FI (14; 19) is adapted to reflect said wavelength λx and to let the other wavelengths of said WDM optical signal (1000) pass.
4. An optical apparatus (4), according to claim 2 or 3 , wherein said first optical device (1) also comprises a first optical waveguide portion (15; 16) adapted to house said at least one optical filter FI (14; 19) .
5. An optical apparatus (4) according to anyone of the preceding claims 1 to 4 , wherein said wavelength selective element is an optical filter F2 (30; 29)
6. An optical apparatus (4) according to claim 5, wherein said optical filter F2 (30; 29) is adapted to reflect said wavelengths λx and λy and to let the other wavelengths of said WDM optical signal (1000) pass.
7. An optical apparatus (4) according to anyone of the preceding claims 1 to 6, wherein said third optical device
(3) comprises at least one optical filter F3 (34; 39).
8. An optical apparatus (4) according to claim 7, wherein said optical filter F3 (34; 39) is adapted to reflect said wavelength λy and to let the other wavelengths of said WDM optical signal (1000) pass.
9. An optical apparatus (4) according to claim 7 or 8 , wherein said third optical device (3) also comprises a first optical waveguide portion (35; 36) adapted to house said at least one optical filter F3 (34; 39) .
10. A WDM optical communication system comprising
* a transmission station (100) adapted to provide a WDM optical signal (1000) comprising a plurality of N signals having wavelengths λl, λ2 .... λN;
* an optical transmission line (300a, 300b) , optically connected to said transmitting station (100) , to transmit said WDM optical signal (1000) ;
* a reception station (200) , optically connected to said optical transmission line (300a, 300b) , to receive said WDM optical signal (1000) ;
* at least one optical apparatus (4) for adding/dropping signals from said WDM optical signal (1000) , comprising in turn
- a first optical device (1) comprising an input (11) for said WDM optical signal (1000) , a first output (12) and a second output (13) , said first device (1) being adapted to drop from said WDM optical signal (1000) at least one signal at wavelength λx, preselected among said wavelengths λl, λ2 .... λN, sending said at least one signal dropped at wavelength λx to said first output (12), and sending said WDM optical signal (1000) to said second output (13) ; - a second optical device (2) comprising a first input (21) in communication with said second output (13) of said first optical device (1) , a second input (22) , a first output
(23), a second output (24) and a portion of waveguide (25; 26) , between said first input (21) and said second input
(22), housing a wavelength selective element (30; 29), said wavelength selective element (30; 29) being adapted to
* drop, from said WDM optical signal (1000) , a possible residual component of said at least one signal at wavelength λx, and
* add to said WDM optical signal (1000) at least one add signal, having a wavelength λxχ substantially equal to said wavelength λx, coming from said second input (22);
characterised in that
• said wavelength selective element (30; 29) of said second optical device (2) is also adapted to drop, from said WDM optical signal (1000) , at least one signal at a wavelength λy, preselected among said wavelengths λl, λ2 ... λN,
• said second optical device (2) is adapted to send the signal dropped at the wavelength λy to said first output
(23) , and said WDM optical signal (1000) to said second output (24) ;
and in that said optical apparatus also comprises
• a third optical device (3) having a first input (31) in communication with said second output (24) of said second optical device (2) , a second input (32) and an output (33) , said third device (3) being adapted to drop from said WDM optical signal (1000) a possible residual component of said at least one signal at wavelength λy; to receive from said second input (32) at least one add signal having a wavelength λxy substantially equal to said wavelength λy and add it to said WDM optical signal (1000) ; and to send said WDM optical signal (1000) to said output (33) .
11. A method for adding-dropping signals from a WDM optical signal (1000) comprising a plurality of N signals having wavelengths λl, λ2 .... λN, said method comprising the steps of
a) dropping, in a first optical device (1) , at least one signal at wavelength λx, preselected among said wavelengths λl, λ2 .... λN, from said WDM optical signal (1000) ;
b) dropping, in a portion of waveguide (25; 26) of a second optical device (2), a possible residual component of said at least one signal at wavelength λx from said WDM optical signal (1000) ;
c) adding, in said portion of waveguide (25; 26) , at least one add signal having wavelength λxx substantially equal to said wavelength λx, into said WDM optical signal (1000) ,
characterised in that it further comprises the following steps
d) dropping, in said portion of waveguide (25; 26) of said second optical device (2) , at least one signal at wavelength λy, preselected among said wavelengths λl, λ2 .... λN, from said WDM optical signal (1000) ;
e) dropping, in a third optical device (3), a possible residual component of said at least one signal at wavelength λy from said WDM optical signal (1000) ; and
f) adding, in said third optical device, at least one add signal, having wavelength λxy substantially equal to said wavelength λy, into said WDM optical signal (1000) .
PCT/EP2000/012788 1999-12-27 2000-12-13 Optical apparatus for dropping and/or adding optical signals WO2001048961A1 (en)

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EP0948153A2 (en) * 1998-04-02 1999-10-06 Fujitsu Limited Optical transmission apparatus, optical transmission system, and optical terminal station

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US5953470A (en) * 1996-12-03 1999-09-14 Nec Corporation Circuit for optical WDM system
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