WO2002103447A1 - Wavelength selective optical device - Google Patents

Wavelength selective optical device Download PDF

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Publication number
WO2002103447A1
WO2002103447A1 PCT/SE2002/001186 SE0201186W WO02103447A1 WO 2002103447 A1 WO2002103447 A1 WO 2002103447A1 SE 0201186 W SE0201186 W SE 0201186W WO 02103447 A1 WO02103447 A1 WO 02103447A1
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WO
WIPO (PCT)
Prior art keywords
optical
waveguide
resonator
rtf
optical device
Prior art date
Application number
PCT/SE2002/001186
Other languages
French (fr)
Inventor
Anders Henriksson
Ulf ÖHLANDER
Bengt Sahlgren
Ulf Olin
Original Assignee
Proximion Fiber Optics Ab
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Proximion Fiber Optics Ab filed Critical Proximion Fiber Optics Ab
Publication of WO2002103447A1 publication Critical patent/WO2002103447A1/en

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Classifications

    • GPHYSICS
    • G02OPTICS
    • G02BOPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B6/00Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
    • G02B6/24Coupling light guides
    • G02B6/26Optical coupling means
    • G02B6/28Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals
    • G02B6/293Optical 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
    • G02B6/29379Optical 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
    • G02B6/29395Optical 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 configurable, e.g. tunable or reconfigurable
    • GPHYSICS
    • G02OPTICS
    • G02BOPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B6/00Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
    • G02B6/24Coupling light guides
    • G02B6/26Optical coupling means
    • G02B6/28Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals
    • G02B6/293Optical 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
    • G02B6/29346Optical 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
    • G02B6/29356Interference cavity within a single light guide, e.g. between two fibre gratings
    • GPHYSICS
    • G02OPTICS
    • G02BOPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B6/00Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
    • G02B6/24Coupling light guides
    • G02B6/26Optical coupling means
    • G02B6/28Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals
    • G02B6/293Optical 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
    • G02B6/29346Optical 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
    • G02B6/29358Multiple beam interferometer external to a light guide, e.g. Fabry-Pérot, etalon, VIPA plate, OTDL plate, continuous interferometer, parallel plate resonator
    • G02B6/29359Cavity formed by light guide ends, e.g. fibre Fabry Pérot [FFP]
    • GPHYSICS
    • G02OPTICS
    • G02FOPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
    • G02F1/00Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics
    • G02F1/01Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour 
    • G02F1/21Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour  by interference
    • HELECTRICITY
    • H04ELECTRIC COMMUNICATION TECHNIQUE
    • H04BTRANSMISSION
    • H04B10/00Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
    • H04B10/25Arrangements specific to fibre transmission
    • H04B10/2589Bidirectional transmission
    • GPHYSICS
    • G02OPTICS
    • G02BOPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B6/00Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
    • G02B6/24Coupling light guides
    • G02B6/26Optical coupling means
    • G02B6/28Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals
    • G02B6/293Optical 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
    • G02B6/29379Optical 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
    • G02B6/29398Temperature insensitivity
    • GPHYSICS
    • G02OPTICS
    • G02FOPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
    • G02F1/00Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics
    • G02F1/01Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour 
    • G02F1/21Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour  by interference
    • G02F1/213Fabry-Perot type
    • GPHYSICS
    • G02OPTICS
    • G02FOPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
    • G02F2201/00Constructional arrangements not provided for in groups G02F1/00 - G02F7/00
    • G02F2201/34Constructional arrangements not provided for in groups G02F1/00 - G02F7/00 reflector
    • HELECTRICITY
    • H04ELECTRIC COMMUNICATION TECHNIQUE
    • H04JMULTIPLEX COMMUNICATION
    • H04J14/00Optical multiplex systems
    • H04J14/02Wavelength-division multiplex systems
    • H04J14/0201Add-and-drop multiplexing
    • HELECTRICITY
    • H04ELECTRIC COMMUNICATION TECHNIQUE
    • H04JMULTIPLEX COMMUNICATION
    • H04J14/00Optical multiplex systems
    • H04J14/02Wavelength-division multiplex systems
    • H04J14/0201Add-and-drop multiplexing
    • H04J14/0202Arrangements therefor
    • H04J14/0209Multi-stage arrangements, e.g. by cascading multiplexers or demultiplexers
    • HELECTRICITY
    • H04ELECTRIC COMMUNICATION TECHNIQUE
    • H04JMULTIPLEX COMMUNICATION
    • H04J14/00Optical multiplex systems
    • H04J14/02Wavelength-division multiplex systems
    • H04J14/0201Add-and-drop multiplexing
    • H04J14/0202Arrangements therefor
    • H04J14/021Reconfigurable arrangements, e.g. reconfigurable optical add/drop multiplexers [ROADM] or tunable optical add/drop multiplexers [TOADM]

Definitions

  • the present invention relates to an optical device for manipulating an optical signal propagating in a waveguide, such as an optical fibre. More particularly, the present invention relates to a highly versatile device capable of manipulating individual channels within a wavelength division multiplexed optical signal.
  • WDM wavelength division multiplexing
  • optical signals may propagate in either direction through the fibre, for which reason the operation of such devices and methods preferably should be invariant to the propagation direction of the signal.
  • the first and the second reflectors are superimposed upon each other within the waveguide, and arranged to deflect light out from the waveguide into two beams having symmetrical propagation directions with respect to the waveguide (i.e. with respect to the propagation of light within the waveguide) .
  • the reflectors deflect light out from the waveguide in two different directions, which have the same angle with respect to the waveguide, i.e. light is deflected out from the waveguide symmetrically.
  • One advantage of an optical device according to the present invention is that spectrally selective manipulation of the optical signal is allowed, while at the same time operational inversion symmetry of the device is maintained.
  • an optical device according to the present invention can be used for manipulating individual wavelength channels within a wavelength division multiplexed optical signal .
  • Another advantage of an optical device according to the present invention is that it provides an optical device for manipulating an optical signal, which device is inherently cascadeable.
  • any number of optical devices according to the present invention can be arranged in series in order to provide a cascaded structure for manipulating any number of wavelength channels simultaneously.
  • a tilted reflector is a reflector which has an inclination with respect to the propagation direction of light in the waveguide. Consequently, light incident upon the tilted reflector from the waveguide will have a non-normal angle of incidence, and will therefore be deflected away from its original propagation direction.
  • the inclination of the tilted reflector is such that light is deflected out from the waveguide.
  • the angle of inclination of the tilted reflectors with respect to the propagation direction of light within the waveguide is close to 45 degrees, so that light is deflected out from the waveguide in a substantially normal direction (i.e. substantially perpendicularly out from the waveguide) .
  • the tilted reflectors are comprised of blazed Bragg gratings. It is to be noted that the angle of deflection from a blazed Bragg grating is determined by the angle of the blaze and of the period of the grating. Perpendicular deflection can be obtained even if the tilt angle of the grating is different from 45 degrees with respect to the waveguide if the appropriate period is chosen.
  • enhanced wavelength selectivity is provided by means of an external Fabry-Perot type resonator, the reflectors in the waveguide being operative to deflect at least some light into a resonant mode in said external resonator.
  • the Fabry-Perot type resonator is typically defined by at least two resonator mirrors. It is to be understood that said resonator mirrors may be comprised of any suitable structure for providing optical reflection, such as a metal layer, a dielectric stack or a distributed Bragg structure .
  • the present invention provides an optical device for selectively transmitting, in a forward-propagating direction, or reflecting back, in a backwards-propagating direction, individual wavelength channels within a wavelength division multiplexed optical signal propagating in an optical fibre.
  • the present invention provides an optical device for manipulating individual channels within a wavelength division multiplexed optical signal, which device is inherently cascadeable.
  • any number of devices can be arranged in series (in cascade) and thereby provide means for manipulating any num- ber of channels simultaneously.
  • the present invention provides a channel manipulation element for manipulation of individual channels within a wavelength division multiplexed optical signal.
  • the present invention can be used as an interferometer, since the present invention allows selective control of spectral resonance and phase relation.
  • the present invention can be used as a variable and spectrally selective optical atten- uator.
  • any degree of transmission along the fibre can be achieved by appropriately altering the characteristics of the external Fabry-Perot type resonator.
  • the present invention can be used as a digital modulator for modulating individual channels within a WDM signal. Such modulation is rendered possible by the present invention at high speeds and with low dispersion. Further advantages are obtained by virtue of the device being wavelength tuneable.
  • the device may also be used as a modulator for lasers.
  • the present invention can serve as an add/drop filter.
  • Individual WDM channels may conveniently be added or dropped by a respective channel manipulation element . Any channel can be made either to be reflected back from the element or to be transmitted through the element along the fibre.
  • Yet another way of tuning the optical device according to the present invention includes tilting at least one of the resonator mirrors.
  • the mirror can either be tilted parallel to the waveguide, or perpendicularly to the waveguide.
  • the resonant wavelength is shifted to a slightly different wavelength, and the resonance is broadened.
  • the mirror is tilted perpendicular to the waveguide, the main effect is to reduce the coupling between the waveguide and the external resonator.
  • the resonance of the resonator will be removed.
  • the resonator will no longer be resonant and, consequently, manipulation of the corresponding channel within the optical signal in the waveguide is interrupted.
  • Another possibility of tuning the device is to tilt both of the resonator mirrors, while keeping them essentially parallel to each other, i.e. while keeping the resonance in the external resonator. Light of a different wavelength will in this case be coupled into the resonant mode of the external resonator. It is to be noted that a slight adjustment of the separation between the mirrors might be necessary in this case.
  • Broad-band spectral selectivity of the device can be achieved by designing the reflectors inside the optical fibre to have a wavelength dependent reflectivity and/or by designing the mirrors of the Fabry-Perot type resonator to have wavelength dependent reflectivity.
  • High- precision, narrow-band spectral selectivity is achieved by the etalon-effect of the Fabry-Perot type resonator.
  • the combined effect of said broad-band and said narrowband selectivity provides a highly versatile and spectrally selective device for manipulating individual chan- ⁇
  • Figure 1 is a schematic illustration of an optical device according to the present invention having deflecting reflectors and an external Fabry-Perot type resonator
  • FIG. 2 is a schematic illustration of another optical device according to the present invention.
  • Figure 3 is a schematic illustration of the core of an optical fibre provided with reflectors in the form of superimposed, blazed Bragg gratings,
  • Figure 4 is a schematic illustration of two paired optical devices in transmitting mode, according to the present invention.
  • Figure 5 is a schematic illustration of two paired optical devices in reflecting mode
  • Figure 6 is a schematic illustration of a plurality of optical devices arranged in cascade for individually manipulating a plurality of wavelength channels simultaneously
  • Figure 7 is a schematic illustration of a wavelength selective, dynamic attenuator according to the present invention.
  • Figure 8 is a schematic illustration of one method of tuning by tilting a resonator mirror
  • Figure 9 is a schematic illustration of another method of tuning by tilting both resonator mirrors
  • Figure 10 shows the filter characteristics of one type of tuning
  • Figure 11 shows the filter characteristics of another type of tuning.
  • FIG. 1 A preferred embodiment of an optical device 1 according to the present invention is schematically shown in figure 1.
  • a cross section of a piece of optical fibre 10 is shown, the light guiding core 11 of which is schematically indicated by a broken line centrally in the piece of fibre 10.
  • the two reflectors are oriented at right an- gles with respect to each other, in order to deflect light impinging upon the two superimposed reflectors into two anti-parallel beams.
  • the device shown in the figure further comprises two external mirrors 14 and 15, forming an external Fabry-Perot type resonator. The resonator is positioned so that the deflecting reflectors 12, 13 are enclosed within the resonator.
  • the deflecting reflectors and the external Fabry-Perot type resonator have such mutual orientations that at least some of the light deflected out from the core of the fibre enters a resonant mode in the resonator.
  • at least some of the light in any resonant mode in the resonator will also be deflected back into the core 11 of the fibre 10 by the crossed reflectors 12, 13.
  • the device shown in figure 1 is sometimes referred to, in this specification, as a channel manipulation element.
  • the Fabry-Perot type resonator will remain resonant to the same wavelength.
  • odl and od2 must be changed while keeping odO constant (i.e. a lateral translation of the external resonator) .
  • Lateral translation means a translation perpendicularly to the longitudinal direction of the waveguide .
  • FIG 2 another embodiment of an optical device according to the present invention is shown.
  • light is not deflected out from the waveguide perpendicularly, but at another angle.
  • the deflected beams are still symmetrical with respect to the longitudinal direction of the waveguide.
  • light cannot be coupled back in the waveguide in the counter propagating direction.
  • the embodiment shown in figure 2 is preferred when a channel within a WDM signal is to be manipulated for attenuation or modulation.
  • the deflecting reflectors shown in figure 2 are comprised of crossed Bragg gratings which have equal periods and equal but opposite blaze angles. In this way, light is deflected symmetrically out from the waveguide.
  • the deflection angle from a blazed grating is determined by both the blaze angle and the period. Hence, a predetermined deflection angle can be achieved by appropriate selections of period and blaze angle .
  • a preferred embodiment of the tilted reflectors is schematically shown in figure 3.
  • the reflectors are comprised of blazed Bragg gratings 21, 22.
  • Each blazed Bragg grating is comprised of a periodic structure of refractive index variations, which are inclined with re- spect to the propagation direction of light within the waveguide (i.e. the grating is blazed). Consequently, the blazed gratings are, in fact, tilted reflectors.
  • the characteristics of blazed Bragg gratings are commonly known in the art.
  • Each of the blazed gratings 21 and 22 shown in figure 3 are arranged to deflect light of a predetermined wavelength out from the waveguide perpendicularly. Hence, it can be said that the blazed gratings are arranged at right angles with respect to each other.
  • multiplexer can be incorporated into an optical fibre-to-fibre router.
  • each wavelength channel Ch.l to Ch.5 there is provided one pair each of elements 1.1 to 1.10, as such shown in figures 4 and 5. Paired elements are utilised ' in order to avoid coherent mixing of channels. Five such pairs (one for each respective channel Ch.l to Ch.5) are ⁇ shown in the figure, the multiplexer as shown thereby being designed for WDM communications on -five channels. However, it is to be understood that any number of paired elements can be cascaded in order to provide a multiplexer for any number of wavelength channels.
  • first optical signal comprised of five wavelength channels in the first fibre ring 41 and a second optical signal, also comprised of five wavelength channels, in the second fibre ring 42.
  • Each of said rings is connected to the length of fibre comprising the cascaded and paired elements 1.1 to 1.10 by means of a respective optical circulator 43, 44.
  • channel number 2 (Ch.2) is to be exchanged between the two fibre rings. Then, all pairs of elements are operated in full reflection, i.e. acting to couple light back in a backwards-propagating direction with respect to the incident direction, except the pair corresponding to the channel to be exchanged, which pair operates in transmission mode.
  • elements 1.3 and 1.4 are switched to transmission mode. Consequently, in the signal in the first fibre ring 41, channels Ch.l, Ch.3, Ch.4 and Ch.5 are back-reflected by the cascaded structure towards the optical circulator 43, and are thereby directed to the output port for further propagation in the fibre ring 41.
  • the wavelength channel Ch.2 is received from the second fibre ring 42, which channel is interleaved with the other channels in ring 41.
  • the second fibre ring 42 Similar interleaving takes place for channel Ch.2 from the first ring 41.
  • any number of channels can be exchanged simultaneously by switching the appropriate pairs to transmission mode.
  • the entire optical signal in each of the fibre rings can be exchanged simultaneously by switching all pairs to transmission mode.
  • a similar configuration can be operated as a wavelength selective, dynamic attenuator, as shown in figure 7.
  • the elements need not be paired, since light is only propagated in one direction through the fibre at any one instant.
  • an optical circulator 51 is only necessary at the input side.
  • tuning of the device 1 can be performed by tilting one of the resonator mirrors 14 parallel to the waveguide, such that the two resonator mirrors are no longer parallel to each other. Tilting parallel to the waveguide means that the mirror is pivoted about an axis perpendicular to the waveguide. Consequently, the length of the external resonator will vary along the same. Hence, the shape of the filter function will be broadened and shifted towards longer wavelengths.
  • This adjustment of the filtering is schematically shown in figure 10, in which the initial filter function is illlustrated by a solid line, and the filter function at tilted resonator mirror is illustrated by a broken line.
  • another method of tuning by tilting the resonator mirror 14 is illustrated.
  • the resonator mirror is tilted perpendicularly to the longitudinal direction of the waveguide (the mirror is pivoted about an axis parallel to the waveguide.
  • the finesse and the quality of the external Fabry-Perot type resonator will be lowered, since less light is coupled back from the resonator into the waveguide when the resonator mirror is tilted in this manner.
  • the resonant wavelength will remain substantially the same.
  • the filter function is still broadened, but with a maintained centre wavelength. Effectively, this type of tuning can be used for modulating the amplitude of the channel to which the external resonator is resonant.
  • This adjustment of the filtering is schematically shown in figure 11, in which the initial filter function is illlustrated by a solid line, and the filter function at tilted resonator mirror is illustrated by a broken line.
  • the present invention provides an optical device for manipulating individual channels within a WDM optical signal, which optical device is tuneable, controllable and configurable, as well as direction invariant as regards propagation of the signal to be manipulated. Furthermore, the inherent possibility to cascade elements according to the present invention provides a virtually unlimited scalability, and may therefore be utilised even if a very large number of wavelength channels are used in a WDM communications system.

Abstract

This invention relates to an optical device for manipulating an optical signal propagating in a waveguide; and to a multiplexer, an attenuator and a modulator incorporating such device. A device according to the invention comprises two superimposed and tilted reflectors within the waveguide, which reflectors are arranged to deflect light out from the waveguide in two individual, substantially counter-propagating beams. The device according to the invention allows manipulation of individual channels within a wavelength dividion multiplexed optical signal. In a preferred embodiment, light is deflected out from the waveguide and into an external Fabry-Perot type resonator, which is tuned and adjusted in order to manipulate individual wavelengh regions of light. The invention also relates to a method for manipulating an optical signal, and to a method of tuning an optical device according to the invention.

Description

WAVELENGTH SELECTIVE OPTICAL DEVICE
Technical field
The present invention relates to an optical device for manipulating an optical signal propagating in a waveguide, such as an optical fibre. More particularly, the present invention relates to a highly versatile device capable of manipulating individual channels within a wavelength division multiplexed optical signal.
Technical background In optical communications based on wavelength division multiplexing (WDM) , information is sent through an optical waveguide, such as an optical fibre, on a multiplicity of wavelength channels simultaneously. Each wavelength channel propagate through the fibre independ- ently of all other wavelength channels. By using WDM, very large data rates can be achieved.
However, as the number of wavelength channels increases, it becomes increasingly important to manipulate each individual channel separately. Therefore, there is a need for devices and methods for manipulating individual channels within a wavelength division multiplexed optical signal.
Furthermore, the optical signals may propagate in either direction through the fibre, for which reason the operation of such devices and methods preferably should be invariant to the propagation direction of the signal.
Summary of the invention
It is an object of the present invention to provide an optical device for manipulating an optical signal propagating in a waveguide, such as an optical fibre, which optical device exhibits operational inversion symmetry with respect to the .propagation direction of said optical signal in said waveguide. In other words, it is an object of the present invention to provide a device for manipulating an optical signal, wherein the operation of the device is the same for both possible propagation directions of the optical signal through the waveguide. The above-mentioned object is met by an optical device of the kind set forth in the appended claims .
An optical device according to the present invention for manipulating an optical signal propagating in a wave- guide comprises a first and a second tilted reflector, which are provided in said waveguide. The first and the second reflectors are superimposed upon each other within the waveguide, and arranged to deflect light out from the waveguide into two beams having symmetrical propagation directions with respect to the waveguide (i.e. with respect to the propagation of light within the waveguide) . In other words, the reflectors deflect light out from the waveguide in two different directions, which have the same angle with respect to the waveguide, i.e. light is deflected out from the waveguide symmetrically. One advantage of an optical device according to the present invention is that spectrally selective manipulation of the optical signal is allowed, while at the same time operational inversion symmetry of the device is maintained.
Furthermore, an optical device according to the present invention can be used for manipulating individual wavelength channels within a wavelength division multiplexed optical signal . Another advantage of an optical device according to the present invention is that it provides an optical device for manipulating an optical signal, which device is inherently cascadeable. In other words, any number of optical devices according to the present invention can be arranged in series in order to provide a cascaded structure for manipulating any number of wavelength channels simultaneously. In the context of the present application, a tilted reflector is a reflector which has an inclination with respect to the propagation direction of light in the waveguide. Consequently, light incident upon the tilted reflector from the waveguide will have a non-normal angle of incidence, and will therefore be deflected away from its original propagation direction. Preferably, the inclination of the tilted reflector is such that light is deflected out from the waveguide. In a preferred embodi- ment, the angle of inclination of the tilted reflectors with respect to the propagation direction of light within the waveguide is close to 45 degrees, so that light is deflected out from the waveguide in a substantially normal direction (i.e. substantially perpendicularly out from the waveguide) . Preferably, the tilted reflectors are comprised of blazed Bragg gratings. It is to be noted that the angle of deflection from a blazed Bragg grating is determined by the angle of the blaze and of the period of the grating. Perpendicular deflection can be obtained even if the tilt angle of the grating is different from 45 degrees with respect to the waveguide if the appropriate period is chosen.
In connection with the present invention, it is preferred that enhanced wavelength selectivity is provided by means of an external Fabry-Perot type resonator, the reflectors in the waveguide being operative to deflect at least some light into a resonant mode in said external resonator.
The Fabry-Perot type resonator is typically defined by at least two resonator mirrors. It is to be understood that said resonator mirrors may be comprised of any suitable structure for providing optical reflection, such as a metal layer, a dielectric stack or a distributed Bragg structure . In one aspect, the present invention provides an optical device for selectively transmitting, in a forward-propagating direction, or reflecting back, in a backwards-propagating direction, individual wavelength channels within a wavelength division multiplexed optical signal propagating in an optical fibre.
In another aspect, the present invention provides an optical device for manipulating individual channels within a wavelength division multiplexed optical signal, which device is inherently cascadeable. In other words, any number of devices can be arranged in series (in cascade) and thereby provide means for manipulating any num- ber of channels simultaneously. In this aspect, the present invention provides a channel manipulation element for manipulation of individual channels within a wavelength division multiplexed optical signal.
In another aspect, the present invention can be used as an interferometer, since the present invention allows selective control of spectral resonance and phase relation.
In another aspect, the present invention can be used as a variable and spectrally selective optical atten- uator. For example, any degree of transmission along the fibre can be achieved by appropriately altering the characteristics of the external Fabry-Perot type resonator.
In another aspect, the present invention can be used as a digital modulator for modulating individual channels within a WDM signal. Such modulation is rendered possible by the present invention at high speeds and with low dispersion. Further advantages are obtained by virtue of the device being wavelength tuneable. The device may also be used as a modulator for lasers.
In another aspect, the present invention can serve as an add/drop filter. Individual WDM channels may conveniently be added or dropped by a respective channel manipulation element . Any channel can be made either to be reflected back from the element or to be transmitted through the element along the fibre.
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Figure imgf000008_0001
Figure imgf000009_0001
guide can be controlled. Consequently, coupling strength or attenuation of the optical device is controlled.
Yet another way of tuning the optical device according to the present invention includes tilting at least one of the resonator mirrors. There are two different ways of tilting the resonator mirror. The mirror can either be tilted parallel to the waveguide, or perpendicularly to the waveguide. When the mirror is tilted parallel to the waveguide, the resonant wavelength is shifted to a slightly different wavelength, and the resonance is broadened. When, on the other hand, the mirror is tilted perpendicular to the waveguide, the main effect is to reduce the coupling between the waveguide and the external resonator. By sufficiently tilting one of the resonator mirrors perpendicularly to the waveguide, the resonance of the resonator will be removed. Hence, the resonator will no longer be resonant and, consequently, manipulation of the corresponding channel within the optical signal in the waveguide is interrupted. Another possibility of tuning the device is to tilt both of the resonator mirrors, while keeping them essentially parallel to each other, i.e. while keeping the resonance in the external resonator. Light of a different wavelength will in this case be coupled into the resonant mode of the external resonator. It is to be noted that a slight adjustment of the separation between the mirrors might be necessary in this case.
Broad-band spectral selectivity of the device can be achieved by designing the reflectors inside the optical fibre to have a wavelength dependent reflectivity and/or by designing the mirrors of the Fabry-Perot type resonator to have wavelength dependent reflectivity. High- precision, narrow-band spectral selectivity is achieved by the etalon-effect of the Fabry-Perot type resonator. The combined effect of said broad-band and said narrowband selectivity provides a highly versatile and spectrally selective device for manipulating individual chan- Φ
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Figure imgf000011_0001
of which will be described below, other which are apparent to the man skilled in the art.
Brief description of the drawings Further features and advantages of the present invention will be apparent from the following detailed description of some preferred embodiments thereof. In the detailed description, reference is made to the accompanying drawings, on which: Figure 1 is a schematic illustration of an optical device according to the present invention having deflecting reflectors and an external Fabry-Perot type resonator,
Figure 2 is a schematic illustration of another optical device according to the present invention,
Figure 3 is a schematic illustration of the core of an optical fibre provided with reflectors in the form of superimposed, blazed Bragg gratings,
Figure 4 is a schematic illustration of two paired optical devices in transmitting mode, according to the present invention,
Figure 5 is a schematic illustration of two paired optical devices in reflecting mode, according to the present invention, Figure 6 is a schematic illustration of a plurality of optical devices arranged in cascade for individually manipulating a plurality of wavelength channels simultaneously,
Figure 7 is a schematic illustration of a wavelength selective, dynamic attenuator according to the present invention,
Figure 8 is a schematic illustration of one method of tuning by tilting a resonator mirror,
Figure 9 is a schematic illustration of another method of tuning by tilting both resonator mirrors,
Figure 10 shows the filter characteristics of one type of tuning, and Figure 11 shows the filter characteristics of another type of tuning.
On the drawings, like parts are designated like reference numerals.
Detailed description of preferred embodiments
A preferred embodiment of an optical device 1 according to the present invention is schematically shown in figure 1. In the figure, a cross section of a piece of optical fibre 10 is shown, the light guiding core 11 of which is schematically indicated by a broken line centrally in the piece of fibre 10. In the core 11 of the fibre 10, there is provided two superimposed reflectors 12 and 13. The two reflectors are oriented at right an- gles with respect to each other, in order to deflect light impinging upon the two superimposed reflectors into two anti-parallel beams. The device shown in the figure further comprises two external mirrors 14 and 15, forming an external Fabry-Perot type resonator. The resonator is positioned so that the deflecting reflectors 12, 13 are enclosed within the resonator. Furthermore, the deflecting reflectors and the external Fabry-Perot type resonator have such mutual orientations that at least some of the light deflected out from the core of the fibre enters a resonant mode in the resonator. By consequence, at least some of the light in any resonant mode in the resonator will also be deflected back into the core 11 of the fibre 10 by the crossed reflectors 12, 13. The device shown in figure 1 is sometimes referred to, in this specification, as a channel manipulation element.
The principle of action of the optical device 1 shown in figure 1 will now be described. In the figure, propagation direction of light is indicated by arrows. Note that the arrows have been displaced for clarity. Assume that an optical signal is incident from the left in the figure (indicated by an arrow 5) . Said signal will impinge upon the two superimposed reflectors 12, 13, 00 UH 4J SH . 4J φ φ 0 rd 0 r rH rβ . 0
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mode there must be a constant constructive interference within the resonator, i.e. the resonator must support a standing wave at the resonant wavelength. Consequently, as long as odO is held constant, the Fabry-Perot type resonator will remain resonant to the same wavelength.
Recall now that, according to the present invention, light is coupled out from the external resonator both when, co ing from the upper portion of the resonator and when coming from the lower portion of the resonator. Therefore, in each propagation direction in the waveguide (left and right) , there will be a superposition of light from the upper and the lower portion of the resonator. Whether constructive or destructive interference is obtained is determined by the phase relation between light from the upper portion and light from the lower portion. The phase relation, in turn, is determined by the optical path lengths odl and od2 of each respective portion of the resonator. Consequently, constructive interference is any desired direction to any desired degree can be achieved by appropriately controlling the optical path lengths odl and od2 , allowing any ratio between transmission and reflection at the optical manipulation element .
It is to be noted that, if the degree of trans- mission (reflection) of the element is to be adjusted without changing the wavelength to which the resonator is resonant, odl and od2 must be changed while keeping odO constant (i.e. a lateral translation of the external resonator) . Lateral translation means a translation perpendicularly to the longitudinal direction of the waveguide .
In figure 2 , another embodiment of an optical device according to the present invention is shown. In this case, light is not deflected out from the waveguide perpendicularly, but at another angle. Note, however, that the deflected beams are still symmetrical with respect to the longitudinal direction of the waveguide. In the shown embodiment, light cannot be coupled back in the waveguide in the counter propagating direction. However, the embodiment shown in figure 2 is preferred when a channel within a WDM signal is to be manipulated for attenuation or modulation.
The deflecting reflectors shown in figure 2 are comprised of crossed Bragg gratings which have equal periods and equal but opposite blaze angles. In this way, light is deflected symmetrically out from the waveguide. As mentioned above, the deflection angle from a blazed grating is determined by both the blaze angle and the period. Hence, a predetermined deflection angle can be achieved by appropriate selections of period and blaze angle . A preferred embodiment of the tilted reflectors is schematically shown in figure 3. Here, the reflectors are comprised of blazed Bragg gratings 21, 22. Each blazed Bragg grating is comprised of a periodic structure of refractive index variations, which are inclined with re- spect to the propagation direction of light within the waveguide (i.e. the grating is blazed). Consequently, the blazed gratings are, in fact, tilted reflectors. The characteristics of blazed Bragg gratings are commonly known in the art. Each of the blazed gratings 21 and 22 shown in figure 3 are arranged to deflect light of a predetermined wavelength out from the waveguide perpendicularly. Hence, it can be said that the blazed gratings are arranged at right angles with respect to each other. In effect, light from the waveguide, which light is incident upon the two blazed gratings, will be perpendicularly deflected out from the waveguide in two opposite directions, i.e. in two beams that are symmetrical with respect to the blazed gratings. In this case, the deflected beams are essentially anti-parallel to each other and perpendicular to the waveguide.
In an optical communications system based on transmission of signals through optical fibres, there is an TJ 1 .. ra 1 J o φ SH β -H rH 1 Φ 0 Φ
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Figure imgf000018_0001
shown multiplexer can be incorporated into an optical fibre-to-fibre router.
For each wavelength channel Ch.l to Ch.5, there is provided one pair each of elements 1.1 to 1.10, as such shown in figures 4 and 5. Paired elements are utilised' in order to avoid coherent mixing of channels. Five such pairs (one for each respective channel Ch.l to Ch.5) are shown in the figure, the multiplexer as shown thereby being designed for WDM communications on -five channels. However, it is to be understood that any number of paired elements can be cascaded in order to provide a multiplexer for any number of wavelength channels.
Although the channel manipulation elements have been described as having a respective set of crossed de- fleeting reflectors, it is also possible to implement the deflecting reflectors as crossed, blazed gratings without any actual division between the reflectors of different elements. Different portions of the chirped gratings will then act as reflectors for different elements. Consider now a first optical signal comprised of five wavelength channels in the first fibre ring 41 and a second optical signal, also comprised of five wavelength channels, in the second fibre ring 42. Each of said rings is connected to the length of fibre comprising the cascaded and paired elements 1.1 to 1.10 by means of a respective optical circulator 43, 44. In this case, it is required that the manipulation elements operate on the zeroth order of deflected light, i.e. that the deflection of light from the waveguide is performed perpendicularly. Consequently, any light reflected back from the cascaded elements 1.1 to 1.10 will continue to propagate in the original fibre ring (41 or 42, depending on its origin) . Now, an exchange of one channel between the two fibre rings is to be executed. Initially, when no exchange of channels is to be executed, each of the pairs in the cascade operates in reflection mode as shown in figure 5, i.e. acts to reflect the corresponding channel back towards each respective circulator. In order to exchange any of the five channels between the two fibre rings, the appropriate pair of the cascaded elements is switched to transmission mode. As a result, the channel corresponding to the pair operating in transmission mode is passed on to the other fibre ring - a channel exchange has been performed.
Assume, for example, that channel number 2 (Ch.2) is to be exchanged between the two fibre rings. Then, all pairs of elements are operated in full reflection, i.e. acting to couple light back in a backwards-propagating direction with respect to the incident direction, except the pair corresponding to the channel to be exchanged, which pair operates in transmission mode. Thus, in this example of exchanging Ch.2, elements 1.3 and 1.4 are switched to transmission mode. Consequently, in the signal in the first fibre ring 41, channels Ch.l, Ch.3, Ch.4 and Ch.5 are back-reflected by the cascaded structure towards the optical circulator 43, and are thereby directed to the output port for further propagation in the fibre ring 41. In addition, the wavelength channel Ch.2 is received from the second fibre ring 42, which channel is interleaved with the other channels in ring 41. For the signal in the second fibre ring 42, similar interleaving takes place for channel Ch.2 from the first ring 41.
Furthermore, any number of channels can be exchanged simultaneously by switching the appropriate pairs to transmission mode. In fact, the entire optical signal in each of the fibre rings can be exchanged simultaneously by switching all pairs to transmission mode.
A similar configuration can be operated as a wavelength selective, dynamic attenuator, as shown in figure 7. In this case, however, the elements need not be paired, since light is only propagated in one direction through the fibre at any one instant. For the same reason, an optical circulator 51 is only necessary at the input side. fH Φ
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Referring to figure 8, tuning of the device 1 can be performed by tilting one of the resonator mirrors 14 parallel to the waveguide, such that the two resonator mirrors are no longer parallel to each other. Tilting parallel to the waveguide means that the mirror is pivoted about an axis perpendicular to the waveguide. Consequently, the length of the external resonator will vary along the same. Hence, the shape of the filter function will be broadened and shifted towards longer wavelengths. This adjustment of the filtering is schematically shown in figure 10, in which the initial filter function is illlustrated by a solid line, and the filter function at tilted resonator mirror is illustrated by a broken line. In figure 9, another method of tuning by tilting the resonator mirror 14 is illustrated. In this case, the resonator mirror is tilted perpendicularly to the longitudinal direction of the waveguide (the mirror is pivoted about an axis parallel to the waveguide. Hence, the finesse and the quality of the external Fabry-Perot type resonator will be lowered, since less light is coupled back from the resonator into the waveguide when the resonator mirror is tilted in this manner. The resonant wavelength, however, will remain substantially the same. In effect, the filter function is still broadened, but with a maintained centre wavelength. Effectively, this type of tuning can be used for modulating the amplitude of the channel to which the external resonator is resonant. This adjustment of the filtering is schematically shown in figure 11, in which the initial filter function is illlustrated by a solid line, and the filter function at tilted resonator mirror is illustrated by a broken line.
In conclusion, the present invention provides an optical device for manipulating individual channels within a WDM optical signal, which optical device is tuneable, controllable and configurable, as well as direction invariant as regards propagation of the signal to be manipulated. Furthermore, the inherent possibility to cascade elements according to the present invention provides a virtually unlimited scalability, and may therefore be utilised even if a very large number of wavelength channels are used in a WDM communications system.
Although specific embodiments are presented in the detailed description above, it is to be understood that the present invention can be implemented differently than described. The detailed description of embodiments is not intended to limit the scope of the invention as defined in the claims.

Claims

1. An optical device for manipulating an optical signal propagating in a waveguide, comprising a first tilted reflector arranged in said waveguide, and a second tilted reflector arranged in said waveguide, wherein said first and said second tilted reflectors are superimposed upon each other, and arranged to deflect light out from said waveguide into two individual beams.
2. An optical device as claimed in claim 1, wherein the first and the second reflectors are arranged to deflect light at equal angles with respect to the propagation direction of light within the waveguide, but in opposite directions .
3. An optical device as claimed in claim 1 or 2, further comprising a Fabry-Perot type resonator defined by at least two resonator mirrors, the first and the second tilted reflectors being positioned within said resonator and arranged to deflect light into a resonant mode in said resonator.
4. An optical device as claimed in claim 3, wherein the resonator mirrors defining the Fabry-Perot type resonator are arranged outside of the waveguide.
5. An optical device as claimed in claim 3 or 4, wherein each resonator mirror comprises a reflecting metal layer.
6. An optical device as claimed in claim 3 or 4, wherein each resonator mirror comprises a reflecting dielectric stack.
7. An optical device as claimed in claim 3 or 4, wherein each resonator mirror comprises a distributed Bragg reflector.
8. An optical device as claimed in any one of the claims 3 to 7, wherein at least one of the resonator mirrors has high reflectivity only for light within a predefined wavelength region, so that the Fabry-Perot type resonator is resonant only to light -within said predefined wavelength region.
9. An optical device as claimed in any one of the preceding claims, wherein each tilted reflector comprises a blazed Bragg grating.
10. An optical device as claimed in claim 9, wherein the blazed Bragg grating has a period such that light within a predefined wavelength range is deflected out from the waveguide .
11. An optical device as claimed in claim 3, further comprising means for changing the optical distance between at least one of the two mirrors of the Fabry- Perot type resonator and the superimposed first and second reflectors.
12. An optical device as claimed in any one of the claims 3 to 11, further comprising means for tilting at least one of the two mirrors of the Fabry-Perot type resonator.
13. An optical device as claimed in claim 11, wherein the means for changing the optical distance is operative to displace at least one of the two mirrors.
14. An optical device as claimed in claim 11, wherein the means for changing the optical distance is operative to change a refractive index between the two mirrors.
15. An optical device as claimed in any one of the preceding claims, wherein the waveguide is a planar waveguide .
16. An optical device as claimed in any -one of the claims 1 to 14, wherein the waveguide is an optical fibre.
17. A method of manipulating an optical signal propagating in a waveguide, comprising the steps of laterally extracting at least part of the light of said optical signal from said waveguide in two individual beams, controlling the phase of at least one of said two individual beams, and bringing the extracted light back into said waveguide for further propagation along the same.
18. A method as claimed in claim 17, further comprising the step of directing the two individual beams into an external resonator, wherein the step of controlling the phase is performed by means of said external resonator.
19. A method as claimed in claim 18, wherein the step of controlling the phase is performed by controlling the optical path length in at least a portion of the external resonator.
20. A method as claimed in any one of the claims 17 to
19. wherein the step of extracting is performed by means of a first and a second deflecting reflector provided in the waveguide .
21. A method as claimed in claim 20, wherein the first and the second deflecting reflectors are further used for performing the step of bringing the extracted light back into the waveguide .
22. A method as claimed in claim 19, wherein the optical path length is controlled by controlling the geometrical path length.
23. A method as claimed in claim 19, wherein the optical path length is controlled by controlling a refractive index in at least a portion of the external resonator.
24. A method of tuning the resonant wavelength of an optical device as defined in any one of the claims 3-8, comprising the step of controlling the optical path length between the two resonator mirrors of the Fabry-
Perot type resonator.
25. A method as claimed in claim 24, wherein the step of controlling the optical path length is performed by controlling the geometrical distance between the resonator mirrors .
26. A method as claimed in claim 24, wherein the step of controlling the optical path length is performed by controlling a refractive index between the resonator mirrors .
27. A method as claimed in claim 26, wherein the refractive index is controlled by controlling temperature.
28. A method as claimed in claim 26, wherein the refractive index is controlled by controlling electron/hole concentration in a semi-conductor material.
29. A method as claimed in claim 26, wherein the refractive index is controlled by means of an applied electric field.
30. A method of tuning an optical device as defined in any one of the claims 3-8, comprising the step of controlling the optical path length between the tilted reflectors and each of the resonator mirrors, respectively.
31. A method as claimed in claim 30, wherein the separation between the resonator mirrors is kept essentially constant .
32. A method as claimed in claim 30 or 31, wherein the step of controlling the optical path length between the tilted reflectors and either of the resonator mirrors is performed by controlling the geometrical distance.
33. A method as claimed in claim 30 or 31, wherein the step of controlling the optical path length between the tilted reflectors and either of the resonator mirrors is performed by controlling a refractive index between said resonator mirrors .
34. A method as claimed in claim 33, wherein the refractive index is controlled by controlling temperature .
35. A method as claimed in claim 33, wherein the refractive index is controlled by means of an applied electric field.
36. A method as claimed in claim 33, wherein the refractive index is controlled by controlling a band gap in a semi-conductor material.
37. A method of tuning an optical device as defined in claim 12, comprising the step of tilting at least one of the resonator mirrors with respect to the waveguide .
38. A method as claimed in claim 37, wherein the resonator mirror is tilted parallel to the longitudinal direction of the waveguide..
39. A method as claimed in claim 37, wherein the resonator mirror ia tilted perpendicular to the longitudinal direction of the waveguide.
40. An optical add/drop multiplexer comprising a length of optical fibre arranged between a first and a second optical circulator, wherein said length of fibre comprises at least one optical device as defined in any one of the claims 3-8, said device being operative to selectively transmit or reflect an associated wavelength channel, and wherein each of said circulators is operative to direct light from an input terminal into said length of optical fibre, and to direct light from said length of optical fibre to an output terminal.
41. An optical multiplexer as claimed in claim 40, comprising a plurality of optical devices arranged in cascade, each of said devices being operative to selectively transmit or reflect a respective associated wavelength channel .
42. A wavelength selective, variable attenuator comprising a length of optical fibre having an input end and an output end, said length of fibre comprising at least one optical device as defined in any one of the claims 3-16, which device is controllable to provide a desired level of transmission of light within said length of optical fibre.
43. An attenuator as claimed in claim 42, further comprising an optical isolator that is operatively connected to the input end of the length of fibre.
44. An attenuator as claimed in claim 42, further comprising an optical circulator that is operatively connected to the input end of the length of fibre.
45. An attenuator as claimed in any one of the claims 42 to 44, wherein the at least one optical device is adjustable between full transmission and zero transmission, in order to allow digital modulation of a carrier wavelength delivered to the input end of the fibre to provide a modulated output ..
46. A modulator for lasers comprising at least one optical device as defined in any one of the claims 3-8 provided in a resonant cavity of a laser, which device is configured to transmit a desired resonance wavelength.
PCT/SE2002/001186 2001-06-20 2002-06-19 Wavelength selective optical device WO2002103447A1 (en)

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US7697801B2 (en) 2002-07-10 2010-04-13 Proximion Fiber Systems, Ab Wavelength selective switch
WO2005047942A1 (en) * 2003-11-12 2005-05-26 Engana Pty Ltd Wavelength manipulation system and method
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