WO1991015906A1

WO1991015906A1 - Optical communications system

Info

Publication number: WO1991015906A1
Application number: PCT/GB1991/000541
Authority: WO
Inventors: David Brian Payne; Jonathan Richard Armitage
Original assignee: British Telecommunications Public Limited Company
Priority date: 1990-04-06
Filing date: 1991-04-08
Publication date: 1991-10-17
Also published as: AU7671391A; GB9007897D0

Abstract

An optical communications system includes a central station (2) coupled to a plurality of outstations (Oi) by an optical waveguide network (4) which can transmit optical signals to the outstations on a plurality of wavelengths (μ1 to n). There is included a refractive grating (6) formed within an optical waveguide of the network (4) immediately before the outstation O1 which is reflective to optical signals of a wavelength (μi) that propagate as a single mode in the network (4). The grating (6) permits optical signals of wavelengths greater and smaller than μi to pass so selectively blocking an optical wavelength channel from the outstation O1.

Description

OPTICAL COMMUNICATIONS SYSTEM

This invention relates to optical communications systems and particularly, but not exclusively, to passive optical fibre communication systems. One of the attractions of an optical fibre network is that it can transmit on many wavelength channels simultaneously. One example of such a network is a passive optical network in which wavelength multiplexed channels are distributed from a central station to each of a number of outstations by passive coupling from a single fibre from the central station. The channels could carry various broadband subscriber services such as entertainment TV, video library sources as well as narrowband services such as telephony and data. The channels available to an outstation are determined by which wavelengths the outstation is configured to detect. If all the channels reach an outstation it is possible that an outstation may gain access to a channel which it is not permitted to receive. These non-permitted channels may vary as permissions to access them are given and withdrawn.

EP286, 350A published on the 12th August 1988 shows an optical fibre communication system that operates at a wavelength of λ₀ and comprises an optical fibre having a cut-off wavelength λ_c > λ₀, a fibre grating with parameters chosen such that the grating causes resonant coupling between a predetermined guided mode (eg LP.. ) and a tunneling-leakey mode. This is said to be able to be used advantageously to remove unwanted guided modes which can substantially improve the bandwidth of such a system.

According to a first aspect of the present invention an optical communications system includes a central station coupled to a plurality of outstations by an optical waveguide network characterised in that there is at least one refractive grating formed within an optical waveguide of the network which is reflective to optical signals of a wavelength that propagate as a single mode in the network.

The system may have a central station which has transmitting means for transmitting optical signals at a plurality of wavelengths on the waveguide network and at least one of the refractive gratings is reflective to one of the plurality of wavelengths. The grating can be selected to reflect totally at a wavelength which is to be made unavailable to outstations farther down the transmission path thereby preventing unauthorised access to the channel.

The invention is based on the applicant' s discovery that refractive index gratings formed within an optical waveguide, as opposed to surface gratings on the waveguide, are transmissive at wavelengths shorter than the Bragg wavelength reflected by the grating. This means only the reflected channels are blocked.

The fibre gratings may be formed by any convenient method, for example as disclosed in O86/01303 published on the 27th February 1986. Preferably, the sidewriting technique of a co-pending application GB9007912.0 filed on 6th April 1990, of the present applicants is used.

A further advantage of the present invention is that such internal gratings are readily erasable, by heating for example, so a channel block can be readily removed without affecting the optical network whilst doing so.

The refractive index gratings can be prepared away from the optical network and subsequently optically coupled into a network (for example by splicing if the network is an optical fibre network) if a few minutes disruption to the network can be tolerated. In this case the grating may be formed by techniques other than sidewriting if more convenient. In a passive optical network in which a single fibre successively branches to a number of outstations, the grating can be placed at positions which prevent one outstation or a limb of outstations from receiving the selected channel. Several gratings may be formed independently and at different positions if desired, each blocking selected channels thus allowing independent control of the distributions of a channel on the multiplexed network.

The blocking grating is invisible, that is there are no external grating structures or bulk filters, so security is high in that location of the grating for unauthorised erasing is difficult.

Another use of the present invention is to permit monitoring an optical network. Refractive index gratings reflective at a wavelength outside the communications band or bands can be formed at various points in the network. These are transparent to the user of the network yet provide reflectors for probe signals at the appropriate wavelengths. Again, these gratings can be formed and erased without affecting the concurrent operation of the optical network.

According to a second aspect of the present invention, a method of forming an optical communications system includes a central station coupled to a plurality of outstations by an optical waveguide network and at least one refractive grating is formed within an optical waveguide of the network which is reflective to optical signals of a wavelength that propagate as a single mode in the network in which at least one of the gratings is formed by sidewriting. It is possible by this method to apply the invention at any accessible position on a previously installed optical fibre network without affecting the network' s operation at any of the other wavelengths during formation of the grating. A specific embodiment of the invention will now be described, by way of example only, with reference to the accompanying drawing which is a schematic diagram of a passive optical network to which the invention is applicable. Referring to Figure 1 an exemplary optical communications network is shown to which the method of the present invention according to its first aspect is applicable. A central station or exchange 2 when operative transmits a wavelength multiplex of channels 1 to n on respective wavelengths λ. and λ_n. The central station 2 is linked by a passively branching optical network 4 to outstations O_j to 0_. For the purposes of illustration it will be assumed that outstations O_j and 0₂ are initially permitted to receive the channels broadcast on each of the wavelengths λ_j to λ_n.

Should for any reasons it be necessary to deny access to the channel λ _t to an outstation O_j, say, then a refractive index grating 6 is provided at any position between the outstation O_j and the next passive coupler 8 upstream reflective at λ,. All the other channels from the central station 2 will propagate through the grating to station O_j and so will continue to be accessible during and after formation of the grating. Should the channel access need to be reinstated the grating can be erased, by for example suitable heat treatment. The preferred method of forming the grating is described in the applicants above reference patent application GB9007912 but other sidewriting techniques may be applicable (see for example WO86/01303. ) If both O_j and 0₂, only, are to be denied access to the channel on wavelength λ_j then the grating is provided on the portion of the network common to only those two outstations, at position 10, for example. Clearly, not all subgroups of the outstations can be denied access at a chosen channel wavelength by a single grating in the network. In general more than one will be necessary. Only groups sharing access via a common fibre of the network can be affected simultaneously.

If a break in operation of this exemplary network to be controlled by installation of a grating can be tolerated, a reflective grating can be made in a piece of fibre and later spliced in position. Other optical waveguide networks employing non-fibre waveguides might also be open to modification by optically coupling in a portion of a waveguide incorporating a grating.

If it is known before laying the network that certain channels or monitoring signals will need to be reflected from certain positions, the gratings can be provided during initial installation of the network. The invention is also applicable in the above described example to providing wavelength blocks on channels propagating from outstations towards the central station.

It will be appreciated that the invention is applicable to other topologies of network. For example, a 1-to-l optical waveguide link may usefully be provided with one or more gratings.

The invention also finds application with waveguides other than optical fibres, for example germania doped planar waveguides formed by flame hydrolysis deposition. The phrase optical waveguide communications system in this application is intended to include any system capable of or actually propagating intelligence by means of optical signals, ie. in the LTV, visible or infrared regions of the electromagnetic spectrum. It includes for example passive sensor systems in which a passive sensor is interrogated remotely by an optical signal. In such systems, gratings may be installed within the network to reflect optical monitoring signals which gratings are transparent to the interogating optical signals.

Claims

CLAI MS

1. An optical communications system including a central station (2) coupled to a plurality of outstations (0_|) by an optical waveguide network (4) characterised in that there is at least one refractive grating (6) formed within an optical waveguide of the network (4) which is reflective to optical signals of a wavelength that propagate as a single mode in the network (4).

2. A system as claimed in claim 1 in which the central station (2) has transmitting means for transmitting optical signals at a plurality of wavelengths (λ.) on the waveguide network (4) and at least one of the refractive gratings is reflective to one of the plurality of wavelengths. (λ_j).

3. A system as claimed in claim 2 in which there is at least one further grating formed within and optical waveguide of the network (4) reflective to optical signals of a wavelength that propogate as a single mode in the network (4), the wavelength being other than one of the plurality of wavelengths (λ_j).

4. A system as claimed in any preceding claim in which the optical waveguide network comprises an optical fibre network.

5. A system as claimed in claim 4 in which the optical fibre network (4) is a passive optical network (4).

6. A method of forming an optical communications system which system includes a central station (2) coupled to a plurality of outstations (0,) by an optical waveguide network (4) and at least one refractive grating (6) is formed within an optical waveguide of the network (4) which is reflective to optical signals of a wavelength that propagate as a single mode in the network (4) in which at least one of the gratings (6) is formed by sidewriting.