GB2185589A - Optical switching arrangement - Google Patents

Abstract

An optical switching arrangement includes a first optical fibre bundle 3 from which a multiple light beam is directed via a lens system 4 on to an array 5 of multiple quantum well (MQW) devices. After passing through the array the beam is focussed by another lens system 6 on to an output fibre bundle 7. The array allows switching between the beam's input configuration and a desired output configuration. The MQW devices are operable in a detection mode, whereby a desired re-configuration pattern is input optically over one of the fibres and the detected interference pattern that results (which can form a hologram) is conveyed over electrical connections from each device to an electronic store. When switching is required the pattern is recalled and applied as modulation to the devices. The devices may use the self-electro-optic effect or a variant comprising stacked, alternate p-type and n-type layers. <IMAGE>

Description

SPECIFICATION Optical switching arrangement This invention relates to optical switching arrangements using devices each of which can function both as a detector and as a modulator.

Examples of such devices include multiple quantum well (MOW) devices.

One example of such an MOW device is described in a paper "Novel hybrid optically bistable switch: the quantum well self-electro-optic device" by D.A.B.

Milleretal, at pp 13-16 ofApplied Physics Letters, 45(1), 1 July, 1984. This device, referred to as the SEED uses a sandwich structure of alternate layers of GaAs and GaAlAs in a p-i-n diode structure. That is, the sandwich structure is in effect the "active" portion ofthe device. If a light beam falls on this device and passes through it, it can be modulated in accordance with electrical potentials applied across the SEED. Hence we get an output beam which is a modulated version of an input beam.

Another device which has somewhat similar properties is described in our Patent Application No.

8517534(P.A. Kirkby 8). This specification describes an optical fibre network component for connection between an electro-optical source and a detector, which component includes an optical fibre coupling in whichthe end of one optical fibre is optically coupled with the end of another, wherein interposed between the coupled ends ofthetwofibres is a transmissive detector and modulator (TDAM).

Such a TDAM is constituted by a multilayer semiconductive device having a stack of electrically commoned sub-micron thickness p-type layer interleaved with a stack of electrically commoned sub-micron thickness n-type layers.

Thus the two devices specifically mentioned above have similarities, but the SEED uses layers which, according to Miller et al, are not intentionally doped, whereas the TDAM uses alternate p-type and n-type layers. Thus the SEED is a compositional superlattice (CS) device, while the TDAM isadopingsuperlattice (DS) device. The CS device was the first of these to be investigated. H owever, th e TDAM has certain fabrication advantages, and also some operational advantages, e.g. awiderwavelength range.

An object ofthe present invention is to extend the usefulness of components each of which can function both as a detector and as a modulator. As mentioned, examples of such devices include the SEED andtheTDAM,which will be collectively referred to herein as MQW devices.

According to the present invention there is provided an optical switching arrangement which includesafirst o ptica I fibre bundlefrom which a multiple light beam is directed via a first lens system on to one side of a multiple assembly of devices each of which can function both as a detector and as a modulator, wherein a second lens system directs the light leaving that assembly on to a second optical fibre bundle, wherein electrical connections extend between the individual devices ofthe assembly and associated circuitry via which selected beams of light from thefirstfibre bundle can be modulated and re-routed to appropriate ones of the fibres of the second bundle, said modulation and re-routing being effected underthe control of electrical signals applied via said electrical interconnections.

According to the present invention there is also provided an optical switching arrangmentwhich includes a first optical fibre bundlefrom which a multiple light beam is directed via a first lens system on to one side of a multiple assembly of MQW devices, wherein a second lens system on the other side of the multiple assembly directs the light leaving that assembly on to a second optical fibre bundle, wherein electrical connections extend between the individual devices of the assembly and associated circuitry via which selected beams of light from the first fibre bundle can be modulated and re-routed to appropriate ones of the fibres of the second bundle, said modulation and re-routing being effected under the control of electrical signals applied via said electrical interconnections.

The MOW devices as already indicated can function both in the detection and in the modulation mode, so that the pattern of re-routing, or reconfiguration of the input beams which is needed, can be signalled to the assembly of MOW devices by interference between two light beams. This pattern is then recorded in storage means included in the associated electronic circuitryforfuture use. A number of such patterns can be thus recorded, in which case means is provided to select whichever is needed for use.

Embodiments ofthe invention will now be described with reference to the accompanying drawing, in which Figure lisa plan view of an assembly of MQW devices, in the present instance TDAM's, as used in a system embodying the invention, Figure 2 is an enlarged view of one ofthe elements of Figure 1, Figure 3 is a perspective view of the "optics" of a system embodying the invention, and Figure4 is a similarviewofanotherembodiment ofthe invention.

The assembly, see Figure 1, is in the present case circular, and each element is square. Figure 2 shows one ofthe elements enlarged,from which it will be seen that the optically active area 1 is considerably largerthan the associated electronic function 2. Such an array may include a large number of elements, e.g. 1000 x 1000 in a matrix. The electrical connections to these functions are not shown; they can be made in a similarwayto that used forthe connections to individual elements of matrix-addressed active silicon backed liquid crystal devices.

Figure 3 shows the optical arrangements of the system. Hereweseean inputfibrebundle3whose end, i.e. the ends of the individual fibres ofthat bundle, is "aimed" art a lens system represented by the lens 4. Next in line is the multiple device assembly 5; note that the associated electronics and the connections between that and the assembly 5are not shown. The focussing can be such that each individual beam from the fibre ofthe bundle is directed on to different sets of MOW elements.

On the other side of the assembly 5, we see another lens assembly, represented by the lens 6, which focusses the beams emergent from the assembly 5 on to the ends ofthefibres of another bundle7. The focussing effected by the lens system 6 can be altered to focus the beams from different sets of elements on to the ends of the fibres 7.

Thetwo lens systems 4 and 6 could in some cases each be single lenses, or in other cases multi-lens systems.

Thus the pattern ofthe beams from the fibre bundle 3 can be reconfigured before the beams reach the output bundle 7. This reconfiguration is effected underthecontrol oftheelectrical connections to the devices ofthe assembly 5, by altering the modulation pattern within each set of MQW elements so that any desired reconfiguration is possible.

As will beseen from, for instance, our above-mentioned patentapplication,the MOW devices can also function in a detection mode. This is exploited in the present arrangement; thus the desired reconfiguration pattern can be signalled in optically via one ofthefibre bundles, and such signals detected by the MOW devicesfunctioning in their detection mode. The results of these detections are sent via the above-mentioned electrical connections to storage means so th at they are stored in the associated electronics. The interference patternsthussignalled in are then, in effect, stored as mathematical functions.

The fibre bundles referred to can be a bundle whose fibres and the light they convey are related to each other, or they can be individual (e.g.

monomode) fibres.

Thusthearrangementprovidestheabilityto generate the hologram for'a desired interconnection pattern by optical means and to store the information therefor electrically. Thus there is a considerable saving in time and energy compared with a mathematical derivation ofthe pattern.

One reconfiguration that can be effected in the above manners an incremental rotation ofthe pattern defined by the fibres ofthe bundle. This rotation can be continuous or discontinuous, and may be applied to thetopological manipulation of two-dimensional wavefronts which are not derived from optical fibres e.g. expanded plane wavefronts.

This involves co-ordinate transormation on freely propagating image wavefronts, which is important in such applications as pattern classification and image processing.

We now consider extensions and applications of the techniques referred to above. There is a requirement in both telecommunications and computing for devices which can reconfigure, in the general case, N input channels into M output channels. Practical applications include network routing and alignment of data in an array processor.

The traditional solution is the NXM crossbar switch.

The time required by this switch is O(log N) orO(log M) gate delays, and the number of gates required is N.M. The term 0 (log N) means the order of log N ratherthan the order of N.O.

The number of gates is reduced in the Batcher sorting networkto O(N(log N)1),butO(log N)1) time units are required. Alternatively, the Omega network can be used. This consists of log N stages, where each stage is a perfect shuffle followed by N/2 switching elements. Therefore, the total number of switching elements is N/2 log N, and the time delay is log N. The greater efficiency ofthe Omega network is gained by processing the channels in parallel.

Unfortunately, it is only unidimensional parallelism and is not entirely conflict free. It may be profitableto consider the two-dimensional parallelism, i.e.

left-right-up-down switching, which is possible with (laterally) extended light beams and optical switching arrays.

In one such array, Figure 4, a relationship exists between a point in the input plane 20 and a point, or group of points, in the output plane 21 such that a translation of the input point results in a corresponding translation ofthe output point(s). The input plane 20 represents the end of an optical fibre bundle, i.e. the end from which incoming light reaches the lens system 22, while the plane 21 represents the end of the output optical fibre bundle.

Here also we have afirst lens (or lens system) 22,a hologram 23 and a second lens (or lens system) 24.

As an interconnection strategy this is most profitably exploited as a one-to-many switch in conjunction with ancillary beamsto enable or disable the desired or unwanted points. The number of resolution elements required forthe hologram is equal to the number output channels (multiplexed by coding and beam-shaping factors) forthe whole ofthe hologram The hologram forthe arrangement of Figure 4 can be written on an electrically addressed spatial light modulator (SLM). the mathematical form ofthe hologram is generated bycomputercalculation.

Alternatively, an optically addressed SLM with some form of erasable memory could be used. The hologram is written by interfering a beam issuing from the required channels in the outputarray,with a beam which is conjugatetothe input beam. Thus, a second input array 30 is placed in the image plane of the original input array 21, Figure 6. When a new interconnect is required, the hologram is erased and a new one is formed. The wavelength of the light used to form the hologram must differ from that of the lightto be switched, in orderthatthe hologram is not erased in use. This leads to aberrations in the performance of the hologram, and is not entirely satisfactory forthis reason. Hence a device which exhibits degenerate fourwave mixing is preferred, and the beams from the two input arrays (now ofthe samewavelength) are maintained during the switching operation ofthe system. Unfortunately, a problem remains with this system, also. The degenerate fourwave mixer is usually of such a thickness (to maintain diffraction efficiency with weak optical non-linearity), thatthe spatial extent of the output array which can be addressed is limited.

Materials with larger non-linearities, such as liquid crystals close to the nematic/isotropictransition, are of considerable advantage in this respect.

A relatively simple (in terms of system enactment rather than device design) alternative exists for the two-dimensional TDAM arrays. The TDAM as already indicated, is a device (e.g. multiple quantum wells on GaAs) which can record an interference pattern, and subsequently reinstate that pattern as an amplitude modulation. In this case, all the required interference patterns could be learnt by the device and stored in an electronic memory. When switching is required, the pattern is recalled from memory. An added attraction of this device is the possibility of analogue modulation which reduces the resolution requirements on the array.

Reverting to the system of Figure 4, as indicated this shows a method of forming the interconnect hologram in a system in which the input and output are fixed. A "dummy" array 30 is placed in a conjugate position to the input bundle 20. In this context, "conjugate position" means the image position if there is nothing in the plane 23. To write the required interconnect, the expected input pattern is composed on 30 and the required output pattern on 21. These interfere atthe hologram plane 23 and the hologram is written. This requires that light can be transmitted in both directions in the output bundle.

To utilise the hologram, the bundle 30 is switched off and the input applied to 20. The bundle 21 then receives the connect output pattern.

Claims (6)

1. An optical switching arrangementwhich includes a first optical fibre bundle from which a multiple light beam is directed via a first lens system on to one side ofa multiple assembly of devices each of which can function both as a detector and as a modulator, wherein a second lens system directs the light leaving that assembly on to a second optical fibre bundle, wherein electrical connections extend between the individual devices of the assembly and associated circuitry via which selected beams of light from thefirstfibre bundle can be modulated and re-routed to appropriate ones ofthe fibres ofthe second bundle, said modulation and re-routing being effected underthecontrol of electrical signals applied via said electrical interconnections.

2. An optical switching arrangement which includes a first optical fibre bundle from which a multiple light beam is directed via a first lens system on to one side of a multiple assembly of MOW devices, wherein a second lens system on the other side ofthe multiple assemblydirectsthe light leaving that assembly on to a second optical fibre bundle, wherein electrical connections extend between the individual devices ofthe assembly and associated circuitry via which selected beams of light from the first fibre bundle can be modulated and re-routed to appropriate ones ofthe fibres ofthe second bundle, said modulation and re-routing being effected under the control of electrical signals applied via said electrical interconnections.

3. An arrangement as claimed in claim 1 or 2, and in which a hologram is formed on said multiple assembly when the arrangement is in use.

4. An arrangement as claimed in claim 3, in which the hologram is supplied to the arrangement via one ofthe optical fibre bundles so as to be detected by the devices of the assembly, and in which signals representative ofthe hologram are applied from the assemblyto electrical storage means.

5. An arrangement as claimed in claim 2, in which an additional optical fibre bundle is located adjacent to the output optical fibre bundle, from which a hologram can be transmitted into the assembly of devices.

6. An optical switching arrangement, substantially as described with reference to Figures 1,2and3,ortoFigures1,2and4ofthe accompanying drawings.