GB2394375A

GB2394375A - Integrated optical monitor with polarisation scrambling

Info

Publication number: GB2394375A
Application number: GB0224336A
Authority: GB
Inventors: Owen Burke; Ivan Evans; Daniel Kitcher; Jonathan Bradley Luff
Original assignee: Bookham Technology PLC
Current assignee: Lumentum Technology UK Ltd
Priority date: 2002-10-19
Filing date: 2002-10-19
Publication date: 2004-04-21
Also published as: GB0224336D0

Abstract

The invention relates to optical monitors for monitoring signals independently of the polarisation of the signal. A polarisation scrambler 1 comprises an input waveguide 2, a splitter 5, two intermediate waveguides 3, 4, a combiner 7 and an output waveguide 6. One intermediate waveguide incorporates a polarisation rotator 9 and the other incorporates a phase modulator 10. In operation an input optical signal is equally split with one component being rotated through 90{ and the other component subjected to a phase shift controlled by a periodic electrical drive signal. The combined output is a polarisation scrambled signal having a time average substantially independent of the polarisation of the input signal. One or more scrambled signals are passed to a multiplexing/demultiplexing arrangement 43-48 to supply selected channels to a photodetector array 49 for monitoring purposes. Several embodiments are described. The device can achieve accurate and low polarisation-dependence monitoring, whilst being fabricated on a single chip.

Description

1 2394375

"Optical Devices" The present invention relates to optical devices and particularly to optical monitors for monitoring an optical signal substantially independently of the polarization of the optical signal.

The accurate monitoring of optical signals provides a difficult challenge for integrated optical systems because of the need to minimise the polarization dependence of the monitoring signal. Typically multichannel optical monitors, such as optical channel monitors (OCM) or optical spectrum analysers (OSA), also known as optical performance monitors (OPM) , comprise an arrayed waveguide grating (AWG) for splitting the multichannel input signal into its constituent channel signals and a series of photodiodes for detecting the channel signals.

Furthermore optical monitors using waveguide-based optical technology have been developed for wavelength division multiplexing (WDM) in optical communication systems. In WDM systems different data channels within a light signal transmitted along a single optical fibre or waveguide are differentiated according to the wavelength band of the transmitted light corresponding to that channel. Signal processing in such WDM systems involves multiple combination and/or separation of the individual multiplex channels using multiplexer/demultiplexer devices. Similar technology can also be used to produce small-sized spectrometers for use in substance analysis outside the laboratory.

WO 99/57834 (University of Maryland) discloses a real time wavelength monitoring circuit for monitoring signals in a WDM optical communication system using a phased array waveguide grating (PAWG), such a circuit being used to monitor the wavelength distribution of signals at different nodes within the communication system. In order to increase the resolution of the resulting spectrum analyser the PAWG has multiple inputs, and the input signal to the circuit is switched so as to be supplied successively to a first centre wavelength offset input and a second centre wavelength offset input, the corresponding outputs from the PAWG being detected by

r detectors which in turn supply signals by way of amplifiers and an A/D converter to a signal processor. The signal processor analyses the output signals produced in each measurement phase, and accurately monitors the wavelength of the signals using discrimination curves based on the ratios between pairs of output signals resulting from input signals supplied to different inputs of the PAWG. However such apparatus requires the use of input switches for switching between successive measurement phases, and this introduces both complexity and possible losses.

Standard single mode, non-polarisation-maintaining (PM) optical fibres do not preserve the launched polarisation, with the result that the output polarisation varies as a function of time and wavelength. The time variation of the polarisation would be expected to take place on a slow time scale of the order of seconds to days (or longer) varying with external conditions such as stress and temperature.

Furthermore it is extremely challenging to manufacture planar lightwave circuits (PLC) with ultra-low polarisation dependent losses (PDL). The inherent waveguide asymmetry results in geometric birefringence, and residual stress in the waveguide layers can give rise to further stress birefringence. This residual birefringence is one of the causes of polarisation dependence in PLCs. In such optical monitors both the optical splitting arrangement and the optics of the photodiodes exhibit birefringent properties, in that they provide a refractive index for light of one polarisation mode, for example transverse electric (TE) polarised light, which differs from the refractive index for light of another polarisation mode, for example transverse magnetic (TM) polarised light. US 5911016 discloses a polarisation scrambler comprising a Yjunction splitter for dividing an optical input signal between first and second branch waveguides, a polarisation rotator for adjusting the relative polarisation for the signals supplied to the two branch waveguides, and a Yjunction combiner for combining together the signals from the first and second branch waveguides to supply a scrambled optical output signal to an output waveguide. A polarisation mode dispersion compensator operating on similar principles is disclosed in T. Saida et al, "Planar Lightwave Circuit Polarisation

Mode Dispersion Compensator", IEEE Photonics Technology Letters, Vol.14, No. 4, April 2002. K. Takada et al, "Optical Spectrum Analyzer using Cascaded AWGs with Different Channel Spacings", IEEE Photonics Technology Letters, Vol. 11, No. 7, July 1999 discloses an OPM using multiple AWGs and multiple AWG inputs with a switching arrangement.

It is an object of the invention to provide an optical device having reduced polarization dependence that may be formed in a straightforward manner, during fabrication of an integrated device based on silicon-oninsulator (SOI) technology, for example.

According to the present invention there is provided an optical device comprising an input waveguide for receiving an optical input signal, splitting means for dividing the optical input signal between first and second branch waveguides, polarization conversion means for converting the optical signal supplied to the first branch waveguide to a differently polarised optical signal, modulation means for varying the optical signal supplied to the second branch waveguide with respect to the optical signal supplied to the first branch waveguide in accordance with a time-varying drive signal, combining means for combining the optical signals from the first and second branch waveguides to supply a scrambled optical output signal to an output waveguide having a time average which is substantially independent of the polarization of the optical input signal, and multiplexing/demultiplexing means having at least one input for receiving the scrambled optical output signal and a plurality of outputs for supplying channel output signals.

Such arrangement is advantageous since the device can achieve accurate and low polarisation-dependence optical monitoring, whilst being capable of being fabricated on a single chip utilising silicon-on-insulator (SOI) technology, so that the device can be produced at a relatively low unit cost.

Preferably at least the waveguides, the polarization conversion means and the phase modulation means are integrally formed on a substrate using SOI (silicon-on

insulator) technology. In one embodiment of the invention the device is an optical monitor incorporating photodetector means, for example, photodiodes. In this case the photodiodes may be in the form of discrete photodiodes or a photodiode array, and the photodiodes or other monitoring means may be monolithically integrated or provided externally of the substrate.

The invention also provides an optical scrambler comprising a plurality of input waveguides for receiving optical input signals, a corresponding plurality of splitting means for dividing the optical input signals between first and second branch waveguides, polarization conversion means associated with each first branch waveguide for converting the optical signal supplied to the first branch waveguide to a differently polarised optical signal, modulation means associated with each second branch waveguide for varying the optical signal supplied to the second branch waveguide with respect to the optical signal supplied to the first branch waveguide in accordance with a time-varying drive signal, combining means for combining the optical signals from the first and second branch waveguides to supply a respective scrambled optical output signal to each of a plurality of output waveguides having a time average which is substantially independent of the polarization of the optical input signal, and demultiplexing means having a plurality of inputs for receiving scrambled optical output signals from the combining means and a plurality of outputs for supplying demultiplexed output signals.

Preferred and optional features of the invention will be apparent from the subsidiary claims of the specification.

For a better understanding of the present invention and to show how the same may be carried into effect, various embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 diagrammatically shows an integrated optical polarization scrambler for a particular wavelength band for use in a first embodiment of the invention;

Figure 2 diagrammatically shows four polarization scramblers for use in a second embodiment of the invention; Figure 3 diagrammatically shows four polarization scramblers and two offset selectors for use in a third embodiment of the invention; Figures 4, 5 and 6 show three alternative polarization switching devices for use in further embodiments of the invention; Figures 7 to 13 diagrammatically show further arrangement for use in further embodiments of the invention.

The embodiments of the optical device to be described below may be optical channel monitors (OCM), optical performance monitors (OPM), optical spectral analyser monitors (OSAM) or any other type of optical device, and it will be well understood to persons skilled in the art how such devices will be adapted to suit their particular applications. Each of the described embodiments incorporates one or more optical scramblers in order to render the monitoring substantially independent of the polarization of the input optical signal, and to thereby increase the measurement accuracy as compared with conventional devices which do not incorporate such optical scrambling. Furthermore each embodiment is suitable for being fabricated using SOI technology with at least the components of the optical scrambler being fully integrated on a single silicon optical chip having V-groove structures for attachment of optical fibres, for example. Where more than one optical channel is provided, all the optical channel components are integrated on a single chip.

Figure 1 shows the simplest case in which the integrated device 1 is provided for a particular wavelength band and comprises an input waveguide 2, two intermediate waveguides 3 and 4, optically coupled to the input waveguide by a 2 x 2 coupler 5, and an output waveguide 6 optically coupled to the intermediate waveguides 3 and 4 by a 2 x 2 coupler 7. A second output of the coupler 7 is optically coupled to a deep etched beam dump 8. Furthermore the intermediate waveguide 3 incorporates a 90

polarisation rotator 9, as described in US 5703977 for example, and the intermediate waveguide 4 incorporates a phase modulator lO. The phase modulator 10 may be a carrier injection based phase modulator as disclosed in US 5757986 for example, or a thermal phase modulator. Such a thermal phase modulator may be formed by thin metal tracks patterned near the waveguide and formed at the wafer level as part of the fabrication process. Then patterned tracks are dimensioned such that a suitable current and voltage gives a suitable heating effect, typically having a thickness of 0. 1-5 microns and a width of 2-100 microns, the metal of the tracks typically being aluminium, gold, chromium, etc. It will be appreciated that the couplers 5 and 7 together form a Mach-Zehnder interferometer having the polarisation rotator 9 in one path and the phase modulator lO in the other path.

In operation of the monitor an optical input signal supplied to the input waveguide 2 is equally split by the coupler 5 between the two intermediate waveguides 3 and 4. The optical signal supplied to the intermediate waveguide 3 has its polarisation rotated through 90 by the polarisation rotator 9 that comprises a periodic etched pattern in a cladding layer on the substrate causing an abrupt small change (for example 10 ) in the polarisation of the signal at each pattern change. Thus, for example, the polarisation rotator 9 may serve to change TE light supplied to the waveguide 3 to TM light as a result of the 90 rotation. Furthermore the proportion of the optical input signal which is supplied to the intermediate waveguide 4 is subjected to a phase shift by the phase modulator 10 which is controlled by a periodic electrical drive signal to ensure that the optical output signal of the scrambler has a time average which is substantially independent of the polarisation of the optical input signal. The signals supplied to the coupler 7 from the intermediate waveguides 3 and 4 are combined so as to supply the optical output signal to the output waveguide 6 and a further optical output signal which is dumped by the beam dump 8.

In the case of an ideal 90 polarisation rotator 9, 50% of the light supplied to the coupler 7 is dumped, and the remaining 50% of the light is supplied to the output

waveguide 6. When the phase modulator 10 has a balanced setting, TE light is split 50:50 between the two intermediate waveguides 3, 4, and the resulting TM light outputted by the polarisation converter 9 is combined with the TE light outputted by the phase modulator 10, thus supplying an optical output signal to the output waveguide 6 having a linear space plus 45 polarisation. If the phase modulator 10 is advanced by \/4, then the polarisation of the optical output signal is circular, whereas, if the phase modulator 10 is advanced by \/2, then the polarisation of the optical output signal is linear- 45 .

Considering now the case of left-circular polarised light split 50:50 between the two intermediate waveguides 3, 4, the right-circular polarised light outputted by the polarisation rotator 9 is combined by the coupler 7 with the left-circular polarised light outputted by the phase modulator 10, with the result that linear TE polarised light is supplied at one output of the coupler 7 and liner TM polarised light is supplied at the other output of the coupler 7. If the phase modulator is advanced by \/4, then the polarisation of the light at both outputs of the coupler 7 is circular. If the phase modulator 10 is advanced by \/2, then the linear TM and TE polarizations at the outputs of the coupler 7 are interchanged. Where the optical input signal has opposite circular polarisation, TE and TM polarised light is again supplied at the outputs of the coupler 7, but in this case corresponding outputs are obtained with modulator settings which are reversed as compared with those previously described.

The above examples assume that the birefringence in each waveguide arm is zero or has been compensated for. Even in the absence of zero birefringence the device will still operate as a scrambler but with a different output polarisation to that described above. If the phase modulator 10 is supplied with an AC drive signal of sufficient frequency and amplitude, then the optical output signal supplied to the output waveguide 6 has the property that its time average is either i/2 TE + I/2 TM or /4 + 45 + [/2 circular + '/. - 45 if averaged over a sufficient number of cycles. If the phase modulator 10 receives appropriate periodic drive signals to cause the phase to follow a

ramp waveform up and down one cycle then the optical output signal supplied to the output waveguide 6 has the property that its time-average is either i/ TE or a /z TM.

Such a ramp waveform has the advantage that it is compatible with thermal phase modulators driven with substantially square waveforms. Offsets of the phase modulator caused by manufacturing imperfections (imperfectly balanced Mach-

Zehnder) are eliminated and no phase bias is needed. Also drift of the phase modulator bias point is eliminated. Only the amplitude of the phase modulator needs to be controlled. Although it has been shown above for four polarisation states of the optical input signal that the time average of the optical output signal is substantially independent of the polarisation of the optical input signal, such an arrangement is not a full polarisation scrambler because it will not achieve full polarisation changing with respect to time for some input polarisation states. However it will scramble the light in a manner which should average the polarisation dependent losses (PDL) of further integrated optical components on the remaining part of the chip. However this might fail if later paths on the same chip can contain accidental polarisation changes.

In a non-illustrated variation of the arrangement of Figure 1 the inputs of the coupler 5 may be coupled to two separate input waveguides for receiving two different optical input signals, for example signals from different monitor tap-off points of a WDM system. In this case a selector VOA (variable optical attenuator) is provided on each of the input waveguides, and the two VOAs are controlled such that, at any one time, one VOA is in a blocking state and the other VOA is in a non-blocking state so that the output waveguide passes light originating from the input waveguide having the VOA which is not blocked.

An extension of such an arrangement to an arrangement having eight input waveguides is shown diagrammatically in Figure 2 where each pair of input waveguides 2, 12 incorporates a pair of VOAs 14, 15 for alternately supplying the corresponding optical input signal to the coupler 5. Such an arrangement effectively comprises four optical scramblers in parallel, and the same reference numerals are used to denote like parts in the four scramblers. However the eight VOAs 14, 15 are controlled so that, at

any one time, only one of the VOAs is not blocked and the other seven VOAs are set to > 50dB attenuation, for example. In this case only one of the phase modulators needs to be driven at a time, although it may be preferable to leave all modulators running for the whole of the time. The optical output signals supplied to the four output waveguides 6 may be supplied to four offset inputs of an AWG to enable these optical output signals to be separately monitored by photodiodes optically coupled to the outputs of the AWG.

Each of the scramblers has a 3 dB loss associated with it in addition to the coupler loss, which can be arranged to be small.

A further alternative arrangement is diagrammatically shown in Figure 3 in which the two outputs of each coupler 7 are coupled to respective output waveguides 6 and 16 and the beam dump is dispensed with. In this case the optical output signals are supplied to the left and right offset inputs of the AWG, and this enables the number of photodiodes in a single stage OPM to be halved or the number of selections in the first AWG of a two-stage OPM to be halved. Such an arrangement would be suitable for a monitor monitoring the power and wavelength of 120 DWDM optical signals using 128 photodiodes. The input VOAs 14 and 15 are controlled so as to select one or other of the input signals supplied to the input waveguides 2 and 12, and corresponding output VOAs 17 and 18 are controlled so as to select which of the two offset outputs, to which the output waveguides 16, 6 are coupled, the output signal is to be directed. As previously only one of the input VOAs 14, 15 is in a non-blocking state at a time so that the photodiodes monitoring the output of the AWG provide outputs corresponding to the input channels consecutively.

In each of the embodiments described above the polarisation scrambling is achieved by Mach-Zehnder arrangements incorporating polarization converters and phase shifters between two n x n couplers so as to produce an output signal by interference between the signals outputted by the two arms of the arrangement.

However each scrambler may instead incorporate polarization switching arrangements for providing an output signal which alternates between two polarization states.

l One possible polarisation switching arrangement is shown diagrammatically in Figure 4 and comprises an input Yjunction 20 to which the input signal is supplied along an input waveguide 21, and an output Yjunction 22 for supplying an output signal to an output waveguide 23. Between the Yjunctions 20 and 22 are two intermediate waveguides 24 and 25 incorporating respective VOAs 26 and 27 which serve as polarisation selectors by virtue of the fact that a 90 polarisation converter 29 is incorporated in the waveguide 24. At any one time one of the VOAs 26, 27 is set to a blocking state, and the other VOA 26, 27 is set to its nonblocking state, so that the output signal supplied to the output waveguide 23 alternates between the polarisation state of the input signal and the opposite polarisation state as a result of the input signal passing through the polarisation converter 29. An etched or absorbing beam dump 28 is provided to dump any stray light which is not conducted along the output waveguide 23.

In this case the output signal is not produced as a result of interference between the signals supplied along the intermediate waveguides. Half of the light intensity is lost due to the fact that one of the VOAs 26, 27 will always be in the blocking state, and additional losses are associated with the Yjunctions 20, 22.

In operation of such an arrangement, the input signal is polarised in the TE mode, and the output signal is polarised in the TE mode for half of the time and in the TM mode for the other half of the time (having been rotated through 90 ). If the input signal is + 45 linearly polarised, then the output signal will be + 45 linearly polarised for half of the time and - 45 linearly polarised for the other half of the time. Generally, for any polarisation state of the input signal, the output signal may be represented as ATE + /TM, and the required scrambled output signal is obtained. Such an arrangement is suitable for a slow scrambling rate provided that an even number of VOA switchings occur during the measurement period.

The arrangement of Figure 5 is similar to the arrangement of Figure 4 except that two input waveguides 30 and 31 provided with respective input VOAs 32 and 33 are provided for selectively supplying one of two optical input signals to a 2 x 2 coupler 34. In this case the input VOAs 32 and 33 are alternately cycled between their blocking and non-blocking states so as to admit only one of the input signals at a time to the

scrambler. Such an arrangement enables scrambling of two input signals simultaneously. Figure 6 shows a development of the arrangement of Figure 5 in which a second 2 x 2 coupler 35 is provided in place of the Yjunction 22 for coupling to two output waveguides 36 and 37 having respective output VOAs 38 and 39. The output waveguides 36 and 37 are coupled to + and - offset inputs of an AWG, and the output VOAs 38, 39 are controlled so as to alternately block and unblock the output signal transmitted along the corresponding output waveguide 36, 37. Thus only one output signal is supplied to the AWG at a time. The control of the input and output VOAs 32, 33, 38, 39 may be such as to cycle through eight VOA settings comprising every permutation of inputs and offsets to give two useful measurement points per photodiode (2 inputs x 2 offsets, ATE + /TM or circular).

Monitors incorporating any of the polarisation scramblers described above may incorporate demultiplexers, and optionally also multiplexers, for selectively applying the scrambled output signals to the photodetector array.

Figure 7 diagrammatically illustrates an optical performance monitor (OPM) 40 having an input waveguide 41 for receiving a WDM input signal and a polarisation scrambler 42 for supplying a scrambled output signal to a demultiplexer 43 for demultiplexing the signal to supply channel signals to a series of intermediate waveguides 44 and fine selection VOAs 45 which are operated so as to supply one channel signal at a time to a multiplexer 46. The output of the multiplexer 46 is coupled by a waveguide 47 or a polarisation maintaining (PM) fibre to the input of a demultiplexer 48. The demultiplexers 43 and 48 are AWGs such that the free spectral range of the demultiplexer 43 maps onto the channel spacing of the demultiplexer 48, and thus the VOAs 45 provide fine selection of the output signals of the demultiplexer 48 supplied to a photodetector array 49 comprising a large number of photodiodes.

Thus the electrical output signals from the photodiodes provide a large number of measurements of the light intensity over the complete spectral range of the input signal.

Figure 8 diagrammatically shows an OPM 50 which is a development of the OPM 40 of Figure 7, with like parts being denoted by the same reference numerals as in Figure 7. However, in this case, eight input waveguides 51 of four polarization scramblers 52, which may be configured as shown in Figure 2 for example, are provided for supplying input signals to four offset inputs of a demultiplexer 53. The input waveguides 51 are provided with respective VOAs 54 which are controlled in such a manner as to apply only one input signal at a time to the scrambler 52. As with the embodiment of Figure 7, the VOAs 45 are operated so as to supply one input at a time to a multiplexer 55 for supplying output signals by way of four waveguides 56 or PM fibres to a second demultiplexer 57 having four offset inputs.

More particularly the VOAs 54 are controlled so that, in a first phase, the first VOA 54 is controlled to pass an input signal along the first waveguide 51 whereas the input signals applied to the other seven waveguides 51 are attenuated by the corresponding VOAs 54. During this first phase, a first channel signal is passed by the first of the VOAs 45, whilst the other VOAs 45 act to attenuate the other channel signals so as to provide the required fine selection. The resulting optical output signals are then read by the photodiodes of the array 49. The second channel signal is then passed by the second VOA 45 and the other channel signals are attenuated by the other VOAs 45 and the resulting output signals are read by the photodiodes, this sequence of operations being repeated until all the channel signals have been read in the first phase.

The second VOA 54 then operated to pass the input signal on the second input waveguide 51, the other input signals being attenuated by the other VOAs 54 in a second phase, and the VOAs 45 are used to pass each channel signal in turn. This sequence of operations is repeated until all the input signals supplied to the input waveguides 51 have been processed, the whole sequence of operations corresponding to a single measurement cycle.

It will be appreciated that the effect of changing the offset input of the demultiplexer 58 to which an input signal is supplied is to shift the output signals of the demultiplexer 53 or 57 so that the channel centres are shifted by a known amount. The amount of this shift can be chosen such that it equals the output channel spacing of the

l AWG and that the measurement points are the same irrespective of which input is selected. Figure 9 shows a variant 60 of the OPM of Figure 8 in which the multiplexer 55 is dispensed with, and instead the waveguides 44 are coupled so as to supply input signals directly to multiple offset inputs of a demultiplexer 58. As before the AWG constituting the demultiplexer 53 has a free spectral range corresponding to the channel spacing of the AWG constituting the demultiplexer 58. The number of photodiodes in the photodetector 49 corresponds to the sum of the number of channels of the demultiplexer 58 and the number of offsets and inputs.

Figure 10 diagrammatically shows a further variant of OPM 70 in which the demultiplexer 53 is dispensed with and the outputs of the polarization scramblers 52 are connected directly to the waveguides 44. The number of outputs of the scramblers 52 corresponds to the number of inputs as in the arrangement shown in Figure 3. In this case the number of measurements obtained from the photodetector array 49 corresponds to the number of outputs of the demultiplexer 58 less the number of inputs of the demultiplexer 58. Thus, where the photodetector array 49 comprises 128 photodiodes, the monitor may be adapted to measure the power and wavelength of up to 120 channels in a sequence of two offsets. Allmeasurements are preferably taken with the scramblers running at maximum rate.

Figures 11 and 12 show a further OPM 80, Figure 12 showing the monitor diagrammatically in similar form to Figure 10 but with the addition of Yjunctions 61 and further selector VOAs 62 for supplying signals to a demultiplexer 63 having additional offset inputs. In this case the number of measurements provided by the photodetector array 49 may be equivalent to the product of (the number of offsets) and (the number of outputs less the number of inputs of the demultiplexer 63 plus one).

Thus, for example, a photodetector array 49 incorporating 128 photodiodes may provide 4 x (128 - 16 + l) = 453 measurements. Such a monitor may be used to detect detailed spectra or to measure the power and wavelengths with redundancy to protect against missing photodiodes and related damage. In the more detailed diagram of Figure 11

each of the polarisation scramblers is shown as having the same form as in Figures 1, 2 and 3 and like reference numerals are used to denote the same parts as in those figures.

The demultiplexer 63 and photodetector array 49 are omitted from Figure 11.

Figure 13 diagrammatically shows an OPM 90 having a generally similar configuration to that of Figure 11, but with the Yjunctions 61 and VOAs 62 omitted and a further demultiplexer 71 in the form of an AWG provided, as in the arrangement of Figure 9. In this case the demultiplexer 71 has a very small free spectral range, and the outputs of the demultiplexer 71 are coupled to intermediate waveguides 44 incorporating VOAs 45 for supplying input signals to the demultiplexer 63 (not shown in Figure 13). As before the free spectral range of the demultiplexer 71 maps onto the channel spacing of the demultiplexer 63. In this case the number of measurements obtained from the associated photodetector array may be equivalent to twice the product of (the number of offsets from the demultiplexer 71) x (the number of inputs to the demultiplexer 63 less the number of offsets) x (the number of photodiodes less the number of inputs to the demultiplexer 63).

If 32 photodiodes are provided then up to 512 measurement point spectra can be obtained. If 128 photodiodes are provided then up to 3,300 measurement point spectra can be obtained. However such a monitor is preferably used as a power wavelength monitor with redundancy, in which case typically 1,600 points (5 GHz point spacing) might be obtained using 128 photodiodes.

It will be appreciated that various modifications of the above-described arrangements can be contemplated within the scope of the invention. For example the demultiplexers may be in the form of devices other than AWGs, and the VOAs may be replaced by optical switches where appropriate. Many other embodiments in accordance with the invention will be apparent to those skilled in the art.

Claims

l CLAIMS:

1. An optical device comprising an input waveguide for receiving an optical input signal, splitting means for dividing the optical input signal between first and second branch waveguides, polarisation conversion means for converting the optical signal supplied to the first branch waveguide to a differently polarised optical signal, modulation means for varying the optical signal supplied to the second branch waveguide with respect to the optical signal supplied to the first branch waveguide in accordance with a time-varying drive signal, combining means for combining the optical signals from the first and second branch waveguides to supply a scrambled optical output signal to an output waveguide having a time average which is substantially independent of the polarization of the optical input signal, and multiplexing/demultiplexing means having at least one input for receiving the scrambled optical output signal and a plurality of outputs for supplying channel output signals.

2. An optical device according to claim l, including photodetector means for detecting the channel output signals and for supplying an electrical output indicative of said channel output signals.

3. An optical device according to claim l or 2, wherein the demultiplexing means comprises a wavelength dispersive element.

4. An optical device according to claim 3, wherein the wavelength dispersive element is an arrayed waveguide grating (AWG).

5. An optical device according to any preceding claim, wherein the waveguides, the polarization conversion means and the phase modulation means are integrally formed on a substrate.

6. An optical device according to claim 5, wherein the substrate is a SOI (silicon-

on-insulator) substrate.

7. An optical device according to any one of claims I to 6, wherein the modulation means comprises phase modulation means for varying the phase of the optical signal supplied to the second branch waveguide with respect to the optical signal supplied to the first branch waveguide in accordance with the time-varying drive signal.

8. An optical device according to any one of claims l to 6, wherein the modulation means comprises attenuation means for alternately blocking the optical signal supplied to the first branch waveguide and the optical signal supplied to the second branch waveguide in accordance with the time-varying drive signal.

9. An optical device according to claim 8, wherein the attenuation means comprises a first attenuator for alternately blocking and unblocking the optical signal supplied to the first branch waveguide and a second attenuator for alternately blocking and unblocking the optical signal supplied to the second branch waveguide, the first and second attenuators being controlled in antiphase to one another by the time-varying drive signal.

10. An optical device according to claim 7, wherein the phase modulation means comprises heating means, such as a thin film heater, for locally heating the branch waveguide in response to an applied electrical control signal to change the refractive index in accordance with the thermo-optic effect.

11. An optical device according to claim 7, wherein the phase modulation means comprises carrier injection means, such as a p-i-n diode, for changing the local refractive index in the branch waveguide in response to an applied electrical control signal by virtue of the plasma dispersion effect.

12. An optical device according to any preceding claim, wherein the polarisation conversion means comprises a structure providing a varying refractive index in the direction of propagation for asymmetrically perturbing the optical signal.

13. An optical device according to any preceding claim, wherein the polarisation conversion means comprises a periodically varying structure in the direction of propagation for asymmetrically perturbing the optical signal.

14. An optical device according to any one of claims I to 13, wherein the splitting and/or combining means comprises a Y junction.

15. An optical device according to any one of claims 1 to 13, wherein the splitting andlor combining means comprises an evanescent coupler.

16. An optical device according to any one of claims 1 to 13, wherein the splitting and/or combining means comprises a multi-mode interference (MMI) coupler.

17. An optical device according to any one of claims 1 to 16, wherein the combining means comprises a Y junction having its output coupled to the output waveguide and a beam dump for absorbing light dispersed at the Y junction.

18. An optical device according to any one of claims 1 to 16, wherein the combining means comprises a 2 x 2 optical coupler having a first output coupled to the output waveguide and a second output coupled to a beam dump.

l 9. An optical device according to any one of claims 1 to 16, wherein the combining means comprises a 2 x 2 optical coupler having a first output coupled to a first output waveguide and a second output coupled to a second output waveguide, selection means being provided for selectively applying to a further optical device a first optical output signal by way of the first output waveguide or a second optical output signal by way of the second output waveguide.

20. An optical device according to any preceding claim, wherein the splitting means comprises a 2 x 2 optical coupler having a first input coupled to a first input waveguide

l and a second input coupled to a second input wavcguide, selection means being provided for selectively applying to the optical coupler a first optical input signal by way of the first input waveguide or a second optical input signal by way of the second input waveguide.

21. An optical device according to claim 19 or 20, wherein the selection means comprises at least one variable optical attenuator (VOA) for selecting a required optical signal in dependence on wavelength.

22. An optical device according to any preceding claim, comprising a plurality of input waveguides for receiving optical input signals, a corresponding plurality of splitting means for dividing the optical input signals between first and second branch waveguides, polarization conversion means associated with each first branch waveguide for converting the optical signal supplied to the first branch waveguide to a differently polarised optical signal, modulation means associated with each second branch waveguide for varying the optical signal supplied to the second branch waveguide with respect to the optical signal supplied to the first branch waveguide in accordance with a time-varying drive signal, and combining means for combining the optical signals from the first and second branch waveguides to supply a respective scrambled optical output signal to each of a plurality of output waveguides having a time average which is substantially independent of the polarization of the optical input signal.

23. An optical device according to claim 22, wherein the demultiplexing means is provided with a plurality of offset inputs for receiving scrambled optical output signals from the combining means, and selection means for sequentially applying selected optical output signals from the demultiplexing means to the photodetector means.

24. An optical device according to claim 22 or 23, wherein the demultiplexing means comprises an arrayed waveguide (AWG) having a first centre wavelength offset input for receiving a first offset output signal from the combining means and a second centre wavelength offset input for receiving a second offset output signal from the

l combining means and a plurality of outputs for supplying selected optical output signals in dependence on which of the offset inputs is selected by offset selection means.

25. An optical device according to claim 23 or 24, wherein multiplexing means is provided having a plurality of inputs for receiving optical output signals from the demultiplexing means and for supplying at least one multiplexed optical signal.

26. An optical device according to claim 23, 24 or 25, wherein further demultiplexing means is provided having a plurality of offset inputs for receiving optical output signals from the multiplexing means or the firstmentioned demultiplexing means and for supplying optical output signals.

27. An optical device according to any one of claims 23 to 26, wherein the waveguides, the polarisation conversion means, the modulation means and multiplexing/demultiplexing means are integrally formed on a substrate.

28. An optical device according to claim 27, wherein photodetector means is integrally formed on the substrate or is in the form of a hybridised array.

29. An optical device according to any one of claims 23 to 28, wherein further splitting means is provided for dividing the optical output signal from the or each device between two branch output waveguides, selection means being provided for selecting the signal from a selected one of the branch output waveguides for onward transmission.

30. An optical device according to claim 29, wherein the selection means is provided for sequentially applying selected optical output signals to the photodetector means.

31. An optical device according to any one of claims 1 to 30, which is a spectral analyser for analysing the spectral components of an optical input signal.

32. An optical device according to any one of claims 1 to 30, which is an optical channel monitor (OCM) for monitoring the channels of a multichannel optical input signal.

33. An optical scrambler comprising a plurality of input waveguides for receiving optical input signals, a corresponding plurality of splitting means for dividing the optical input signals between first and second branch waveguides, polarization conversion means associated with each first branch waveguide for converting the optical signal supplied to the first branch waveguide to a differently polarised optical signal, modulation means associated with each second branch waveguide for varying the optical signal supplied to the second branch waveguide with respect to the optical signal supplied to the first branch waveguide in accordance with a time-varying drive signal, combining means for combining the optical signals from the first and second branch waveguides to supply a respective scrambled optical output signal to each of a plurality of output waveguides having a time average which is substantially independent of the polarization of the optical input signal, and demultiplexing means having a plurality of inputs for receiving scrambled optical output signals from the combining means and a plurality of outputs for supplying demultiplexed output signals.

34. An optical scrambler or monitor substantially as hereinbefore described with reference to the accompanying drawings.