GB2391954A

GB2391954A - Optical device for reducing polarisation dependence

Info

Publication number: GB2391954A
Application number: GB0219233A
Authority: GB
Inventors: Ivan Evans; Stephen Roberts; Stephen Day
Original assignee: Bookham Technology PLC
Current assignee: Lumentum Technology UK Ltd
Priority date: 2002-08-17
Filing date: 2002-08-17
Publication date: 2004-02-18
Also published as: GB0219233D0

Abstract

A device such as A Dynamic Gain Flattening Filter (DGFF) 1 may be adapted to reduce polarisation dependence. It consists of an input/output waveguide 2 for receiving an optical input signal from an optical fibre and for outputting an optical output signal to the optical fibre. A bidirectional optical structure may consist of an arrayed waveguide (AWG) 3 for demultiplexing the optical input signal from the waveguide 2 and for supplying the resulting channel signals to intermediate waveguides 4, the optical structure having substantially the same optical properties for identical optical signals transmitted through the optical structure in opposite directions. The optical structure further incorporates a reflecting facet 5 for returning the channel signals back along the intermediate waveguides 4, and a polarisation converter 6 associated with the reflecting facet 5 for converting the received channel signals to differently polarised signals to be transmitted back through the optical structure prior to being multiplexed by the AWG 3 to produce the output signal. In this manner a filter is provided which compensates for polarisation dependent losses (PDL) and which can be fabricated on a single chip to reduce complexity, cost and size.

Description

, "Optical Devices for Reducing Polarisation Dependence" The present

invention relates to optical devices for reducing polarisation dependence, for example in optical filters or other two-port optical devices, and is concerned more particularly, but not exclusively, with such devices for use in optical communication systems.

Optical flattening filters are commonly used to reduce the variation in the amplitude of an optical signal over the range of wavelengths of the signal. For example, the degree of amplification of an optical signal by an optical amplifier generally varies with wavelength, and accordingly gain-flattening filters are frequently used to reduce the variation in the gain produced by such amplifiers. A gain-flattening filter is disclosed in US 5351317 which splits an input signal into a plurality of channel signals within waveguide branches in dependence on wavelength, controls the amplitude and phase of each channel signal individually, and recombines the channel signals. Another gain-flattening filter is described in WO 02/14934. Such filters may be passive, i.e. their spectral transmittance may be fixed and invariable, or they may be active (that is "reconfigurable" or " tunable") so that signal fluctuations may be actively compensated for to ensure optimum gain-flattening. However such digital gain-flattening filters (DGFFs) can suffer from polarisation dependent losses (PDL) with the result that their filtering effect may itself vary with the polarisation of the optical signal.

US 6304380 discloses a spectral analyser in which such PDL is compensated for by supplying the input optical signal to a polarising beam splitter by way of a circulator which splits the signal into two different polarisation components which are in turn supplied to two ports of the spectral equaliser by way of polarisation maintaining (PM) optical fibres with one of the optical fibres being twisted so as to convert the corresponding polarisation component to polarised light of the same polarisation mode as the other polarisation component. After transmission through the spectral analyser the polarisation component applied to each port is outputted from the other port and returned to the beam splitter by way of the optical fibre connected to that port with the result that differently polarised polarisation components (as a result of the rotation of

the polarisation of one of these polarisation components by the twisted fibre) are combined within the beam splitter to produce an optical output signal which is routed by the circulator to an output port. As a result of such an arrangement any polarisation dependencies of the polarisation components are cancelled when these components are combined. However such a polarisation compensating arrangement is relatively l complex and costly to produce requiring a number of discrete components Y. Inoue et al., "Polarisation Mode Converter with Polyimide Half Waveplate in Silica Based Planar Lightwave Circuits", IEEE Photonics Technology Letters, Vol. 6, No.5, May 1994 discloses the use of a polyimide half plate in an arrayed waveguide grating (AWG) to reduce the PDL. Although such an arrangement serves to compensate the polarisation dependence of such an AWG, the polyimide wave plate is difficult to fabricate, complicated to hydridise on to a PLC (requiring slots to be etched or cut into the waveguide path) and introduces undesirable insertion losses.

Although the above description is given with reference to optical filters these

represent only one type of optical device to which the invention is applicable, and it will be appreciated from the following description that the invention is applicable to a wide

range of different types of two-port optical device.

It is an object of the invention to provide an optical device for reducing polarisation dependence which may be formed in a straightforward manner during fabrication of an integrated device based on SOI technology, for example.

According to the present invention there is provided an optical device for reducing the polarisation dependence, the device comprising waveguide means for receiving an optical input signal and for outputting an optical output signal along a common conduction path, a bidirectional optical structure for receiving the optical input signal from the waveguide means and for supplying the optical output signal to the waveguide means, the optical structure having substantially the same optical properties for identical optical signals transmitted through the optical structure in opposite directions, return means for returning the optical signal received from the waveguide

means by way of the optical structure back through the optical structure, and polarisation conversion means associated with the return means for converting the received optical signal to a differently polarised optical signal to be transmitted back through the optical structure.

Such an arrangement is advantageous since it does not require the use of a polarisation beam splitter or relatively expensive PM optical fibres as required in the arrangement of US 6304380. Furthermore the device can be fabricated on a single chip utilising silicon-on-insulator (SOT) technology. This reduces the complexity, size and cost of the device.

In a preferred embodiment of the invention the bidirectional optical structure is an optical filter structure, capable of applying dynamic gain flattening. The filter structure may be either an arrayed waveguide (AWG) or a Fourier filter. In either case the device may be fabricated to be of small size and to compensate for polarisation dependent losses (PDL) . In both cases it is also possible for the device to be fabricated so as to compensate for polarisation dependent frequency (PDF).

The polarisation conversion means utilised in the device may be a converter for rotating the polarisation of the optical signal by a total of 90 . Furthennore the conversion means advantageously provides 45 rotation per path so that the polarisation of the optical signal is rotated by 45 before reaching the return means and is rotated by a further 45 after having been returned by the return means. Preferably the polarisation conversion means comprises a stress-inducing structure providing a periodically varying refractive index in the direction of propagation. The layout of the device may be chosen such that the polarisation converter is in a position where the waveguides are sufficiently separated to allow particularly reliable polarisation conversion to be achieved. This arrangement also means that it is not necessary to fabricate a polyimide wave plate so that the insertion losses and additional complexity of such a wave plate are avoided.

It is also a particular advantage of the device of the invention that only a single fibre attachment to the device is required for both the supply of the optical input signal and the return of the optical output signal. This arrangement exploits true symmetry by returning the light back along its original path, unlike the scheme used in the Y. Inoue et al reference discussed above In one embodiment of the invention the return means comprises a reflecting surface for reflecting the optical signal from the optical structure back to the optical structure. The reflecting surface may be constituted by a highly reflecting coated polished facet.

In an alternative embodiment the return means may comprise an optical fibre loop having an input for receiving the optical signal from the optical structure and an output for returning the optical signal to the optical structure after it has passed around the loop. In this case the polarisation conversion means may comprise polarisation splitting/combining means for supplying an optical signal of one polarisation mode to the input of the loop and for returning an optical signal of a different polarisation mode received from the output of the loop, a twist being provided in the loop so that the optical signal at the output is differently polarised to the optical signal at the input. The I polarisation splitting/combining means may be a Mach-Zehnder polarisation splitter/combiner or some other type of splitter/combiner, such as a Yjunction for example. The twisted fibre loop could also be replaced an integrated equivalent, namely by a waveguide loop with a 90 degrees polarisation convertor (TE to TM conversion i and vice versa).

For a better understanding of the present invention and to show how the same may be carried into effect, a number of embodiments of dynamic gain flattening filter (DOFF) in accordance with the invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a schematic diagram of a first embodiment of the invention utilising an AWG-based structure;

Figure 2 is a schematic diagram of a second embodiment of the invention utilising a Fourier filter structure; Figure 3 is a schematic diagram of a third embodiment of the invention utilising an AWG-based structure; Figure 4 is a schematic diagram of a fourth embodiment of the invention utilising a Fourier filter structure; Figure 5 is an explanatory diagram showing how PDL and PDF compensation occur in such embodiments; Figure 6 is a schematic diagram of a fifth embodiment of the invention utilising a Fourier filter structure and a PM fibre loop for the return of the optical signal; and Figures 7A to 7E are explanatory diagrams showing the operation of a polarization converter which may be used in embodiments of the invention; Figure 8 is a graph illustrating the operation of such a polarization converter; Figures 9A and 9B are graphs illustrating the operation of two 45 polarization converters which may be used in embodiments of the invention; and Figure 10 is a schematic diagram of a sixth embodiment of the invention.

Various embodiments of DOFF in accordance with the invention will be described below with reference to the drawings by way of example. However it should be understood that the invention is also applicable to other types of two-port optical device, such as a channel balancing filter, a dispersion compensator and an interleaved, for example. Such an optical device may comprise an optical attenuator, a diffraction grating or diffraction elements, and may be, for example, a finite impulse response

filter, an infinite impulse response filter, a gain flattening filter, a dispersion compensating filter, a filter incorporating single or multiple Mach-Zehnder units or a filter incorporating a Sagnac interferometer. It will be apparent to those skilled in the art how the following teaching may be applied to other types of optical device.

The DOFF 1 diagrammatically shown in Figure 1 comprises an input/output waveguide 2, an AWG 3, intermediate waveguides 4 incorporating variable optical attenuators (VOAs), and a polarization converter 6 having a highly reflecting (HR) coated polished facet 5.

In operation an input optical signal incorporating a range of wavelengths is supplied from an optical fire to the waveguide 2 by way of a suitable fibre connection, and the resulting demultiplexed channel signals produced within the AWG 3 are supplied to respective intermediate waveguides 4 within which the signals are selectively attenuated by the VOAs so as to provide the required wavelength profile in the optical output signal.

The channel signals transmitted along the intermediate waveguides 4 are received by the polarization converter 6 and reflected from the facet 5 so that the signals are transmitted back through the polarization converter 6 and the intermediate waveguides 4 to the AWG 3 which serves to multiplex the signals to provide the optical output signal having a required wavelength profile which is transmitted along the waveguide 2 to the optical fibre. The polarization converter 6 is such as to rotate the polarization of the channel signals through an angle of substantially 45 before they reach the reflecting facet 5 and through a further angle of 45 after reflection from the facet 5 and prior to the chaMe1 signals being supplied to the intermediate waveguides 4 for transmission back to the AWG 3.

It will be appreciated that the channel signals which are passed through the AWG 3 in the opposite direction are polarised at 90 relative to the polarization of the channel signals supplied to the intermediate waveguides 4 by the AWG 3. Assuming that the optical characteristics of the AWG 3 are the same for both directions of

propagation through the AWG, it will be appreciated that both TE and TM input signals will be subjected to the same polarisation dependent losses in their passage through the device, comprising the losses associated with the TE mode during one of the passes through the AWG and the losses associated with the TM mode during the other pass through the AWG. Since both TE and TM input signals are subjected to the same overall transfer function the effect of PDL is compensated for. However such a device does not compensate PDF.

Figure 2 shows a second embodiment of DOFF 10 in accordance with the invention comprising an input/output waveguide 11, a series of MachZehnder splitters/combiners 12, intermediate waveguides 14 constituting variable delay lines, and a polarization converter 16 having a HR coated polished facet 15.

In operation of this embodiment the input optical signal is supplied to the waveguide 11 and to one input of the first splitter/combiner 12 which serves to divide the signal between its two outputs. Preferably substantially the entire range of wavelengths of the input optical signal is present at each of the outputs, that is the optical signal is divided by amplitude rather than by wavelength. Each of the optical signals outputted by the first splitter 12 is then supplied to an input of a respective further splitter 12 which in turn divides each signal into two output signals which are supplied to the intermediate waveguides 14 which are different lengths. As in the preceding embodiment the signals pass through the polarization converter 16 and are reflected by &e facet IS back along the intermediate waveguides 14 with the signals being rotated through 45 by the polarization converter 16 before reaching the facet 15 and by a further 45 after being reflected by the facet 15 and before being returned to the intermediate waveguides 14.

Furthermore the signals reflected back along the intermediate waveguides 14 are recombined by the splitters 12 now acting as combiners to produce the optical output signal having the required wavelength profile for supplying to the optical fibre by way of the waveguide 11. The intermediate waveguides 14 of different lengths introduce phase differences between the signals which are arranged to be such that, when the

signals are combined, they interfere in such a manner as to provide an output signal of the required wavelength profile. Reference may be made in this regard to the description ofthe functions of the filters of US 5351317 and WO 02/14934.

The third embodiment of DGPF 20 shown in Figure 3 comprises an input/output waveguide 21, a first AWG 22, intermediate waveguides 23 incorporating VOAs, a second AWG 24, a further waveguide 25 and a polarization converter 26 having a mirror facet 27.

Such an embodiment operates in a similar manner to the embodiment of Figure 1 except that the channel signals transmitted along the intermediate waveguides 23 are recombined by being passed to the second AWG 24 which supplies a multichannel signal 25 to the polansation converter 26 which rotates the signal by 45 prior to reflection of the signal by the mirror 27 and then rotates the reflected signal through a further 45 before returning the signal to the AWG 24 by way of the waveguide 25. The AWG 24 then demultiplexes the signal supplied along the waveguide 25 to provide channel signals which are transmitted back along the intermediate waveguides 23 to the AWG 22 which combines the signals to provide the output signal of the required wavelength profile to be passed back to the optical fibre by way of the waveguide 21.

Such an arrangement compensates not only for PDL but also for PDF.

The fourth embodiment of DOFF 30 shown in Figure 4 comprises an input/output waveguide 31, a first series of Mach-Zehnder splitters/combiners 32, intermediate waveguides 34 of variable lengths, a second series of Mach-Zehnder splitters/combiners 35, a further waveguide 36 and a polarization converter 37 having a mirror facet 38.

This embodiment operates in a similar manner to the embodiment of Figure 2 except that the signals supplied to the intermediate waveguides 34 are recombined by the further splitters 35 acting as combiners so as to produce a multichannel signal which is supplied along the further waveguide 36 to the polarization converter 37 which again rotates the signal through 45 prior to reflection by the mirror 38 and by a further 45

before supplying the signal back to the splitters 35 by way of the waveguide 36. The splitters 35 then divide the signal into signal components which are supplied along the variable length waveguides 34 to the splitters 32 acting as combiners to produce the output signal of a required wavelength profile. This embodiment again provides both PDL and PDF compensation.

The function of the DOFF in providing polarisation compensation is best understood by considering each polarisation mode separately with reference to the explanatory diagram of Figure S in which the upper series of blocks denotes the passage of the transverse electric (TE) polarisation mode, through the DOFF, and the lower series of blocks denotes the passage of the transverse magnetic (TM) polarisation mode through the DOFF. During the outward pass of the TE signal through the AWG 3 and associated intermediate waveguides 4 a transfer function TE is supplied to the signal as indicated by the block 50, and the signal is then converted to TM polarised light by being passed through the polarisation converter 6 prior to being subjected to the transfer function TM in its return pass through the waveguides 4 and the AWG 3 as shown by the block 51.

On the other hand the TM signal is subjected to the transfer function TM in its outward pass through the AWG 3 and intermediate waveguides 4 as shown by the block 52 before being converted to TE polarised light on its passage through the polarisation converter 6. The TE light transmitted back along the intermediate waveguides 4 and the AWG 3 is then subjected to the transfer function TE as shown by the block 53.

Comparison of the upper and lower series of blocks in Figure 3 shows that both the TE signal and the TM signal are subjected to the same overall transfer function (which is the same irrespective of the direction in which the light passes through the chain of filters with the result that the effect of PDL is compensated for.

The fifth embodiment of DOFF 60 shown in Figure 6 comprises an input/output waveguide 61, a first series of Mach-Zehnder splitters/combiners 62, intermediate waveguides 64 of variable length, a second series of Mach-Zehnder splitters/combiners 65, a further waveguide 66, an on-chip polarisation splitter 67, and a polarisation

converter 68 in the form of a twisted PM optical fibre. In a variant of this embodiment the twisted PM fibre is replaced by a looped waveguide and a polansation converter in combination. The operation of the DOFF 60 of Figure 6 is similar to that of the embodiment of Figure 4 except that a different arrangement is provided for converting the signal supplied to the waveguide 66 to orthogonally polarised light and for returning the signal back along the waveguide 66. This alternative arrangement comprises a polarization splitter 67, which may be a Mach-Zehnder splitter or Y splitter for example, for supplying the input polarised light to a selected one of the two outputs 69 and 70, and a PM loop fibre coupled between the two outputs 69 and 70 and twisted through 90 so that the light outputted by the fibre 68 is rotated through 90 relative to the light inputted to the fire 68. The polarised light returned to the other output of the splitter 67 is thus returned to the waveguide 66 with its polarisation rotated through 90 relative to the optical signal originally supplied along the waveguide 66.

Such an arrangement requires coupling of the two ends of the PM fibre loop 68 to the two output facets of the polarisation splitter 67, and permits PDL and PDF compensation without requiring an integrated polarization converter as used in the other embodiments described. Such an arrangement relies for its utility on the fibre-to-

waveguide coupling loss being small.

The polarization converters used in the embodiments of Figures 1 to 4 may be constituted by asymmetric stressed portions of the waveguides, formed, for example, by a thennally stressed oxide layer extending partially over the waveguide in the lateral direction and possibly being patterned so that the stress varies along the waveguide axis.

In this regard it should be appreciated that, in a symmetric and unperturbed waveguide, such as a SOI waveguide structure, the optical modes are a hydrid of TE and TM modes. For example the mode which is predominantly orientated in the x plane, which is generally referred to as TE, contains a small field component which is orientated in

the y plane. In this case the x field component is termed the major component of the TE

mode and the y field component is termed the minor component. However these minor

field components are ordinarily very small, typically 1000 times smaller than the major

component, and have an antisymmetric mode profile, giving an overlap of zero to a symmetrically shaped mode. Therefore the TE and TM modes are almost purely orientated in the x and y planes extending respectively, that is within planes extending laterally within the waveguide 71 in the direction indicated by the arrow 72 in Figure 7A and vertically within the waveguide (upwardly in a direction normal to the direction 72 in Figure 7A).

However, if asymmetric stress is applied to the waveguide, the axis of the modes is tilted with the result that the two modes are no longer largely TE and TM modes orientated in the x and y planes. Instead strongly hybrid modes are formed, namely a TE mode having a major component orientated in the x plane and a much smaller component orientated in the y plane, and a TM mode having a major component orientated in the y plane and a much smaller component orientated in the x plane.

Significantly the minor field components are no longer antisymmetric and have a non-

zero overlap relative to a symmetric mode profile. Thus the application of stress effectively tilts the axis of the waveguide so that, although the two fundamental modes are still polarised orthogonally, the net direction of each of the x and y axes is tilted as a result of the fact that the minor polarization components are significantly enhanced by the asymmetry.

The manner in which polarization rotation can be achieved by butting together of a symmetric waveguide section and a waveguide to which asymmetric stress is applied will now be described with reference to Figures 7A to 7E. It will be appreciated that, if the symmetric waveguide section 71 shown in Figure 7A is butted together axially with the asymmetrically stressed waveguide section 73 shown in Figure 7B and light polarized in the direction 72 is launched from the section 71 into the section 73, such light is decomposed into TE and TM modes, and net polarization rotation through an angle O occurs after appropriate phase matching, as indicated in the diagram of Figure 7C. The amount of rotation that can be achieved by two such butted sections is usually less than 90 , so that a series of such sections may be butted together in sequence to achieve a total desired rotation of 90 if required, with the period between

sections being chosen to ensure that the successive polarisation rotations add up constructively. Since rotations in opposite directions effectively occur at the input and the output of each asymmetrically stressed waveguide section 73, in practice the phase angle between the modes must rotate by 180 over half a period in order for the rotations to add constructively.

Considering the transition from the symmetric waveguide section 71 shown in Figure 7A to the asymmetrically stressed waveguide section 73 shown in Figure 7B, the light of pure TE mode launched from the section 71 both the TE and the TM modes to be excited in the section 73 producing a largely TE mode comprising a major component 74 along the x plane and a minor component 75 along the y plane orthogonal to the x plane, as well as a largely TM mode comprising components 75A and 76 along the x and y planes. Furthermore Figure 7C indicates the effect of the propagation through the section 73 providing a 180 phase shift between the modes to provide a net polarization rotation through an angle of at the output of the section 73 as a result of the addition of the components 75 and 76. The subsequent transition on passing of the light from the output of the section 73 to a further symmetric waveguide section 78 provides a net effect as illustrated in Figure 7D from which it will be appreciated that the further transition converts the light into TE and TM modes 80 and 81 along the x and y planes (ignoring the minor components which are small). Figure 7E is a plan view showing the butted together sections 71, 73 and 78, and indicating at A, B and C the locations of the cross-sectional views shown in Figures 7A, 7B and 7C.

The variation in rotation which occurs with variation in phase angle shift can be used to ensure that two 45 rotators (or two successive passes through a single device in opposite directions) can achieve either no net polarization conversion or TE-TM conversion, depending on the length of propagation that occurs between the two rotators (or two passes) .

Figure 8 is a graph showing, along the vertical axis, the percentage of polarization power converted as a Function of the applied phase shift, the curve 82

showing the percentage for a one period rotator, and the curve 83 showing the percentage for a two period 90 rotator.

On the other hand Figure 9A and 9B show the matrix model of two 45 polarisation converters, for (i) the case where there is no phase shift between the polarisation states at the input of the rotator and (ii) the case where there is a rr phase shift between the polarisation states at the input of the rotator, respectively. In Figure 9A 83 indicates the percentage polarisation conversion for a one period device, whereas 84 shows the percentage polarisation conversion for the case of two passes through the device, as experienced by light reflected back through the device at a mirror facet, for example. Depending on the phase relationship between the polarisation states at the input of the rotator on the reverse pass, it will be appreciated that the optical signal emerges either unrelated or rotated by 90 . A number of variations of the above-

described arrangements are possible within the scope of the invention. For example a circulator may be provided having an input port, and output port and a common transfer port. An input optical signal supplied to the input port of the circulator may be transferred to the optical fibre by way of the transfer port to which the fibre is connected. The output optical signal from the DGFF is returned along the optical fibre to the transfer port of the circulator which in turn transfers the output optical signal to the output port, thus separating the input and output signals.

Figure 10 schematically shows a further DGFF 90 in accordance with the invention. In this case an on-chip interferometer is provided in place of the circulator in order to separate the input and output signals, with an isolator being provided in the input path to block any return leakage signal. The DGFF 90 comprises an input 2X2 coupler 91 integrally formed on the PLC chip 92 and having one arm 93 arranged to receive an input signalfrom an input optical fibre and another arm 94 arranged to transmit an output signal to another optical fibre. Further arms of the coupler 91 are connected respectively to the two-port filter 95 to be compensated, and a loss element 96 to compensate for the loss of the filter 95. In both cases the signal which has passed through the filter 95 or loss element 96 is reflected back by a mirror 97 and converted to the opposite polarisation state by an integrated polarisation rotator 98 before the two

signals are recombined within the coupler 91 to provide the output signal compensated for the effect of PDL and PDF at the output provided by the arm 94. As previously indicated any leakage signal outputted along the arm 93 can be blocked by an isolator provided in the input path.

It will be appreciated that the optical filter 95 in Figure 10 can be any appropriate two-port device having the symmetry property for signals passing through the ports in opposite directions which can be used to reduce PDL and PDF in the manner described. Such a device may for example be one of the arrangements used in E the previous embodiments described with reference to Figures 1, 2, 3, 4 and 6.

In another variation the filter itself may be such as to provide some polarization conversion in which case the polarization converter may be chosen to give a different level of conversion appropriate to provide the required overall conversion of the signals reflected back through the device. ' In a further variant the interferometer comprises a 2x2 variable Mach-Zehnder t coupler with an adjustable phase modulator being provided in one or both arms to adjust I the coupling ratio.

In another variation a polarization splitter and 90 converter may be used in a configuration broadly similar to that described above with reference to Figure 6 (but not t including a twisted PM loop fibre) in place of the reflecting facet and two-pass 45 3 polarization conversion used in the embodiments of Figures 1 to 4. i The above-described embodiments of DGFF in accordance with the invention enable doubling of the dynamic range whilst substantially completely eliminating PDL.

Such an arrangement does not require a polarising beam splitter, and requires only a single optical fibre for the input and output optical signals which need not be a PM loop fibre. This has the advantage of decreasing cost and simplifying the fibre-to-chip connection. Whilst the on-chip insertion losses are increased as compared with a single pass configuration, such losses can be maintained at acceptable levels for many

applications' particularly when using the arrangements of Figures 2 and 4. Whilst such arrangements will also tend to increase the dispersion characteristics, suitable design of the devices can be used to minimise such dispersion.

Claims

CLAIMS:

1. An optical device for reducing the polarization dependence, the device comprising waveguide means for receiving an optical input signal and for outputting an optical output signal along a common conduction path, a bidirectional optical structure for receiving the optical input signal from the waveguide means and for supplying the optical output signal to the waveguide means, the optical structure having substantially the saline optical properties for identical optical signals transmitted through the optical structure in opposite directions, return means for returning the optical signal received from the waveguide means by way of the optical structure back through the optical structure, and polarization conversion means associated with the return means for converting the received optical signal to a differently polarised optical signal to be transmitted back through the optical structure.

2. A device according to claim 1, wherein the waveguide means and the optical structure are integrally formed on a substrate.

3. A device according to claim 2, wherein the substrate is a SOI (siliconon-

insulator) substrate.

4. A device according to claim 1, 2 or 3, wherein the polarization conversion means is adapted to convert the received optical signal to an orthogonally polarized optical signal to be transmitted back through the optical structure.

5. A device according to any preceding claim, wherein the optical structure is a two-port optical device

6. A device according to any preceding claim wherein the optical structure is an optical attenuator.

7. A device according to any preceding claim, wherein the optical structure is an optical filter structure.

l

8. A device according to claim 7, wherein the optical filter structure comprises one or more arrayed waveguides (AWGs).

9. A device according to claim 7, wherein the optical filter structure comprises a Fourier filter.

10. A device according to claim 7, wherein the optical filter structure incorporates a diffraction grating or diffraction elements, a finite impulse response filter, an infinite impulse response filter, a gain flattening filter, a dispersion compensating filter, a filter incorporating single or multiple Mach-Zehnder units or a filter incorporating a Sagnac interferometer.

11. A device according to any preceding claim, wherein the polarization conversion means is a converter for rotating the polarization of the optical signal by a total of 90 .

12. A device according to claim 11, wherein the polarization conversion means provides 45 rotation per pass so that the polarization of the optical signal is rotated by 45 before reaching the return means and is rotated by a further 45 having been returned by the return means.

13. A device according to any preceding claim, wherein the polarization conversion means comprises a stress-inducing structure providing a periodically varying refractive index in the direction of propagation for asymmetrically perturbing the optical signal.

14. A device according to any one of claims 1 to 13, wherein the return means comprises a reflecting surface for reflecting the optical signal from the optical structure back to the optical structure.

15. A device according to any one of claims 1 to 13, wherein the return means comprises a polarisation-maintaining (PM) optical fibre loop having an input for

receiving the optical signal from the optical structure and an output for returning the optical signal to the optical structure after it has passed around the loop.

16. A device according to claim 15, wherein the polarisation conversion means incorporates polarisation splitting/combining means for supplying an optical signal of one polarisation mode to the input of the loop and for returning an optical signal of a different polarisation mode received from the output of the loop.

17. A device according to claim 16, wherein the polarisation conversion means incorporates a twist in the loop so that the optical signal at the output is differently polarised to the optical signal at the input.

18. A device according to claim 17, wherein the polarisation splitting/combining means comprises a Mach-Zehnder polarisation splitter/combiner.

19. A device according to claim 17, wherein the polarisation splitting/combining means comprises an adiabatic polarisation splitter/combiner.

20. A device according to any preceding claim, wherein the waveguide means comprises a common waveguide for receiving the optical input signal from an optical fibre coupled to the waveguide and for outputting the optical output signal to the optical fibre.

21. A device according to claim 20, wherein circulator means is provided for supplying light from an input terminal to the optical fibre and for supplying light from the optical fibre to an output terminal.

22. A device according to any one of claims 1 to 19, wherein the waveguide mear;,s comprises an interferometer having an input waveguide for receiving the optical input signal and an output waveguide for outputting the optical output signal.

23. A device according to claim 227 wherein the interferometer is arranged to split the optical input signal into intermediate signals supplied to intermediate waveguides and to combine together returned signals supplied back by the return means along the intermediate waveguides to form the optical output signal.

24. A device according to claim 23, wherein one of the intermediate waveguides incorporates the bi-directional optical structure, and the other intermediate waveguide incorporates a compensating element for applying a polarisation dependent loss (PDL) which compensates for the PDL of the bi-directional optical structure.

25. A device according to claim 22, 23 or 24, wherein the interferometer comprises a Mach-Zehnder interferometer having two arms and an adjustable phase control in one or both arms to form a variable coupler.

26. An optical device for reducing polarisation dependence, the device being substantially as hereinbefore described with reference to the accompanying drawings.