GB2391955A

GB2391955A - Integrated optical devices for reducing polarisation dependence

Info

Publication number: GB2391955A
Application number: GB0219237A
Authority: GB
Inventors: Ivan Evans; Stephen Roberts
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: GB0219237D0

Abstract

An integrated polarisation dependence compensator 1 may consist of, on what may be a silicon-on-isolator (Sol) substrate, a Mach-Zehnder polarisation splitter 2, a 2-port optical device 3 and a waveguide structure 4 interconnecting the polarisation splitter 2 and the optical device 3. An optical input signal is supplied by way of a circulator 5 and an optical fibre pigtail 6 to one input of the polarisation splitter 2 which serves to split the signal into first and second polarisation components TM and TE which are supplied to branch waveguides 8 and 9 of the waveguide structure 4. The first polarisation component TM is transmitted by the waveguide 8 to an integrated polarisation converter or rotator 11 which converts the component TM to TE polarised light for supplying to a first port 12 of the device 3 by way of a waveguide 13. The second polarisation component TE is passed directly to a second port 14 of the device 3. The polarisation splitter 2 also serves as a polarisation combiner for light received back along the waveguides 8 and 9. Thus the light supplied along the waveguide 13 to the first port 12 of the device 3 is returned along the waveguide 9 to the splitter 2 after passing through the device 3, and the light supplied along the waveguide 4 to the second port 14 of the device 3 is passed by the waveguide 13 to the polarisation convertor 11 (after the light has passed though the device 3) which converts the light to TM polarised light, which is in turn returned to the splitter 2 by way of the waveguide 8. Thus the splitter 2 serves to combine third and fourth polarisation components TM and TE of the returned light to produce an output signal which is supplied by the optical fibre 6 to the circulator 5. A device may also incorporate two identical devices 63 which receive polarised light from splitter 64 and the light is later combined in combiner 65.

Description

r "Integrated Optical Devices for Reducing Polarisation Dependence" The

present invention relates to integrated optical devices for reducing polarisation dependence, and is concerned more particularly, but not exclusively, with such devices for use in optical communication systems.

Various difficulties are encountered in the field of optical communication by

virtue of the fact that the polarisation of an optical signal is altered on transmission through an optical element. This is as a result of the birefringent properties of the optical element providing 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 a different polarisation mode, for example transverse magnetic (TM) polarised light. Such birefringence may be large and may be affected by ambient conditions such as temperature and mechanical stress.

Various methods have been proposed for controlling such polarisation dependence. US 6304380 describes a polarisation dependence compensator comprising a polarising beam splitter to which an input optical signal is supplied by way of a circulator, and two polarisation-maintaining (PM) optical fibres coupling the splitter to opposite ports of a planar lightwave chip (PLC) exhibiting polarisation dependence, one of the fibres being twisted so that polarised light supplied to one end of the fibre is rotated through 90 at the other end of the fibre. In operation of such a compensator the optical input signal is split by the splitter so that a first polarisation component, for example a TM polarisation component, is supplied to one of the fibres and a second polarisation component, for example a TE polarisation component is supplied to the other fibre. As a result of the twist in the fibre the TE component is converted to TM polarised light prior to being applied to the chip. The resulting TM polarised light is passed through the chip and returned along the untwisted fibre to the splitter, whereas the TM component applied to the other port of the chip is passed through the chip and supplied by way of the twisted fibre to the splitter so that it is rotated to fonn TE

..>u2GB 2 polarised light at the splitter. The splitter than acts as a combiner to combine the TM and TE light and to supply an output signal by way of the circulator...DTD: Because the light passed through the chip in both directions is similarly polarised, the overall effect of passing the light through the chip will be substantially the same regardless of the polarisation of the input optical signal, with the result that such an arrangement compensates the polarisation dependence of the device. However such an arrangement requires the use of a polarising beam splitter and two PM fibres having axes which have to be angularly aligned relative to the chip to ensure that the input polarizations have the appropriate extinctions. Thus the arrangement is relatively expensive to produce and requires complex fibre alignment processes.

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

According to the present invention there is provided an integrated optical device for reducing polarisation dependence, the device comprising a substrate, inputloutput waveguide means on the substrate for receiving an optical input signal and for outputting an optical output signal of reduced polarisation dependence, polarisation splitting/combining means on the substrate for splitting the input signal into first and second polarisation components and for combining together third and fourth polarisation components to produce the output signal, and polarisation conversion means on the substrate for converting the first polarisation component to differently polarised light for supplying to the polarisation splitting/combining means as the third polarisation component.

Such an arrangement is advantageous since the device does not require the use of a polarisation beam splitter or relatively expensive PM optical fibres as required in the arrangement of US 63043280. Furthermore the device can be fabricated on a single chip utilising silicon-on-insulator (SOI) technology. This reduces the complexity, size and cost of the device.

In a preferred embodiment of the invention polarisation dependent means is provided on the substrate having a first port for receiving an input polarisation component and for supplying an output polarisation component and a second port for receiving the second polarisation component and for supplying the fourth polarisation component, the polarisation conversion means converting the first polarisation component to differently polarised light for supplying to the polarisation dependent means as the input polarisation component and converting the output polarisation component from the polarisation dependent means to differently polarised light for supplying to the polarisation splitting/combining means as the third polarisation component. The polarisation means may be any two-port optical device requiring polarisation dependence compensation, so that the integration of the specified components with such a device on a chip ensures that the performance of the chip is not dependent on the polarisation of the optical signal.

Advantageously the input/output waveguide means on the substrate comprises a common waveguide for receiving an optical input signal from an optical fbre coupled to the waveguide and for outputting an optical output signal to the optical fibre, and the polarisation splitting/combining means comprises a common integrated element which performs both splitting and combining. This enables the optical input and output signals to be supplied to the device by a single optical fibre which need not be a polarisation mode (PM) fibre. This reduces the complexity of the fibre alignment process and reduces the expense of the fibre assembly involved.

Circulator means may be provided for supplying light from an input terminal to the optical fibre and for supplying light from the optical fibre to an output terminal, thus ensuring that the optical input and output signals are separated.

The invention also provides an integrated optical device for reducing polarisation dependence, the device comprising a substrate, input waveguide means on the substrate for receiving an optical input signal, output waveguide means on the substrate for outputting an optical output signal of reduced polarisation dependence,

polarisation splitting means on the substrate for splitting the input signal into first and second polarisation components, polarisation combining means on the substrate, which is a separate element to the polarisation splitting, for combining together third and fourth polarisation components to produce the output signal, first polarisation dependent means having a first port for receiving the first polarisation component and a second port for supplying the third polarisation component, and second polarisation dependent means, substantially identical to the first polarisation dependent means, having a first port for receiving the second polarisation component and a second port for supplying the fourth polarisation component.

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 is a schematic diagram of a first embodiment of the invention; Figures 2 and 3 are diagrams of possible polarisation splitters which may be used in the first embodiment of the invention; Figure 4 is a schematic diagram of a second embodiment of the invention; Figure 5 is a schematic diagram of a third embodiment of the invention; Figures 6 is a schematic diagram of a fourth embodiment of the invention; and Figures 7A, 7B, 7C and 7D are explanatory diagrams showing a preferred fabrication method.

: s Figure 1 diagrammatically shows an integrated polarisation dependence compensator I comprising, on an SOI substrate, a polarisation splitter 2, a 2-port optical device 3 requiring polarisation dependence compensation and a waveguide structure 4 interconnecting the polarisation splitter 2 and the optical device 3. In operation of such a compensator 1, an optical input signal is supplied by way of a circulator 5 and an optical fibre pigtail 6 which supplies the input signal to an input/output waveguide 7 of the compensator 1. The input signal supplied by the optical fibre pigtail 6 to the waveguide 7 is passed to one input of the polarisation splitter 2 which serves to split the signal into first and second polarisation components which are supplied to branch waveguides 8 and 9 of the waveguide structure 4 and may, for example, be the polarisation components.

The first polarisation component TM is transmitted by the waveguide 8 to an integrated polarisation converter 11 which converts the first polarisation component TM to TE polarised light for supplying to a first port 12 of the device 3 by way of a waveguide 13 of the waveguide structure 4. The second polarisation component TE supplied to the waveguide 4 is passed directly to a second port 14 of the device 3.

The polarisation splitter 2 also serves as a polarisation combiner for light received back along the waveguides 8 and 9. Thus the light supplied along the waveguide 13 to the first port 12 of the device 3 is resumed along the waveguide 9 to the splitter 2 after passing through the device 3, and the light supplied along the waveguide 4 to the second port 14 of the device 3 is passed by the waveguide 13 to the polarisation convertor 11 (after the light has passed through the device 3) which converts the light to orthogonally polarised light, in this case TM polarised light, which is in turn returned to the splitter 2 by way of the waveguide 8. Thus the splitter 2 serves to combine third and fourth polarisation components TM and TE of the returned light to produce an output signal which is supplied by the optical fibre 6 to the circulator 5, which in turn outputs the output signal at a different port to the port to which the input signal was originally supplied.

But 6 It will be appreciated that, due to the presence of the polarisation converter 11, all the light passed through the device 3 is of the same polarisation, for example TE polarised light, and accordingly any polarisation dependence of the device 3 will be compensated for by virtue of the fact that all the light signals passed through the device 3 will be similarly affected by the polarisation dependence of the device. This will be the case regardless of the proportion of the different polarisation components in the input signal, and regardless of the polarisation components into which the signal is split by the splitter 2, provided that the polarisation converter 11 is such as to convert the polarisation of the first polarisation component to the polarisation of the second polarisation component.

The above described compensator is advantageous in that it does not require the use of expensive polarisation-maintaining (PM) optical fibre and instead ordinary optical fibre can be used for conducting the signals between the circulator 5 and the waveguide 7. In addition the use of a separate polarising beam splitter is not required, and it is only necessary for one optical fibre connection to be made to the chip since the same optical fibre is used for conducting the input and output signals. This considerably simplifies the fibre-to-chip connection.

In a variant of the above-described compensator the input and output signals are supplied to and from the compensator by means of separate input and output optical fibres, in which case it is no longer necessary to provide a circulator. In this case, instead of the splitter 2 also performing the function of a combiner, a separate polarisation combiner is supplied for receiving the component signals after they have been passed through the device 3 and for combining these signals to produce the output signal outputted along the output optical fibre.

The polarisation splitter 2 used in the embodiment of Figure 1 is preferably a Mach-Zehnder polarisation splitter as shown diagrammatically in Figure 2 and comprising a first coupler 20 for splitting an input signal supplied to one of its inputs into equal parts between two branch waveguides 21 and 22 incorporating birefringence altering patches 23 and 24. The patches 23 and 24 may be areas of thermal oxide

applied to the substrate so as to overlap the branch waveguides 21 and 22, in which case these patches will be of different extents (or alternatively one of the patches may be dispensed with altogether) in order that the optical signals supplied along the waveguides 21 and 22 experience different birefringent effects. In addition a variable phase shifter 25 is provided along the branch waveguide 21 in order to shift the phase of the signal applied along that waveguide 21 by is relative to the signal in the other waveguide 22 prior to supply of both signals to a second coupler 26.

As is well known, the effect of combining the phase shifted signal of the waveguide 21 with the signal supplied by the waveguide 22 to the second coupler 26 is to provide the required TM and TE component signals at the two outputs of the coupler 26. It will be appreciated that a required birefringence imbalance must be provided so that the TM and TE components of the signals supplied to the coupler 26 interfere with different phases within the coupler 26 so that the TM and TL components are routed to different outputs of the coupler 26. The preferred method of achieving the differential birefringence is to leave patches 23 and 24 of thermal oxide of carefully selected lengths covering one or both or the branch waveguides 21 and 22 during an etching step within the fabrication process as shown in Figure 2.

However, in order to avoid stress hotspots associated with the sharp edges of such patches, it is preferred to use a patch 28 in the form of an oxide layer with rounded edges which may be of such an extent as to cover both of the branch waveguides 21,22, as shown in Me modified arrangement 2' of Figure 3. The lengths of the waveguides 21 and 22 covered by such patches are typically in the range of 0.1-5 man, and the maximum spacing apart of the branch waveguides 21 and 22 is typically in the range of 50-250 Em in order to enable the birefringence of one waveguide to be controlled independently of the birefringence of the other waveguide. Such a splitter arrangement is based on the disclosure of M. Oquno et al., "Birefringence Control of Silica

Waveguides on Si and its Application to a Polarisation-Beam Splitter/Switch", Journal of Lightwave Technology, Vol.12, No.4, April 1994, pages 625-33. An alternative splitter arrangement is described in US 5293436.

The phase shifter 25 may be adjustable to provide variable polarisation splitting in order to enable the splitter to be tuned to provide accurate splitting between the TM and TE components. For example the phase shifter may be constituted by a small p-i-n diode which changes the local refractive index in response to an applied electrical control signal by virtue of the plasma dispersion effect, or a thin film heater for locally heating the waveguide to change the refractive index in accordance with the therrno- 5 optic effect. Alternatively the variable phase shifter may be omitted and a static path imbalance or phase shift employed, for example by designing the arms of the Mach_Zehnder arrangment to be of different lengths.

The couplers 20 and 26 may consist of one or more of the following arrangements: (i) a Y branch, (ii) a tapered Y branch as disclosed in US 5818989, (iii) a MMI coupler, and (iv) an evanescent coupler.

Instead of utilising the above-described Mach-Zehnder polarisation splitter to perform the polarisation splitting andlor combining functions described above with reference to Figure 1, a Y branch polarisation splitter/combiner may be used, for example as disclosed by R.M. de Ridder et al., "An Integrated Optic Adiabatic TE/TM Mode Splitter on Silicon", Journal of Lighhvave Technology, Vol. 11, No. 11, November 1994, pp. 18711. Alternatively a Y branch polarisation splitter may be used in which one of the branches of the splitter is provided with a stress-controlling oxide layer providing the required differential birefringence to produce splitting apart of the TM and TE components. Such a device can also be operated in reverse to act as a polarisation combiner.

The polarisation converter 11 of Figure 1 is fabricated so as to act as the integrated equivalent of a bulk optics retardation wave plate, a 90 converter (converting TE to TM or TM to TE) being equivalent to a \/2 wave plate. The polarisation converter 11 may be formed by a periodically asymmetric waveguide structure comprising, for example, an asymmetrically patterned thermal oxide layer overlying the waveguide. For a 90 converter the period of such asymmetric patterning is typically between 1, 000 and 4,000 Am repeated for 2-4 periods and giving an overall length

between 2 and 16 mm. For example the integrated polarisation rotator structure may be as described in US 5185828, US 5243669 or US 5703977, although certain other arrangements may be preferred for silicon waveguides.

Apart from the integrated components described above, the polarisation compensator 1 of Figure 1 requires a fibre optic circulator 5 and an optical fire pigtail 6 aligned with the chip using one of a range of fibre-to-chip alignment technologies.

Possible alignment technologies make use of (i) a fibre block attached to the polished chip edge, or (ii) fibre locating features such a V or U grooves located on chip or by means of a flip chip arrangement, the fibre attachment being either active or passive depending on the application.

Figure 4 diagrammatically shows an integrated optical device in the form of a polarisation scrambler 30 for providing an optical output signal which is substantially independent of the polarisation of the optical input signal to the device. In this case the scrambler 30 comprises, on an SOI substrate, a 1X2 or 2X2 optical switch 31, a polarisation converter 32 and a 2X2 optical switch 33. Each of the optical switches 31 and 33 may be in the form of a Mach-Zehnder switch incorporating a phase biasing arrangement, preferably in the form of a small pen diode 34 or 37 in one of its branch waveguides 35 or 36. The switching may be controlled such that TE polarised light is supplied to the branch waveguide 35 and TM polarised light is supplied to the branch waveguide 36 or vice versa. The TE (or TM) polarised light supplied to the branch waveguide 36 is converted by the polarisation converter 32 to TM (or TE) polarised light before being supplied to one input of the 2X2 optical switch 33, the TM polarised light supplied to the branch waveguide 36 being supplied to the other input of the 2X2 optical switch 33.

It should be appreciated that the above described arrangement is only one example of a polarisation scrambler in accordance with the invention and that other polarisation scrambler arrangements can be contemplated in accordance with the invention in which the elements are provided in a different configuration and/or in which equivalent elements are used to perform similar functions.

Electrical signals supplied to the diodes 34 and 37 may be used to control the coupling so that a scrambled output signal having substantially randomly varying polarisation is supplied along the output waveguide 38. In one possible drive arrangment the switches are identical and are alternately driven such that the light passes, in a first phase, all along the waveguide 36 (through state) and then to the output waveguide 38 and, in a second phase, all along the waveguide 35 (cross state) and then to the output waveguide 38. The drive signal required for maximum throughput is a square wave with a steep falling edge (compared to its period) although other periodic drives signals, such as sinusoidal or triangular waveforms, are possible Figure 5 diagrammatically shows an integrated optical device in the form of a fixed polariser 40 comprising, on an SO! substrate, a polarization splitter 42 similar to the polarization splitter 2 of the embodiment of Figure 1, a phase shifter 44 similar to the phase shifter 25 described above, and a 2X2 optical switch 45 incorporating a phase biasing arrangement, which may be in the form of a small pen diode 46. The 2X2 optical switch 45 is preferably in the form of a Mach-Zehnder switch which is actively controlled by an electrical signal so as to maximise the optical power supplied to the output waveguide 47. The phase shifter 44 will need to be adjusted on initial setting up in order to ensure that the TE and TM components applied along the branch waveguides 49 and 50 are in phase with one another, and the electrical signal to the diode 46 being actively controlled to provide continual phase adjustment as the signal power varies. In this case power monitoring may be performed using a tap-off coupler on the output waveguide 47 and an associated photodetector.

In operation of the polariser 40 an optical input signal is supplied to the input waveguide 48, and the splitter 42 serves to split this signal into TE and TM polarization components which are applied to the branch waveguides 49 and 50. The TE component applied to the branch waveguide 49 is converted to TM light by passing through the converter 43 and the phase of the resulting TM light is adjusted relative to the TM I component in the branch waveguide SO by means of the phase shifter 44. As a result of such adjustment and adjustment of the operation of the switch 45 by the electrical control signals supplied to the diode 46, TM light of maximum power is outputted along

_JB 1l the output waveguide 47. Such a polariser provides an output of constant power and polarisation regardless of the relative phases and intensities of the polarisation components of the input signal.

Figure 6 diagranunatically shows an integrated polarisation dependence compensator 60 providing polarisation dependence compensation in a similar manner to that provided by the compensator 1 of Figure 1 but using an arrangement in which the input and output signals are supplied to and from the compensator by means of separate input arid output waveguides 61 and 62. In this case two identical devices 63 requiring polarisation dependence compensation are integrated in parallel. Furthermore the compensator 60 comprises, on an SOI substrate, a polarisation splitter 64 and a polarisation combiner 65, each of which may be in the fonn of a Mach-Zehnder device as described above with reference to Figures 2 and 3. In this case the TE component supplied to the branch waveguide 66 is passed through one of the devices 63, and the TM component supplied to the branch waveguide 67 is passed through the other device 63, and the components outputted by the devices 63 are then combined within the combiner 65 to provide an output signal which is compensated for the polarisation dependent action of the device 63. Such an arrangement is particularly applicable where the the PDL of the device is not a strong function of wavelength or where the component operates over a narrow band of fixed wavelength, in which case the operation of the device in the lower and upper arms can be selected to match.

a variant of the arrangement of Figure 6, which the PDL of the two devices is identical, a first rotator is provided at the input of one of the devices 63 in one arms and a second rotator is provided at the output of the device 63 in order to reduce the PDL dependence. The preferred method of fabrication of the integrated optical device as described above will now be briefly described with reference to Figure 7A, 7B, 7C and 7D based on a SOI rib/ridge waveguide formed on a silicon substrate. I

1 2 As shown in Figure 7A the starting material is a SOI wafer 70 comprising a 100-

1000 Em (and preferably about 500 m) thick silicon substrate 71, a 0.05-4. 0,um (and preferably about 0.4 m) thick buried oxide layer 72, and a 0.520.0 rim (and preferably about 5,um) thick epitaxial layer 73 of silicon. The substrate 71 provides the lower confinement of the waveguide mode, and the upper confinement is provided by the interface between the epitaxial layer 73 and air (although optionally a suitable cladding layer, such as a thermally grown silicon dioxide or deposited silicon nitride or oxynitride, may be provided). Confinement in the lateral direction is provided by a rib/ridge structure which can be designed to ensure single mode operation or to ensure that higher order modes are not localised within the structure.

Figure 7B shows the formation of a rib 74 on the epitaxial layer 73 which has been Romped by photolithography by wet or dry etching, preferably by dry etching using chlorine or fluorine based chemistry. The rib 74 preferably has a width in the range 0.5-

30 m, and preferably about 4.0 m, and a height of between 0.5 1lm and 20 m, and preferably about 5 m, with a slab height of between 0.2 and] 7 m, and preferably about 2.5 1lm.

The various polarisation manipulation structures can be formed on top of such a waveguide structure which can be patterned by photolithography and etching to provide the required waveguide pattem. A number of such structures require the creation of one or more stressing layers, for example by thermal oxidation. For example a layer 75 of silicon oxide is grown on top of the layer 73 and the rib 74, as shown in Figure 7C, in a wet atmosphere at a temperature of between 700 and 1200 C for a period of between 0.2. and 10 hours. The oxide layer 75 is typically grown to a depth of 0.05 to 3.0 An, preferably about 0.4 m, in order to optimise the birefringence to the required device characteristic. The oxide layer 75 may be patterned as shown in plan view in Figure 7D by photolithography and etching in known manner.

Claims

CLAIMS:

1. An integrated optical device for reducing polarisation dependence, the device comprising a substrate, inputloutput waveguide means on the substrate for receiving an optical input signal and for outputting an optical output signal of reduced polarisation dependence, polarisation splitting/combining means on the substrate for splitting the input signal into first and second polarisation components and for combining together third and fourth polarisation components to produce the output signal, and polarisation conversion means on the substrate for converting the first polarisation component to differently polarized light for supplying to the polarisation splitting/combining means as the third polarisation component.

2. A device according to claim 1, wherein the input/output waveguide means, the polarisation splitting/combining means and the polarisation conversion means are integrally formed on the substrate.

3. A device according to claim 1 or 2, wherein the substrate is a SOI (silicon-on-

insulator) substrate.

4. A device according to claim 1, 2 or 3, wherein polarisation dependent means is provided on the substrate having a first port for receiving an input polarisation component and for supplying an output polarisation component and a second port for receiving the second polarisation component and for supplying the fourth polarisation component, the polarisation conversion means converting the first polarisation component to differently polarised light for supplying to the polarisation dependent means as the input polarisation component and converting the output polarisation component from the polarisation dependent means to differently polarised light for supplying to the polarisation splitting/combining means as the third polarisation component.

5. A device according to any one of claims I to 4, wherein the inpuVoutput waveguide means on the substrate comprises a common waveguide for receiving an

optical input signal from an optical fibre coupled to the waveguide and for outputting an optical output signal to the optical fibre, and the polarisation splitting/combining means comprises a common integrated element which performs both splitting and combining.

6. A device according to claim 5, 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.

7. A device according to any one of claims 1 to 4, wherein the polarisation splitting/combining means comprises polarisation splitting means for receiving an optical input signal from an input waveguide on the substrate and polarisation combining means, which is a separate element to the polarisation splitting means, for outputting an optical output signal to an output waveguide on the substrate.

8. A device according to claim 7, which is a polarisation scrambler for averaging polarisation losses with respect to time, wherein the polarisation splitting/combining means is adapted to provide an optical output signal in which the first and second polarisation components alternate with respect to time.

9. A device according to claim 8, wherein the polarisation splitting means incorporates optical switching means for shifting the phase of the first and second polarisation components with respect to time.

10. A device according to claim 9, wherein the polarisation splitting means comprises a Mach-Zehnder switch, and the phase shifting means comprises a pen diode arranged in one am1 of the switch.

11. A device according to claim 8, 9 or 10, wherein the polarisation combining means is a tunable recombiner adapted to provide an output averaging the third and fourth polarisation components.

12. A device according to claim 7, wherein the polarisation combining means is a tunable recombiner for combining the third and fourth polarisation components to provide an optical output signal of a predetermined polarisation mode.

13. A device according to claim 12, wherein phase shifting means are provided for controlling the relative phases of the third and fourth polarisation components to ensure that the optical output signal of the polarisation combining means has a predetermined polarisation mode.

14. A device according to claim 7, wherein first polarisation dependent means is provided on the substrate having a first port for receiving the first polarisation component and a second port for supplying the third polarisation component, and second polarisation dependent means, substantially identical to the first polarisation dependent means, is provided on the substrate having a first port for receiving the second polarisation component and a second port for supplying the fourth polarisation component.

15. A device according to any preceding claim, wherein the polarisation conversion means applies an integrated total polarisation rotation of 90 .

16. A device according to any preceding claim, wherein the polarisation conversion means comprises a stress-inducing structure providing a periodically varying refractive index in the direction of propagation.

17. A device according to any one of claims 1 to 16, wherein the polarisation splitting and/or combining means comprises a Mach-Zehnder polarisation splitter or combiner.

18. A device according to any one of claims 1 to 16, wherein the polarisation splitting and/or combining means comprises an adiabatic polarisation splitter or combiner.

19. An integrated optical device for reducing polarisation dependence, the device comprising a substrate, input waveguide means on the substrate for receiving an optical input signal, output waveguide means on the substrate for outputting an optical output signal of reduced polarisation dependence, polarisation splitting means on the substrate for splitting the input signal into first and second polarisation components, polarisation combining means on the substrate, which is a separate element to the polarisation splitting, for combining together third and fourth polarisation components to produce e the output signal, first polarisation dependent means having a first port for receiving the first polarisation component and a second port for supplying the third polarisation component, and second polarisation dependent means, substantially identical to the first polarisation dependent means, having a first port for receiving the second polarisation component and a second port for supplying the fourth polarisation component.

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