WO2012120306A2

WO2012120306A2 - Polarisation control device

Info

Publication number: WO2012120306A2
Application number: PCT/GB2012/050519
Authority: WO
Inventors: Barry Michael HOLMES; David Crichton HUTCHINGS; Anthony Edward Kelly; John Haig Marsh
Original assignee: The University Court Of The University Of Glasgow
Priority date: 2011-03-10
Filing date: 2012-03-08
Publication date: 2012-09-13
Also published as: WO2012120306A3; GB201104114D0

Abstract

A polarisation control device having a polarisation converter in a first section of optical waveguide with a gradually varying asymmetrical cross-sectional (transverse) profile and an active phase shifter in a second section of optical waveguide whose birefringence is adjustable by a relative phase controller. The polarisation converter converts an orthogonal fundamental mode (TE or TM) into a hybridised signal present in the active phase shifter. The first and second sections of waveguide are joined to one another to form a single continuous waveguide, which can significantly reduce interfacial reflections. The invention teaches the introduction of a selectable degree of asymmetry to a section of planar optical waveguide after growth of that waveguide to form the polarisation converter. This feature enables the use of lower efficiency mode convertors to give far greater control over fabrication tolerances, leading to increased yield.

Description

POLARISATION CONTROL DEVICE

TECHNICAL FIELD The invention relates to a device for controlling the polarisation of light within a planar optical waveguide structure, e.g. for subsequent use in optoelectronic devices and photonic integrated circuits. BACKGROUND TO THE INVENTION

Most designs of waveguide exhibit polarisation

dispersion, and the gain and refractive index of quantum well semiconductor based structures in particular are highly polarisation dependent. As quantum well structures are integral to many modern device designs, including lasers, optical amplifiers and QCSE modulators, polarisation

sensitivity is of major importance.

The ability to control and manipulate polarisation underpins the operation of everyday items from LCD displays to

Polaroid spectacles. When combined with lasers, control of polarisation has a diverse range of applications including metrology [1], polarimetry [2], time of flight range finding [3,4] and polarisation mode division multiplexing [5,6]. In a more recent development, a novel switched-polarisation method is being used for the in-vivo detection of malaria through the use of a magnetic dichroism effect [7], eliminating the time involved in sending samples of blood for analysis.

Known polarisation control techniques are cumbersome. Typically, bulk components, such as polarisers, analysers and electro-optic cells (phase retarders) must be used in various combinations in order to achieve control over polarisation. However, the assembly of these expensive, discrete, optical components in complex modules is an intricate, difficult and time-consuming process that adversely impacts both form factor and production costs.

The prospect of optoelectronic modules that have integrated active control of polarisation and yet are capable of mass production is attracting considerable attention for existing and potential applications, in particular within the optical communications industry.

In addition to traditional systems assembled using bulk optical components, polarisation control has recently been identified as an enabling technology for the utilisation of photonic integrated circuits in applications including all optical computing & ultra-high speed data routers [8] and "lab on a chip" sensing systems [9] . In particular, the ability to effectively control and manipulate polarisation states "on- chip" has the potential to offer practical solutions to existing problems, such as polarisation dependency, which severely compromises performance and limits functionality.

In bulk optics, the combination of optical phase shifters (retarders) with polarisation converters (wave plates) is a well understood method to provide control over polarisation.

There have already been attempts to transfer these principles into a waveguide-based format. However, such attempts have encountered difficulties, e.g. interfacial reflections between the various components of the waveguide, which limit their application.

SUMMARY OF THE INVENTION

At its most general, the present invention discloses a technique that enables smooth transitions between phase shifting and polarisation converting components in a waveguide to be provided through the use of regions of varying

asymmetry. With this technique, it is possible to apply the principles of the bulk optics approach because phase shifting and polarisation converting elements may be joined to one another in a single continuous waveguide, e.g. in a contiguous manner, which can significantly reduce interfacial

reflections .

A key aspect of the technique is the ability to introduce a selectable degree of asymmetry to a section of planar optical waveguide after growth (e.g. epitaxial growth) of that waveguide. The ability to select the asymmetrical

configuration is in stark contrast to many existing

arrangements in which the asymmetry is restricted to that offered by the crystallographic structure of the material system being used. This feature enables the use of lower efficiency mode convertors to give far greater control over fabrication tolerances, leading to increased yield.

According to the invention, there may therefore be provided a polarisation control device for controllably adjusting the polarisation of an optical signal input to an planar optical waveguide, the device comprising: a

polarisation converter comprising: a first optical waveguide section arranged to receive the input optical signal, the first optical waveguide section having a longitudinal portion with an asymmetrical cross-sectional (transverse) profile for converting an input signal occupying either one of the orthogonal fundamental modes (TE or TM) of the planar optical waveguide into a hybridised signal occupying both orthogonal fundamental modes of the longitudinal portion, wherein the longitudinal portion includes a variation region in which the degree of asymmetry of the cross-sectional (transverse) profile gradually varies along the first optical waveguide section; and an active phase shifter optically connected to the polarisation mode converter, the active phase shifter comprising: a second optical waveguide section arranged to receive an output signal from the first optical waveguide section; and a relative phase controller arranged to adjust birefringence exhibited by the second optical waveguide section .

The variation regions may be used to create smooth transitions into and out of the longitudinal portion. Such smooth transitions may ensure minimal loss, e.g. via radiation modes, higher order modes or back-reflections. In one embodiment, the longitudinal portion may have terminal variation regions in which the asymmetry is reduced to zero, i.e. the cross-sectional profile gradually returns to that of the original planar optical waveguide. The first and second optical waveguide sections may thus be contiguous and may form a single waveguide. In other words, the first and second waveguide sections are integrated (i.e. contiguous) with each other in series along a common waveguide without creating a discontinuity in the outer surface of the waveguide in direction of light propagation (referred to hereinafter as "propagation direction") .

By providing smooth transitions between the waveguide- based phase shifting and polarisation converting components defined above, the invention facilitates the fabrication of devices having these components assembled in various

combinations in series along a waveguide to provide complete control over the orientation and ellipticity of a polarised signal traversing through the waveguide. The order of the components typically depends on the application in which the device is to be used. Accordingly, the invention may provide means to control and exploit polarisation effects that conventionally are achieved using bulk optical elements, such as polarisers, phase retarders (wave plates) and analysers, in various combinations.

For example, one combination of the waveguide-based phase shifting and polarisation converting components may provide a device having the ability to take an optical signal having any polarisation and ellipticity and to convert it to any desired polarisation and ellipticity.

Furthermore, fabrication of one or both of the waveguide- based phase shifting and polarisation converting components using an active gain medium results in the ability to produce a laser, an optical amplifier, or any other optical waveguide based components, with integrated polarisation control.

Additionally, the ability to rapidly alter the polarisation of a laser source using relatively small currents can enable rapid modulation of the laser through polarisation. This feature is discussed in more detail below.

As defined above, the asymmetry of the longitudinal portion of the polarisation converter creates a hybridised signal occupying both orthogonal fundamental modes of the longitudinal portion. The asymmetry is configured such that the degree of hybridisation of the signal, i.e. the relative share of the hybridised signal that is in each of the

orthogonal fundamental modes of the longitudinal portion, varies along the propagation direction. This variation in the degree of hybridisation may be achieved through a

predetermined variation in the degree of asymmetry of the cross-sectional (transverse) profile of the first waveguide section. To improve or maximise manufacturing yield, it may be desirable to select the degree of asymmetry such that the mode conversion efficiency (i.e. the percentage proportion of output light that is in a second (e.g. TM) mode generated from input light in a first (e.g. TE) mode) is relatively low, e.g. 50% or less. Fabricating a polarisation mode converter with this level of efficiency may require more relaxed tolerances than the fabrication of a highly resonant (e.g. 100%

efficient) mode converter. Low efficiency devices are available for use in the invention because the degree of asymmetry, i.e. the offset of the optical axes in the

asymmetric section, is controllable in a manner that enables a hybridised signal (i.e. a signal comprising components in both fundamental modes) to be output to generate the "converted" signal in the waveguide beyond the (asymmetrical) first waveguide section. In contrast, devices in which the degree of asymmetry is constrained by the crystal structure of the waveguide may be required to exhibit very high efficiency. For example, as discussed below, if the degree of asymmetry provides a 45° offset in the optical axes, the hybridised signal may need to be highly resonant in order to exhibit the 100% mode conversion efficiency that is needed because the "converted" signal in the waveguide beyond the asymmetrical section is generated from only one of the fundamental modes in the asymmetrical section.

The invention may also be expressed as a polarisation control device for controllably adjusting the polarisation of a transverse electric (TE) or transverse magnetic (TM) polarised input optical signal, the device comprising: a polarisation mode converter comprising a first optical waveguide section having an asymmetric refractive index profile connected to receive the TE or TM polarised input optical signal and rotate its plane of polarisation to generate a mixed output having both TE and TM polarisation components; an active phase shifter comprising: a second optical waveguide section connected to receive the mixed output; and a relative phase controller arranged to electro- optically adjust the birefringence of the second optical waveguide section to control the ellipticity of the

polarisation of the output from the polarisation mode

converter, wherein the first optical waveguide section is substantially mode matched (or impedance matched) with the second optical waveguide section.

The substantial mode matching between the first and second waveguide sections may be achieved by a physically smooth transition between the first and second waveguide sections. In one embodiment, the smooth transition may be characterised by a smooth (e.g. constant) evolution of effective modal index with respect to lateral position on the waveguide as the modal parameters of the waveguide change in the propagation direction. The smooth transition between the first and second optical waveguide sections may improve efficiency by avoiding unwanted reflections at discontinuities in between separate components. The smooth transition in this embodiment may be characterised by the absence of

discontinuities the evolution of the outline of the cross- sectional area of the optical waveguide sections. The cross- sectional area (and its outline) may change between and within the first and second waveguide sections, but, according to the invention, this change occurs in a smooth fashion, e.g.

without step changes in the cross-sectional area of the

. dA

waveguide A or such that the derivative remains finite,

dz

where z is the propagation direction. In one embodiment, the waveguide sections may be integrated into a ridge waveguide. The side walls of the waveguide ridge in and between the first and second waveguide sections may be continuous, i.e. exhibit no discontinuities. However, an increase or decrease in cross-sectional area may be achieved by providing tapering transition regions between the first and second waveguide sections .

The device effectively provides two integrated waveguide- based modules which provide different types of control. The first module is the polarisation mode converter (PMC), which may operate passively or actively in a manner analogous to a bulk waveplate, i.e. to rotate the plane of polarisation. The second module is the active phase shifter (APS), which may apply a current or voltage to the waveguide to operate in manner analogous to a bulk electro-optic phase retarder.

The first and second waveguide sections may be integrally formed with each other. The phrase "integrally formed" may mean that the first and second waveguide sections exist as a common element, e.g. so that optical radiation propagating therein does not cross a material boundary (e.g. pass into free space) when passing from the first waveguide section to the second waveguide section. The first and second waveguide section may be part of a monolithic structure, e.g. formed on a common substrate. Monolithic integration may be facilitated by ensuring these modules are specifically designed to be compatible with existing semiconductor laser device

fabrication processing. Integrally forming all or parts of the first and second waveguide sections may facilitate efficient mass fabrication on a miniature scale. For example, each waveguide section may be part of a common waveguide that is etched following a single alignment and masking procedure.

The first and second waveguide sections may share a common layer of core material bounded in at least one

dimension by common layers of cladding material, the

refractive indices of the core material and cladding material being arranged to cause the core material to guide the propagating light. The core and cladding materials may be semiconductor materials arranged in a double heterostructure configuration (referred to herein as a "common

heterostructure"). The core semiconductor material may include an intrinsic region, whilst the bounding cladding semiconductor material may be p-doped and n-doped on opposite sides of the core to create a p-n, or p-i-n type structure. As mentioned below, the common layer of core semiconductor material (especially in the active phase shifter) may be arranged to exhibit quantum confinement effects. For example, the common core layer may comprise one or more quantum wells or other quantum confinement structure (e.g. quantum dots or the like) . Each quantum well may comprise a well layer between a pair of barrier layers made of semiconductor materials with band gaps selected to create the quantum well. Conventional semiconductor materials and/or superlattice based semiconductors, as used in, for example, lasers, amplifiers, LEDs and quantum cascade lasers, may be used.

The active phase shifter comprises a relative phase controller which may be arranged to inject current carriers or apply a bias voltage to the second waveguide section in order to controllably induce electro-optic and/or free carrier effects which alter the birefringence exhibited by the second waveguide section to induce a change in the relationship between a propagation constant for the TE polarisation component and a propagation constant for the TM polarisation component in the second waveguide section. For example, the relative phase controller may be an adjustable voltage source connected to the second optical waveguide section to

controllably induce therein a relative change between a propagation constant for the TE polarisation component and a propagation constant for the TM polarisation component. One or more electrodes may be fabricated on the second optical waveguide section to provide an electrical connection to the adjustable voltage source.

The relative phase controller may thus make use of natural birefringence in the second waveguide section, the electro-optic and/or free carrier effects being utilised to produce variations in the natural birefringence. Natural birefringence may result from the materials and/or geometry of the second waveguide section, and cause the TE and TM

polarised modes of light to propagate along the second waveguide section at different speeds, whereby a phase difference is introduced between the modes. The rate of the phase change depends on the difference between the propagation constants for the TE and TM polarisation components in the second waveguide section. Varying the propagation constants causes the phase difference introduced for propagation through a given length of the second waveguide section to change accordingly. The phase change seen over a given length of the second optical waveguide section is therefore controllable.

Varying the TE and TM propagation constants may be achieved by applying a voltage to forward bias or reverse bias the p-i-n structure in the common heterostructure to induce electro-optic and/or free carrier effects.

Double heterostructure waveguide phase modulators are known to operate using both electro-optic and free carrier effects [10] . Typical electro-optic effects include the Pockels effect and the Kerr effect. Carrier effects may include plasma effects and band filling/ shifting . The extent to which each electro-optic or free carrier effect occurs depends on the material used and whether forward or reverse bias is applied.

If a reverse bias voltage is applied across the common heterostructure, electro-optic effects may dominate. The overall effect is an increase in electric field within the core semiconductor material which causes a change in

refractive index. Applying a reverse bias may be preferred as it is a fast effect because the carriers are already depleted in the relevant direction. This may permit modulation of the polarisation at 1 GHz or more.

If the heterostructure is forward biased, carriers are injected. This may vary the electric field in the core semiconductor material (and hence vary the associated electro- optic effects) and may increase free carrier effects to cause differential changes in the propagation constants for the TE and TM modes as the quantum mechanical selection rules will tend to favour the TE mode (HH transition) , which leads to non-equal carrier contributions (i.e. band filling). The effective refractive index for the TE mode varies at a higher rate than the effective refractive index for the TM mode because the effective refractive index for the TE mode is nearer to the material band edge than the effective refractive index for the TM mode.

Since the TE mode is favoured, a side effect of

forwarding biasing may be polarization-dependent amplification in the second optical waveguide section, which may need to be compensated for elsewhere in the device.

As mentioned above, the core semiconductor material may be arranged to exhibit quantum confinement effects, i.e.

restrict carriers (electrons and holes) to certain discrete energy states. The electric field represented by the carriers is related to the refractive index of the material. As the TE and TM polarised modes are affected differently by the electric field, the phase difference between the TE and TM modes can be controlled by altering that field.

Quantum confinement effects may be produced by

incorporating one or more quantum dots and/or quantum wires and/or quantum wells in the core semiconductor material.

Providing a quantum confinement structure in the core layer of the common heterostructure can provide an anisotropy to the absorption band edges between TE and TM modes, due to the breaking of the valence band degeneracy. Changing the band edge will therefore influence the modes differently, which can be utilised to introduce a differential phase shift.

The band gap of any quantum confinement structure may be modified so that its edge is moved away from the wavelength of the light propagating through the waveguide. For a quantum well, band gap modification may be achieved by known techniques, such as selective area growth (SAG) , selected area re-growth (i.e. growth of a new structure on a partially etched structure), offsetting the quantum well in the core, or by forming a vertical twin guide with a dual core with quantum wells of different wavelengths. Quantum well intermixing

(QWI) techniques may also be used [11], e.g. using pulsed- photoabsorption-induced disordering (PPAID) , ion implantation, impurity free vacancy diffusion (IFVD) and the like.

For forward biased arrangements, shifting the band gap in this way may prevent the differential effect described above from dominating. However, the magnitude of the effect may still be useful within the amount of band edge shifting associated with normal QWI. For example, a TE mode π phase change may be introduced using a current injection in the order of milliamps.

Three advantages arise from using a quantum confinement structure (QCS ) -modified optical waveguide section.

Firstly, losses due to absorption of the guided mode in the waveguide material are reduced, which loosens any

restrictions on the length of the waveguide section.

Secondly, the shift in the band edge position enables bipolar operation of the device (i.e. selective application of forward or reverse bias across the boundary between the core and cladding semiconductor materials in the second optical waveguide section) without fear of disrupting the propagating signal. This advantage means that the magnitude of current required to make available a 180° range phase shifts is lower than in non-QCS-modified arrangement, e.g. by using a bipolar current to provide a ± 90° phase shift range.

Thirdly, carrier lifetimes in a QCS-modified optical waveguide, modified through the use of QWI, may be reduced compared with a non-QCS-modified arrangement. This may facilitate rapid switching of the active phase shifter, which may be important in modulation applications. Indeed, in one embodiment discussed below, the switching speed of the active phase shifter may mean it is suitable for use as a modulator.

The operation of the polarisation mode converter may be passive. For example, the polarisation mode converter and comprise a waveguide that is fabricated to give it a physical cross-section with no line of symmetry that extends through the centre of the waveguide (i.e. no plane of mirror symmetry containing the axis of the waveguide) . The waveguide may be subjected to an etching process to obtain this configuration. Conventional techniques of created asymmetric waveguides require realignment and re-masking steps to be performed before that etching process can take place. However, the present invention is preferably used with a technique of forming an asymmetric waveguide without the realignment and re-masking steps .

As mentioned above, the polarisation mode converter may be formed in a common heterostructure with the active phase shifter. The asymmetry may be provided in the core material and/or cladding material in the common heterostructure. For example, in a ridge waveguide, the ridge may be treated to provide the asymmetric refractive index profile of the first optical waveguide section.

In one embodiment, the first waveguide section may comprise one or more etched sub-features arranged

asymmetrically therein. The etched sub-features may be formed by one or more subsequent etching steps after the waveguide is formed. In a preferred embodiment, however, the may be formed with the waveguide in a reactive ion etch lag (RIE Lag) process during the etching of the ridge. RIE Lag is a

phenomenon associated with plasma etching techniques commonly used in semiconductor device fabrication. RIE Lag manifests itself as a variation in the depth of an etched feature that is proportional to the width of the feature. It arises primarily as a result of a reduction in the number of ions incident upon the feature to be etched, (due to the multiple angular trajectories of the incident ions) so that as the feature size is reduced, so too is the number of incident ion trajectories. This results in smaller features being etched less quickly than larger features. The extent of this effect is dependent upon the plasma etching parameters employed and may be reduced or increased dependent upon the application. In one embodiment, each sub-feature comprises a trench formed in and extending along one of the cladding layers, the trenches increasing in width and, due to RIE Lag, depth across the guide until a predetermined width is attained, at which point the width (and depth) may remain constant for a pre-determined length and may then reduce, causing the depth to reduce also.

There may be one or more trenches, arranged to provide a desired asymmetry. In one embodiment, there may be a

plurality of trenches, each trench being separated from an adjacent trench by an upstanding rib. The dimensions of each rib may be selected depending on the desired asymmetry.

Moreover, the trenches may be dimensioned to facilitate mode matching between the first and second waveguide sections. For example, each trench may vary in width and/or depth as a function of length in the propagation direction.

An advantage of the etched sub-features formed within a waveguide section is that the degree of asymmetry may be varied along the length of the guide. This may be

particularly useful for the creation of smooth transition regions which may transfer the integrally formed optical waveguide from a symmetric configuration to an asymmetric configuration. A transition region may be provided at the input to the first optical waveguide section and between the first and second optical waveguide sections. In one

embodiment, each trench tapers to a close in a transition region integrally formed between the first and second optical waveguide sections.

The purpose of the asymmetric refractive index profile is to provide an angular offset of the optical axes of the first optical waveguide section with respect to the input polarised radiation to cause the input polarised radiation to excite both fundamental orthogonally polarized modes in the first optical waveguide section. The length of the first optical waveguide section in the laser propagation direction may be determined based on the angular offset to generate a desired mixed output for a given input. In one embodiment, the angular offset may be 22.5° and the length L of the first optical waveguide section may be

(Δ?)

where (Αβ)_ΡΜΙ= is the difference between the propagation constant of the TE polarisation component and the propagation constant of the TM polarisation in the first optical waveguide section. In this embodiment, an input TE polarised signal effectively has its plane of polarisation rotated by 45° by exciting a mixed (hybrid) signal in the first waveguide section which yields 50% TE polarised and 50% TM polarised light when it leaves the first waveguide section (i.e. when the offset of optical axes is removed) . The same effect may be achieved for different angular offsets by making suitable alterations to the length of the first optical waveguide section so that the variation of the relative proportions of the components of the hybrid signal in a zone exhibiting an asymmetric refractive index profile reaches a point where when the asymmetry is removed the required proportion of TE polarised and TM polarised light is present. One advantage of the use of the RIE Lag process is that it permits the angular offset to be selectable, rather than constrained to a single value by the crystal structure of the waveguide, as it is for embodiments fabricated by conventional wet etching step to create the angled wall. This means that the first waveguide section may comprise a plurality of sub-sections, the optical axes of each adjacent sub-section being angularly offset from each other. This arrangement may provide the variation in the degree of hybridisation described above.

For example, if the angular offset is 45°, the length L of the first optical waveguide section may be

to achieve an output having 50% TE polarised and 50% TM polarised light when the asymmetry is removed.

The fabrication tolerances required to achieve a 45° rotation with a device having a 45° offset in the optical axes may be very demanding, however, because it requires the hybrid modes to convert very efficiently between each other (i.e. at output ideally 100% of the mode in the asymmetric section needs to be in one of the fundamental modes) . The alternative approach, described above, is to design the waveguide

asymmetry to give an offset in the optical axis of 22.5°. In this case, after propagating along a length

the polarisation will again consist of 50% TE and 50% TM components. The benefit of this second approach is that the fabrication tolerances are vastly improved when compared to those of a highly resonant structure with 45° offset. Fine tuning of the asymmetry may be provided by controllable carrier injection into the asymmetric waveguide sections, or through the addition of a heating element in close proximity to the asymmetric waveguide section.

Additional polarisation mode converters and/or active phase shifters may be coupled in series with the polarisation control device to provide additional layers of control. For example, the polarisation control device may include a second polarisation mode converter comprising a third optical waveguide section having an asymmetric refractive index profile, the third optical waveguide section being connected to receive the output from the active phase shifter and rotate the major axis of polarisation of that output through a predetermined angle. This component allows rotation of a polarisation configuration having a set phase difference.

The device may further include a second active phase shifter comprising: a fourth optical waveguide section connected to receive the output from the second polarisation mode converter; and a second relative phase controller arranged to electro-optically adjust the birefringence of the fourth optical waveguide section to control the ellipticity of the polarisation of the output from the second polarisation mode converter. This component may permit fine tuning of the phase difference between TE and TM components of a signal.

The additional components may be integrated in the same device, i.e. the third and/or fourth optical waveguide sections may be integrally formed with the first and second optical waveguide sections.

The combination of two polarisation mode converters and two active phase shifters in an alternating configuration (i.e. first polarisation mode converter - first active phase shifter - second polarisation mode converter - second active phase shifter) enables any TE or TM input signal to be adjusted to any mix of TE and TM polarisation and ellipticity.

Furthermore, if this combination of elements is reversed, it can be seen that any combination of TE and TM polarised input light, of any ellipticity, may be transformed to either linear TE or linear TM output. A device having this

configuration may further include a third active phase shifter at its output, the third active phase shifter comprising: a fifth optical waveguide section connected to receive the output; and a third relative phase controller arranged to electro-optically adjust the birefringence of the fifth optical waveguide section to control the ellipticity of the polarisation of that output. With this five-component configuration, the polarisation control device may

controllably adjust the polarisation and ellipticity of any polarised input optical signal consisting of transverse electric (TE) and/or transverse magnetic (TM) components, with any degree of ellipticity.

One important advantage of the polarisation control device discussed above is its ability to be monolithically integrated in an optical waveguide device. Accordingly, the device may provide in-built (e.g. "on-chip") polarisation control for photonic integrated circuits and the like. In particular, the polarisation control device may be

incorporated into a semiconductor laser to enable the

polarisation orientation of the laser output signal to be fully controllable.

Accordingly, another aspect of the invention may provide a semiconductor laser comprising a laser cavity arranged to emit polarised optical radiation that is optical coupled to a polarisation control device as described above.

In one embodiment, the laser cavity itself may be integrally formed with any one or more of the first and/or second and/or third and/or forth and/or fifth optical

waveguide sections described above. For example, the laser cavity may comprise a conventional double heterostructure arrangement, e.g. with one or more quantum wells confined in a core layer set between two cladding layers . The core and cladding layers may be integral with the core and cladding layers of the heterostructure of the first and second optical waveguides. The laser cavity may be optically coupled to the first optical waveguide section via a partially reflective grating structure, e.g. a distributed Bragg reflector grating.

The polarisation control device in the semiconductor laser may have any one or more of the properties discussed above .

BRIEF DESCRIPTION OF THE DRAWINGS Examples of the invention are described in detail below with reference to the accompanying drawings, in which: Fig. 1 is a schematic view of a semiconductor laser with an integral polarisation control device that is an embodiment of the invention;

Fig. 2 is a schematic perspective view of a waveguide section that is part of a polarisation mode converter in the polarisation control device that is an embodiment of the invention, together with a simulated contour plot of the TE component of the fundamental guided mode when viewed along the waveguide axis;

Fig. 3A is a scanning electron microscope (SEM) image of a cross-section through a waveguide section that is part of a polarisation mode converter in the polarisation control device that is an embodiment of the invention;

Fig. 3B is a SEM image of a top view through a waveguide section that a transition region at the end of a polarisation mode converter in the polarisation control device that is an embodiment of the invention;

Fig. 4 is a graphical representation of the phase difference between equal TE and TM polarisations propagating through a birefringent waveguide, to illustrate the effect of an active phase shifter in the polarisation control device that is an embodiment of the invention;

Fig. 5A is a schematic representation of the relative orientations of a laser array and a non-linear medium in a known frequency doubling apparatus; and

Fig. 5B is a schematic representation of the relative orientations of a laser array and a non-linear medium in a frequency doubling apparatus that incorporates a plurality of polarisation control devices that are embodiments of the invention .

DETAILED DESCRIPTION; FURTHER OPTIONS AND PREFERENCES

A semiconductor laser 10 incorporating a polarisation control device that is an embodiment of the invention is depicted in Fig. 1. The laser 10 has five sections A to E monolithically formed on a common substrate 11. Sections B and C are the fundamental building blocks of the polarisation control device and are discussed in detail below.

In this embodiment, all five sections of the laser are integrally formed in a double heterostructure optical waveguide 15 fabricated on the substrate 11. The double heterostructure optical waveguide 15 comprises a core

semiconductor layer 17 (e.g. made of a semiconductor such as (Al)GaAs or the like) surrounded on a top and bottom surface thereof by top and bottom cladding layers 18, 20 made from a material having a lower refractive index than the core semiconductor layer (e.g. made of a semiconductor such as AlGaAs with a higher percentage of Al, or the like) . The top cladding layer 18 can be p-doped and the bottom cladding layer 20 is n-doped to form a p-i-n type structure. Alternatively, the top cladding layer 18 can be n-doped and the bottom cladding layer 20 is p-doped to form a n-i-p type structure.

Section A comprises a laser cavity 12, e.g. gain section, located between a reflecting surface 14 (e.g. a cleaved facet) on the left of the device and a distributed Bragg reflection grating 16 at the right of Section A. The invention is applicable to many conventional types of laser, so the illustrated details should not be viewed as limiting. The laser may comprise a bulk material core, or one or more quantum wells, quantum wires or quantum dots (not shown) extending along the waveguide may be confined in the core layer within the laser cavity. The laser cavity is pumped with energy (e.g. via current injection) from a pump source (not shown) to cause the cavity to emit lasing radiation along the waveguide. As a consequence of quantum mechanical selection processes governing the gain of a semiconductor laser, the output of the laser cavity is linearly polarised. For a conventional, lattice-matched (non-strained) or

compressively strained quantum well semiconductor laser, the polarisation is in the plane of the wafer, known as the transverse electric (TE) direction. If the quantum well is subjected to a suitable tensile strain, the electric field may be orthogonal to the plane of the wafer, i.e. in the

transverse magnetic (TM) direction. The following discussion assumes that the laser output is polarised in the TE

direction, but is equally applicable for an output polarised in the TM direction.

Section B comprises a polarisation mode converter (PMC) 22, which in this embodiment is a passive device arranged to rotate the plane of polarisation of the signal outputted from the laser cavity 12. In this embodiment, the PMC 22 is arranged to rotate the plane of polarisation by 45°, so that the TE polarised mode that exists in the laser cavity is manipulated such that upon entering section C the mode consists of 50% polarisation in the TE state and 50% in the orthogonal TM state.

The PMC 22 comprises a first section 24 of the optical waveguide 15 which is fabricated to exhibit an asymmetric refractive index profile. This asymmetry has the effect of offsetting the optical axes of the waveguide. This means that each component of incident polarised radiation (i.e. a TE polarised component and/or a TM polarised component) excites both of the orthogonally polarised fundamental modes in the asymmetric section upon entering the asymmetric section. As a consequence of the offset of the optical axes, the modes in the asymmetric section become hybrid in nature, leading to beating as they propagate along the waveguide. Beating of the hybridised modes in the asymmetric section can be viewed as rotation of the plane of polarised light in the non-asymmetric section. If the optical axes are offset by 45°, the modes are essentially fully hybridized when the input signal is

received. Accordingly, at each half beat length ^_/2 the modes in the asymmetric section correspond to a 90° rotation of the polarised li ht in the non-asymmetric section, where

' which is the difference between the propagation constants of the two fundamental ( single-lobed) modes in the asymmetric section. For a given waveguide, the angle of rotation thus depends of the offset angle (effectively the relative proportion of hybrid modes at signal input) and the length of the asymmetric section. A polarisation rotation of 45° could therefore be achieved by designing the length of a device with a 45° offset to be equal to 0.5x(L_1/2)_PMC .

Several techniques for realising waveguides with asymmetric profiles are known. The conventional approach makes use of ribs with slanted walls and has been used to fabricate waveguide-based PMC devices [12, 13, 14] . In order to create the slanting walls, intricate processing is required including multiple realignment and masking steps together with a combination of crystallographic wet etching to produce the angled wall, together with dry etching to achieve the straight wall .

This technique has been improved by using an angled sample holder to ensure the ion etching takes place in the required direction, eliminating the wet etch stage and associated requirement of the intricate realignment and re- masking stages. Using this approach, the first demonstration of an integrated laser and mode converter has been reported

[15] .

A preferred approach, however, is based upon a

development of another fabrication technique [16]. This technique utilises the phenomenon of reactive ion etch lag (RIE Lag) to produce multiple-depth waveguides with

controllable profiles, in a simple, single-stage, dry-etch process. It is this inherent simplicity that makes the approach ideally suited for inclusion in the existing

integrated-optical system design suite.

RIE Lag manifests itself as a variation in the depth of an etched feature, proportional to its width. Whilst

historically viewed as a problem, RIE Lag enables the

fabrication of waveguides with sub-features having a size less than the mode guided by the waveguide. Fig. 2 shows a simple waveguide 26 that exhibits the effects of RIE Lag. In Fig. 2, a cladding layer 28 of GaAs, fabricated as an elongate ridge on a core layer 29 of AlGaAs, has three trenches 30, 31, 32 etched at one side of its upper surface. The shape of the trenches may be defined in a lithographic step (e.g. using e- beam lithography or nano-imprint lithography or photolithography depending on the dimensions) . The trenches are then formed by etching. Each trench extends along the elongate guide. The depth of each trench may be proportional to its width, and in this embodiment the trenches are equally spaced by upstanding ribs 33, 34.

Because the dimensions of the ribs and trenches produced using this method are much smaller than the wavelength of the propagating light, as the mode propagates it effectively experiences an averaged refractive index profile that is equivalent a guide with an asymmetric profile. The inset of Fig. 2 illustrates this point by showing a simulated contour plot of the TE component of the lowest order guided mode.

One, two, three or more trenches may be used. Fig. 3A shows cross-section view of a normally rectangular cross- section waveguide that has been subjected to RIE Lag treatment to create two trenches 35.

In the embodiment, the RIE Lag technique may be used to introduce asymmetry into the core semiconductor material. In comparison with alternative approaches, fabrication of a PMC using the RIE Lag technique is extremely simple, e.g.

requiring only one direct-write mask step followed by one dry etch process. As a result, the time taken to produce such devices is reduced significantly and the fabrication

tolerances greatly improved.

Furthermore, the RIE Lag technique permits more control of the degree of asymmetry than the alternative techniques, which can be limited by crystallographic constraints, i.e. only certain angled surfaces may be etched onto a crystal structure .

Moreover, the RIE Lag technique has an addition advantage in providing the ability to grade the asymmetry as a function of propagation length. This permits the creation of "smooth" transition regions between asymmetric waveguide sections and symmetric waveguide sections. An example of a transition region is shown in Fig. 3B, where it can be seen that the two trenches 36, 37 taper to a close along the waveguide. This ability to grade the asymmetry reduces interfacial reflections to an absolute minimum, and thereby addresses a critical concern for many integrated optical devices and a major problem with previous attempts to provide polarisation control in a waveguide-based format.

Returning to Fig. 1, Section C comprises an active phase shifter (APS) 38, which is a current-controlled section of the waveguide 15 arranged to select a phase difference between the TE and TM components output from the PMC 22. The APS 38 operates using the natural birefringence of waveguide

structures, wherein the TE and TM components propagate at different speeds, resulting in a phase difference between the TE and TM waves as the mode travels along the guide, in a manner as depicted in Fig 4. The function of the APS 38 is to permit this phase difference to be increased/decreased upon demand .

Expressed generally, the APS 38 is a birefringent waveguide connected via electrodes (not shown) mounted thereon to an adjustable voltage source (not shown) arranged to inject carriers into the waveguide, or apply a reverse bias voltage across the waveguide, to cause variations in the propagation constants {β_ΤΕ)_ΑΡ8 and {β_ΤΜ)_Αρ f°^r the TE and TM components propagating therein respectively. As explained above, the propagation constants are related to the beat length {L_I/2)_APS ·

Because the length of the APS 38 is fixed, any change in the beat length results in a phase shift between the TE and TM components of the guided mode, which effectively results in the direction of the major axis of the polarisation being adjusted.

To enhance and provide control over these effects, the core semiconductor layer 17 may be arranged to exhibit quantum confinement effects, e.g. by having one or more quantum wells, quantum wires, or quantum dots formed therein. To form these quantum confinement structures, the core layer 17 may

comprises a plurality of sub-layers (not shown) of suitable dimension. For example, the core layer 17 may comprise a pair of barrier layers sandwiching a well layer. The materials and methods of fabrication may be conventional.

The injection of carriers causes free carrier (and to a lesser extent electro-optic effects) in the waveguide, which change the anisotropy of the material in a way that causes changes in the propagation constants. Two approaches are available to produce this electro-optic change in the beat length (controllable variation of birefringence) within the

APS .

The first approach is based on current injection which induces differential changes in the propagation constants of the TE and TM modes, due to non-equal carrier depletion. The resultant change in (Δ?)_Α/¾ facilitates an ability to precisely control the beat length {L_1/2)_aps r albeit with an amplification of the TE component, which would need to be accounted for through a variation of the input power proportions of the TE and TM modes . The preferred, second approach relies upon the fact that the band edge of the quantum well or wells in the APS section may be "blue shifted" through various band gap modification techniques, such as selective area growth or quantum well intermixing (QWI). The amplification is now negligible because the free carrier effects are reduced as the band gap moves away from the active wavelength, and the predominant effect is that the refractive index changes with drive current. The drive current thus causes a relative change of (P_TE)_APS with respect to {β_ΤΜ )_APS , thereby causing (L_l/2)_Aps to change .

Whilst this approach offers a more simple design, the smaller absolute changes in the propagation constant means that the length of the APS needs to be longer than the APS that operates using polarisation-dependent amplification.

However, an increase in length can be accommodated because the band gap modification process shifts the band edge away from the active wavelength, thereby reducing absorption losses. Moreover, the shift in band edge also enables a bipolar current to be applied, i.e. application of both forward bias where free carrier effects dominate and reverse bias where electro-optic effects dominate, which reduces the current required to induce a 180° phase shift. An additional advantage is that some processes (e.g. QWI) can lead to reduced carrier lifetimes, enabling superior switching speeds, a critical concern in high-speed modulation applications.

In a development of the invention, the arrangement described in the first approach above may also be used in reverse, resulting in an output intensity that is dependent upon the input polarization (and power) . In conjunction with a simple detector, this would enable the measurement of an incoming signal's relative TE and TM components.

Returning again to Fig. 1, the output of the APS 38 may be passed through a second PMC 40 (Section D) . This section functions in exactly the same way as the PMC 22 discussed above. Consequently, the second PMC 40 would act to rotate the major axis of the output signal's polarisation depending on the phase difference between the TE and TM components. As the input phase difference is controllable (by the APS 38), the rotation in the second PMC 40 is therefore indirectly controllable .

Section E is a second APS 42 that enables the ellipticity of the output of the second PMC 40 to be controlled, i.e. to fine tune the ellipticity of the output signal 44. The second

APS 42 functions in exactly the same way as the APS 38 in Section C. Thus, full control is achieved over both the plane and the ellipticity of the polarisation. In turn, this enables the output polarisation to be tuned, switched, or even rotated at will.

The combination of the modular components (the PMC and APS) discussed above, especially together with a semiconductor laser, may offer unprecedented control over output

polarisation states and enable the realisation of a multitude of potential applications.

In one example, a laser (or an array of lasers)

integrated with polarisation controller ( s ) can be used to illuminate a sample or target with various known polarisations and the reflected signal analysed through a simple imager or detector to obtain polarisation dependent contrast images and

(or) measurement of the Stokes parameters (which describe polarised light) leading to compositional information, surface smoothness, material properties and slope angles.

An additional advantage of the proposed technique is that the laser's polarisation may be switched through relatively small currents applied to the APS section(s), enabling modulation of the output power of lasers through the addition of a simple polarising element (e.g. polarising beam

splitter). This offers a simple alternative to direct

modulation of the lasers or laser arrays, which involves switching large currents at speed.

Modulation of the polarisation of the beam could also be used in laser range finding applications (e.g. LIDAR or

LADAR) , where the speed of the transition between different polarisation states would enable pulses of variable length and duty-cycle, with rapid rise times to be produced. The

characteristics of the laser (s), in particular pulse length and rise/fall times, determine the accuracy of LIDAR systems. Additionally, in Doppler LIDAR applications, one polarisation could be emitted and the reflected beam "mixed" with the orthogonal polarisation in a heterodyne (or homodyne) detection system. In either the time-of-flight or Doppler approach, one polarisation could be used as a reference and the orthogonal polarisation as the signal.

In another example, one or more lasers, each with an integrated polarisation controller, could be used to produce a three dimensional laser projection system. Projection of images with alternating orthogonal polarisation in conjunction with suitably configured polarising eyewear, i.e. with one polarising lens orthogonally orientated with respect to the other, would result in alternating polarisation projected images being received by each eye individually, one

requirement for three dimensional viewing.

In another aspect, the invention may facilitate the fabrication of an array of frequency doubled optical signals. Many conventional non-linear media used for frequency doubling require input TM polarised input signals. However, many conventional laser outputs at frequencies useful for frequency doubling to generate e.g. blue or green light are TE

polarised. However, in this aspect of the invention, the waveguide-based polarisation mode converter described above may be arranged to convert each TE polarised output from an array of lasers into a respective TM polarised input for a non-linear medium. The polarisation mode convertor rotates the polarisation to permit direct optical coupling (i.e.

optical coupling without the use of auxiliary optical

elements) of each laser output to a non-linear medium used for frequency doubling. Indeed, the array of lasers with their respective polarisation mode converters may be arranged to feed an array of non-linear waveguides to yield an output comprising a plurality of frequency doubled optical signals.

This arrangement may be particularly suited for small scale blue and green lasers, e.g. for use in colour pico-pro jectors or the like, or in high power array applications, such as laser projectors for digital cinema.

Fig. 5A shows a known array 50 of semiconductor lasers 52 formed monolithically on a common substrate 54. The outputs 56 of the lasers 52 is TE polarised (depicted graphically as being in the plane of the substrate 54) . To achieve the greatest power conversion in a non-linear crystal 58 used for frequency doubling, the input light should be TM polarised

(i.e. perpendicular to the plane of the substrate) . As shown in Fig. 5A this leads to a conflict between the orientation of the non-linear crystal and the laser outputs from the array. Conventional manipulation of the output polarisations to solve this problem was cumbersome and often costly, e.g. requiring several lenses and polarisation-controlling wave plates to rotate the polarisation to the required plane; an intricate and time consuming process, which adds considerably to fabrication costs.

The polarisation control device as set out above may represent a disruptive innovation which overcomes these difficulties. The solution is depicted in Fig. 5B, where each semiconductor laser bar 52 has an polarisation rotation section 60 integrated therewith. At its simplest, each polarisation rotation section 60 may be a PMC only, e.g.

having one or more sections arranged to rotate the input polarisation through 90°. Each polarisation rotation section 60 may be a polarisation control device as set out above, e.g. having a PMC and APS connected in series, to enable a TM polarisation output to be output from a standard TE emitting wafer. When coupled with a non-linear wafer 58, this may lead to cheap, reliable, high-power red, green and blue laser bars for application in laser projection systems. The non-linear wafer 58 may be fabricated as a plurality of non-linear waveguides. A monolithic array of frequency doubling lasers may therefore be achieved.

In one embodiment, an active phase shifter (APS), as described previously, may be formed in one or more of the non-linear waveguides. The ellipticity of the output optical signal may thus be controlled to ensure efficient operation of the frequency doubling media. Moreover, as mentioned above, the active phase shifter may be used as a modulator. In the array, the active phase shifter may thus be arranged to switch the output polarisation of the laser to modulate the output of frequency doubling light from the non-linear medium.

In other embodiments, controllable polarisation mode modulation may be used in telecommunication, e.g. to carry data itself or to enable polarisation multiplexing, or for the production of pulses with rapid rise times for laser range finding and sensing applications.

The polarisation control device of the invention may present other advantages. In particular, the polarisation control device may be used as an interface between received signals of unknown polarisation and devices that exhibit polarisation dependence, such as, polarisation dependent semiconductor optical amplifiers (SOA) . Currently, since optical fibres do not preserve the polarisation state of light propagating therethrough, components which interact with signals from optical fibres preferably exhibit polarisation independence, especially if they are small scale components where conventional polarisation control is too bulky to be usable.

The polarisation control device of the invention may enable the requirement for polarisation independent operation to be bypassed. In one example, the alternating arrangement of three phase shifters and two polarisation mode convertors described above may be used to take an input optical signal of any mix of TE and TM polarisation and any degree of

ellipticity, and convert it to any required mix of TE and TM polarisation and ellipticity.

The device finds applications in telecommunications networks where the incoming polarisation state of an optical signal to an optical device is generally not known and due to the properties of optical fibre can be in any polarisation state. In one example, the polarisation control device may be used to control the polarisation of a local signal relative to an incoming signal of unknown polarisation, to enable further processing and/or demodulation to take place. This aspect of the invention may be expressed as polarisation alignment apparatus comprising: a local laser source arranged to output optical radiation having a first polarised state; a

polarisation control device as set out above connected to receive the optical radiation as an input and output a controlled signal having a controllable polarisation state according to a feedback signal indicative of a known

relationship between the second polarised state and the controlled polarisation state.

This apparatus may be of particular use in an optical heterodyne detection and/or polarisation multiplexed systems.

Furthermore, the polarisation control device may be used in polarisation modulation and demodulation apparatus for carrying data according to the state of polarisation of each bit in the data stream. For example a 'mark' could be represented by a particular polarisation state and a 'space' by the orthogonal polarisation state. One example of such an apparatus comprises, for the transmitter, a laser source arranged to output optical radiation having a first polarised state; a polarisation control device as set out above

connected to receive the optical radiation as an input and output a signal having its polarisation state modulated according to an input electronic signal to the polarisation control elements. For the receiver apparatus, alignment of the incoming signal to a particular polarisation state

suitable for demodulation can be carried out as described above .

REFERENCES

Handbook of Optical Metrology: Principles and

Applications. Toru Yoshizawa CRC 1st ed (Feb 25 2009) . [2] Principles and Applications of Optical Polarimetry.

Azzam. John Wiley & Sons (March 2, 2009) .

[3] M.-C. Amann et al . , "Laser ranging: a critical

review of usual techniques for distance

measurement", Opt. Eng. 40 (1), 10 (2001).

[4] J. Clark, A.M. Wallace, B.Liang, E. Trucco.

"Eliminating Multiple Reflections in a Laser

Rangefinder Using Polarized Light", 1997 The

Institution of Electrical Engineers.

[5] Optical communications based on optical polarization multiplexing and demultiplexing. United States Patent 7343100

[6] MIN Chul-Ki., LEE Manseop., "Simulation of link

property on 40Gbit/s polarization division multiplexing transmission", Proceedings of SPIE, the International Society for Optical Engineering ISSN 0277-786X [7] D. M. Newman, J. Heptinstall, R. J. Matelon, L.

Savage, M. L. Wears, J. Beddow, M. Cox, H. D.F.H. Schallig, P. F. Mens, " A Magneto-Optic Route toward the In Vivo Diagnosis of Malaria: Preliminary Results and Preclinical Trial Data" Biophysical

Journal, 95 (2), 994-1000, (July 2008) .

[8] Optical Computing. F.A.P Tooley (Ed), B.S Wherrett

(Ed) Taylor & Francis 1st ed (January 1, 1989)

[9] V. K. Yadavalli, M. V. Pishko, "Biosensing in

microfluidic channels using fluorescence

polarization", Analytica Chimica Acta, 507, (1) 123- 128 (April 2004) .

[10] S. S. Lee, R. V. Ramaswamy, V. S. Sundaram,

"Analysis and design of high-speed high-efficiency GaAs=AlGaAs double-heterostructure waveguide phase modulator", IEEE Journal of Quantum Electronics, Vol. 27, No. 3 (March 1991).

[11] J. H. Marsh, "Quantum well intermixing", Semicond.

Sci. Technol. 8, pp. 1136-1155 (1993). [12] P. Tzolov and M. Fontaine, "A passive polarization converter free of longitudinally-periodic

structure," Opt. Comraun. , vol. 127, pp. 7-13, 1996.

[13] J. Z. Huang, R. Scarmozzino, G. Nagy, M. J. Steel, and R. M. Osgood, "Realization of a compact and single-mode optical passive polarization converter," IEEE Photon. Technol. Lett., vol. 12, no. 2, pp. 317-319, Feb. 2000. [14] F. H. Groen,Y. C. Zhu, and J. J. G. M. van der Tol,

"Compact polarization converter on InP/InGaAsP using an asymmetrical waveguide," in Proc.ECIO, Prague, Czech Republic, 141-144, 2003.

[15] J. J. Bregenzer, S. McMaster, M. Sorel, B. M.

Holmes, and D. C. Hutchings, "Compact Polarization Mode Converter Monolithically Integrated within a Semiconductor Laser" J. Light. Tech., accepted for publication (Feb 2009) . [16] B. M. Holmes, D. C. Hutchings, "Realisation of Novel

Low-Loss Monolithically Integrated Passive Waveguide Mode Converters", IEEE Photon. Technol . Lett., 18, 43 (2006) .

Claims

1. A polarisation control device for controllably adjusting the polarisation of an optical signal input to an planar optical waveguide, the device comprising:

a polarisation converter comprising:

a first optical waveguide section arranged to receive the input optical signal, the first optical waveguide section having a longitudinal portion with an asymmetrical cross-sectional (transverse) profile for converting an input signal occupying either one of the orthogonal fundamental modes (TE or TM) of the planar optical waveguide into a hybridised signal occupying both orthogonal fundamental modes of the longitudinal portion,

wherein the longitudinal portion includes a variation region in which the degree of asymmetry of the cross-sectional (transverse) profile gradually varies along the first optical waveguide section; and

an active phase shifter optically connected to the polarisation mode converter, the active phase shifter

comprising :

a second optical waveguide section arranged to receive an output signal from the first optical waveguide section; and

a relative phase controller arranged to adjust birefringence exhibited by the second optical waveguide section .

2. A polarisation control device according to claim 1, wherein the first and second optical waveguide sections are contiguous and form a single waveguide.

3. A polarisation control device according to claim 1 or 2, wherein the asymmetrical cross-sectional profile of the longitudinal portion is selected to exhibit a mode conversion efficiency of 50% or less.

4. A polarisation control device according to any preceding claim, wherein the relative phase controller is an adjustable voltage source connected to the second optical waveguide section to controllably induce therein a relative change between the propagation constants of both orthogonal fundamental modes of the second optical waveguide section.

5. A polarisation control device according to any one of claims 1 to 4, wherein the first and second optical

waveguide sections are parts of a common structure formed by a core semiconductor material sandwiched between an p-doped cladding semiconductor material and a n-doped cladding

semiconductor material.

6. A polarisation control device according to claim 5, wherein the core semiconductor material includes one or more quantum confined structures.

7. A polarisation control device according to claim 6, wherein the quantum confined structures comprise one or more band gap modified quantum structures.

8. A polarisation control device according to any one of claims 5 to 7, wherein the relative phase controller is arranged to apply a reverse bias voltage across the common structure in the second optical waveguide section.

9. A polarisation control device according to any one of claims 5 to 7, wherein the relative phase controller is operable to inject current carriers into the common structure at the second optical waveguide section.

10. A polarisation control device according to any one of claims 5 to 9, wherein the cladding and/or core material in the first optical waveguide section comprises one or more etched sub-features arranged asymmetrically on the guide.

11. A polarisation control device according to claim 10, wherein the one or more etched sub-features exhibit a reactive ion etch lag (RIE Lag) -generated variation in depth as a function of width.

12. A polarisation control device according to claim 10, wherein the one or more etched sub-features comprise one or more trenches formed in and extending along the first optical waveguide section, the trenches increasing in depth and width across the waveguide.

13. A polarisation control device according to claim 10 wherein each trench tapers to a close in a junction region between the first and second optical waveguide sections.

14. A polarisation control device according to any one of claims 10 to 13, wherein the asymmetrical arrangement of etched sub-features provides a variation in the degree of asymmetry of the longitudinal portion of the first optical waveguide section to cause a polarised input signal to excite both orthogonal fundamental modes of the longitudinal portion.

15. A polarisation control device according to claim 14 wherein the length L of the longitudinal portion of the first optical waveguide section is

π

(Δ?)

where (Αβ)_ΡΜΙ= is the difference between the propagation constant of the TE polarisation component and the propagation constant of the TM polarisation in the first optical waveguide section .

16. A polarisation control device according to any preceding claim comprising a second polarisation mode

converter connected to receive the output from the active phase shifter to control the orientation of a major axis of polarisation of an output signal, the second polarisation mode converter comprising:

a third optical waveguide section arranged to receive an input signal that is the output from the active phase shifter, the third optical waveguide section having a second

longitudinal portion with an asymmetrical cross-sectional (transverse) profile for converting the input signal into a hybridised signal occupying both orthogonal fundamental modes of the second longitudinal portion,

wherein the second longitudinal portion includes a variation region in which the degree of asymmetry of the cross-sectional (transverse) profile gradually varies along the third optical waveguide section.

17. A polarisation control device according to claim 16, wherein the first, second and third optical waveguide sections are contiguous and form a single waveguide.

18. A polarisation control device according to claim 16 comprising a second active phase shifter connected to receive the output from the second polarisation mode converter to control the ellipticity of the polarisation of the output signal, the second active phase shifter comprising:

a fourth optical waveguide section arranged to receive an output signal from the third optical waveguide section; and a second relative phase controller arranged to adjust birefringence exhibited by the fourth optical waveguide section .

19. A polarisation control device according to claim 18 wherein the first, second, third and fourth optical waveguide sections are contiguous and form a single waveguide.

20. A polarisation control device according to claim 18 comprising a third active phase shifter placed before the first optical waveguide section to control the ellipticity of the input optical signal before it enters the first optical waveguide section, the third active phase shifter comprising a fifth optical waveguide section arranged to receive the input optical signal; and

a third relative phase controller arranged to adjust birefringence exhibited by the fifth optical waveguide

section .

21. A semiconductor laser comprising a laser cavity arranged to emit polarised optical radiation that is optically coupled to a polarisation control device according to claim 1.

22. A semiconductor laser according to claim 21, wherein the laser cavity emits TE (or TM) polarised optical radiation, and the polarisation mode converter is operable to transform the TE polarised optical radiation to a TM (or TE) polarised output signal.

23. A semiconductor laser according to claim 21, wherein the laser cavity emits TE (or TM) polarised optical radiation, and the polarisation mode converter and active phase shifter is operable to transform the TE polarised optical radiation to a mixture of TM and TE polarised output signal.

24. A semiconductor laser according to any one of claims 21 to 23, wherein the active phase shifter is operable to modulate the polarisation state of the output signal.

25. A semiconductor laser comprising a laser cavity arranged to emit polarised optical radiation that is optically coupled to a polarisation control device according to claim 18 or 19, wherein the laser cavity emits TE (or TM) polarised optical radiation, and the polarisation mode converters and active phase shifters are operable to transform the TE (or TM) polarised optical radiation into any mixture of TM and TE polarised output, with any degree of ellipticity between the polarised signals.

26. A semiconductor laser according to claim 25, wherein one of the active phase shifters is operable to modulate the polarisation state of the output signal.

27. A semiconductor laser according to any one of claims 21 to 26 coupled directly to a non-linear medium used for frequency doubling.

28. An array of semiconductor lasers comprising:

a plurality of semiconductor laser cavities arranged to emit a plurality of plane polarised input optical signals;

a plurality of polarisation mode converters, each polarisation mode converter comprising:

a first optical waveguide section arranged to receive a plane polarised input optical signal from a

respective semiconductor laser cavity, the first optical waveguide section having a longitudinal portion with an asymmetrical cross-sectional (transverse) profile for converting the plane polarised input signal into a hybridised signal occupying both orthogonal fundamental modes of the longitudinal portion,

a plurality of waveguides made of non-linear material for frequency doubling, each non-linear waveguide being optically coupled to the first optical waveguide section of a respective polarisation mode converter,

wherein the first optical waveguide section of each polarisation mode converter has an asymmetry and length selected such that the relative proportion of the TE and TM polarisation components of the hybridised signal yield an output optical signal for their respective waveguide made of non-linear material that is a mixture of both orthogonal fundamental modes of that waveguide made of non-linear material .

29. An array of semiconductor lasers according to claim 28, wherein each polarisation mode converter is formed in its respective non-linear waveguide or its respective laser cavity .

30. An array of semiconductor lasers according to claim 28 or 29 further comprising a plurality of active phase shifters, each active phase shifter comprising:

a second optical waveguide section optically connected to receive an output signal from the first optical waveguide section of a respective polarisation mode converter; and

a relative phase controller arranged to adjust

birefringence exhibited by the second optical waveguide section .

31. An array of semiconductor lasers according to claim 30 further comprising a plurality of second polarisation mode converters, each second polarisation mode converter

comprising :

a third optical waveguide section optically connected to receive an input signal that is the output from a respective active phase shifter, the third optical waveguide section having a second longitudinal portion with an asymmetrical cross-sectional (transverse) profile for converting the input signal into a hybridised signal occupying both orthogonal fundamental modes of the second longitudinal portion,

32. An array of semiconductor lasers according to claim 30 or 31, wherein each active phase shifter is formed in its respective non-linear waveguide or laser cavity.

33. Polarisation control apparatus comprising:

a local laser source arranged to output optical radiation having a first polarised state;

a polarisation control device according to any one of claims 1 to 19 connected to receive the optical radiation as an input and output a controlled signal having a controllable polarisation state;

a controller comprising a closed feedback loop arranged to control the state of polarisation via the active phase shifter (s) of the polarisation control device.

34. Polarisation control apparatus according to claim

33, wherein the controller includes a comparator device arranged to mix a remotely generated signal having a second polarised state and the controlled signal, and generate a feedback signal indicative of a match between the second polarised state and the controllable polarisation state.

35. Polarisation control apparatus according to claim

34, wherein polarisation control device is operable based on the feedback signal to dynamically tune the controllable polarisation state to the second polarised state.

36. Polarisation modulation apparatus comprising an polarisation control device according to any one of claims 1 to 20 arranged to generate an output signal carrying data by modulating the polarisation state of an input optical signal.

37. A polarisation control device for transforming, in an planar optical waveguide, an input optical signal having any polarisation state into an output signal polarised in one of the orthogonal fundamental modes of the optical waveguide, the device comprising:

a first active phase shifter comprising:

a first optical waveguide section arranged to receive the input signal; and

a relative phase controller arranged to adjust birefringence exhibited by the first optical waveguide section;

a first polarisation converter comprising a second optical waveguide section optically coupled to receive an output optical signal from the first optical waveguide section, the second optical waveguide section having a first longitudinal portion with an asymmetrical cross-sectional (transverse) profile for converting the received output optical signal into a hybridised signal occupying both orthogonal fundamental modes of the first longitudinal portion, wherein the first longitudinal portion includes a variation region in which the degree of asymmetry of the cross-sectional (transverse) profile gradually varies along the second optical waveguide section;

a second active phase shifter comprising:

a third optical waveguide section optically coupled to receive an output signal from the second optical waveguide section; and

a relative phase controller arranged to adjust birefringence exhibited by the third optical waveguide section; and

a second polarisation converter comprising a fourth optical waveguide section optically coupled to receive an output optical signal from the third optical waveguide section, the fourth optical waveguide section having a second longitudinal portion with an asymmetrical cross-sectional (transverse) profile for converting the received output optical signal into a hybridised signal occupying both orthogonal fundamental modes of the second longitudinal portion, wherein the second longitudinal portion includes a variation region in which the degree of asymmetry of the cross-sectional (transverse) profile gradually varies along the second optical waveguide section,

wherein the first and second active phase shifters are arranged to adjust the birefringence of the first and third optical waveguide sections such that the polarisation

conversion performed in the fourth optical waveguide section yields an output signal that is plane polarised in the planar optical waveguide.

38. Projection apparatus for projecting three- dimensional images, the apparatus comprising one or more semiconductor lasers according to claim 26 arranged to project one of more images, wherein each laser is arranged to modulate the polarisation of its output optical signal such that the projected images alternate between orthogonal polarisations.