Patent Description:
A TE/TM-mode converter with a laterally tilted rib waveguide fabricated on a stepped substrate is described in "<NPL>.

<CIT> relates to a semiconductor polarisation rotating element.

<CIT> relates to a semiconductor wavelength plate type polarisation element consisting of two semiconductor waveguides formed on a semiconductor substrate and arranged in parallel to each other in different heights from the substrate surface.

Examples described herein relate to a semiconductor structure for a PIC. More specifically, the examples described herein relate to a light polarisation converter for a PIC.

In some examples, a PIC is constructed from basic building blocks intended for the construction of the PIC. The basic building blocks include various components, each having a particular function. An example of a basic building block is a waveguide structure. Basic building blocks may have a particular effect on light incident thereon. The examples described herein relate to a light polarisation converter that can be used as a basic building block for a PIC.

A light polarisation converter is a component for an optical system (such as a PIC) which converts between different polarisations of light. For example, a suitably configured light polarisation converter can be used to convert between a first linear polarisation of light and a second linear polarisation of light (e.g. convert horizontally polarised light into vertically polarised light and vice versa). In another example, a suitably configured light polarisation converter can be used to convert between linearly polarised light and circular polarised light. A light polarisation converter may also be referred to as a light polarisation rotator or a birefringence rotator.

Light polarisation converters can be manufactured as a basic building block for a PIC by, for example, wet etching a waveguide to have angled side walls. However, forming one or more boundaries of a waveguide to obtain an angled surface using a wet etching technique may be disadvantageous. This is because etching to create a surface may result in small irregularities in that surface. The irregularities can have an impact on light propagation in the waveguide. Furthermore, the control over the manufacturing process with etching techniques for the waveguide may not be enough to obtain desired manufacturing tolerances for the light polarisation converter. For example, it may be desired that the wet etching continues to a particular depth into a substrate on which the waveguide is supported. However, this etch depth may not be controllable to a desired level.

For better control over light propagation in the waveguide and better manufacturing tolerances, a light polarisation converter is desired in which the waveguide is not wet etched to define an angled side wall, for example.

<FIG> illustrates schematically a side cross-section of a light polarisation converter <NUM> according to examples. The light polarisation converter <NUM> is for a PIC. The light polarisation converter <NUM> comprises a substrate <NUM>. The substrate <NUM> comprises a first surface <NUM> and a second surface <NUM>. In some examples, the substrate <NUM> comprises a so-called III-V semiconductor compound such as Indium Phosphide (InP), gallium arsenide (GaAs), gallium nitride (GaN) or gallium antimonide (GaSb). In other examples, the substrate comprises a Nitride based material or a Silicon based material.

The light polarisation converter <NUM> comprises a waveguide <NUM>. The waveguide <NUM> comprises a first waveguide portion <NUM> in contact with the first surface <NUM>, and a second waveguide portion <NUM> in contact with the second surface <NUM>.

The second surface <NUM> is offset from the first surface <NUM> along a first axis <NUM> and a second axis <NUM> each perpendicular to a light propagation direction for converting polarisation of light, such that the second waveguide portion <NUM> is offset from the first waveguide portion <NUM>. The first axis <NUM> is perpendicular to the second axis <NUM>.

In the examples of <FIG>, a side-cross section is shown such that the light propagation direction is into the page, as indicated by symbol <NUM>. The first axis <NUM> is the horizontal axis with respect to the orientation shown in <FIG>, and the second axis <NUM> is the vertical axis with respect to the orientation shown in <FIG>. For example, the first axis <NUM> extends in a direction from the first surface <NUM> towards the second surface <NUM>. As referred to herein, widths of parts of the light polarisation converter <NUM> are along the first axis <NUM>. As referred to herein, lengths of various parts described herein are along the light propagation direction as indicated by the symbol <NUM>. As referred to herein, the terms height, upper and lower are with respect to the second axis <NUM>. In the orientation shown in <FIG>, the first surface <NUM> is a lower surface of the substrate <NUM>, and the second surface <NUM> is an upper surface of the substrate <NUM>. The upper surface <NUM> provides a step upwards (in a direction parallel to the vertical axis) from the lower surface <NUM>. Therefore, in these examples, the first surface <NUM> and the second surface <NUM> are offset in position along the vertical axis <NUM>, e.g. so that the first surface and the second surface are displaced and/or vertically spaced from each other.

The waveguide <NUM> having the first waveguide portion <NUM> in contact with the first surface <NUM> and the second waveguide portion <NUM> in contact with the second surface <NUM> means that there is provided a single waveguide <NUM> in which light can propagate in the light propagation direction indicated by the symbol <NUM> with different portions offset from one another in a direction parallel to the vertical axis <NUM>.

A single waveguide with portions offset in a direction parallel to the vertical axis <NUM> provides for conversion of the polarisation of light as described in further detail below.

Because of the structure of the substrate <NUM> with a first and a second surface offset in a direction parallel to the vertical axis <NUM>, the waveguide <NUM> can be defined to have an arrangement for polarisation conversion without performing wet etching on the waveguide <NUM>, e.g. to define an angled side wall. For example, one or more materials for the waveguide <NUM> can be epitaxially grown on the substrate <NUM>, and because of the offset arrangement of the substrate <NUM>, the waveguide <NUM> with portions offset in a direction parallel to the vertical axis <NUM> is formed.

The waveguide <NUM> comprises a material which has a higher refractive index than the material of the substrate <NUM>. For example, the waveguide <NUM> comprises Indium Gallium Arsenide Phosphide (InGaAsP). More generally, in some examples, the waveguide <NUM> comprises (Al)InGaAs(P). The elements indicated in the parentheses can be interchangeable and the composition of the different elements is selected depending on the desired function. For example, the composition of Ga and As in InGaAs can be selected according to the desired bandgap. In some examples, the waveguide <NUM> is a layer of (Al)InGaAs(P). In other examples, the waveguide <NUM> comprises a plurality of sub-layers. In some such examples, the waveguide <NUM> comprises a (Al)InGaAs(P)/(Al)InGaAs(P) multiple quantum well structure in contact with the substrate <NUM>. In some examples, the sub-layers are between <NUM> and <NUM> nanometres thick. The sub-layer stack of the waveguide <NUM> has a band gap selected in accordance with the desired application of the light polarisation converter <NUM>.

The bandgap and therefore, as will be appreciated by those skilled in the art, the refractive index of the InGaAsP, for example, can be tuned. In some examples, the bandgap of the InGaAsP of the waveguide <NUM> is tuned to a wavelength of <NUM> nanometres (e.g. for propagation of light of wavelength <NUM> nanometres) or <NUM> nanometres (e.g. for propagation of light of wavelength <NUM> nanometres). In other examples, the wavelength to which the bandgap is tuned is different.

The waveguide <NUM> is for guiding light. In use, light propagates within the waveguide <NUM> and is confined within the waveguide <NUM>, due to reflection at the boundaries of the waveguide <NUM>. The waveguide <NUM> has a refractive index higher than the refractive index of material in contact with the waveguide <NUM> at the boundaries at which confinement of light is desired. For example, due to this refractive index difference at the boundaries at which confinement of light is desired, total internal reflection takes place when the angle of incidence at these boundaries of the waveguide <NUM> is greater than the critical angle. In this manner, the waveguide <NUM> guides the propagation of the light. For a particular optical mode to propagate in the waveguide <NUM>, it is desired that the light reflected at the boundaries of the waveguide <NUM> fulfils the conditions for constructive interference, as will be appreciated by the skilled person.

For example, particular optical modes of light are desired to propagate through the waveguide <NUM> depending on the desired application of the light polarisation converter <NUM>. The direction in which the optical modes propagate within the waveguide <NUM> is herein referred to as the light propagation direction. The light propagation direction is the general direction in which the energy of the optical mode travels through the waveguide <NUM> and is not necessarily, for example, the direction defined by the angle of incidence at a boundary of the waveguide <NUM>.

In the examples of <FIG>, the substrate <NUM> comprises a third surface <NUM> between the first surface <NUM> and the second surface <NUM>. The third surface <NUM> is sloped relative to the first surface <NUM> and/or to the second surface <NUM>. As referred to herein, one surface being sloped relative to another surface means that there is a non-zero angle between the two surfaces in question. As such, reference can be made to one such surface being at a particular angle relative to the other in the context of one surface being sloped relative to another. In such contexts, reference can also be made to a surface being tilted relative to the other. The terms "tilt", "angle" and "slope" are used interchangeably herein to refer to the angle of an entity relative to another entity or relative to a given axis. As referred to herein, that the third surface <NUM> is "between" the first surface <NUM> and the second surface <NUM> means, for example, that the third surface <NUM> is interposed between the first surface <NUM> and the second surface <NUM> and is for example immediately adjacent to each of the first surface <NUM> and the second surface <NUM>. Thus, the first, second and third surfaces together can in examples be considered together to constitute the top surface of the waveguide.

In the examples of <FIG>, the first axis <NUM> (also referred to as the horizontal axis <NUM>) is substantially parallel (e.g. parallel within acceptable tolerances) to the plane of the first surface <NUM>. In these examples, the third surface <NUM> is at an angle less than <NUM> degrees relative to the first surface <NUM>. In these examples, the second surface <NUM> is offset from the first surface <NUM> along the first axis <NUM> by an amount greater than a width <NUM> of the first surface <NUM> along the first axis <NUM>. As referred to herein, the offset in a direction parallel to an axis is relative to a position along that axis where the edge of the entity in question lies (the edge which lies at the earliest position along that axis). For example, in <FIG>, the edge of the first surface <NUM> lies at a position <NUM> along the first axis <NUM>. It will be appreciated that a zero offset from the first surface <NUM> along the first axis <NUM> would mean that edges of each the first and the second surfaces <NUM>, <NUM> lie at the same position, the position <NUM>, along the first axis <NUM>. In the examples of <FIG>, the offset along the first axis <NUM> is such that the second surface <NUM> does not overlap the first surface <NUM> and the third surface <NUM> slopes upwards (with respect to the orientation of <FIG>). In these examples, the position <NUM> along the first axis <NUM> at which the edge of the second surface <NUM> lies is a distance greater than the width <NUM> of the first surface <NUM> away from the position <NUM>.

In the examples of <FIG>, the waveguide <NUM> comprises an intermediate waveguide portion <NUM> in contact with the third surface <NUM> and between the first waveguide portion <NUM> and the second waveguide portion <NUM>. In some examples, the substrate <NUM> with surfaces offset in a direction parallel to the second axis <NUM> and a sloped third surface <NUM>, as described, is at least partly formed before the waveguide <NUM> on top of the substrate <NUM>. In this manner, due to the sloped third surface <NUM>, there is formed an intermediate portion of the waveguide at an angle corresponding to the angle of the sloped third surface <NUM> of the substrate. Thus, the first waveguide portion, the second waveguide portion and the intermediate waveguide portion can in examples be considered together to constitute the waveguide.

As described, in the examples of <FIG>, the first waveguide portion <NUM> is in contact with the first surface <NUM> of the substrate <NUM>. On the other hand, the second waveguide portion <NUM> is in contact with the second surface <NUM> which is offset (in a direction parallel to the second axis <NUM> from the first surface <NUM>. This means that the waveguide <NUM> comprises two portions which are offset from one another in a direction parallel to the second axis <NUM>. As described, in these examples, the waveguide <NUM> also comprises the intermediate waveguide portion <NUM> which is at an angle relative to the second waveguide portion <NUM> and the first waveguide portion <NUM> as shown in <FIG>.

This arrangement of the waveguide <NUM> provides for conversion of the polarisation of light. The following description is in the context of linearly polarised light incident on the light polarisation converter <NUM> as indicated by the symbol <NUM>.

<FIG> illustrates a transverse electric (TE) polarisation axis <NUM> and a transverse magnetic (TM) polarisation axis <NUM>. The symbol <NUM> is included in <FIG> to indicate the light propagation direction, which is into the page (perpendicular to both the TE polarisation axis <NUM> and the TM polarisation axis <NUM>). With respect to the cross section of <FIG>, the TE polarisation axis <NUM> is parallel to the first axis <NUM>, and the TM polarisation axis <NUM> is parallel to the second axis <NUM>.

For linearly polarised light, the direction of the electric field of light propagating as indicated by the symbol <NUM> can be indicated with respect to the TE polarisation axis <NUM> and the TM polarisation axis <NUM>. The arrow <NUM> indicates linearly polarised light that is TE polarised.

The waveguide <NUM> having portions offset in a direction parallel to the second axis <NUM> (and therefore in a direction parallel to the TM axis <NUM>) and the intermediate waveguide portion <NUM> being at an angle relative to the second waveguide portion <NUM> and the first waveguide portion <NUM> provides a single waveguide with portions offset in a direction parallel to the second axis <NUM>. This causes there to be hybrid modes within the waveguide <NUM>. This arrangement of the waveguide <NUM> provides a "tilted" or sloped boundary condition for the light propagating within the waveguide <NUM>, providing for the hybrid modes. This is because, the light is propagating within a single waveguide which has portions offset in a direction parallel to the vertical axis <NUM>.

The tilted boundary condition causes there to be a first hybrid mode which has an electric field tilted with respect to the TE axis because of the geometry of the waveguide <NUM> with portions offset in a direction parallel to the second axis <NUM>. The first hybrid mode occupies the first waveguide portion <NUM>, second waveguide portion <NUM> and the intermediate waveguide portion <NUM>. The terms "tilt", "angle" and "slope" are used interchangeably herein to refer to the angle of the first hybrid mode relative to the TE axis <NUM>. There is also a second hybrid mode orthogonal to the first hybrid mode. A hybrid mode, as referred to herein, is a mode of light which has an electric field with a non-zero component along the TE polarisation axis <NUM> and a non-zero component along the TM polarisation axis <NUM>.

In the examples of <FIG>, there is shown a first hybrid mode <NUM> and a second hybrid mode <NUM>. The first and second hybrid modes <NUM>, <NUM> illustrate an example of hybrid modes that may exist within the waveguide <NUM> when light is propagating within the waveguide <NUM>. In this example, the first and second hybrid modes <NUM>, <NUM> arise from light with TE polarisation (with the electric field along the TE polarisation axis <NUM>), as shown by arrow <NUM>, incident on the light polarisation converter <NUM> for propagation through the waveguide <NUM>.

In the examples of <FIG>, the tilt angle (relative to the TE axis <NUM>) for the first hybrid mode <NUM> is assumed to be <NUM> degrees. Such a tilt angle for the first hybrid mode <NUM> arises as a result of the particular arrangement of the first and second waveguide portions <NUM>, <NUM> relative to the second axis <NUM> and the angle of the sloped third surface <NUM>, for example. In these examples, the angle with respect to the TE axis <NUM> of the second hybrid mode <NUM> is also <NUM> degrees and the first and second modes <NUM>, <NUM> have electric fields with equal magnitude. Those skilled in the art will appreciate that the first and second modes <NUM>, <NUM>, with the tilt angle of <NUM> degrees and the phase relationship shown in <FIG>, have equal and opposite components along the TM polarisation axis <NUM> and in combination correspond to TE polarised light.

Furthermore, the described arrangement of the waveguide <NUM> causes a different propagation constant for the first and second hybrid modes <NUM>, <NUM>. The arrangement of the waveguide <NUM> results in birefringence such that the first and second hybrid modes <NUM>, <NUM> experience different effective refractive indices to one another when propagating within the waveguide <NUM>. This means that the phase difference between the first and second hybrid modes <NUM>, <NUM> changes as the first and second hybrid modes <NUM>, <NUM> propagate through the waveguide <NUM>. In other words, the phases of the first and second modes <NUM>, <NUM> evolve differently as the first and second modes <NUM>, <NUM> propagate within the waveguide <NUM>. The presence of the hybrid modes and their different propagation constants provide that the arrangement of the waveguide <NUM> can be used to convert the polarisation of light input into the light polarisation converter <NUM>. The way in which polarisation is converted is described in further detail below.

In some examples, the light polarisation converter <NUM> is for converting between a first linear polarisation of a given wavelength of light and a second linear polarisation of the given wavelength of light. In some such examples, a length of the waveguide <NUM> in the light propagation direction is substantially equal to an odd integer multiplied by half of a beat length for the given wavelength of light.

<FIG> illustrates schematically a side cross-section of the light polarisation converter <NUM> according to examples. The examples of <FIG> correspond to the examples of <FIG>. The cross-section of <FIG> is taken at line A-A indicated in <FIG>. It should be noted that <FIG> is a schematic illustration and should not be taken to indicate precise proportions with respect to <FIG>, which is also schematic. In these examples, the waveguide <NUM> has a length <NUM> as shown in <FIG>. The light propagation direction is indicated by arrow <NUM>.

A length of the waveguide <NUM> for the phase of the modes of light propagating therein to be restored is referred to as the beat length. For example, if the first and second modes <NUM>, <NUM> start their propagation within the waveguide <NUM> in phase, the modes will be back in phase after propagating an integer multiple of the beat length within the waveguide <NUM>. By selecting the waveguide length in the light polarisation converter <NUM> based on the beat length, the relative phase of the modes of light propagating therein can be controlled for light output from the light polarisation converter <NUM>.

As discussed above, the described arrangement of the waveguide <NUM> causes a different propagation constant for the first and second hybrid modes <NUM>, <NUM>. The described arrangement of the waveguide <NUM> (with portions offset in a direction parallel to the vertical axis <NUM>), causes there to be birefringence in that the first and second hybrid modes <NUM>, <NUM> experience a different effective refractive index to one another. The propagation constant of the first hybrid mode <NUM> in the waveguide <NUM> can be represented by β<NUM> and the propagation constant of the second hybrid mode <NUM> can be represented by β<NUM>. The difference in these propagation constants can be represented as Δβ = β<NUM> - β<NUM>. Equation <NUM> below shows the beat length Lλ for the waveguide <NUM> for the first and second hybrid modes <NUM>, <NUM>. Those skilled in the art will appreciate that β represents phase propagation. In Equation (<NUM>) below, λ is the given wavelength and Δn represents the difference in the effective refractive indices of the first and second hybrid modes <NUM>, <NUM>: Δn = n<NUM> - n<NUM>.

In some examples, the light polarisation converter <NUM> is for rotating the linear polarisation of the given wavelength of light. As described above, in some such examples, the waveguide length <NUM> is substantially (within acceptable tolerances) equal to an odd integer multiplied by half of the beat length. This is the case in examples in which the tilt angle of the first mode <NUM> is <NUM> degrees relative to the TE axis <NUM>. Factors determining the tilt are described further below. In other words, the waveguide length can be <NUM>/<NUM> of the beat length, <NUM>/<NUM> of the beat length, <NUM>/<NUM> of the length and so on, as indicated by Equation <NUM> below. In Equation <NUM>, m represents an odd integer such as <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, etc. <MAT>.

This means that the relative phase of the first and second modes <NUM>, <NUM> after propagating through the waveguide <NUM> is shifted by π radians. Those skilled in the art will appreciate that when TE polarised light as indicated by the arrow <NUM> is incident on the waveguide <NUM>, and the first and second modes <NUM>, <NUM> arise at the beginning of the waveguide <NUM>, the first and second modes <NUM>, <NUM> will be in phase with one another. In other words, after propagating through the waveguide <NUM> with a waveguide length of <NUM>/<NUM> the beat length (or <NUM>/<NUM> the beat length, or <NUM>/<NUM> of the beat length, etc.) the first and second modes <NUM>, <NUM> are out of phase with each other by <NUM> degrees.

<FIG> illustrates the TE and TM polarisation axes <NUM>, <NUM> shown in <FIG> relates to the examples where linear TE polarised light is incident on the waveguide <NUM> as indicated by <FIG>, and the tilt angle of the first hybrid mode <NUM> relative to the TE axis <NUM> is <NUM> degrees. <FIG> illustrates the first and second modes <NUM>, <NUM> after having propagated through the waveguide <NUM> in the direction indicated by the symbol <NUM> for an integer multiple of half the beat length. Those skilled in the art will appreciate that the electric field for light oscillates between opposing quadrants of the space indicated by the TE and TM polarisation axes <NUM>, <NUM>. Accordingly, in comparison to the state shown in <FIG>, in <FIG>, the first mode <NUM> has completed an integer multiple of its electric field oscillations, whereas the second mode <NUM> has completed an odd integer multiple of half of its electric field oscillation (to end up in the top left quadrant).

Those skilled in the art will appreciate that the first and second modes <NUM>, <NUM> with the phase relationship shown in <FIG> have equal and opposite components along the TE polarisation axis <NUM>. The first and second modes <NUM>, <NUM> with the phase relationship shown in <FIG> would provide TM polarised light as indicated by the arrow <NUM>, if the first and second modes <NUM>, <NUM> exit the waveguide <NUM> with the phase relationship shown in <FIG>. In other words, if the phases of the first and second modes <NUM>, <NUM> no longer evolve differently from the point shown in <FIG>, the result is TM polarised light. It should be noted in the examples of <FIG>, the first and second modes <NUM>, <NUM> have a <NUM> degree angle relative to the TM axis <NUM>.

In these examples, where the first hybrid mode <NUM> has a <NUM> degree angle relative to the TE axis <NUM>, by selecting the waveguide length to be an odd integer multiplied by half of the beat length, linear polarisation of the given wavelength of light can be rotated as described above. For example, a first linear polarisation (TE polarisation in the above examples) can be converted to a second linear polarisation (TM polarisation in the above examples).

The above examples are in the context of the first hybrid mode <NUM> having a <NUM> degree tilt relative to the TE axis <NUM>. In some examples, the arrangement of the substrate <NUM> and the waveguide <NUM> is such that when hybrid modes arise within the waveguide <NUM>, the first hybrid mode <NUM> (e.g. the hybrid mode substantially aligned with the intermediate waveguide portion <NUM>) does not have a <NUM> degree angle relative to the TE axis <NUM>. Factors influencing the tilt of the first hybrid mode are discussed further below. In such examples, a <NUM> degree rotation of polarisation (e.g. from TE polarisation to TM polarisation) does not take place upon propagating through a length <NUM> equal to an odd integer multiplied by half of the beat length.

In some examples, a light polarisation converter (e.g. the light polarisation converter <NUM>) is provided in which the length <NUM> of the waveguide <NUM> is equal to an odd integer multiplied by a quarter of the beat length. A pair of such light polarisation converters can be used where the tilt is not <NUM> degrees, in order to obtain a <NUM> degree linear polarisation rotation, as described in the following. A first light polarisation converter of the pair has a length <NUM> of the waveguide <NUM> which achieves <NUM> degree (π/<NUM> radians) phase difference between the first and second hybrid modes. This means that the length <NUM> can be <NUM>/<NUM> of the beat length, <NUM>/<NUM> of the beat length, <NUM>/<NUM> of the beat length and so on. A second light polarisation converter of the pair has a length <NUM> of the waveguide <NUM> which achieves a <NUM> degree (<NUM>/<NUM>π radians) phase difference between the first and the second hybrid modes. This means that the length <NUM> of the waveguide in the second light polarisation converter can be <NUM>/<NUM> of the beat length, <NUM>/<NUM> of the beat length, <NUM>/<NUM> of the beat length and so on.

<FIG> is a sketch of the equator plane of the Poincare sphere. Those skilled in the art will appreciate that all polarisation states can be mapped onto the surface of the so-called Poincare sphere. Points lying on the equator of the Poincare sphere represent all angles of linear polarisation. The poles of the Poincare sphere represent clockwise and anticlockwise circular polarisations. The points corresponding to TE polarisation and TM polarisation can be seen in <FIG>.

Points corresponding to the first and second hybrid modes <NUM>, <NUM> lie on the equator of the Poincare sphere. The position of the hybrid modes on the equator depends on the tilt, in other words the angle, of the first hybrid mode relative to the TE axis <NUM>.

In the case of the first hybrid mode having a <NUM> degree angle relative to the TE axis <NUM>, the first hybrid mode corresponds to point M1 and the second hybrid mode corresponds to the point M2. An axis crossing M1 and M2 is perpendicular to an axis crossing the TE and TM polarisation points on the equator of the Poincare sphere. Propagation of the hybrid modes through the waveguide <NUM>, where their phases evolve differently from one another, corresponds to rotation of a point that represents the polarisation when the hybrid modes recombine, about the axis crossing M1 and M2. A <NUM> degree rotation about an axis crossing M1 and M2 leads to e.g. polarisation conversion from TE polarisation to TM polarisation.

In the case of the first hybrid mode having an acute angle different to <NUM> degrees relative to the TE axis <NUM>, points corresponding to the first and second hybrid modes are not points M1 and M2. In some examples, the first hybrid mode <NUM> corresponds to point <NUM> and the second hybrid mode <NUM> corresponds to point <NUM>. In these examples, a <NUM> degree rotation about an axis crossing points <NUM> and <NUM> does not arrive at the TM polarisation point. As described above, when the tilt angle is different to <NUM> degrees, a <NUM> degree phase difference between the first and second hybrid modes does not provide a <NUM> degree rotation of linear polarisation.

However, a <NUM> degree rotation about the axis crossing points <NUM> and <NUM> arrives at point <NUM>. Point <NUM> is a point on the surface of the Poincare sphere above the page of <FIG>. In some examples, the first light polarisation converter is used to obtain a polarisation corresponding to point <NUM> on the Poincare sphere. In these examples, the first light polarisation converter provides hybrid modes that have polarisations corresponding to points <NUM> and <NUM>. In other words, the first polarisation converter provides a tilt such that the hybrid modes have polarisations corresponding to points <NUM> and <NUM>.

The second polarisation converter of the discussed examples provides a tilt in the opposite direction such that the hybrid modes in the second light polarisation converter correspond to points <NUM> and <NUM>. This can be achieved by the second polarisation converter having a side cross-section (corresponding to the side cross-section shown in <FIG>) which is the mirror image of the side cross-section (corresponding to the side cross-section shown in <FIG>) of the first light polarisation converter. For example, in <FIG>, the first surface <NUM> (lower surface) is on the left hand side of <FIG> and the second surface <NUM> is on the right hand side. As used with respect to the side cross-section, mirror image means that, the first surface <NUM> (lower surface) is instead on the left hand side and the second surface <NUM> (upper surface) is on the right hand side with the light propagation direction remaining the same (into the page as indicated by symbol <NUM>). In examples where <FIG> represents a side cross-section of the first light polarisation converter, <FIG> illustrates schematically the same side cross-section as that of <FIG> for examples <NUM> of the second light polarisation converter. Features corresponding to those shown in <FIG> are labelled with similar reference numerals with the additional numeral "-<NUM>" added at the end in <FIG>.

An axis crossing points <NUM> and <NUM> is a mirror image of the axis crossing points <NUM> and <NUM>, relative to the axis crossing points M1 and M2. Line <NUM> represent a trajectory from the TE point to point <NUM> after a <NUM> degree rotation about the axis crossing points <NUM> and <NUM>. It should be noted that line <NUM> is a straight line projection of the trajectory on the Poincare sphere which would follow the surface of the Poincare sphere.

A rotation about the axis crossing points <NUM> and <NUM> including the point <NUM> traces a circle on the surface of the Poincare sphere which crosses the TM point. A rotation in the opposite direction from point <NUM> and about the axis crossing points <NUM> and <NUM> can be used to arrive at the TM point. The second polarisation converter having a mirrored side-cross section provides rotation relative to the Poincare sphere in the opposite direction to the first light polarisation converter. Line <NUM> represents a straight line projection of a trajectory starting from point <NUM>, corresponding to rotation about the axis crossing points <NUM> and <NUM> in the opposite direction to the described <NUM> degree rotation to arrive at point <NUM>. The trajectory <NUM> crosses the TM point after a <NUM> degree rotation about the axis crossing points <NUM> and <NUM>.

In this manner, even in examples where the tilt of the first hybrid mode is not <NUM> degrees, a <NUM> degree rotation of the linear polarisation can be achieved by propagation light through the first light polarisation converter and then the second light polarisation converter.

In some examples, the light polarisation converter <NUM> is for converting between linear and circular polarisation of the given wavelength of light. In such examples, the waveguide length <NUM> is substantially (within acceptable tolerances) equal to an odd integer multiplied by a quarter of the beat length. In such examples, the tilt of the first hybrid mode is <NUM> degrees relative to the TE axis <NUM>. The waveguide length <NUM> according to such examples is represented by Equation <NUM> below.

The first and second hybrid modes <NUM>, <NUM> propagating in the waveguide <NUM> for an odd integer multiple of a quarter of the beat length would result in a phase different of <NUM>/<NUM>π radians (or <NUM> degrees). Those skilled in the art will appreciate that introducing a phase difference of <NUM> degrees about the axis crossing points M1 and M2 provides for conversion between linear and circular polarisations. Taking the example of linearly polarised light in the horizontal direction as indicated in <FIG> incident on the waveguide <NUM>, the light polarisation converter <NUM> with a waveguide length <NUM> according to Equation <NUM> would convert the TE polarised light into circularly polarised light. In examples where circularly polarised light is incident on the light polarisation converter for converting between linear and circular polarisations, the circularly polarised light would be converted into linearly polarised light.

In the example of <FIG> and according to the claimed invention, the substrate <NUM> comprises a first substrate layer <NUM> and a second substrate layer <NUM>. The first substrate layer <NUM> comprises the first surface <NUM> and the second substrate layer <NUM> comprises the second surface <NUM>. In some examples, both the first substrate layer <NUM> and the second substrate layer <NUM> comprise the same material. In the examples of <FIG>, the substrate <NUM> comprises a third layer <NUM> between the first substrate layer <NUM> and the second substrate layer <NUM>. The first, second and third layers of the substrate <NUM> are stacked such that the first layer <NUM> is in contact with the third layer <NUM> and the third layer <NUM> is in contact with the second layer <NUM>. In other words, the second layer <NUM> is on the third layer <NUM> and the third layer <NUM> is on the first layer <NUM> with respect to the orientation shown in <FIG>. The third layer <NUM> comprises a material different to that of the first substrate layer <NUM> and the second substrate layer <NUM>.

In these examples, the offset of the second surface <NUM> from the first surface <NUM> in a direction parallel to the second axis <NUM> is defined by the total thickness of the third layer <NUM> and the second substrate layer <NUM>. The total thickness of the third layer <NUM> and the second substrate layer <NUM> may be referred to as a step thickness. The total thickness of whichever layers that defines the offset of the second surface <NUM> from the first surface <NUM> in a direction parallel to the second axis <NUM> can be referred to as the step thickness.

As described above, the first hybrid mode <NUM> occupies the first waveguide portion <NUM>, second waveguide portion <NUM> and the intermediate waveguide portion <NUM>. In some examples, the first hybrid mode <NUM> is substantially aligned with the intermediate waveguide portion <NUM> so as to have a similar tilt relative to the TE axis as the angle of the intermediate waveguide portion <NUM>. It should be noted that the tilt of the first hybrid mode <NUM> will depend on the width of the waveguide <NUM> and the step thickness (assuming a fixed given angle for the third surface <NUM>). In examples, the difference in effective refractive index Δn of the first and second hybrid modes <NUM>, <NUM> will also depend on the width of the waveguide <NUM> and the step thickness (assuming a fixed given angle for the third surface <NUM>). Accordingly, the width of the waveguide <NUM> and the step thickness can be selected to obtain the desired tilt angle and effective refractive index difference. As apparent from Equation (<NUM>), the effective refractive index difference relates to the beat length Lλ and will therefore impact the length <NUM> of the waveguide <NUM> depending on the desired application.

In the examples of <FIG> and <FIG>, the light polarisation converter comprises one or more cladding layers <NUM> that cover the waveguide <NUM>. In some examples, the one or more cladding layers <NUM> comprise an uneven surface on a first side opposite to a second side closer to the waveguide <NUM> than the first side. For example, there is a cladding layer <NUM> in contact with the top surface of each portion of the waveguide <NUM>. The one or more cladding layers comprise an uneven surface <NUM> on the first side opposite to the second side closer to, e.g. facing, the waveguide <NUM>. The light polarisation converter <NUM> may comprise other layers such as an electrical contact layer <NUM> on top (with respect to the orientation of <FIG>) of the cladding layer <NUM>. As can be seen from the examples of <FIG>, as a consequence of the offsets in a direction parallel to the vertical axis <NUM> of the substrate <NUM> and the waveguide <NUM>, the top surface of the light polarisation converter <NUM> is not completely flat.

<FIG> is a graph <NUM> indicating tilt angle of the first hybrid mode as a function of waveguide width W (see <FIG>). It should be noted that the waveguide width is along the first axis <NUM>. The vertical axis indicates tilt angle of the first hybrid mode <NUM> with respect to the TE axis <NUM> and the horizontal axis indicates waveguide width W. The graph <NUM> shows a plurality of relationships for a respective plurality of step thicknesses. The step thicknesses are indicated in <FIG> in units of nanometres. Line <NUM> indicates a tilt angle of the first hybrid mode being <NUM> degrees. The relationships shown in the graph <NUM> are for an example of a light polarisation converter similar to the light polarisation converter <NUM> of the examples of <FIG>. The third layer <NUM> and the electrical contact layer <NUM> were not included. The thickness of the waveguide <NUM> (in the vertical direction with respect to the orientation shown in <FIG>) was <NUM> nanometres, the refractive index of the substrate <NUM> was <NUM> and the refractive index of the cladding layer <NUM> was <NUM>. The refractive index of the waveguide <NUM> was <NUM>, and the refractive index either side of the light polarisation converter (in the orientation shown in <FIG>) was <NUM>. The angle of the intermediate waveguide portion was <NUM> degrees.

<FIG> is a graph <NUM> indicating Δn as a function of waveguide width. The vertical axis indicates Δn, and the horizontal axis indicated waveguide width. The graph <NUM> shows a plurality of relationships for a respective plurality of step thicknesses as indicated. The relationships shown in the graph <NUM> are for the same parameters described above with respect to <FIG>.

The skilled person will appreciate that the desired characteristics of the light polarisation converter can be obtained by selecting e.g. waveguide width, step thickness, etc. based on e.g. data such as that shown in <FIG> and <FIG>.

<FIG> illustrates schematically a side cross-section of a light polarisation converter <NUM> according to examples. In <FIG>, features corresponding to those shown in <FIG> are labelled with similar reference numerals with the additional numeral "-<NUM>" added at the end.

The light polarisation converter <NUM> corresponds to the light polarisation converter <NUM> (and may comprise any combination of the features described above in relation to the light polarisation converter <NUM>), except for the following differences. The substrate <NUM>-<NUM> of the light polarisation converter <NUM> has a third substrate surface <NUM>-<NUM> between the first surface <NUM>-<NUM> and the second surface <NUM>-<NUM>, wherein the third substrate surface <NUM>-<NUM> is sloped relative to the first surface <NUM>-<NUM>. The third surface <NUM>-<NUM> is substantially at a <NUM> degree angle (within acceptable tolerances) relative to the first surface <NUM>-<NUM>. In these examples, the third substrate surface <NUM>-<NUM> corresponds to a side wall of the second substrate layer <NUM>-<NUM>. In the examples of <FIG>, there is no third layer corresponding to the third layer <NUM> described above.

As a consequence of the angle of the third surface <NUM>-<NUM> in these examples, the light polarisation converter <NUM> of <FIG> does not comprise an intermediate waveguide portion. The first waveguide portion <NUM>-<NUM> is continuous with the second waveguide portion <NUM>-<NUM> instead of having an intermediate, sloped waveguide portion therebetween. As in the case of the examples of <FIG>, the waveguide <NUM>-<NUM> of the light polarisation converter <NUM> is a single waveguide for propagation of light.

The waveguide <NUM>-<NUM> of the examples of <FIG> comprises the second waveguide portion <NUM>-<NUM> offset relative to the first waveguide portion <NUM>-<NUM> in a direction parallel to the second axis <NUM>-<NUM>, and continuous with the first waveguide portion <NUM>-<NUM> as shown in <FIG>.

The arrangement of the waveguide <NUM>-<NUM> causes there to be hybrid modes within the waveguide <NUM>-<NUM>. Similarly, to the waveguide <NUM> of the examples of <FIG>, this arrangement of the waveguide <NUM>-<NUM> provides a "tilted" or sloped boundary condition for the light propagating within the waveguide <NUM>-<NUM>, providing for the hybrid modes. This is because, light propagating within the waveguide <NUM>-<NUM> occupies waveguide regions continuous with one another that are offset in a direction parallel to the second axis <NUM>. In addition, the described arrangement of the waveguide <NUM>-<NUM> causes a different propagation constant for different hybrid modes. The arrangement of the waveguide <NUM>-<NUM> provides birefringence such that the first and second hybrid modes experience different refractive indices to one another within the waveguide <NUM>-<NUM>. For example, for light with TE polarisation as shown by arrow <NUM> of <FIG> incident on the light polarisation converter <NUM> for propagation through the waveguide <NUM>-<NUM>, the first and second hybrid modes <NUM>, <NUM> arise with different propagation constants (and consequently different phase evolution).

Both the waveguide <NUM> of the examples of <FIG> and the waveguide <NUM>-<NUM> of the examples of <FIG> give rise to hybrid modes and a different propagation constant for the hybrid modes. As described above, the presence of the hybrid modes and their different propagation constants provide for conversion of the polarisation of light. Therefore, similarly to the light polarisation converter <NUM> of the examples of <FIG>, the light polarisation converter <NUM> of the examples of <FIG> can be used to convert the polarisation of incident light.

Either the examples of <FIG> or the examples of <FIG> may be used depending on various factors such as complexity and cost of manufacturing and the desired level of control over the hybrid modes and their propagation constants. For example, the presence of the intermediate waveguide portion <NUM> being at angle (sloped) provides finer tuning of the characteristics (such as tilt) of the hybrid modes. However, the examples of <FIG> may be simpler and/or cheaper to manufacture as there is no sloped surface to manufacture.

<FIG> is a block diagram illustrating a method <NUM> of manufacturing a light polarisation converter for a PIC. The method <NUM> is for example a method of manufacturing a light polarisation converter according to any of the examples described above. At block <NUM> of the method <NUM>, a substrate comprising a first surface and a second is at least partly formed. The second surface is offset from the first surface along a first axis and a second axis each perpendicular to a light propagation direction for converting polarisation of the light. The substrate is for example the substrate <NUM>, the substrate <NUM>-<NUM> or the substrate <NUM>-<NUM>.

At block <NUM> of the method <NUM>, a waveguide comprising a first waveguide portion in contact with the first surface, and a second waveguide portion in contact with the second surface is at least partly formed. The second surface is offset from the first surface such that the second waveguide portion is offset from the first waveguide portion. The waveguide is, for example, the waveguide <NUM>, the waveguide <NUM>-<NUM> or the waveguide <NUM>-<NUM> described above. More specific aspects of the method <NUM> are further described below.

According to the claimed invention, at least partly forming the substrate comprises at least partly forming a first substrate layer, and at least partly forming a second substrate layer, wherein the first substrate layer comprises the first surface and the second substrate layer comprises the second surface. <FIG> is a block diagram illustrating a method <NUM>. The method <NUM> relates to the examples of <FIG>, <FIG> and <FIG> and can be used to at least partly form the second substrate layer <NUM> or the second substrate layer <NUM>-<NUM> shown in <FIG> and <FIG>, respectively. The following description of the method <NUM> is with reference to the examples of <FIG> and <FIG>, however, corresponding blocks can be performed for the examples of <FIG>.

At block <NUM> of the method <NUM>, a third layer (e.g. the third layer <NUM>) is deposited on a top surface of the first substrate layer <NUM> to function as a stop layer. In the context of the method <NUM>, at least partly forming the second substrate layer comprises the following. At block <NUM>, a substrate material is deposited on top of the stop layer <NUM>. The substrate material is any material described above with respect to the substrate <NUM>. The substrate material is the same material comprised in the first substrate layer <NUM>, for example.

<FIG> illustrates schematically a side cross-section of a structure as arrived at from performing blocks <NUM> and <NUM>. <FIG> is schematic and should not be regarded as represent exact proportions with respect to the examples described above.

At block <NUM> of the method <NUM>, a mask layer is deposited to cover a first portion of the substrate material on top of the stop layer <NUM>. <FIG> illustrates schematically a side cross-section of a structure as arrived at from performing block <NUM>. In the examples of <FIG>, there is shown the mask layer <NUM> deposited to cover a first portion <NUM> of the substrate material. In some examples, the mask layer <NUM> comprises a dielectric material. In some examples, the mask layer <NUM> is a hard mask comprising SiNx or SiOx, where the "x" indicates that different compositions may be used e.g. Si<NUM>N<NUM>. In some examples, the mask layer <NUM> is a photoresist.

At block <NUM> of the method <NUM>, a wet etch procedure is performed to remove material from a second portion <NUM> of the substrate material different to the first portion <NUM> and not covered by the mask layer. In examples, the stop layer <NUM> comprises material that is not etchable by the etchant used for etching to remove material not covered by the mask layer. In some examples, the stop layer <NUM> comprises a quaternary material. <FIG> illustrates schematically a side cross-section of a structure as arrived at from performing block <NUM>. Block <NUM> is performed to remove material from the second portion <NUM> to provide an exposed portion <NUM> of the stop layer <NUM>.

At block <NUM> of the method <NUM>, the exposed portion <NUM> of the stop layer <NUM> is removed. <FIG> illustrates schematically a side cross-section of a structure as arrived at from performing block <NUM>. Removal of the exposed portion <NUM> of the stop layer <NUM> results in the second surface <NUM> of the substrate being at least partly formed. In the specific examples of <FIG>, all exposed parts of the stop layer <NUM> are removed.

In some examples, the substrate material is InP. In some such examples, the wet etch procedure is performed using HCl:H3PO4:H2O. In some examples, a mixture of HCL, H3PO4 and H2O is selected which etches the desired material (in these examples, InP). In other examples, a mixture of HCL and H2O only is used as etchant. Performing a wet etch procedure, as described, provides an intermediate substrate surface (such as the third substrate surface <NUM>) which is at an angle less than <NUM> degrees relative to the first surface <NUM>. Those skilled in the art will appreciate that the angle of the third substrate surface <NUM> would depend on the combination of the crystal structure of the substrate material, the etching chemicals and the particulars of the procedure used.

In some examples, examples of block <NUM> for at least partly forming the waveguide <NUM> are employed on the structure resulting from the method <NUM>. In some examples, the mask layer <NUM> is removed as shown in the schematic illustration of <FIG>. Removal of the mask layer <NUM> provides the second surface <NUM> comprised in the second substrate layer <NUM>.

In such examples, one or more materials for the waveguide <NUM> is deposited on top of the substrate <NUM>. For example, the waveguide <NUM> is epitaxially grown using the materials described above in relation to the waveguide. Due to the arrangement of the substrate <NUM> (e.g. as shown in <FIG>), the waveguide <NUM> has portions offset in a direction parallel to the second axis <NUM> and has an intermediate waveguide portion at an angle as a consequence of the third substrate surface <NUM> being at an angle as described above.

In some examples, other layers are deposited on top of the waveguide <NUM>, such as one or more cladding layers <NUM> and an electrical contact layer <NUM>. These layers can be seen in the schematic illustration of <FIG>.

In examples, the structure of <FIG> undergoes e.g. a dry etching procedure to remove material from either side of the structure, up to a particular depth as desired according to the intended application. For example, a dry etching procedure is performed to remove material and form the light polarisation converter <NUM> of the examples of <FIG> and <FIG>. <FIG> show the same cross-section as shown in <FIG> for the resultant light polarisation converter <NUM>.

<FIG> is a block diagram illustrating a method <NUM> for at least partly forming the second substrate layer. The method <NUM> relates to the examples of <FIG> and can be used to at least partly form the second substrate layer <NUM>-<NUM> shown in <FIG>. The following description of the method <NUM> is with reference to the examples of <FIG>. As used herein, a layer refers to material occupying a volume within a given range of values relating to the vertical axis <NUM>-<NUM>.

<FIG> illustrates schematically a side-cross section of a structure in relation to the method <NUM>. At block <NUM> of the method <NUM>, a mask layer <NUM> is deposited to cover a portion <NUM> of a top surface <NUM> of the first substrate layer (e.g. the first substrate layer <NUM>-<NUM>) and to leave an exposed portion <NUM> of the top surface <NUM>. <FIG> illustrates schematically a side-cross section of a structure as arrived at from performing block <NUM>.

At block <NUM> of the method <NUM>, a substrate material is deposited on the exposed portion <NUM> of the top surface <NUM> to at least partly form the second substrate layer <NUM>-<NUM>. The substrate material is any material described above with respect to the substrate <NUM> or the substrate <NUM>-<NUM>. The substrate material is the same material comprised in the first substrate layer <NUM>-<NUM>, for example. <FIG> illustrates schematically a side-cross section of a structure as arrived at from performing block <NUM>. At block <NUM> of the method <NUM>, the mask layer <NUM> is removed. <FIG> illustrates schematically a side-cross section of a structure as arrived at from performing block <NUM>. Removing the mask layer <NUM> exposes a part of the surface of the first substrate layer <NUM>-<NUM> to at least partly form the first surface <NUM>-<NUM>.

<FIG> is a block diagram illustrating a method <NUM> for at least partly forming the substrate not being part of the claimed invention. The method <NUM> relates to the examples of <FIG> and can be used to at least partly form the first substrate layer <NUM>-<NUM> and the second substrate layer <NUM>-<NUM> shown in <FIG>. The following description of the method <NUM> is with reference to the examples of <FIG>. The method <NUM> is an alternative to the method <NUM> described above. As used in this context, a layer refers to material occupying a volume within a given range of values relating to the vertical axis <NUM>-<NUM>.

<FIG> illustrates schematically a side-cross section of a structure in relation to the method <NUM>. At block <NUM> of the method <NUM>, a mask layer <NUM> is deposited to cover a first portion <NUM> of a top surface <NUM> of a layer of substrate material <NUM>. <FIG> illustrates schematically a side-cross section of a structure as arrived at from performing block <NUM>.

At block <NUM> of the method <NUM>, material from a second portion <NUM> of the layer of substrate material <NUM> different to the first portion <NUM> and not covered by the mask layer <NUM> is removed, to provide the first substrate layer <NUM>-<NUM> comprising the first surface <NUM>-<NUM>. <FIG> illustrates schematically a side cross-section of a structure as arrived at from performing block <NUM>.

At block <NUM> of the method <NUM>, the mask layer <NUM> is removed to provide the second substrate layer <NUM>-<NUM> comprising the second surface <NUM>-<NUM>. <FIG> illustrates schematically a side cross-section of a structure as arrived at from performing block <NUM>.

It will be appreciated that the first and second substrate layers <NUM>-<NUM>, <NUM>-<NUM> are provided from the layer of substrate material <NUM>. It will be understood that the starting point of the method <NUM> is the layer of substrate material <NUM> of a thickness corresponding to the total desired thickness of the first substrate layer <NUM>-<NUM> and the second substrate layer <NUM>-<NUM>.

In some examples, examples of block <NUM> for at least partly forming the waveguide <NUM> are employed on the structure resulting from the method <NUM> or the method <NUM>. In such examples, one or more materials for the waveguide <NUM>-<NUM> is deposited on top of the substrate <NUM>-<NUM>. For example, the waveguide <NUM>-<NUM> is epitaxially grown using the materials described above in relation to the waveguide. Due to the arrangement of the substrate <NUM>-<NUM> (e.g. as shown in <FIG>), the waveguide <NUM>-<NUM> has portions offset (in a direction parallel to the vertical axis <NUM>-<NUM> and in contact with one another as described above in relation the waveguide <NUM>-<NUM>. One or more cladding layers (e.g. cladding layers <NUM>) and an electrical contact layer <NUM> are deposited on top of the waveguide <NUM>-<NUM>, for example.

In examples, a dry etching procedure is performed to remove material from either side of the structure, up to a particular depth as desired according to the intended application. For example, a dry etching procedure is performed to remove material and form the light polarisation converter <NUM> of the examples of <FIG>.

In examples, the method <NUM> illustrated in <FIG> comprises manufacturing the light polarisation converter such that the waveguide has a waveguide length as a function of the beat length (as described above) for the given wavelength of light in the waveguide, wherein the waveguide length is along an axis parallel to a light propagation direction within the waveguide. In some examples, the waveguide length is substantially equal to an odd integer multiplied by half of the beat length. In some examples, the waveguide length is substantially equal to an odd integer multiplied by a quarter of the beat length. One or more of the polarisation converters manufactured using the described methods can be used, as described above, to achieve polarisation conversions, as desired.

In some examples, there is provided a PIC comprising a light polarisation converter according to any of the described examples.

In the above description, reference is made to at least partly forming layers and the like. In some examples, a layer referred to in this manner is simply formed by depositing the relevant material, without requiring further steps. In other examples, further steps are performed to complete the formation of a layer (for example, a curing step, an etching step to define the extent of a layer, etc.). In some examples, the further steps to complete the formation of a layer are performed before further material is deposited on top of the layer in question. In other examples, the further steps to complete the formation of a layer are performed after further material is deposited on top of the layer in question.

In the described Figures, dashed lines are included at the edges of certain parts to indicate continuation of the parts in question beyond what is schematically illustrated in the Figures. The Figures include schematic illustrations of structures related to the described examples of the light polarisation converters. None of the Figures should be taken to indicate precise proportions with respect to any other Figure.

As the skilled person will appreciate, various techniques may be used to deposit a layer of semiconductor material in accordance with examples described herein. Such a technique may be known as a regrowth technique, for example a metalorganic vapour-phase epitaxy (MOVPE) or a molecular beam epitaxy (MBE) process may be used.

Claim 1:
A light polarisation converter (<NUM>, <NUM>, <NUM>) for a photonic integrated circuit, comprising:
A substrate (<NUM>, <NUM>-<NUM>, <NUM>-<NUM>) comprising a first surface (<NUM>, <NUM>-<NUM>, <NUM>-<NUM>) and a second surface (<NUM>, <NUM>-<NUM>, <NUM>-<NUM>), the substrate comprising a first substrate layer (<NUM>, <NUM>-<NUM>, <NUM>-<NUM>) and a second substrate layer (<NUM>, <NUM>-<NUM>, <NUM>-<NUM>), the first substrate layer comprising the first surface and the second substrate layer comprising the second surface; and
a waveguide (<NUM>, <NUM>-<NUM>, <NUM>-<NUM>) comprising a first waveguide portion (<NUM>, <NUM>-<NUM>, <NUM>-<NUM>) in contact with the first surface, and a second waveguide portion (<NUM>, <NUM>-<NUM>, <NUM>-<NUM>) in contact with the second surface,
wherein the second surface is offset from the first surface along a first axis (<NUM>, <NUM>-<NUM>, <NUM>-<NUM>) and a second axis (<NUM>, <NUM>-<NUM>, <NUM>-<NUM>) each perpendicular to a light propagation direction (<NUM>, <NUM>-<NUM>, <NUM>-<NUM>) for converting polarisation of the light, such that the second waveguide portion is offset from the first waveguide portion.