Patent Description:
<CIT> relates to a photonic integrated device comprising two or more vertically stacked asymmetric waveguides.

<CIT> relates to a distributed Bragg reflector laser, and systems and methods for thermal tuning of a distributed Bragg reflector.

<CIT> relates to a thermally wavelength tuneable laser having selectively activated gratings.

Examples described herein relate to a waveguide structure as defined by appended claim <NUM> and a method of manufacturing a waveguide structure as defined by appended claim <NUM>.

Depending on the application, a waveguide structure may be required to provide a particular optical length for the wavelength of light it is to be used for. The optical length is the product of the geometric length of the path followed by light and the refractive index the light is subject to when following that path. The optical length may be varied by varying the length (in the direction of light propagation) or the width of the waveguide structure, for example. However, varying the physical dimensions of the waveguide structure may require careful consideration of the positional arrangement of the various components of a photonic integrated circuit (PIC), which may reduce flexibility in PIC design.

The examples described herein comprise one or more waveguide modifier layers which for example modify the effective refractive index for certain modes of light in a waveguide layer (in which light is confined and propagates). This means that the optical length for those modes can be tuned without the need to vary the physical dimensions of the waveguide structure. This gives greater flexibility in PIC design as compared to the case where the physical dimensions of the waveguide structure are varied in order to tune the optical length. Also, the one or more waveguide modifier layer can be arranged to affect particular modes of light within the waveguide layer and not substantially to affect other modes in the same manner, as desired. This enables further flexibility in the applications of the waveguide structure.

<FIG> illustrates schematically a side cross-section of a waveguide structure <NUM> according to an example. The waveguide structure <NUM> comprises a substrate <NUM> and a waveguide layer <NUM> on the substrate <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 following examples are in the context of the substrate comprising InP.

In the examples described herein the substrate <NUM> comprises mainly InP. In some examples, the substrate <NUM> is purely InP (within acceptable purity tolerances). In other examples, the substrate <NUM> comprises other materials such as dopants or impurities with the material comprising at least <NUM>% InP. For example, the substrate <NUM> is doped with a dopant material so that the substrate is considered n-doped or the substrate <NUM> is doped with a dopant material so that the substrate <NUM> is considered p-doped, or the substrate <NUM> is doped with a dopant material so that the substrate <NUM> is considered semi-insulating.

The waveguide layer <NUM> comprises a material which has a higher refractive index than the material of the substrate <NUM>. For example, the waveguide layer <NUM> comprises Indium Gallium Arsenide Phosphide (InGaAsP). More generally, in some examples, the waveguide layer <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 layer <NUM> is a layer of (Al)InGaAs(P). In other examples, the waveguide layer <NUM> comprises a plurality of sub-layers. In some such examples, the waveguide layer <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 layer <NUM> has a band gap selected in accordance with the desired application of the waveguide structure <NUM>. In examples, the waveguide layer <NUM> has a thickness (in the vertical direction with respect to <FIG>) of <NUM> nanometres (although, it will be appreciated that the thickness and the composition of the waveguide layer <NUM> depends on the desired application).

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 layer <NUM> is tuned to a wavelength of <NUM> nanometres. In other examples, the wavelength to which the bandgap is tuned is different.

The waveguide layer <NUM> is for guiding light. In use, light propagates within the waveguide layer <NUM> and is confined within the waveguide layer <NUM>, for example in vertical and horizontal direction as shown in <FIG>, due to reflection at the boundaries of the waveguide layer <NUM>. The waveguide layer <NUM> has a refractive index higher than the refractive index of material in contact with the waveguide layer <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 layer <NUM> is greater than the critical angle. In this manner, the waveguide layer <NUM> guides the propagation of the light. For a particular optical mode to propagate in the waveguide layer <NUM>, it is desired that the light reflected at the boundaries of the waveguide layer <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 layer <NUM> depending on the desired application of the waveguide structure <NUM>. The direction in which the optical modes propagate within the waveguide layer <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 layer <NUM> and is not necessarily, for example, the direction defined by the angle of incidence at a boundary of the waveguide layer <NUM>.

The waveguide structure <NUM> comprises a cladding layer <NUM> in contact with a first side <NUM> of the waveguide layer <NUM>, the waveguide layer <NUM> between the cladding layer <NUM> and the substrate <NUM>. The first side <NUM> of the waveguide layer <NUM> is the side opposite to the side of the waveguide layer <NUM> in contact with the substrate <NUM>. With reference to the orientation shown in <FIG>, the first side <NUM> of the waveguide layer <NUM> is hereafter referred to as the top side <NUM> of the waveguide layer <NUM>.

For example, the cladding layer <NUM> comprises a III-V semiconductor compound such as Indium Phosphide (InP), gallium arsenide (GaAs), gallium nitride (GaN) or gallium antimonide (GaSb), depending on the substrate used. In these examples, the cladding layer <NUM> comprises mainly InP. As for the substrate <NUM>, this means that the material of the cladding layer <NUM> comprises mainly InP. As in the case of the substrate <NUM>, the material of the cladding layer <NUM> comprises at least <NUM>% InP, and is, for example, considered intrinisic (un-doped), n-doped, p-doped or semi-insulation. In some examples, where the desired application is with active PIC components, the cladding layer is doped (e.g. p-doped) to enable transmission of electrical power, for example. In the examples described herein, the cladding layer <NUM> comprises intrinsic InP.

The waveguide structure <NUM> comprises a first waveguide modifier layer <NUM> comprising a first material for modifying an effective refractive index of the waveguide layer <NUM>. The presence of the waveguide modifier layer <NUM> modifies the way in which light propagates within the waveguide layer <NUM> as compared to when the waveguide modifier layer <NUM> is not present. The presence of the first waveguide modifier layer (which comprises the first material) modifies the effective refractive index of the waveguide layer <NUM>. As used herein, the effective refractive index is the refractive index which the light propagating within the waveguide layer <NUM> experiences. The effective refractive index is not necessarily the refractive index of the waveguide layer in isolation (e.g. the refractive index of the material comprised in the waveguide layer <NUM>), for example. Other material in the vicinity of the waveguide layer <NUM> can also influence the refractive index experienced by light propagating within the waveguide layer <NUM> since part of the optical mode overlaps with this material. Depending upon what material is in the vicinity of the waveguide layer <NUM>, the effective refractive index for light within the waveguide layer <NUM> can be different. For example, the effective refractive index depends on the waveguide structure <NUM> as a whole. The skilled person will appreciate that the effective refractive index depends on parameters including the wavelength of light and also depending on the particular optical mode in question.

By introducing a layer comprising the first material (e.g. the first waveguide modifier layer <NUM>) close to the waveguide layer <NUM>, the effective refractive index for light propagating within the waveguide layer <NUM> is modified. As used herein, the modified effective refractive index is the effective refractive index as modified due to the presence of a waveguide modifier layer such as the first waveguide modifier layer <NUM>.

For example, when the first waveguide modifier layer <NUM> is present, light propagates within the portion of the waveguide layer <NUM> underneath the first waveguide modifier layer <NUM> (with respect to the orientation shown in <FIG>) as if the refractive index it is subject to is the modified effective refractive index. For example, an optical mode propagating through the waveguide layer <NUM> for which constructive interference occurs in the portion of the waveguide layer <NUM> underneath the first waveguide modifier layer <NUM> propagates as if the refractive index it is subject to is the modified effective refractive index. Modifying the effective refractive index does not mean changing physically the waveguide layer <NUM> in any manner. Instead, the effective refractive index is the refractive index according to which the light propagates in the portion of the waveguide layer <NUM> underneath the first waveguide modifier layer <NUM> due to the presence of the first waveguide modifier layer <NUM> close to the waveguide layer <NUM>.

The first material has a different refractive index to the material of the cladding layer <NUM>. In the examples described herein, the waveguide modifier layer <NUM> comprises the same material (the first material) as the waveguide layer <NUM>. In other examples, the first material is a different material to the material of the waveguide layer <NUM>. In some examples, the first material is different to the material of the waveguide layer <NUM>, and has substantially (within acceptable tolerances) the same refractive index as the material of the waveguide layer <NUM>. In some examples, the first material has a higher refractive index than the refractive index of the material of the waveguide layer <NUM>. In other examples, the first material has a lower refractive index than the refractive index of the material of the waveguide layer <NUM>.

The first waveguide modifier layer <NUM> is in contact with the cladding layer <NUM>. The first waveguide modifier layer <NUM> has a width (W1) along a first axis <NUM> less than a width (W2), parallel to the first axis <NUM>, of the cladding layer <NUM>. The first axis <NUM> is perpendicular to a second axis (indicated by reference numeral <NUM> in <FIG> and <FIG>) corresponding with a light propagation direction within the waveguide layer <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>.

As a consequence of the width of the waveguide modifier layer <NUM> along the first axis <NUM> being less than the width of the cladding layer <NUM> parallel to the first axis <NUM>, there is a region of the waveguide layer <NUM> which has the first material above it (in the orientation shown in <FIG>), and a region of the waveguide layer <NUM> which does not have the first material above it. Accordingly, the effective refractive index for the optical modes for which constructive interference occurs in the region of the waveguide layer <NUM> with the first material above it is substantially (within acceptable tolerances) different to the effective refractive index for the optical modes for which constructive interference occurs in the region of the waveguide layer <NUM> with no first material above it.

The skilled person will appreciate that different modes of light will have constructive interference peaks at different locations of the waveguide layer <NUM> along a direction parallel to the first axis <NUM>. The first waveguide modifier layer <NUM> can be positioned and dimensioned in terms of width along the first axis <NUM> according to the particular modes of light for which the effective refractive index is to be modified.

In the example of <FIG>, the first waveguide modifier layer <NUM> is positioned above a left hand side portion of the waveguide layer <NUM> (with respect to the orientation shown in <FIG>). Therefore, the effective refractive index for the modes of light for which constructive interference occurs in that left hand side portion of the waveguide layer <NUM> will be modified in these examples. It should be noted that the position of the first waveguide modifier layer <NUM> along the first axis <NUM> is not limited to these examples. The number, position, size or shape of waveguide modifier layers is not limited to the example shown in <FIG>. As one of many examples, the first waveguide modifier layer <NUM> is positioned above the centre (with respect to a direction parallel to the first axis <NUM>) of the waveguide layer <NUM>.

In the example of <FIG>, the waveguide structure <NUM> comprises a plurality of waveguide modifier layers on the first axis <NUM> comprising the first material. The plurality of waveguide modifier layers on the first axis <NUM> comprise the first waveguide modifier layer <NUM>. The plurality of waveguide modifier layers on the first axis <NUM> are in contact with the cladding layer <NUM> and spaced apart from one another. More specifically, in the example of <FIG>, there are two waveguide modifier layers on the first axis <NUM> including the first waveguide modifier layer <NUM>. In other examples, the waveguide modifier layers on the first axis are positioned and dimensioned in terms of width along the first axis <NUM> differently to the example of <FIG>, depending upon the application of the waveguide structure <NUM>. In some examples, there are three or more waveguide modifier layers on the first axis <NUM> in contact with the cladding layer <NUM> and spaced apart from one another.

In the example of <FIG>, and in accordance with the claimed invention, the waveguide structure <NUM> comprises the first waveguide modifier layer <NUM> and a second waveguide modifier layer <NUM> on the first axis <NUM> comprising the first material for modifying an effective refractive index of the waveguide layer, the first and the second waveguide modifier layers on the first axis <NUM> in contact with the cladding layer <NUM> and laterally spaced apart from one another. The second waveguide modifier layer <NUM> is positioned above the right hand side portion of the waveguide layer <NUM> (with respect to the orientation shown in <FIG>). Accordingly, the modes of light for which constructive interference occurs in the waveguide layer <NUM> in the left hand side and the right hand side portions will experience a modified effective refractive index.

In other examples, described here as comparative examples which help put the claimed invention into context, there is only one waveguide modifier layer. For example, <FIG> illustrates schematically a side cross-section of a waveguide structure <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 waveguide structure <NUM> comprises only the first waveguide modifier layer <NUM>-<NUM> on the first axis <NUM>-<NUM> and does not comprise other waveguide modifier layers on the first axis <NUM>-<NUM>. In these examples, the first waveguide modifier layer <NUM>-<NUM> is positioned centrally with respect to the waveguide layer <NUM>-<NUM> in the direction of the first axis <NUM>-<NUM>. The position and width of the first waveguide modifier layer <NUM>-<NUM> is selected based on the particular application of the waveguide structure <NUM>.

In the examples described herein, the waveguide structure <NUM> comprises a second material in contact with one or more portions <NUM> of a side of the cladding layer <NUM> overlapping the top side <NUM> of the waveguide layer <NUM>, the one or more portions <NUM> not in contact with the first material. The side of the cladding layer <NUM> which overlaps the top side <NUM> of the waveguide layer <NUM> is the top side of the cladding layer <NUM> as shown in <FIG>. In other words, the one or more portions <NUM> are the portions of the cladding layer <NUM> which do not have the first material above them as shown in <FIG>. The second material, therefore, is positioned on the first axis <NUM> in contact with the first waveguide modifier layer <NUM>. In examples with a plurality of waveguide modifier layers on the first axis <NUM>, the second material fills the space between the plurality of waveguide modifier layers on the first axis <NUM>.

In the examples described herein, the cladding layer comprises the second material. In other examples, the second material is a different material to the material of the cladding layer <NUM>. The second material extends up to the same height as a top side of the first waveguide modifier layer <NUM>. In other examples, the second material is omitted. For example, the first waveguide modifier layer <NUM> forms a boundary with air or other material along a position along the first axis <NUM>.

Furthermore, in the examples described herein, the second material is in contact with a side <NUM> of the first waveguide modifier layer <NUM> overlapping the top side <NUM> of the waveguide layer <NUM>. The side <NUM> is the top side <NUM> of the first waveguide modifier layer <NUM>. In examples comprising a plurality of waveguide modifier layers on the first axis <NUM>, the second material is in contact with respective top sides of the waveguide modifier layers of the plurality of waveguide modifier layers on the first axis <NUM>. In some examples, there is a material other than the second material in contact with the top side <NUM> of the first waveguide modifier layer <NUM>. In some examples, a material with a different concentration of dopant is in contact with the top side <NUM> of the first waveguide modifier layer <NUM>. In some examples, there is a material in contact with the top side <NUM> of the first waveguide modifier layer <NUM> with a substantially (within acceptable tolerances) homogeneous dopant concentration. In other examples, a top section in contact with the top side <NUM> of the first waveguide modifier layer <NUM> has a concentration gradient of dopant. For example, the dopant concentration in the top section increases with distance from the waveguide layer <NUM>.

In some particular examples, the top section comprises (not shown in the Figures) an intrinsic semiconductor layer (e.g. InP with a thickness of <NUM> nanometres) in contact with the top side <NUM> of the first waveguide modifier layer <NUM>, a first p-doped top section layer in contact with a top surface of the intrinsic semiconductor layer (e.g. p-doped InP of thickness <NUM> nanometres) and a second p-doped top section layer (e.g. p-doped InP with a higher dopant concentration than the first p-doped top section layer with a thickness of <NUM> nanometres) in contact with a top surface of the first p-doped top section layer. In some such examples, the top section comprises a contact layer in contact with a top surface of the second p-doped top section layer. The contact layer is for injecting charge carriers into the semiconductor structure.

In other examples, there is no semiconductor material in contact with the top side <NUM> of the first waveguide modifier layer <NUM>. For example, the top side <NUM> of the first waveguide modifier layer <NUM> forms a boundary with air, dielectric material, metal or magnetic material.

<FIG> illustrates schematically a plan view cross-section of a waveguide structure 100A according to examples. The waveguide structure 100A illustrates specific examples of the waveguide structure <NUM> shown in <FIG>. The cross-section of <FIG> is taken along line A-A shown in <FIG>, at the top surface <NUM> of the first waveguide modifier layer <NUM>. In <FIG>, the specific examples of the features shown in <FIG> are labelled with similar reference numerals with the letter "a" added at the end.

The second axis corresponding with the light propagation direction within the waveguide layer <NUM> is shown in <FIG> and labelled with reference numeral <NUM>. In the examples of <FIG>, the first waveguide modifier layer 110a has a width (W3) along a third axis <NUM> different to the width (W1a) of the first waveguide modifier layer 110a along the first axis 114a. The third axis <NUM> is perpendicular to the second axis <NUM> and spaced from the first axis 114a. In other words, the third axis <NUM> is at a different position with respect to the second axis <NUM> than is the first axis 114a. In this manner, the width of the first waveguide modifier layer 110a tapers with respect to position along the second axis <NUM>.

In the examples of <FIG>, the width of the second waveguide modifier layer 116a on the first axis 114a also tapers with respect to position along the second axis <NUM>. In these examples, the amount by which the width of the first waveguide modifier layer 110a tapers is substantially (within acceptable tolerances) the same as the amount by which the width of the second waveguide modifier layer 116a tapers. In other examples, the widths of the first waveguide modifier layer 110a and the second waveguide modifier layer 116a taper differently (or one width tapers while the other does not, for example). In some examples where the width of the first waveguide modifier layer 110a tapers, there is no further waveguide modifier layers.

For example, the described taper is selected in accordance with the particular modification of the effective refractive index for the light propagating in the waveguide layer 104a that is desired. In the example of <FIG>, the taper can be considered to be linear in that there is a linear transition from one width to a different width of the modifier layer. However, in other examples a change in width of the first waveguide modifier layer 110a can be non-linear, for example stepped, depending on the desired application.

<FIG> illustrates schematically a plan view cross-section of a waveguide structure 100B according to examples. The waveguide structure 100B illustrates specific examples of the waveguide structure <NUM> shown in <FIG>. The cross-section of <FIG> is taken along the line A-A shown in <FIG>, at the top surface <NUM> of the first waveguide modifier layer <NUM>. In <FIG>, features corresponding to those described above are labelled with similar reference numerals with the letter "b" added at the end.

The waveguide structure 100B comprises a third waveguide modifier layer <NUM> comprising the first material. The third waveguide modifier layer <NUM> is in contact with the cladding layer 106b. The third waveguide modifier layer <NUM> is located on the third axis 204b. In these examples, the width (W4) of the third waveguide modifier layer <NUM> along the third axis 204b is less than the width, parallel to the third axis 204b, of the cladding layer 106b. In the examples of <FIG>, the width (W1b) of the first waveguide modifier layer 110b is substantially (within acceptable tolerances) the same as the width (W4) of the third waveguide modifier layer <NUM> (the width being measured along the first axis 114b in the case of the first waveguide modifier layer 110b, and along the third axis 204b in the case of the third waveguide modifier layer <NUM>, which is parallel to the first axis 114b). The examples of <FIG> may be used where filtering of certain optical modes is desired.

<FIG> illustrates schematically a side cross-section of a waveguide structure 100C. In <FIG>, features corresponding to those described above are labelled with the same reference numerals with the letter "c" added at the end. In these examples, the width (W1c) of the first waveguide modifier layer 110c is different to the width (W4c) of the third waveguide modifier layer 302c. In examples, the third waveguide modifier layer 302c has any width in accordance with the intended application of the waveguide structure in question. In the examples of <FIG>, the position of the first waveguide modifier layer 110b is aligned with the position of the third waveguide modifier layer <NUM> with respect to a direction parallel to the first axis 114b. In other examples, the first waveguide modifier layer 110b and the third waveguide modifier layer <NUM> are not so aligned.

Referring again to <FIG>, in these examples, the waveguide structure 100B comprises a plurality of waveguide modifier layers on the third axis 204b comprising the first material, in contact with the cladding layer 106b and spaced apart from one another. The plurality of waveguide modifier layers on the third axis 204b comprise the third waveguide modifier layer <NUM>. More specifically, in the examples of <FIG>, the waveguide structure 100B comprises a fourth waveguide modifier layer <NUM> on the third axis 204b comprising the first material for modifying the waveguide function of the waveguide layer, the third and the fourth waveguide modifier layers in contact with the cladding layer and spaced apart from one another. In other examples, the waveguide structure 100B comprises only the third waveguide modifier layer <NUM> on the third axis 204b (and no other waveguide modifier layers). In some examples, the waveguide structure 100B comprises more than two waveguide modifier layers on the third axis 204b spaced apart from one another.

<FIG> illustrates schematically a side cross-section of a waveguide structure 100D according to examples. In <FIG>, features corresponding to those described above are labelled with the same reference numerals with the letter "d" added at the end. In these examples, the waveguide structure 100D comprises only the first waveguide modifier layer 110d and the second waveguide modifier layer 116d. In these examples, the first waveguide modifier layer 110d and the second waveguide modifier layer 116d extend along the entire length of the waveguide structure 100D. In these examples, the width of the first waveguide modifier layer 110d in a direction parallel to the first axis 114d and the width of the second waveguide modifier layer 116d in a direction parallel to the first axis 114d does not change with respect to position along the second axis 202d.

Various examples of one or more waveguide modifier layers have been described. In the described examples, the space above the cladding layer <NUM> which is not covered by the first material is filled with the second material along the entire length of the waveguide structure parallel to the second axis. For example, in the case of waveguide structure 100A, the space between the first waveguide modifier layer <NUM> and the second waveguide modifier layer <NUM> on the first axis <NUM> contains the second material. For example, in the case of the waveguide structure 100B, the space between the first waveguide modifier layer 110b, the second waveguide modifier layer 116b on the first axis 114b, the third waveguide modifier layer <NUM> and the fourth waveguide modifier layer <NUM> on the third axis 204b is filled with the second material. The space in the same plane as the waveguide modifier layers not containing the first material contains the second material.

Referring again to <FIG>, the waveguide structure <NUM> represents examples of what may be referred to as a deep waveguide structure. For example, in order to manufacture a deep waveguide structure, such as the waveguide structure <NUM>, material is removed to form sides of the waveguide structure parallel to the second axis <NUM>, starting from the top section, and beyond a top surface of the substrate <NUM>.

<FIG> illustrates schematically a side cross-section of a waveguide structure <NUM> according 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 waveguide structure <NUM> may comprise any combination of the features described above in relation to the waveguide structure <NUM>, except that the waveguide structure <NUM> is what may be referred to as a shallow waveguide structure. For example, in order to manufacture a shallow waveguide structure, such as the waveguide structure <NUM>, material is removed to form sides of the waveguide structure parallel to the second axis <NUM>, starting from the top section, and at least until a top surface of the waveguide layer <NUM>-<NUM> without removing all of the waveguide layer <NUM>-<NUM> down to its bottom surface (for example, material is removed until slightly below the top surface of the waveguide layer <NUM>-<NUM>).

In the waveguide structure <NUM>, parts of the top side <NUM>-<NUM> of the waveguide layer <NUM>-<NUM> are not covered by the cladding layer <NUM>-<NUM> and the waveguide layer <NUM>-<NUM> is wider as compared to the deep waveguide structure <NUM>. The skilled person will appreciate that the deep waveguide structure <NUM> or the shallow waveguide structure <NUM> may be used depending on the desired light confinement within the waveguide layer <NUM>-<NUM> and/or the desired modes of light propagating within the waveguide layer <NUM>-<NUM>. The skilled person will also appreciate that a deep waveguide structure may be manufactured using etching methods. A shallow waveguide structure may be desired where lower electron hole non-radiative recombination is desired. This is because etching away less of the waveguide layer may produce less damage to the surface of the waveguide layer and reduce non radiative recombination due to defects in the damaged material. This may be desired for applications such as amplifiers and detectors. A shallow waveguide structure also has less of the surface of the waveguide layer that is etched. This can mean lower optical losses in a shallow waveguide structure, and a shallow waveguide structure may be desired in view of this, depending on the application.

On the other hand, a deep waveguide structure can offer greater lateral confinement (in a direction parallel to the first axis <NUM>-<NUM>) and may be used in view of this characteristic, depending on the application. For example, greater lateral confinement may result in different tuning characteristics of the optical modes. In some examples, a deep waveguide structure may provide flexibility in PIC design due to a smaller width of the waveguide layer and smaller bend radius for changing the direction of the propagation of light.

Modifying the effective refractive index using waveguide modifier layers as described facilitates the optical length of a waveguide structure to be tuned without a need to alter the physical dimensions of the waveguide structure (e.g. length in the light propagation direction). This allows much greater flexibility in the design of PICs as compared to, for example, varying the length of the waveguide structures in accordance with the desired optical length.

Furthermore, the effective refractive index can be tuned for particular modes of light positioned at different positions along a direction parallel to the first axis <NUM> within the waveguide layer <NUM>. Therefore, the principles elucidated by the described examples facilitate the effective refractive index to be varied for the differently positioned modes of light in a direction parallel to the first axis <NUM> within the waveguide layer <NUM>.

Various examples of waveguide structures have been described above. However, the number, size(s), shape(s) and arrangement of the waveguide modifier layers is not limited to the described examples. The waveguide structure may comprise any number, shape and arrangement of waveguide modifier layers in a manner so that, at least at one position in a direction parallel to the second axis <NUM>, a waveguide modifier layer has a width, parallel to the first axis <NUM>, less than a width, parallel to the first axis <NUM> of the cladding layer <NUM>. In this manner, the effective refractive index can be modified for those modes located in the waveguide layer <NUM> along a direction parallel to the first axis <NUM> underneath the first material. Accordingly, numerous different patterns of waveguide modifier layers can be provided depending upon the particular application of the waveguide structure.

The distance between the waveguide layer <NUM> and the first waveguide modifier layer <NUM> is selected according to a desired magnitude by which the waveguide function of the waveguide layer <NUM> is to be modified. This distance is along a direction perpendicular to the first axis <NUM> and perpendicular to the second axis <NUM>. In other words, this distance is defined by the thickness of the cladding layer <NUM> between the first waveguide modifier layer <NUM> and the waveguide layer <NUM>. The closer a waveguide modifier layer is to the waveguide layer <NUM>, the greater the modification of the effective refractive index of the waveguide layer <NUM>. For the greatest modification of the effective refractive index, the waveguide modifier layers is positioned as close to the waveguide layer <NUM> as permitted by manufacturing tolerances. For example, the distance between the first waveguide modifier layer <NUM> and the waveguide layer <NUM> is around <NUM> nanometres. For example, if the waveguide modifier layer <NUM> is defined using dry etching techniques, it is desired that the cladding layer <NUM> is thick enough to compensate variation on the etching rate such that the waveguide layer <NUM> is not affected by the etching.

In some examples, the thickness of the waveguide modifier layers affects the magnitude of the modification of the effective refractive index at a position parallel to the first axis <NUM> in the waveguide layer <NUM> above which the waveguide modifier layers in question are present. Accordingly, in some examples, the thickness of one or more of the waveguide modifier layers is selected in accordance with the desired magnitude of the modification of the effective refractive index. In some examples, the thickness of the waveguide layers is <NUM> nanometres.

In some examples, further waveguide modifier layers are provided at different distances from the waveguide layer <NUM> in a direction perpendicular to the first axis <NUM> and perpendicular to the second axis <NUM>. The further waveguide modifier layers can be positioned so as to modify the effective refractive index of the desired modes of light. As discussed above, the distance of a waveguide modifier layer from the waveguide layer <NUM> affects the magnitude by which the effective refractive index is modified. Accordingly, the effective refractive index for different modes of light can be modified to varying degrees.

In some examples, the waveguide modifier layers at a particular distance from the waveguide layer <NUM> are close to or in contact with waveguide modifier layers at a distance from the waveguide layer <NUM> different to that particular distance. In other examples, the waveguide modifier layers at a particular distance from the waveguide layer <NUM> are spaced apart in a direction perpendicular to the first axis <NUM> and perpendicular to the second axis <NUM> from other waveguide modifier layers.

<FIG> illustrates schematically a side cross-section of a waveguide structure <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 waveguide structure <NUM> comprises any combination of the features of the examples of waveguide structure <NUM> described above. In some examples, the waveguide structure <NUM> comprises any combination of features of the examples of waveguide structure <NUM> described above. In addition, in these examples, the waveguide structure <NUM> comprises waveguide modifier layers at a distance from the waveguide layer <NUM>-<NUM> (perpendicular to the first axis <NUM>-<NUM> and perpendicular to the second axis corresponding to a direction (as indicated by <NUM>-<NUM>) of light propagation) different to the first waveguide modifier layer <NUM>-<NUM>. In these examples, the waveguide structure <NUM> comprises a fifth waveguide modifier layer <NUM> and a sixth waveguide modifier layer <NUM> at a distance from the waveguide layer <NUM>-<NUM> further away than the first waveguide modifier layer <NUM>-<NUM> and the second waveguide modifier layer <NUM>-<NUM>. In other examples, there may be one or more waveguide modifier layers at any number of distances from the waveguide layer <NUM>-<NUM>, according to the intended application.

In the examples of <FIG>, the space between the waveguide modifier layers at different distances from the waveguide layer <NUM>-<NUM> comprises the second material. In some examples, the waveguide modifier layers at a first distance from the waveguide layer <NUM>-<NUM> comprise a different material to the waveguide modifier layers at a second distance from the waveguide layer <NUM>-<NUM>. The number, size(s), shape(s), arrangement and material(s) of the various waveguide modifier layers is selected according to the application of the waveguide structure <NUM>.

Various examples of arrangements of the waveguide modifier layers are described above. In addition, in some examples in accordance with the claims, waveguide modifier layers may be arranged so that they repeat (along a direction parallel to the first axis and/or the second axis) periodically with a periodicity selected depending on the application. For example, the periodicity is selected to correspond to a particular structure (e.g. a photonic crystal). Periodicity may be included for applications such as to provide filters, reflectors, etc, in addition to modifying the effective refractive index.

The boundaries of the described examples of the waveguide structure may be in contact with air, dielectric material, metal or magnetic material. In some examples, the waveguide structure comprises layers not described above.

<FIG> is a flow diagram illustrating a method <NUM> of manufacturing a waveguide structure, such as a waveguide structure according to any of the examples described above. The method <NUM> is described with reference to the above described examples of waveguide structures. The substrate is for example the substrate <NUM>. At block <NUM>, a first layer is deposited on a substrate to at least partly form the waveguide layer <NUM> on the substrate <NUM>. For example, the first layer comprises the material of the waveguide layer <NUM> which is deposited on the substrate <NUM> to at least partly form the waveguide layer <NUM>. In examples where the waveguide layer <NUM> comprises more than one material (e.g. in the case of the waveguide layer <NUM> comprising a plurality of sub-layers such as a (Al)InGaAs(P)/(Al)InGaAs(P) multiple quantum well structure) the relevant materials are e.g. deposited in the appropriate order to at least partly form the waveguide layer <NUM>.

At block <NUM> of the method <NUM>, a second layer material is deposited in contact with the first layer to at least partly form the cladding layer <NUM> in contact with the waveguide layer <NUM>. The second layer material is, for example, the material comprised in the cladding layer <NUM> described above. For example, the second layer material is the second material referred to above in the context of the waveguide structures <NUM> and <NUM>. The second layer material is hereafter referred to as the second material. The second material is deposited such that, once formed, the waveguide layer <NUM> is between the cladding layer <NUM> and the substrate <NUM>.

At block <NUM>, a waveguide modifier layer material is deposited in contact with the second layer. The waveguide modifier layer material is, for example, the first material described above. The waveguide modifier layer material is hereafter referred to as the first material. The first material is deposited to at least partly form the first waveguide modifier layer <NUM> in contact with the cladding layer <NUM> such that the first waveguide modifier layer <NUM> has a width along the first axis <NUM> (which is perpendicular to the second axis <NUM> corresponding with the light propagation direction within the waveguide layer <NUM>, as described above) less than a width, parallel to the first axis <NUM>, of the cladding layer <NUM>.

<FIG> is a flow diagram illustrating more specific examples <NUM> of the method <NUM>. Method <NUM> illustrates specific examples of at least partly forming the first waveguide modifier layer <NUM>. In these examples, an amount of second material is deposited on the first layer such that the height of the second material equates substantially (within acceptable tolerances) to the height of the cladding layer <NUM> plus the height of the first waveguide modifier layer <NUM> (see <FIG>). At block <NUM>, prior to depositing the first material for the first waveguide modifier layer <NUM> (between blocks <NUM> and <NUM>), one or more portions are removed from the second material. As used herein, a thickness of a layer is the dimension in a direction perpendicular to the first axis <NUM> and perpendicular to the second axis <NUM>. The one or more portions that are removed have a thickness less than the thickness of the second material deposited in contact with the first layer. The thickness of the one or more portions that are removed is therefore such that the first layer is not exposed as a result of the removal of the one or more portions.

The thickness of the one or more removed portions is selected in accordance with the desired thickness of the first waveguide modifier layer <NUM>. At block <NUM> of the method <NUM>, the first material is deposited in one or more spaces created by removing the one or more portions of the second material. For example, the first material is deposited so that the thickness of the first material is substantially (within acceptable tolerances) the same as the thickness of the second material at a position from which the second material is not removed at block <NUM>.

The number of portions of the second material that are removed depends upon the number of desired waveguide modifier layers. The size(s), shape(s) and arrangement of the portions of the second material that are removed depends on the desired size(s), shape(s) and arrangement of the waveguide modifier layers. In examples in which the waveguide structure comprises only the first waveguide modifier layer <NUM>, one portion of the second material is removed and the corresponding space filled with the first material.

<FIG> is a flow diagram illustrating more specific examples <NUM> of the method <NUM>. The examples according to the method <NUM> are alternatives to the examples according to the method <NUM> described with reference to <FIG>. Method <NUM> illustrates specific examples of at least partly forming the first waveguide modifier layer <NUM>. In these examples, the first material is deposited over the entire top surface of the second material that was deposited in contact with the first layer. At block <NUM> of the method <NUM>, one or more portions of the first material (the waveguide modifier layer material) are removed to create respective one or more exposed portions of the second material (the second layer material). At block <NUM> of the method <NUM>, the second material is deposited onto the one or more exposed portions of the second layer material.

Either of the methods <NUM> of <FIG> and <NUM> of <FIG> are used to at least partly form the portion of the waveguide structure comprising the waveguide modifier layers with the second material therebetween. As with the method <NUM>, in the method <NUM>, the number, size, shape and arrangement of the one or more portions of the waveguide layer material (the first material) that are removed depends upon the desired number, size, shape and arrangement of waveguide modifier layers according to an application of the waveguide structure.

In some examples where the top surfaces of the waveguide modifier layers are covered with a material, the second layer material is deposited on the first material to at least partly form the top section of the waveguide structure comprising the second material.

In the above description with reference to <FIG> and <FIG>, reference is made to at least partly forming layers. In some examples, a layer referred to in this manner is simply formed by depositing the relevant material. For example, the cladding layer <NUM> is formed simply by depositing the second material without requiring further steps. In other examples, further steps are performed to complete the formation of a layer (for example, a curing step, 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 order to manufacture a deep waveguide structure, such as the waveguide structure <NUM>, material is removed to form sides of the waveguide structure parallel to the second axis <NUM>, starting from the top section, and beyond a top surface of the substrate <NUM>. In order to manufacture a shallow waveguide structure, such as the waveguide structure <NUM>, material is removed to form sides of the waveguide structure parallel to the second axis <NUM>, starting from the top section, and at least until a top surface of the waveguide layer <NUM>-<NUM> without removing all of the waveguide layer <NUM>-<NUM> down to its bottom surface (for example, material is removed until slightly below the top surface of the waveguide layer <NUM>-<NUM>).

As the skilled person will appreciate, various techniques can be used to deposit the material in accordance with the described examples. Such techniques include, for example, chemical vapour deposition techniques such as metalorganic vapour-phase epitaxy (MOVPE) or molecular beam epitaxy (MBE). The skilled person will appreciate that etching techniques are used to remove material in accordance with the described examples. For example, a dry etching technique or a wet etching technique is used. For example, a patterned mask is used.

In some examples, there is provided a PIC comprising the waveguide structure according to any of the described examples.

Claim 1:
A waveguide structure (<NUM>) comprising:
a substrate (<NUM>);
a waveguide layer (<NUM>) on the substrate;
a cladding layer (<NUM>) in contact with a first side (<NUM>) of the waveguide layer, the first side on an opposite side of the waveguide layer to the substrate, the waveguide layer between the cladding layer and the substrate;
a first waveguide modifier layer (<NUM>) comprising a first material of a different refractive index to a refractive index of the cladding layer, the first material arranged to modify an effective refractive index of the waveguide layer, the first waveguide modifier layer in contact with the cladding layer and having a width along a first axis (<NUM>) less than a width, parallel to the first axis, of the cladding layer, the first axis perpendicular to a second axis (<NUM>) corresponding with a light propagation direction within the waveguide layer; and
a second waveguide modifier layer (<NUM>) comprising the first material to modify the effective refractive index of the waveguide layer, the second waveguide modifier layer laterally spaced apart from the first waveguide modifier layer along the first axis and in contact with the cladding layer.