Light polarisation converter and method of manufacture

Abstract

This invention discloses an optical polarization converter for a photonic integrated circuit, comprising: a substrate and a waveguide. The substrate includes a first surface and a second surface. The waveguide includes a first waveguide portion in contact with the first surface and a second waveguide portion in contact with the second surface. The second surface is offset from the first surface along a first axis and a second axis. Each axis is perpendicular to a light propagation direction for converting the polarization of light. The second waveguide portion is offset from the first waveguide portion.

Description

technical field

[0001] Embodiments of the present invention relate to an optical polarization converter and a method for manufacturing the optical polarization converter. prior art

[0002] Semiconductor structures can be used in photonic integrated circuits (PICs) to perform various functions. Semiconductor structures can be fabricated for use in specific applications within a PIC. A semiconductor structure for use in a PIC includes materials and is fabricated according to the intended application of the semiconductor structure within that PIC. For example, a PIC may include a waveguide structure to allow light to propagate in a desired manner from one portion of the PIC to another. Contents of the invention

[0003] An embodiment of the present invention provides a light polarization converter for a photonic integrated circuit, the light polarization converter includes: a substrate including a first surface and a second surface; and a waveguide, which comprising a first waveguide portion in contact with the first surface and a second waveguide portion in contact with the second surface, wherein the second surface is along a first axis perpendicular to a light propagation direction and a second waveguide portion respectively An axis is offset from the first surface for converting the polarization of the light such that the second waveguide portion is offset from the first waveguide portion.

[0004] Another embodiment of the present invention provides a photonic integrated circuit including the optical polarization converter according to the above-mentioned embodiments.

[0005] Yet another embodiment of the present invention provides a method of manufacturing a light polarization converter for a photonic integrated circuit, the method comprising: at least partially forming one of a first surface and a second surface a substrate; and at least partially forming a waveguide comprising a first waveguide portion in contact with the first surface and a second waveguide portion in contact with the second surface, wherein the two surfaces are along directions each perpendicular to a light propagation direction A first axis and a second axis are offset from the first surface for converting the polarization of the light such that the second waveguide portion is offset from the first waveguide portion. Brief description of the diagram

[0006] Figure 1 schematically illustrates a first side-view cross-section of a first light polarization converter according to an example; Figure 2A illustrates a first polarization of light; Figure 2B illustrates a second polarization of light; Figure 3 schematically illustrates a second side-view cross-section of one of the first light polarization converters; Fig. 4 schematically illustrates a side view cross-section of a second light polarization converter according to an example; Figure 5 is a flowchart illustrating a first set of steps as part of a method of manufacturing an optical polarization converter according to an example; Figure 6 is a flow diagram illustrating a second set of steps as part of a method of manufacturing a light polarization converter according to an example; 7A to 7G each schematically illustrate a first side cross-section of a first light polarization converter at sequential stages during manufacture according to an example; Figure 8 is a flow diagram illustrating a third set of steps as part of a method of manufacturing a light polarization converter according to an example; 9A to 9C each schematically illustrate a first side cross-section of a second light polarization converter according to a first set of examples during manufacture at a first stage, a second stage and a third stage, respectively; Figure 10 illustrates the equatorial plane of a Poincare sphere according to an example; Figure 11 schematically illustrates a side-view cross-section of a third light polarization converter according to examples; Figure 12 is a graph indicating the skew angle of a first mixed mode as a function of waveguide width; Figure 13 is a graph indicating the effective refractive index difference as a function of waveguide width; 14 is a flow diagram illustrating a fourth set of steps as part of a method of making a light polarization converter, according to examples; and 15A-15C each schematically illustrate a first side cross-section of a second light polarization converter according to a second set of examples during fabrication at a first stage, a second stage and a third stage, respectively. Implementation

[0007] The examples described herein relate to a semiconductor structure for a PIC. More specifically, the examples described herein relate to an optical polarization converter for a PIC.

[0008] In some instances, a PIC is constructed from the basic building blocks used to construct the PIC. The basic building block consists of various components, each of which has a specific function. An example of a basic building block is a waveguide structure. A basic building block can have a specific effect on light incident on it. The examples described herein relate to a light polarization converter that can be used as one of the basic building blocks for a PIC.

[0009] A light polarization converter is a component used in an optical system, such as a PIC, to convert between different polarizations of light. For example, a suitably configured light polarization converter can be used to convert between a first linear polarization of light and a second linear polarization of light (e.g., converting horizontally polarized light to vertically polarized light, and vice versa). In another example, a suitably configured light polarization converter can be used to convert between linearly polarized light and circularly polarized light. A polarization converter may also be called a polarization rotator or a birefringence rotator.

[0010] For example, by wet etching a waveguide to have angled sidewalls, a light polarization converter can be fabricated as a basic building block for a PIC. However, using a wet etching technique to form one or more boundaries of a waveguide to obtain an angled surface can be disadvantageous. This is because the etch used to form a surface can cause small irregularities in that surface. Irregularities can have an effect on light propagation in the waveguide. In addition, control of the waveguide fabrication process using etching techniques may not be sufficient to achieve the desired fabrication tolerances of the optical polarization converter. For example, it may be desirable to wet etch to a certain depth in a substrate on which the waveguides are supported. However, it may not be possible to control the etch depth to a desired level.

[0011] For better control of light propagation in the waveguide and better manufacturing tolerances, a light polarization converter in which the waveguide is not wet etched to define an angled sidewall is desired.

[0012] FIG. 1 schematically illustrates a side-view cross-section of a light polarization converter 100 according to an example. The optical polarization converter 100 is for a PIC. The light polarization converter 100 includes a substrate 102 . The substrate 102 includes a first surface 104 and a second surface 106 . In some examples, substrate 102 includes 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 includes a nitride-based material or a silicon-based material.

[0013] The light polarization converter 100 includes a waveguide 108 . The waveguide 108 includes a first waveguide portion 110 in contact with the first surface 104 and a second waveguide portion 112 in contact with the second surface 106 .

[0014] The second surface 106 is offset from the first surface 104 along a first axis 114 and a second axis 116, each perpendicular to a light propagation direction, for converting the polarization of the light such that the second waveguide portion 112 is separated from the first waveguide. Section 110 is offset. The first axis 114 is perpendicular to the second axis 116 .

[0015] In the example of FIG. 1 , a side view cross-section is shown such that the direction of light propagation is into the page, as indicated by symbol 118 . The first axis 114 is a horizontal axis relative to the orientation shown in FIG. 1 , and the second axis 116 is a vertical axis relative to the orientation shown in FIG. 1 . For example, the first axis 114 extends in a direction from the first surface 104 toward the second surface 106 . As referred to herein, the width of the portion of the light polarization converter 100 is along the first axis 114 . As mentioned herein, the lengths of the various portions set forth herein are along the direction of light propagation as indicated by symbol 118 . As referred to herein, the terms height, upper and lower are relative to the second axis 116 . In the orientation shown in FIG. 1 , the first surface 104 is a lower surface of the substrate 102 and the second surface 106 is an upper surface of the substrate 102 . The upper surface 106 provides a step upwards (in a direction parallel to one of the vertical axes) from the lower surface 104 . Thus, in these examples, the first surface 104 and the second surface 106 are offset in position along the vertical axis 116, eg, such that the first and second surfaces are displaced and / or vertically spaced from each other.

[0016] A waveguide 108 having a first waveguide portion 110 in contact with the first surface 104 and a second waveguide portion 112 in contact with the second surface 106 is meant to provide a waveguide having different portions offset from each other in a direction parallel to the vertical axis 116. A single waveguide 108 in which light can propagate in the direction of light propagation indicated by symbol 118.

[0017] A single waveguide with portions offset in a direction parallel to the vertical axis 116 provides polarization conversion of light, as explained in further detail below.

[0018] Since the structure of the substrate 102 has a first surface and a second surface that are offset in a direction parallel to the vertical axis 116, the waveguide 108 can be defined without performing a wet etch on the waveguide 108 to have a characteristic for polarization conversion. One configuration, for example to define an angled side wall. For example, one or more materials for waveguide 108 may be epitaxially grown on substrate 102 and, due to the offset configuration of substrate 102, form a waveguide having portions offset in a direction parallel to vertical axis 116. 108.

[0019] The waveguide 108 includes a material having a higher index of refraction than the material of the substrate 102 . For example, waveguide 108 includes indium gallium arsenide phosphide (InGaAsP). More generally, in some examples, waveguide 108 includes (Al)InGaAs(P). Elements indicated in parentheses are interchangeable, and the composition of different elements is chosen depending on the desired function. For example, the composition of Ga and As in InGaAs can be chosen according to the desired bandgap. In some examples, waveguide 108 is an (Al)InGaAs(P) layer. In other examples, waveguide 108 includes a plurality of sub-layers. In some such examples, waveguide 108 includes an (Al)InGaAs(P) / (Al)InGaAs(P) multiple quantum well structure in contact with substrate 102 . In some examples, the sublayers are between 5 nm and 30 nm thick. The sublayer stack of waveguide 108 has a bandgap selected according to the desired application of light polarization converter 100 .

[0020] As understood by those skilled in the art, the bandgap and thus the index of refraction of InGaAsP can be tuned. In some examples, the bandgap of the InGaAsP of waveguide 108 is tuned to 1250 nm (e.g., for propagating light at a wavelength of 1550 nm) or 1100 nm (e.g., for propagating light at a wavelength of 1310 nm). ) of one wavelength. In other examples, the wavelengths to which the bandgap is tuned are different.

[0021] The waveguide 108 is used to guide light. In use, light propagates within the waveguide 108 and becomes trapped within the waveguide 108 due to reflections at the boundaries of the waveguide 108 . The waveguide 108 has a refractive index higher than the refractive index of the material in contact with the waveguide 108 at the boundary where light confinement is desired. For example, due to this difference in refractive index at the boundaries where it is desired to confine light, total internal reflection occurs when the angle of incidence at these boundaries of the waveguide 108 is greater than a critical angle. In this way, waveguide 108 guides the propagation of light. For a particular optical mode propagating in waveguide 108, light reflected at the boundary of waveguide 108 is expected to satisfy the condition of constructive interference, as will be understood by those skilled in the art.

[0022] For example, depending on the desired application of the light polarization converter 100 , it is desirable for light of a particular optical mode to propagate through the waveguide 108 . The direction in which the optical modes propagate within the waveguide 108 is referred to herein as the light propagation direction. The light propagation direction is the general direction in which the energy of the optical modes travels through the waveguide 108, and is not necessarily a direction defined, for example, by the angle of incidence at one boundary of the waveguide 108.

[0023] In the example of FIG. 1 , the substrate 102 includes a third surface 120 between the first surface 104 and the second surface 106 . The third surface 120 is inclined relative to the first surface 104 and / or the second surface 106 . As referred to herein, the inclination of one surface relative to the other means that there is a non-zero angle between the two surfaces in question. Thus, in the context of a surface being inclined relative to another surface, it may be mentioned that one such surface is at a particular angle relative to the other surface. In such contexts, it may also be mentioned that one surface is skewed relative to the other. The terms "skew", "angled" and "oblique" are used interchangeably herein to refer to the angle of one entity relative to another entity or relative to a given axis. As referred to herein, the third surface 120 being "between" the first surface 104 and the second surface 106 means, for example, that the third surface 120 is interposed between the first surface 104 and the second surface 106, and For example, immediately adjacent to each of the first surface 104 and the second surface 106 . Thus, in an example, the first surface, the second surface, and the third surface together may be considered to constitute the top surface of the waveguide.

[0024] In the example of FIG. 1 , first axis 114 (also referred to as horizontal axis 114 ) is substantially parallel (eg, within acceptable tolerances) to the plane of first surface 104 . In such examples, the third surface 120 is at an angle of less than 90 degrees relative to the first surface 104 . In these examples, the second surface 106 is offset from the first surface 104 along the first axis 114 by an amount greater than a width 122 of the first surface 104 along the first axis 114 . As referred to herein, an offset in a direction parallel to an axis is a position along that axis relative to where the edge of the entity in question is located (the edge located at the earliest position along that axis). For example, in FIG. 1 , the edge of the first surface 104 is located at a location 124 along the first axis 114 . It should be appreciated that a zero offset from the first surface 104 along the first axis 114 would mean that the edges of each of the first surface 104 and the second surface 106 sit at the same position along the first axis 114 ( position 124). In the example of FIG. 1 , the offset along the first axis 114 is such that the second surface 106 does not overlap the first surface 104 and the third surface 120 (relative to the orientation of FIG. 1 ) slopes upward. In these examples, the position 126 along the first axis 114 where the edge of the second surface 106 is located is located at a distance greater than the width 122 of the first surface 104 from the position 124 .

[0025] In the example of FIG. 1 , waveguide 108 includes an intermediate waveguide portion 128 in contact with third surface 120 and between first waveguide portion 110 and second waveguide portion 112 . In some examples, as illustrated, the substrate 102 is at least partially formed with a surface offset in a direction parallel to the second axis 116 and an inclined third surface 120 prior to the waveguide 108 on top of the substrate 102. . In this way, due to the inclined third surface 120, a middle portion of the waveguide is formed at an angle corresponding to the inclined third surface 120 of the substrate. Thus, in an example, it can be considered that the first waveguide portion, the second waveguide portion and the intermediate waveguide portion together form a waveguide.

[0026] As illustrated, in the example of FIG. 1 , the first waveguide portion 110 is in contact with the first surface 104 of the substrate 102 . On the other hand, the second waveguide portion 112 is in contact with the second surface 106 offset from the first surface 104 (in a direction parallel to the second axis 116 ). This means that the waveguide 108 comprises two portions offset from each other in a direction parallel to the second axis 116 . As stated, in these examples, the waveguide 108 also includes an intermediate waveguide portion 128 at an angle relative to the second waveguide portion 112 and the first waveguide portion 110, as shown in FIG. 1 .

[0027] This configuration of waveguide 108 provides polarization conversion of the light. The following description is made in the context of linearly polarized light incident on the light polarization converter 100 , as indicated by symbol 118 .

[0028] FIG. 2 illustrates a transverse electrical (TE) polarization axis 202 and a transverse magnetic (TM) polarization axis 204 . Symbol 118 is included in Figure 2 to indicate the direction of light propagation into the page (perpendicular to both TE polarization axis 202 and TM polarization axis 204). With respect to the cross-section of FIG. 1 , the TE polarization axis 202 is parallel to the first axis 114 and the TM polarization axis 204 is parallel to the second axis 116 .

[0029] For linearly polarized light, the direction of the electric field for light propagation as indicated by symbol 118 may be indicated relative to the TE polarization axis 202 and the TM polarization axis 204 . Arrow 206 indicates TE polarized linearly polarized light.

[0030] The waveguide 108 has sections offset in a direction parallel to the second axis 116 (and thus in a direction parallel to the TM axis 204), and the intermediate waveguide section 128 is aligned with respect to the second waveguide section 112 and the first waveguide section 110. The angle provides a single waveguide with portions offset in a direction parallel to the second axis 116 . This results in mixed modes within the waveguide 108 . This configuration of waveguide 108 provides a "skew" or sloped boundary condition for light propagating within waveguide 108, thereby providing a mixed mode. This is because light propagates within a single waveguide with portions offset in a direction parallel to the vertical axis 116 .

[0031] The skew boundary condition results in the existence of a first mixed mode with an electric field skewed relative to the TE axis, since the geometry of the waveguide 108 has portions offset in a direction parallel to the second axis 116 . The first mixed mode occupies the first waveguide section 110 , the second waveguide section 112 , and the intermediate waveguide section 128 . The terms "skew," "angled," and "oblique" are used interchangeably herein to refer to the angle of the first mixing mode relative to the TE axis 202 . There is also a second blend mode orthogonal to the first blend mode. A mixed mode as referred to herein is an optical mode having an electric field with a non-zero component along the TE polarization axis 202 and a non-zero component along the TM polarization axis 204 .

[0032] In the example of FIG. 2 , a first blending mode 208 and a second blending mode 210 are shown. First mixing mode 208 and second mixing mode 210 illustrate one example of mixing modes that may exist within waveguide 108 as light propagates within waveguide 108 . In this example, the first mixed mode 208 and the second mixed mode 210 are TE-polarized (with an electric field along the TE polarization axis 202 ) having light incident on the optical polarization converter 100 for propagation through the waveguide 108 caused, as shown by arrow 206.

[0033] In the example of FIG. 2, it is assumed that the tilt angle (relative to TE axis 202) of the first hybrid mode 208 is 45 degrees. Such an angle of inclination of the first mixed mode 208 results, for example, from the specific configuration of the first waveguide portion 110 and the second waveguide portion 112 relative to the second axis 116 and the angle at which the third surface 120 is inclined. In these examples, the angle with respect to the TE axis 202 of the second hybrid mode 210 is also 45 degrees, and the first mode 208 and the second mode 210 have electric fields of equal magnitude. Those skilled in the art will appreciate that the first mode 208 and the second mode 210 having a tilt angle of 45 degrees and the phase relationship shown in FIG. 2A have equal but opposite components along the TM polarization axis 204 and in combination correspond to TE polarized light.

[0034] Furthermore, the illustrated configuration of the waveguide 108 results in a different propagation constant for the first mixed mode 208 and the second mixed mode 210 . The configuration of waveguide 108 results in birefringence such that first mixed mode 208 and second mixed mode 210 experience different effective indices of refraction from each other as they propagate within waveguide 108 . This means that when the first mixed mode 208 and the second mixed mode 210 propagate through the waveguide 108, the phase difference between the first mixed mode 208 and the second mixed mode 210 will change. In other words, when the first mode 208 and the second mode 210 propagate in the waveguide 108, the phases of the first mode 208 and the second mode 210 evolve in different ways. The presence of mixed modes and their different propagation constants provides that the configuration of waveguide 108 can be used to convert the polarization of light input into optical polarization converter 100 . The manner of polarization conversion will be described in more detail below.

[0035] In some examples, light polarization converter 100 is used to switch between a first linear polarization of light at a given wavelength and a second linear polarization of light at the given wavelength. In some of these examples, a length of the waveguide 108 in the direction of light propagation is substantially equal to an odd integer multiplied by half the length of a beat frequency for a given wavelength of light.

[0036] FIG. 3 schematically illustrates a side-view cross-section of a light polarization converter 100 according to an example. The example in FIG. 3 corresponds to the example in FIG. 1 . The cross-section of FIG. 3 is taken along the line A-A indicated in FIG. 1 . It should be noted that FIG. 3 is a schematic illustration and should not be used to indicate a precise scale relative to FIG. 1 , which is also a schematic diagram. In these examples, waveguide 108 has a length 302 as shown in FIG. 3 . The direction of light propagation is indicated by arrow 304 .

[0037] The length over which the phase of the propagating optical mode in the waveguide 108 is to be recovered is called the beat length. For example, if the first mode 208 and the second mode 210 start their in-phase propagation within the waveguide 108, for example, after propagating within the waveguide 108 by an integer multiple of the beat frequency length, they will return to the in-phase mode. By selecting the waveguide length in the optical polarization converter 100 based on the beat length, the relative phase of the light modes propagating therein can be controlled for the light output from the optical polarization converter 100 .

[0038] As discussed herein, the illustrated configuration of the waveguide 108 results in a different propagation constant for the first mixed mode 208 and the second mixed mode 210 . The illustrated configuration of the waveguide 108 , with portions offset in a direction parallel to the vertical axis 116 , results in birefringence because the first mixed mode 208 and the second mixed mode 210 experience effective indices of refraction that are different from each other. The propagation constant of the first mixed mode 208 in the waveguide 108 may be represented by β 208 and the propagation constant of the second mixed mode 210 may be represented by β 210. The difference between these propagation constants can be expressed as ∆ β = β 208 - β 210. Equation 1 below shows the beat length Lλ of the waveguide 108 for the first hybrid mode 208 and the second hybrid mode 210. Those skilled in the art will understand that β denotes phase propagation. In equation (1) below, λ is a given wavelength, and Δn represents the difference in effective refractive index of the first mixed mode 208 and the second mixed mode 210: Δn=n 208−n 210. (1)

[0039] In some examples, light polarization converter 100 is used to rotate the linear polarization of light of a given wavelength. As set forth above, in some such examples, waveguide length 302 is substantially (within acceptable tolerances) equal to an odd integer times one-half the beat length. This is the case in the example where the tilt angle of the first mode 208 is 45 degrees relative to the TE axis 202 . The factors for determining skew will be further elaborated below. In other words, the waveguide length may be 1 / 2 the beat length, 3 / 2 the beat length, 5 / 2 the length, etc., as indicated by Equation 2 below. In Equation 2, m represents an odd integer such as 1, 3, 5, 7, 9 and so on. (2)

[0040] This means that the relative phases of the first mode 208 and the second mode 210 after propagating through the waveguide 108 are shifted by π radians. Those skilled in the art will appreciate that when TE polarized light as indicated by arrow 206 is incident on waveguide 108 and first mode 208 and second mode 210 appear at the beginning of waveguide 108, first mode 208 and second mode Patterns 210 will be in phase with each other. In other words, the first mode 208 and the second mode 210 are different from each other after propagating through the waveguide 108 at a waveguide length of 1 / 2 the beat length (or 3 / 2 the beat length, or 5 / 2 the beat length, etc.). Phase 180 degrees.

[0041] Figure 2B illustrates the TE polarization axis 202 and the TM polarization axis 204 shown in Figure 2A. FIG. 2B relates to an example where linear TE polarized light is incident on the waveguide 108 (as indicated by FIG. 2A ) and the tilt angle of the first mixed mode 208 relative to the TE axis 202 is 45 degrees. 2B illustrates the first mode 208 and the second mode 210 after having propagated through the waveguide 108 in the direction indicated by symbol 118 for an integer multiple of half the beat length. Those skilled in the art will understand that the electric field of light oscillates between opposite quadrants of space indicated by the TE polarization axis 202 and the TM polarization axis 204 . Thus, compared to the state shown in FIG. 2A , in FIG. 2B the first mode 208 has completed its electric field oscillation by an integer multiple, while the second mode 210 has completed its electric field oscillation by an odd integer multiple of half ( ends in the upper left quadrant).

[0042] Those skilled in the art will appreciate that the first mode 208 and the second mode 210 having the phase relationship shown in FIG. 2B have equal but opposite components along the TE polarization axis 202 . If the first mode 208 and the second mode 210 exit the waveguide 108 in a phase relationship as shown in FIG. 2B, the first mode 208 and the second mode 210 having the phase relationship as shown in FIG. TM polarized light indicated by 212. In other words, if the phases of the first mode 208 and the second mode 210 no longer evolve differently from the point shown in Figure 2B, the result is TM polarized light. It should be noted that in the example of FIG. 2B , the first mode 208 and the second mode 210 have a 45 degree angle with respect to the TM axis 204 .

[0043] In these examples, where the first hybrid mode 208 has an angle of 45 degrees with respect to the TE axis 202, by choosing the waveguide length to be an odd integer multiple of half the beat length, it is possible as explained above rotates the linear polarization of light of a given wavelength. For example, a first linear polarization (TE polarization in the example above) can be converted to a second linear polarization (TM polarization in the example above).

[0044] The above example is in the context of the first hybrid mode 208 having a 45 degree skew relative to the TE axis 202 . In some examples, the substrate 102 and waveguide 108 are configured such that when a mixed mode occurs within the waveguide 108, the first mixed mode 208 (e.g., the mixed mode substantially aligned with the intermediate waveguide portion 128) has no One of 202 is at an angle of 45 degrees. Factors affecting the skew of the first blend mode will be discussed further below. In such instances, a 90 degree rotation of polarization (eg, from TE polarization to TM polarization) does not occur when propagating through a length 302 equal to an odd integer times one-half the beat length.

[0045] In some examples, an optical polarization converter (eg, optical polarization converter 100 ) is provided wherein the length 302 of the waveguide 108 is equal to an odd integer times a quarter of the beat length. As explained below, to obtain a 90 degree linear polarization rotation, a pair of such optical polarization converters can be used at skews other than 45 degrees. A first optical polarization converter of the pair has a length 302 of the waveguide 108 that achieves a phase difference of 90 degrees (π / 2 radians) between the first mixed mode and the second mixed mode. This means that the length 302 can be 1 / 4 of the beat length, 5 / 4 of the beat length, 9 / 4 of the beat length, and so on. A second optical polarization converter of the pair has a length 302 of the waveguide 108 that achieves a phase difference of 270 degrees (3 / 2 π radians) between the first mixed mode and the second mixed mode. This means that the length 302 of the waveguide in the second optical polarization converter can be 3 / 4 of the beat length, 7 / 4 of the beat length, 11 / 4 of the beat length, and so on.

[0046] Figure 10 is a sketch of one of the equatorial planes of a Poinca sphere. Those skilled in the art will understand that all polarization states can be mapped onto the surface of a so-called Poineck sphere. Points lying on the equator of the Poyinka sphere represent all angles of linear polarization. The poles of the Poyinka sphere represent clockwise circular polarization and counterclockwise circular polarization. Points corresponding to TE and TM polarizations can be seen in FIG. 10 .

[0047] The points corresponding to the first blending mode 208 and the second blending mode 210 lie on the equator of the Poyinca sphere. The position of the mixed mode on the equator depends on the skew (in other words, the angle) of the first mixed mode relative to the TE axis 202 .

[0048] Where the first mixing mode has an angle of 45 degrees relative to the TE axis 202, the first mixing mode corresponds to point M1 and the second mixing mode corresponds to point M2. The axis spanning M1 and M2 is perpendicular to the axis spanning the TE and TM polarization points on the equator of the Poincar sphere. Propagation of the mixed modes through waveguide 108 , where their phases evolve differently from each other, corresponds to a rotation across the axes of M1 and M2 of a point representing the polarization when the mixed modes recombine. A 180 degree rotation about an axis spanning M1 and M2 results in a polarization conversion, for example, from TE polarization to TM polarization.

[0049] In cases where the first hybrid mode has an acute angle other than 45 degrees with respect to the TE axis 202, the points corresponding to the first and second hybrid modes are not points M1 and M2. In some examples, first blend mode 208 corresponds to point 1002 and second blend mode 210 corresponds to point 1004. In these examples, a rotation of 180 degrees about an axis intersection point 1002 and 1004 does not reach the TM polarization point. As explained above, a 180 degree phase difference between the first mixed mode and the second mixed mode does not provide a 90 degree rotation of the linear polarization when the tilt angle is different from 45 degrees.

[0050] However, a rotation of 90 degrees around the intersection of the axes 1002 and 1004 would result in point 1006 . Point 1006 is a point on the surface of the Poinca sphere at the top of the page of FIG. 10 . In some examples, a first light polarization converter is used to obtain a polarization corresponding to a point 1006 on the Poyncar sphere. In these examples, the first light polarization converter provides a mixed mode with polarizations corresponding to points 1002 and 1004. In other words, the first polarization converter provides a skew such that the mixed mode has polarizations corresponding to points 1002 and 1004.

[0051] The second polarization converter of the discussed example provides a skew in the opposite direction such that the mixing mode in the second light polarization converter corresponds to points 1008 and 1010. This can be achieved by means of a second polarization converter whose side-view cross-section (corresponding to the side-view cross-section shown in FIG. 1 ) is a side-view cross-section of the first light polarization converter. (corresponding to the side view cross-section shown in Figure 1). For example, in FIG. 1 , the first surface 104 (lower surface) is on the left-hand side of FIG. 1 and the second surface 106 is on the right-hand side. As used with respect to a side-view cross-section, mirror image means that the first surface 104 (lower surface) is instead on the left-hand side and the second surface 106 (upper surface) is on the right-hand side, where the direction of light propagation remains the same (as indicated by the symbol 118 to enter the page as indicated). In the example in which FIG. 1 shows a side cross-section of a first light polarization converter, FIG. 11 schematically illustrates the same side as that side cross-section of the example 1100 of the second light polarization converter of FIG. View cross-section. In FIG. 11 , features corresponding to those shown in FIG. 1 are labeled with similar reference numerals, with the addition of an extra numeral "-11" at the end.

[0052] An axis intersection point 1008 and 1010 is a mirror image of an axis intersection point 1002 and 1004 with respect to the axis intersection points M1 and M2. Line 1012 represents a trajectory from point TE to point 1006 after a 90 degree rotation around the intersection of axes 1002 and 1004 . It should be noted that line 1012 is a rectilinear projection of a trajectory on the Poinca sphere that would follow the surface of the Poinca sphere.

[0053] A rotation about axis intersections 1008 and 1010 (including point 1006) outlines a circle on the surface of the Poyinca sphere that spans the point TM. Rotation in one of opposite directions from point 1006 and about axis intersections 1008 and 1010 may be used to arrive at point TM. A second polarization converter having a mirrored side-view cross-section provides rotation relative to the Poincar sphere in the opposite direction to that of the first optical polarization converter. Line 1014 represents a rectilinear projection of a trajectory starting at point 1006, corresponding to a rotation about axis intersections 1008 and 1010 in a direction opposite to the illustrated 90 degree rotation at point 1006. After a rotation of 270 degrees about axis intersections 1008 and 1010, trajectory 1014 crosses point TM.

[0054] In this way, even in instances where the skew of the first mixed mode is not 45 degrees, one of the linear polarizations can be achieved by propagating light through the first light polarization converter and then through the second light polarization converter. 90 degree rotation.

[0055] In some examples, light polarization converter 100 is used to switch between linear and circular polarization of light of a given wavelength. In such examples, the waveguide length 302 is substantially (within acceptable tolerances) equal to an odd integer times a quarter of the beat length. In these examples, the first hybrid mode is skewed by 45 degrees relative to the TE axis 202. The waveguide length 302 according to such examples is represented by Equation 3 below. (3)

[0056] Propagating the first mixed mode 208 and the second mixed mode 210 in an odd integer multiple of one quarter of the beat length in the waveguide 108 will result in a phase difference of 1 / 2 π radians (or 90 degrees). Those skilled in the art will appreciate that the introduction of a phase difference of 90 degrees around the axis intersections M1 and M2 provides switching between linear and circular polarization. Taking linearly polarized light in the horizontal direction incident on the waveguide 108 as indicated in FIG. . In instances where circularly polarized light is incident on a light polarization converter for converting between linear and circular polarization, circularly polarized light will be converted to linearly polarized light.

[0057] In the example of FIG. 1 , the substrate 102 includes a first substrate layer 134 and a second substrate layer 136 . The first substrate layer 134 includes the first surface 104 and the second substrate layer 136 includes the second surface 106 . In some examples, first substrate layer 134 and second substrate layer 136 include the same material. In the example of FIG. 1 , the substrate 102 includes a third layer 138 between the first substrate layer 134 and the second substrate layer 136 . The first, second and third layers of the substrate 102 are stacked such that the first layer 134 is in contact with the third layer 138 and the third layer 138 is in contact with the second layer 136 . In other words, relative to the orientation shown in FIG. 1 , second layer 136 is on third layer 138 , and third layer 138 is on first layer 134 . The third layer 138 includes a material different from that of the first substrate layer 134 and the second substrate layer 136 .

[0058] In these examples, the offset of the second surface 106 from the first surface 104 in a direction parallel to the second axis 116 is defined by the combined thickness of the third layer 138 and the second substrate layer 136 . The total thickness of the third layer 138 and the second substrate layer 136 may be referred to as a step thickness. The total thickness of any layers defining the second surface 106 offset from the first surface 104 in a direction parallel to the second axis 116 may be referred to as a step thickness.

[0059] As explained above, the first mixed mode 208 occupies the first waveguide portion 110 , the second waveguide portion 112 and the intermediate waveguide portion 128 . In some examples, the first hybrid mode 208 is substantially aligned with the intermediate waveguide portion 128 so as to have a skew with respect to the TE axis that is similar to the angle of the intermediate waveguide portion 128 . It should be noted that the skew of the first mixed mode 208 will depend on the width of the waveguide 108 and the step thickness (assuming the third surface 120 has a fixed given angle). In an example, the effective refractive index difference Δn of the first mixed mode 208 and the second mixed mode 210 will also depend on the width of the waveguide 108 and the step thickness (assuming the third surface 120 has a fixed given angle). Therefore, the width and step thickness of the waveguide 108 can be selected to obtain a desired tilt angle and effective refractive index difference. It is evident from equation (1) that the effective refractive index difference is related to the beat length Lλ and thus will affect the length 302 of the waveguide 108 depending on the desired application.

[0060] In the example of FIGS. 1 and 3 , the light polarization converter includes one or more cladding layers 130 covering the waveguide 108 . In some examples, the one or more cladding layers 130 include an uneven surface on a first side opposite a second side that is closer to the waveguide 108 than the first side. For example, a cladding layer 130 is in contact with the top surface of each portion of the waveguide 108 . The one or more cladding layers include an uneven surface 140 on a first side opposite a second side closer to (eg, facing) the waveguide 108 . The light polarization converter 100 may include other layers, such as an electrical contact layer 132 on top of the cladding layer 130 (relative to the orientation of FIG. 1 ). As can be seen from the example of FIG. 1 , the top surface of the light polarization converter 100 is not perfectly flat due to an offset in a direction parallel to the vertical axis 116 of the substrate 102 and waveguide 108 .

[0061] FIG. 12 is a graph 1200 indicating the tilt angle of the first mixed mode as a function of waveguide width W (see FIG. 1 ). It should be noted that the waveguide width is along the first axis 114 . The vertical axis indicates the tilt angle of the first mixed mode 208 with respect to the TE axis 202, and the horizontal axis indicates the waveguide width W. Graph 1200 shows a plurality of relationships for respective plurality of step thicknesses. Step thicknesses are indicated in nanometers in FIG. 12 . Line 1202 indicates one of the first blend modes with an inclination angle of 45 degrees. The relationship shown in graph 1200 is for an example of a light polarization converter similar to light polarization converter 100 of the example of FIG. 1 . The third layer 138 and the electrical contact layer 132 are not included. The thickness of waveguide 108 (in the perpendicular direction relative to the orientation shown in FIG. 1 ) is 500 nm, the refractive index of substrate 102 is 3.17, and the refractive index of cladding layer 130 is 3.17. The index of refraction of waveguide 108 is 3.364, and the index of refraction on either side of the light polarization converter (in the orientation shown in FIG. 1 ) is 1.5. The angle of the intermediate waveguide section is 35 degrees.

[0062] FIG. 13 is a graph 1300 indicating Δn as a function of waveguide width. The vertical axis indicates Δn, and the horizontal axis indicates the waveguide width. Graph 1300 shows a plurality of relationships for respective plurality of step thicknesses as indicated. The relationships shown in graph 1300 are for the same parameters set forth above with respect to FIG. 12 .

[0063] Those skilled in the art will appreciate that the desired characteristics of the optical polarization converter can be obtained by selecting, for example, waveguide width, step thickness, etc. based on data such as shown in FIGS. 12 and 13 .

[0064] Fig. 4 schematically illustrates a side view cross-section of a light polarization converter 400 according to an example. In FIG. 4, features corresponding to those shown in FIG. 1 are labeled with similar reference numerals, but with an extra numeral "-4" added at the end.

[0065] Light polarization converter 400 corresponds to light polarization converter 100 (and may include any combination of features set forth above in relation to light polarization converter 100), except for the following differences. The substrate 102-4 of the light polarization converter 400 has a third substrate surface 120-4 between the first surface 104-4 and the second surface 106-4, wherein the third substrate surface 120-4 is opposite to the first surface 104 -4 while tilted. The third surface 120-4 is at a substantially 90 degree angle (within acceptable tolerances) relative to the first surface 104-4. In these examples, the third substrate surface 120-4 corresponds to a sidewall of the second substrate layer 136-4. In the example of FIG. 4, there is no third layer corresponding to third layer 138 set forth above.

[0066] Due to the angle of the third surface 120-4 in these examples, the light polarization converter 400 of FIG. 4 does not include an intermediate waveguide section. The first waveguide section 110-4 is continuous with the second waveguide section 112-4 instead of having an intermediate inclined waveguide section therebetween. As in the case of the example of FIG. 1 , waveguide 108-4 of light polarization converter 400 is a single waveguide for light propagation.

[0067] The waveguide 108-4 of the example of FIG. 4 includes a second waveguide portion 112-4 that is offset relative to the first waveguide portion 110-4 in a direction parallel to the second axis 116-4, and is aligned with the first waveguide portion 110-4. 4 consecutively, as shown in Figure 4.

[0068] The configuration of waveguide 108-4 results in mixed modes within waveguide 108-4. Similar to waveguide 108 in the example of FIG. 1, this configuration of waveguide 108-4 provides a "skew" or sloped boundary condition for light propagating within waveguide 108-4, thereby providing a mixed mode. This is because the light propagating in the waveguide 108 - 4 occupies mutually contiguous waveguide regions offset in a direction parallel to the second axis 116 . Additionally, the illustrated configuration of waveguide 108-4 results in a different propagation constant for one of the different mixed modes. The configuration of waveguide 108-4 provides birefringence such that the first mixed mode and the second mixed mode experience different indices of refraction from each other within waveguide 108-4. For example, for light incident on the light polarization converter 400 for propagation through the waveguide 108-4 having a TE polarization as shown by arrow 206 of FIG. propagation constants (and thus different phase evolutions).

[0069] Both the waveguide 108 of the example of FIG. 1 and the waveguide 108-4 of the example of FIG. 4 produce a mixed mode and a different propagation constant for the mixed mode. As explained above, the presence of mixed modes and their different propagation constants provides the polarization conversion of light. Thus, similar to the light polarization converter 100 of the example of FIG. 1 , the light polarization converter 400 of the example of FIG. 4 can be used to convert the polarization of incident light.

[0070] Either the example of FIG. 1 or the example of FIG. 4 may be used depending on various factors such as complexity and cost of fabrication and desired level of control over the mixing modes and their propagation constants. For example, the presence of an angled (slanted) intermediate waveguide portion 128 provides finer tuning of mixed-mode characteristics such as skew. However, the example of Figure 4 may be simpler and / or cheaper to manufacture since there are no sloped surfaces to manufacture.

[0071] FIG. 5 is a block diagram illustrating a method 500 of fabricating an optical polarization converter for a PIC. Method 500 is, for example, a method of manufacturing a light polarization converter according to any of the examples set forth above. At block 502 of method 500, a substrate including a first surface and a second surface is at least partially 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 the polarization of light. The substrate is, for example, the substrate 102, the substrate 102-4, or the substrate 102-11.

[0072] At block 504 of method 500, 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 partially formed. The second surface is offset from the first surface such that the second waveguide portion is offset from the first waveguide portion. The waveguides are, for example, waveguide 108, waveguide 108-4, and waveguide 108-11 described above. More specific aspects of the method 500 are further described below.

[0073] In some examples, at least partially forming the substrate includes at least partially forming a first substrate layer and at least partially forming a second substrate layer, wherein the first substrate layer includes the first surface, and the second substrate layer includes the second substrate layer. surface. FIG. 6 is a block diagram illustrating a method 600 . Method 600 pertains to the examples of FIGS. 1 , 3 and 11 and may be used to at least partially form second substrate layer 110 or second substrate layer 110 - 11 shown in FIGS. 1 and 11 , respectively. The following description of method 600 refers to the examples of FIGS. 1 and 3 , however, for the example of FIG. 11 , corresponding blocks may be performed.

[0074] At block 602 of method 600, a third layer (eg, third layer 138) is deposited on a top surface of first substrate layer 134 to serve as a stop layer. In the context of method 600, at least partially forming the second substrate layer includes the following steps. At block 604 , a substrate material is deposited on top of the stop layer 138 . The substrate material is any of the materials set forth above with respect to the substrate 102 . For example, the substrate material is the same material included in the first substrate layer 134 .

[0075] FIG. 7A schematically illustrates a side cross-section of a structure obtained from performing blocks 602 and 604 . Figure 7A is a schematic diagram and should not be considered to represent an exact scale with respect to the examples set forth above.

[0076] At block 606 of method 600 , a mask layer is deposited to cover a first portion of the substrate material on top of stop layer 138 . FIG. 7B schematically illustrates a side cross-section of a structure resulting from performing block 606 . In the example of FIG. 7B , a masking layer 702 deposited to cover a first portion 704 of substrate material is shown. In some examples, mask layer 702 includes a dielectric material. In some examples, mask layer 702 is a hard mask comprising SiNx or SiOx, where "x" indicates a different composition that may be used, such as Si3N4. In some examples, mask layer 702 is a photoresist.

[0077] At block 608 of method 600, a wet etch process is performed to remove material from a second portion 706 of the substrate material that is different from the first portion 704 and not covered by the mask layer. In an example, the stop layer 134 includes a material that cannot be etched by an etchant that is used to etch to remove material not covered by the mask layer. In some examples, stop layer 134 includes a quaternary material. FIG. 7C schematically illustrates a side cross-section of a structure resulting from performing block 608 . Block 608 is performed to remove material from the second portion 706 to provide an exposed portion 708 of the stop layer 138 .

[0078] At block 610 of method 600, exposed portion 708 of stop layer 138 is removed. FIG. 7D schematically illustrates a side cross-section of a structure obtained from performing block 610 . Removing the exposed portion 708 of the stop layer 138 results in at least partially forming the second surface 106 of the substrate. In the particular example of FIG. 7D, all exposed portions of stop layer 138 are removed.

[0079] In some examples, the substrate material is InP. In some of these examples, the wet etch procedure was performed using HCl:H3PO4:H2O. In some examples, a mixture of one of HCL, H3PO4, and H2O is selected to etch the desired material (InP, in these examples). In other examples, only a mixture of HCL and HO was used as the etchant. As stated, performing a wet etch process provides an intermediate substrate surface (such as the third substrate surface 120 ) at an angle less than 90 degrees relative to the first surface 104 . Those skilled in the art will appreciate that the angle of the third substrate surface 120 will depend on a combination of the crystal structure of the substrate material, the etch chemistry and details of the process used.

[0080] In some instances, an instance of block 504 for at least partially forming waveguide 108 is employed on the structure resulting from method 600 . In some examples, masking layer 702 is removed, as shown in the schematic illustration of FIG. 7E. Removing the mask layer 702 provides the second surface 106 included in the second substrate layer 136 .

[0081] In these examples, one or more materials for waveguide 108 are deposited on top of substrate 102 . For example, waveguide 108 is epitaxially grown using the waveguide-related materials described above. Due to the configuration of the substrate 102 (eg, as shown in FIG. 7E ), the waveguide 108 has a portion offset in a direction parallel to the second axis 116, and since the third substrate surface 120 is at an angle as set forth above And an intermediate waveguide section with an angle.

[0082] In some examples, other layers are deposited on top of the waveguide 108 , such as one or more cladding layers 130 and an electrical contact layer 132 . These layers can be seen in the schematic illustration of Figure 7G.

[0083] In an example, the structure of FIG. 7G is subjected to, for example, a dry etch process to remove material from either side of the structure to a specific depth desired according to the intended application. For example, a dry etch process is performed to remove material and form the light polarization converter 100 of the example of FIGS. 1 and 3 . 7A-7G show a cross-section of the same final light polarization converter 100 as shown in FIG. 1 .

[0084] FIG. 8 illustrates one block of a method 800 for at least partially forming a second substrate layer. Method 800 relates to the example of FIG. 4 and may be used to at least partially form second substrate layer 136-4 shown in FIG. The following description of method 800 refers to the example of FIG. 4 . As used herein, a layer refers to material occupying a volume within a given range of values ​​relative to the vertical axis 116-4.

[0085] FIG. 9A schematically illustrates a side cross-section of a structure related to method 800 . At block 802 of method 800, a mask layer 902 is deposited to cover a portion 904 of a top surface 906 of a first substrate layer (e.g., first substrate layer 134-4) and leave an exposed portion 908 of the top surface 906 . FIG. 9A schematically illustrates a side cross-section of a structure obtained from performing block 802 .

[0086] At block 804 of method 800, a substrate material is deposited on exposed portion 908 of top surface 906 to at least partially form second substrate layer 136-4. The substrate material is any of the materials described above with respect to substrate 102 or substrate 102-4. For example, the substrate material is the same material included in the first substrate layer 134-4. FIG. 9B schematically illustrates a side cross-section of a structure resulting from performing block 804 . At block 806 of method 800, mask layer 902 is removed. FIG. 9C schematically illustrates a side cross-section of a structure obtained from performing block 806 . Removing the mask layer 902 exposes a portion of the surface of the first substrate layer 134-4 to at least partially form the first surface 104-4.

[0087] FIG. 14 is a block diagram illustrating a method 1400 for at least partially forming a substrate. Method 1400 relates to the example of FIG. 4 and may be used to at least partially form first substrate layer 134-4 and second substrate layer 136-4 shown in FIG. The following description of method 1400 refers to the example of FIG. 4 . Method 1400 is an alternative to method 800 described above. As used in this context, a layer refers to material occupying a volume within a given range of values ​​relative to the vertical axis 116-4.

[0088] FIG. 15A schematically illustrates a side cross-section of a structure related to method 1400 . At block 1402 of method 1400 , a mask layer 1402 is deposited to cover a first portion 1404 of a top surface 1406 of a layer of substrate material 1408 . FIG. 15A schematically illustrates a side cross-section of a structure resulting from performing block 1402 .

[0089] At block 1404 of method 1400, material is removed from a second portion 1410 of the substrate material layer 1408 that is distinct from the first portion 1404 and not covered by the masking layer 1402 to provide the first substrate including the first surface 104-4. Layer 134-4. FIG. 15B schematically illustrates a side cross-section of a structure resulting from performing block 1404 .

[0090] At block 1406 of method 1400, mask layer 1402 is removed to provide second substrate layer 136-4 including second surface 106-4. FIG. 15C schematically illustrates a side cross-section of a structure resulting from performing block 1406 .

[0091] It should be appreciated that the first substrate layer 134 - 4 and the second substrate layer 136 - 4 are provided by the substrate material layer 1408 . It should be appreciated that the starting point for the method 1400 is a layer of substrate material 1408 having a thickness corresponding to the total desired thickness of the first substrate layer 134-4 and the second substrate layer 136-4.

[0092] In some instances, an instance of block 504 for at least partially forming waveguide 108 is employed on the structure resulting from method 800 or method 1400 . In such examples, one or more materials for waveguide 108-4 are deposited on top of substrate 102-4. For example, waveguide 108-4 is epitaxially grown using the waveguide-related materials described above. Due to the configuration of the substrate 102-4 (eg, as shown in FIG. 9C ), the waveguide 108-4 has offset portions (in a direction parallel to the vertical axis 116-4) and as above with respect to the waveguide 108-4 The stated grounds are in contact with each other. For example, one or more cladding layers, such as cladding layer 130, and an electrical contact layer 132 are deposited on top of waveguide 108-4.

[0093] In an example, depending on the intended application, a one-step etch procedure is performed to remove material from either side of the structure, down to a specific depth as desired. For example, a one-step etch process is performed to remove material and form the light polarization converter 400 of the example of FIG. 4 .

[0094] In an example, the method 500 illustrated in FIG. 5 includes fabricating an optical polarization converter such that for a given wavelength of light in the waveguide, the waveguide has a waveguide length that is a function of the beat length (as set forth above), where the length of the waveguide is along an axis parallel to a direction in which light propagates in the waveguide. In some instances, the waveguide length is substantially equal to an odd integer times one-half the beat length. In some instances, the waveguide length is substantially equal to an odd integer times a quarter of the beat length. As set forth above, one or more of the polarization converters fabricated using the described methods can be used to achieve polarization conversion as desired.

[0095] In some examples, there is provided a PIC comprising a light polarization converter according to any one of the illustrated examples.

[0096] In the above description, reference was made to at least partially forming layers and the like. In certain instances, a layer referred to in this way is simply formed by depositing the relevant material without further steps. In other examples, further steps are performed to complete the formation of the layer (eg, a curing step, an etching step to define the extent of the layer, etc.). In certain instances, further steps to complete the formation of a layer in question are performed before further material is deposited on top of that layer. In other examples, further steps to complete the formation of a layer in question are performed after further material is deposited on top of that layer.

[0097] In the illustrated figures, dashed lines are included at the edges of certain parts to indicate the continuation of the part in question beyond that schematically illustrated in the figures. The figures contain schematic illustrations of structures related to illustrated examples of light polarization converters. None of the figures should be considered to be indicative of exact scale relative to any other figure.

[0098] As will be appreciated by those skilled in the art, various techniques can be used to deposit a layer of semiconductor material in light of the examples set forth herein. Such a technique may be called a regrowth technique, for example a metal organic vapor phase epitaxy (MOVPE) or a molecular beam epitaxy (MBE) process may be used.

[0099] The above examples are to be understood as illustrative examples of the invention. It should be understood that any feature described in relation to any one example may be used alone or in combination with other features described, and may also be used in any combination with any other of those examples or any other of these examples One or more of the features used in combination. Furthermore, equivalents and modifications not set forth above may also be employed without departing from the scope of the appended claims.

[0100] 100: Light Polarization Converter / Final Light Polarization Converter 102: Substrate 102-4: Substrate 102-11: Substrate 104: first surface / lower surface 104-4: first surface 106: second surface / upper surface 106-4: second surface 108: waveguide 108-4: Waveguide 108-11: Waveguide 110: second substrate layer / first waveguide part 110-4: The first waveguide part 110-11: Second substrate layer 112: The second waveguide part 112-4: Second waveguide part 114: First axis / horizontal axis 116: Second axis / vertical axis 116-4: Second axis / vertical axis 118: symbol 120: third surface / inclined third surface / third substrate surface 120-4: third surface / third substrate surface 122: width 124: position 126: position 128: Intermediate waveguide part 130: cladding layer 132: electrical contact layer 134: first layer / stop layer / first substrate layer 134-4: The first substrate layer 136: Second layer / Second substrate layer 136-4: Second substrate layer 138:Third layer / stop layer 140: non-flat surface 202: Transverse Electrical Axis / Transverse Electrical (TE) Polarization Axis 204: Transverse Magnetic Axis / Transverse Magnetic (TM) Polarization Axis 206: Arrow 208: First Mode / First Hybrid Mode 210: Second Mode / Second Hybrid Mode 212: Arrow 302: length / waveguide length 304: arrow 400: Light Polarization Converter 500: method 502: block 504: block 600: method 602: block 604: block 606: block 608: cube 610: block 702: mask layer 704: Part I 706: Part II 708: Exposed part 800: method 802: block 804: block 806: cube 902: mask layer 904: part 906: top surface 908: Exposed part 1002: point / axis intersection 1004: point / axis intersection 1006: point 1008: point / axis intersection 1010: point / axis intersection 1012: line 1014: line / track 1100: instance 1200:chart 1300: chart 1400: method 1402: block / mask layer 1404: Blocks / Part 1 1406: block / top surface 1408: substrate material layer 1410: Part Two A-A: line M1: point / axis intersection M2: point / axis intersection W: waveguide width

Claims

1. An optical polarization converter for a photonic integrated circuit, comprising: A substrate including a first surface and a second surface, the substrate including a first substrate layer and a second substrate layer, the first substrate layer including the first surface and the second substrate layer including the second surface; and a waveguide including a first waveguide portion in contact with the first surface and a second waveguide portion in contact with the second surface, wherein the second surface is offset from the first surface along a first axis and a second axis perpendicular to a light propagation direction to convert the polarization of light, such that the second waveguide portion is offset from the first waveguide portion.

2. As in request item 1, an optical polarization converter, wherein: The substrate includes a third surface located between the first surface and the second surface, the third surface being inclined relative to the first surface.

3. As in request item 2, an optical polarization converter, wherein: The third surface is substantially at a 90-degree angle relative to the first surface.

4. As in request item 2, an optical polarization converter, wherein: The first axis system is substantially parallel to the plane of the first surface; the third surface system forms an angle of less than 90 degrees with respect to the first surface; the second surface is offset from the first surface along the first axis by an amount greater than the width of the first surface along the first axis; and the waveguide includes an intermediate waveguide portion that contacts the third surface and is located between the first waveguide portion and the second waveguide portion.

5. An optical polarization converter as described in any of requests 1 to 4, wherein: The optical polarization converter is used to convert between a first linear polarization of light of a given wavelength and a second linear polarization of light of the same wavelength; and the length of the waveguide in the direction of light propagation is substantially equal to an odd integer multiplied by half the beat frequency length of light of the given wavelength.

6. An optical polarization converter as described in any of requests 1 to 4, wherein: The length of the waveguide in the direction of light propagation is substantially equal to an odd integer multiplied by one-quarter of the beat frequency length of a given wavelength of light.

7. An optical polarization converter as described in any of requests 1 to 4, wherein: The substrate includes a third layer between the first substrate layer and the second substrate layer. The first layer, the second layer and the third layer are stacked such that the first layer is in contact with the third layer and the third layer is in contact with the second layer. The third layer includes a material different from the materials of the first substrate layer and the second substrate layer.

8. An optical polarization converter as described in any of claims 1 to 4, comprising: One or more cladding layers cover the waveguide, wherein: the one or more cladding layers include a non-flat surface on a first side opposite to a second side, the second side being closer to the waveguide than the first side.

9. A photonic integrated circuit comprising an optical polarization converter as claimed in any one of claims 1 to 8.

10. A method for manufacturing an optical polarization converter for a photonic integrated circuit, the method comprising: A substrate comprising at least partially a first surface and a second surface is formed, comprising at least partially forming a first substrate layer comprising the first surface and at least partially forming a second substrate layer comprising the second surface; and at least partially forming a waveguide comprising a first waveguide portion in contact with the first surface and a second waveguide portion in contact with the second surface, wherein the second surface is offset from the first surface along a first axis and a second axis perpendicular to a light propagation direction to convert the polarization of light, such that the second waveguide portion is offset from the first waveguide portion.

11. The method of claim 10, comprising: Depositing a third layer on the top surface of one of the first substrate layers to serve as a stop layer, and at least partially forming the second substrate layer includes: depositing a substrate material on top of the third layer; depositing a mask layer to cover a first portion of the substrate material on top of the third layer; performing a wet etching process to remove material from a second portion of the substrate material that is different from the first portion and is not covered by the mask layer, thereby providing an exposed portion of the stop layer; and removing the exposed portion of the stop layer and the mask layer.

12. The method of claim 10, wherein forming at least partially the second substrate layer comprises: Deposit a masking layer to cover a portion of the top surface of one of the first substrate layers and leave an exposed portion of the top surface; deposit a substrate material on the exposed portion of the top surface to at least partially form the second substrate layer; and remove the masking layer.

13. The method of claim 10, wherein forming the substrate at least partially comprises: A masking layer is deposited to cover a first portion of a top surface of a substrate material layer; material is removed from a second portion of the substrate material layer that is different from the first portion and is not covered by the masking layer to provide the first substrate layer including the first surface; and the masking layer is removed to provide the second substrate layer including the second surface.

14. The method of any one of claims 10 to 13, comprising manufacturing: the optical polarization converter such that a length of the waveguide in the direction of light propagation is substantially equal to an odd integer multiplied by half the beat frequency length of a given wavelength of light.

15. The method of any one of claims 10 to 13, comprising manufacturing the optical polarization converter such that a length of the waveguide in the direction of light propagation is substantially equal to an odd integer multiplied by one-quarter of the beat frequency length of a given wavelength of light.

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