Polarization Control

Abstract

A polarization control device for a photonic integrated circuit includes a first polarization converter and a second polarization converter. The first polarization converter has a first cross-sectional structure, different effective refractive indices, and supports first and second modes with different polarization orientations. The second polarization converter has a second cross-sectional structure, different effective refractive indices, and supports third and fourth modes with different polarization orientations. A control element changes the effective refractive indices in response to a signal.

Description

[Background technology]

[0001] Polarization control systems can be used in photonic integrated circuits (PICs) to perform polarization control of light. Polarization control systems can be designed and manufactured for use in specific applications within the PIC. For example, a PIC may include a polarization control system for changing light of a first polarization to light of a second polarization. [Brief explanation of the drawings]

[0002] [Figure 1] 1A and 1B show schematic top views of a first light polarization control device according to a first embodiment; [Figure 2] 2A and 2B are schematic diagrams illustrating a cross-sectional structure of a first polarization converter of an optical polarization control device according to a first embodiment; [Figure 3] 2A and 2B are schematic cross-sectional views of first and second polarization converters of an optical polarization control device according to a first embodiment; [Figure 4] 4a and 4b schematically illustrate a first polarization of light and a second polarization of light guided by a polarization converter, according to an embodiment. [Figure 5] 5a and 5b show schematic side cross-sectional views of first and second optical polarization converters according to a first embodiment. [Figure 6] 6a and 6b illustrate schematically the control of the polarization state of light using a polarization control device, according to an embodiment. [Figure 7] 7a and 7b illustrate schematically the control of the polarization state of light using a polarization control device, according to an embodiment. [Figure 8] 10A and 10B schematically show a top view of a polarization control device according to a second embodiment, and a diagram of the Poincaré sphere illustrating the polarization control of the polarization control device according to a second embodiment. [Figure 9] 10A and 10B schematically show top views of light polarization control devices according to further embodiments; [Figure 10] 10A and 10B schematically illustrate cross-sectional structures of polarization converters of optical polarization control devices according to further embodiments; [Figure 11]10 also shows a schematic cross-sectional structure of a polarization converter of an optical polarization control device according to a further embodiment. [Figure 12] 10A and 10B schematically illustrate cross-sectional structures of polarization converters of optical polarization control devices according to still further embodiments; [Figure 13] 10 also shows a schematic cross-sectional structure of a polarization converter of an optical polarization control device according to a still further embodiment. [Figure 14] 2 illustrates a schematic diagram of a system including a photonic integrated circuit featuring the polarization control device of FIG. 1, according to an embodiment. [Figure 15] FIG. 1 is a flow diagram illustrating a method for controlling the polarization state of light in a photonic integrated circuit using a polarization control device described herein, according to an embodiment. [Figure 16] FIG. 10 is a flow diagram illustrating a method of manufacturing a polarization control device as described herein, according to a further embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0003] The embodiments described herein relate to semiconductor structures for PICs. In particular, the embodiments described herein relate to optical polarization control devices for PICs. Such optical polarization control devices may be, for example, polarization scramblers or polarization controllers, or may simply be referred to as polarization control devices.

[0004] In some examples, PICs are constructed from basic building blocks intended to construct PICs. The basic building blocks include various components, each with a specific function. An example of a basic building block is a waveguide structure. The basic building blocks have a specific effect on light incident thereon. Examples described herein relate to light polarization control devices that can be used as basic building blocks of PICs.

[0005] An optical polarization control device is, for example, a device that can be used to actively control the polarization of light. Active control refers to a state of optical polarization that can be dynamically changed during use of the device. In this way, the function of the device can be dynamically changed during use, as opposed to a passive effect in which the function of the device is fixed at the time of device manufacture. For example, a suitably configured optical polarization control device can be used to change the polarization of any input polarization to a specific output polarization. For example, any input polarization (e.g., linear, elliptical, or circular) can be changed to vertical polarization by a suitably configured optical polarization control device. In one such example, the polarization control device functions as a polarization controller. In another example, the input polarization can be changed to a random output polarization. In a first example, a vertically polarized input can be changed to elliptical polarization, in a second example, a vertically polarized input can be changed to circular polarization, and in a third example, a vertically polarized input can be changed to linear polarization. In one such example, the polarization control device functions as a polarization scrambler.

[0006] Optical polarization control devices can be fabricated from a combination of a polarization converter, which is typically a passive device that produces a fixed change in polarization state, and a phase shifter, arranged in series with an interferometer. The order of components (polarization converter, interferometer, and phase shifter) required to realize a polarization controller generally differs from the order of components required to realize, for example, a polarization scrambler. In such instances, the accuracy of the polarization scrambler and controller may depend on the performance of the constituent polarization converters.

[0007] In the examples described herein, the polarization control devices herein include a combination of actively controlled polarization converters, such that the same combination of components can be used to achieve multiple different functions, such as the function of a polarization controller and / or a polarization scrambler.

[0008] In embodiments described herein, the polarization control device includes a first polarization converter having a first cross-sectional structure supporting a first mode and a second mode having different polarization orientations and different effective refractive indices, and a second polarization converter having a second cross-sectional structure supporting a third mode and a fourth mode having different polarization orientations and different effective refractive indices. At least one control element is configured to vary the effective refractive indices of the first, second, third, and fourth modes. In embodiments, this combination of features provides a polarization control device that is more compact than known polarization control devices, thereby reducing the footprint (occupied surface area) of the polarization control device on a PIC. Using appropriate control signals, the polarization control device can further fulfill the functions of both a polarization controller and a polarization scrambler.

[0009] FIG. 1 is a schematic top view of a first embodiment of an optical polarization control device 10. The optical polarization control device 10 is for use in a PIC. In FIG. 1, the optical polarization control device 10 includes an input waveguide 20, a first polarization converter 100, a connecting waveguide 50, a second polarization converter 200, and an output waveguide 40 arranged in series. That is, in use, the output of the input waveguide 20 is received at the input of the first polarization converter 100, the output of the first polarization converter 100 is received at the input of the connecting waveguide 50, the output of the connecting waveguide 50 is received at the input of the second polarization converter 200, and the output of the second polarization converter 200 is received at the input of the output waveguide 40. Those skilled in the art will understand that the reverse relationship, in which light propagates in the opposite direction to that described above, also applies to components described in series. More generally, an optical polarization control device according to embodiments herein comprises at least two polarization converters arranged in series, and in embodiments may comprise an input waveguide, an output waveguide, and a plurality of connecting waveguide sections joining the input waveguide, the output waveguide, and the plurality of polarization converters. In use, light can be thought of as propagating through the optical polarization control device 10 in a direction substantially parallel to the optical propagation axis, which is indicated by arrow 18 in FIG.

[0010] FIG. 2 schematically illustrates a side cross section of the first polarization converter 100 of the polarization control device 10 of FIG. 1 , showing the cross-sectional structure of the first polarization converter 100 as viewed in a cross section perpendicular to the first light propagation axis LPA, which is within the page as indicated by symbol 118 in FIG. 2 . The optical polarization converter 100 is for a PIC, and the substrate plane of the PIC 1001 is below the optical polarization converter 100 in FIG. 2 . The optical polarization converter 100 includes a substrate 102. In some examples, the 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 102 includes a nitride-based material or a silicon-based material, such as silicon carbide.

[0011] The cross-sectional structure of a polarization converter refers to the composition of the polarization converter in a cross section perpendicular to the optical propagation axis of the polarization converter. That is, it refers to the shape and material composition of the constituent layers that form the polarization converter, and refers to the shape as represented by a plane perpendicular to the optical propagation axis. More generally, the cross-sectional structure of a component configured to guide light herein refers to the shape and material composition of the constituent layers that form the component in a cross section perpendicular to the optical propagation axis of the component. The cross-sectional structure of the optical polarization converter 100 includes a first waveguide layer 108 between and in contact with a first cladding layer 130 and a second cladding layer, which in this example is the substrate 102. An electrical contact layer 132 is on top of the cladding layer 130. The electrical contact layer 132 is an element for applying an electric field to the substrate across the first and second cladding layers and the first waveguide layer 108 in response to a signal. This modifies the refractive index of the first and second cladding layers and the first waveguide layer 108 due to electro-optic effects such as the Pockels effect, the Kerr effect, the plasma and / or the band-filling effect, the uses of which are described below.

[0012] The first waveguide layer 108 includes a first portion 110 that is partially bounded by and in contact with a first surface 104a of the substrate 102 and a second surface 104b of the first cladding layer 130, and a second portion 112 that is partially bounded by and in contact with a third surface 106a of the substrate and a fourth surface 106b of the first cladding layer 130. The first surface 104a, the second surface 104b, the third surface 106a, and the fourth surface 106b are parallel to one another.

[0013] The first surface 104a is offset from the third surface 106a along a first axis 114 and a second axis 116, each perpendicular to the light propagation axis, to convert the polarization of the light. The first surface 104a is connected to the third surface 106a by a first bonding surface 120a of the substrate 102. The second surface 104b is offset from the fourth surface 106b along the first axis 114 and the second axis 116. The second surface 104b is connected to the fourth surface 106b by a second bonding surface 120b of the first cladding layer 130. This offsets the second portion 112 from the first portion 110. The first axis 114 is perpendicular to the second axis 116. In this example, the first bonding surface 120a is parallel to the second bonding surface 120b.

[0014] The first axis 114 is a horizontal axis relative to the orientation shown in FIG. 2, and the second axis 116 is a vertical axis relative to the orientation shown in FIG. 2. For example, the first axis 114 extends in a direction from the first surface 104a toward the third surface 106b. As referred to herein, the width of a portion of the optical polarization converter 100 is along the first axis 114. As referred to herein, the lengths of the various portions described herein are along the first optical propagation axis, as indicated by symbol 118. As referred to herein, the terms height, top, and bottom are relative to the second axis 116. In the orientation shown in FIG. 2, the first surface 104a is the bottom surface of the substrate 102, and the third surface 106a is the top surface of the substrate 102. The top surface 106a provides a step upward (in a direction parallel to the vertical axis) from the bottom surface 104a. Thus, in these examples, the first surface 104a and the third surface 106a are offset in position along the vertical axis 116, e.g., such that the first surface 104a and the third surface 106a are displaced and / or vertically spaced apart from one another, and the second surface 104b and the fourth surface 106b are displaced and / or vertically spaced apart from one another.

[0015] The first waveguide layer 108 having a first portion 110 in contact with the first surface 104a and the second surface 104b, and a second portion 112 in contact with the third surface 106a and the fourth surface 106b, means that the different portions offset from each other in a direction parallel to the vertical axis 116 provide the first waveguide layer 108 in which light can propagate along a first optical propagation axis indicated by symbol 118.

[0016] In the example of FIG. 2 , the first mating surface 120a and the second mating surface 120b are inclined surfaces. As referred to herein, a surface being inclined with respect to another surface means that there is a non-zero angle between the two surfaces, and the angle between the two surfaces is either obtuse or acute. In such a context, a surface may be referred to as being inclined with respect to another surface. The terms “inclination,” “angle,” and “tilt” are used interchangeably herein to refer to the angle of an entity with respect to another entity or with respect to a given axis. For example, in FIG. 2 , the first mating surface 120a has an obtuse interior angle with the first surface 104a and the third surface 106a. In other words, the first mating surface 120a is non-orthogonal to the first surface 104a and the third surface 106a. The second mating surface 120b has an obtuse interior angle with the second surface 104b and the fourth surface 106b. In other words, the second bonding surface 120b is non-orthogonal to the second surface 104b and the fourth surface 106b. As referred to herein, the first bonding surface 120a being between the first surface 104a and the third surface 106a means, for example, that the first bonding surface 120a is located between the first surface 104a and the third surface 106a, e.g., immediately adjacent to each of the first surface 104a and the third surface 106a. Thus, the first bonding surface 120a, the first surface 104a, and the third surface 106a can be considered to collectively constitute the top surface of the substrate 102 in this embodiment.

[0017] In the examples of FIG. 2, the first axis 114 (also referred to as the horizontal axis 114) is substantially parallel (e.g., parallel within an acceptable tolerance) to the plane of the first surface 104a. In these examples, the first mating surface 120a is non-orthogonal to the first surface 104a. In these examples, the third surface 106a is offset from the first surface 104a along the first axis 114 by an amount greater than the width 122 of the first surface 104a along the first axis 114. As referred to herein, an offset in a direction parallel to an axis is relative to the position along that axis at which the edge of the entity is located (the edge that is most forward along that axis). For example, in FIG. 2, the edge of the first surface 104a is at position 124a along the first axis 114. It will be appreciated that a zero offset from the first surface 104a along the first axis 114 means that the edges of each of the first surface 104a and the third surface 106a are at the same location, location 124a, along the first axis 114. In the example of FIG. 2, the offset along the first axis 114 is such that the third surface 106a does not overlap the first surface 104a and the first mating surface 120a is tilted vertically (relative to the orientation of FIG. 2). In these examples, the location 126 along the first axis 114 at which the edge of the third surface 106a resides is a distance greater than the width 122 of the first surface 104a away from location 124a.

[0018] The distance between opposing surfaces of the first cross-sectional structure is defined by horizontal positions 124a and 124b, which are points along the horizontal axis 114. The first waveguide layer 108 spans horizontal positions 124a-124b relative to the horizontal axis 114 and has a first outer surface 144a having horizontal position 124a and a second outer surface 144b having horizontal position 124b, the first outer surface and second outer surface being opposing surfaces of the first waveguide layer 108. The second cladding layer 130 also spans horizontal positions 124a-124b relative to the horizontal axis 114. In other words, the first waveguide layer 108 lies between horizontal positions 124a and 124b relative to the first axis 114, or all structures positioned vertically above the substrate 102 lies between horizontal positions 124a and 124b. In this sense, horizontal positions 124 a and 124 b define a first cross-sectional width 150 of the first cross-sectional structure, and define the distance relative to a horizontal axis 114 that is parallel to substrate 102 .

[0019] 2, the first waveguide layer 108 contacts the first and second bonding surfaces 120a and 120b and includes a first intermediate waveguide portion 128 between the first portion 110 and the second portion 112. In some examples, the substrate 102, having the surfaces 104a, 106a offset in a direction parallel to the second axis 116 and the tilted bonding surface 120a, is at least partially formed before the first waveguide layer 108 is formed on top of the substrate 102, as described. In this manner, the tilted bonding surface 120a forms an intermediate portion of the waveguide at an angle corresponding to the angle of the substrate bonding surface 120a. Thus, the first portion 110, the second portion 112, and the first intermediate waveguide portion 128 can be considered to collectively comprise the first waveguide layer 108 in some examples.

[0020] 2, first portion 110 contacts first surface 104a of substrate 102, while second portion 112 contacts third surface 106, which is offset from first surface 104a in a direction parallel to second axis 116. This means that first waveguide layer 108 includes two portions offset from each other in a direction parallel to second axis 116. As described, in these examples, first waveguide layer 108 also includes first intermediate waveguide portion 128, which is angled relative to second portion 112 and first portion 110, as shown in FIG. 2.

[0021] 2 shows a polarization converter 100 having a cross-sectional structure that includes a portion that is offset in a direction parallel to the vertical axis 116 and an inclined surface relative to the surface of the cross-sectional structure. In other examples described below, the polarization converter may include a cross-sectional structure that features a portion that is offset in a direction parallel to the vertical axis 116 but does not feature an inclined surface, or the polarization converter may include a cross-sectional structure that features an inclined surface but does not feature an offset portion. By providing a polarization converter with a cross-sectional structure that includes at least one of an offset portion and / or an inclined surface, the polarization of the light is converted, as described in more detail below.

[0022] 2 having first and second surfaces offset in a direction parallel to the vertical axis 116, the first waveguide layer 108 can be defined to have an arrangement for polarization conversion without performing wet etching on the first waveguide layer, for example, to define angled sidewalls. For example, one or more materials of the first waveguide layer 108 can be epitaxially grown on the substrate 102, resulting in the first waveguide layer 108 having a portion offset in a direction parallel to the vertical axis 116 due to the offset arrangement of the substrate 102.

[0023] The first waveguide layer 108 includes a material having a higher refractive index than the material of the substrate 102. For example, the first waveguide layer 108 may include or consist of indium gallium arsenide phosphide (InGaAsP). However, in other examples, the first waveguide layer 108 may include or consist of indium aluminum gallium arsenide (InAlGaAs), which has efficient electrorefractive properties. More generally, in some examples, the first waveguide layer 108 includes (Al)InGaAs(P). The elements shown in parentheses are interchangeable, and various element compositions are 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 first waveguide layer 108 is a layer of (Al)InGaAs(P). In other examples, the first waveguide layer 108 includes multiple sublayers. In some such examples, the first waveguide layer 108 includes an (Al)InGaAs(P) / (Al)InGaAs(P) multiple quantum well structure in contact with the substrate 102. In some examples, the sublayers are 5 to 30 nanometers thick. The sublayer stack of the first waveguide layer 108 has a bandgap selected according to the desired application of the optical polarization converter 100.

[0024] The bandgap, and therefore the refractive index of the InGaAsP, can be tuned, for example, as one skilled in the art will appreciate. In some examples, the bandgap of the InGaAsP of the first waveguide layer 108 is tuned to a wavelength of 1250 nanometers (e.g., for propagation of light with a wavelength of 1550 nanometers) or 1100 nanometers (e.g., for propagation of light with a wavelength of 1310 nanometers). In other examples, the wavelength at which the bandgap is tuned is different.

[0025] The first waveguide layer 108 is for guiding light. The properties of the waveguide layer, including, for example, the material refractive index and structural geometry, as well as the properties of any surrounding cladding layers, limit the spatial region, e.g., the first waveguide layer 108, through which light can propagate. The first waveguide layer 108 acts as a core layer and has a refractive index higher than the refractive index of the surrounding cladding layer material, in this example, the first cladding layer 130 and the substrate 102. The core-cladding boundary, in this case formed by the surfaces 106 a,b, 120 a,b, 104 a,b in contact with the first waveguide layer 108, can be thought of as resulting in constructive interference of light that confines light propagating within the first waveguide layer 108.

[0026] For example, a particular optical mode of light may be desired to propagate through the first waveguide layer 108 depending on the desired application of the optical polarization converter 100. The direction in which the optical mode propagates within the first waveguide layer 108 is referred to herein as the optical propagation axis. The optical propagation axis is parallel to the Poynting vector and the negative of the Poynting vector of the light propagating through the waveguide. The optical propagation axis is the general direction in which the energy of the optical mode travels through the waveguide 108. As used herein, the term "mode" refers to an optical mode that may be considered, for example, an electromagnetic propagation mode. A particular waveguide mode is described herein as being "supported" by the waveguide.

[0027] This arrangement of the first waveguide layer 108 provides a conversion of the polarization of light. The following description is in the context of linearly polarized light incident on the first light polarization converter 100, as indicated by symbol 118, which denotes the first light propagation axis. However, it should be understood that similar principles apply to light having different polarizations.

[0028] FIG. 3 shows a top view of the first example optical polarization control device 10, the cross-sectional structure of the first polarization converter 100 described in FIG. 2, and the cross-sectional structure of the second polarization converter 200 described below.

[0029] The second polarization converter 200 has a second cross-sectional structure in a plane perpendicular to the second light propagation axis 218 of the second polarization converter (see symbol 218 on the page), and the cross-section of the second polarization converter 200 is taken in a plane parallel to the direction line 2X. The cross-section of the first polarization converter 100 is taken in a plane parallel to the direction line 1X. The cross-sectional structure of the first polarization converter has been described with respect to the first axis 114 and the second axis 116. The cross-sectional structure of the second polarization converter 200 will be described below with respect to the third axis 214 and the fourth axis 216. In FIG. 3 , the first axis 114 and the third axis 214 are substantially parallel (within an acceptable tolerance), the first axis 114 is perpendicular to the first light propagation axis 118, and the third axis 214 is perpendicular to the second light propagation axis 218. In this example, the first light propagation axis 118 is parallel to the second light propagation axis 218. In other examples, the two polarization converters may not be parallel to each other, and therefore the first axis 114 and the third axis 214 may not be parallel, and the first light propagation axis 118 and the second light propagation axis 218 may not be parallel. However, within the cross section of, for example, the first polarization converter, the first axis is still perpendicular to the first light propagation axis 118, and within the cross section of, for example, the second polarization converter, the third axis 214 is still perpendicular to the second light propagation axis 218.

[0030] The cross-sectional structure of the second optical polarization converter 200 includes a second waveguide layer 208 between and in contact with a third cladding layer 230 and a fourth cladding layer, which in this example is a substrate 202. Similar to the first polarization converter 100, here the second waveguide layer 208 acts as a core layer and has a refractive index higher than that of the surrounding cladding layer materials, which in this example are the third cladding layer 230 and the fourth cladding layer, the substrate 202. The second waveguide layer 208 can include a similar or the same material composition as the first waveguide layer 108.

[0031] The second waveguide layer 208 includes a third portion 210 that is partially bounded by and in contact with a fifth surface 204a of the substrate 202 and a sixth surface 204b of the third cladding layer 230, and a fourth portion 212 that is partially bounded by and in contact with a seventh surface 206a of the substrate and an eighth surface 206b of the first cladding layer 130. The fifth surface 204a, the sixth surface 204b, the seventh surface 206a, and the eighth surface 206b are parallel to one another.

[0032] The fifth surface 204a is offset from the seventh surface 206a along the third axis 214 and the fourth axis 216. The fifth surface 204a is connected to the seventh surface 206a by a third bonding surface 220a of the substrate 202. The sixth surface 204b is offset from the eighth surface 206b along the third axis 214 and the fourth axis 216. The sixth surface 204b is connected to the eighth surface 206b by a fourth bonding surface 220b of the third cladding layer 230. This offsets the fourth portion 212 from the third portion 210. The third axis 214 is perpendicular to the fourth axis 216. In this example, the third bonding surface 220a is substantially parallel to the fourth bonding surface 220b (e.g., within acceptable manufacturing tolerances). The third bonding surface 220a, the fifth surface 204a, and the seventh surface 206a can be considered to collectively comprise the top surface of the substrate 202 in this embodiment.

[0033] The third and fourth bonding surfaces 220a and 220b are inclined surfaces, similar to the first and second bonding surfaces 120a and 120b of the first polarization converter 100. The third bonding surface 220a has an obtuse interior angle with the fifth and seventh surfaces 204a and 206a. In other words, it is non-orthogonal to the fifth and seventh surfaces 204a and 206a. The fourth bonding surface 220b has an obtuse interior angle with the sixth and eighth surfaces 204b and 206b. In other words, it is non-orthogonal to the sixth and eighth surfaces 204b and 206b.

[0034] 3 , second waveguide layer 208 contacts third bonding surface 220a and fourth bonding surface 220b and includes second intermediate waveguide portion 228 between third portion 210 and fourth portion 212. In some examples, substrate 202 having surfaces 204a, 206a offset in a direction parallel to second axis 116 and tilted bonding surface 220a is at least partially formed before second waveguide layer 208 is formed on top of substrate 202, as described. In this manner, tilted bonding surface 220a forms second intermediate portion 228 of second waveguide layer 208 at an angle corresponding to the angle of substrate bonding surface 220a. Thus, third portion 210, fourth portion 212, and second intermediate waveguide portion 228 can be considered collectively to comprise second waveguide layer 208 in some examples.

[0035] The distance between opposing surfaces of the first cross-sectional structure is defined by horizontal positions 224a and 224b, which are points along the horizontal axis 214. The first waveguide layer 208 spans horizontal positions 224a-224b relative to the horizontal axis 214 and has a first outer surface 244a having horizontal position 224a and a second outer surface 244b having horizontal position 224b, the first outer surface and second outer surface being opposing surfaces of the first waveguide layer 208. The second cladding layer 230 also spans horizontal positions 224a-224b relative to the horizontal axis 214. In other words, the first waveguide layer 208 lies between horizontal positions 224a and 224b relative to the first axis 214, or all structures positioned vertically above the substrate 202 lies between horizontal positions 224a and 224b. In this sense, horizontal positions 224 a and 224 b define a first cross-sectional width 250 of the first cross-sectional structure, and define the distance relative to a horizontal axis 214 that is parallel to substrate 202 .

[0036] Although the first cross-sectional structure and the second cross-sectional structure are topologically similar and have a similar overall structure, the specific dimensions of the constituent surfaces and characteristics of the second cross-sectional structure differ from the first cross-sectional structure of the first polarization converter 100, as described below.

[0037] 3 , the first cross-sectional structure has a first portion 110, which may be considered a lower portion because the first portion 110 is lower relative to the vertical axis 116 and the second portion 112 is higher relative to the vertical axis 116, and a second portion 112, which may be considered a higher portion of the first cross-sectional structure. The lower portion 110 is located further along the horizontal axis 114 than the upper portion 112. In the second cross-sectional structure, the upper portion 212 is located further along the horizontal axis 214 than the lower portion 210. Thus, the second intermediate portion 228 of the second waveguide layer 208 is angled in an opposite direction from the first intermediate portion 128 of the first waveguide layer 108. In other words, as shown in FIG. 3, the first intermediate portion 128 can be considered to be slanted upward (along the vertical axis 116) in a right-to-left direction (opposite the horizontal axis 114), while the second intermediate portion 228 can be considered to be slanted upward (along the vertical axis 216) in a left-to-right direction (along the horizontal axis 214).

[0038] The width 222 of the fifth surface 204a and the sixth surface 204b along the third axis 214 is less than the width 122 of the first surface 104a along the first axis 114. Similarly, the widths (not labeled) of the seventh surface 206a and the eighth surface 206b are less than the equivalent widths of the third surface 106a and the fourth surface 106b. The width of the first intermediate portion 128 along the first axis 114 is the same as the width of the second intermediate portion 228 along the third axis 214. Although the intermediate portions 128, 228 have the same width, as described above, they are angled in opposite directions; therefore, the second intermediate portion 228 can be considered a mirror image of the first intermediate portion 128. Thus, in this example, the first cross-sectional width 150 of the first polarization converter is greater than the second cross-sectional width 250 of the second polarization converter. However, this is by way of example only.

[0039] The connecting waveguide 50 connects the first polarization converter 100 to the second polarization converter 200. In the example of the polarization control device 10, this connecting waveguide 50 is a planar waveguide with a waveguide layer at the same height (relative to the vertical axis 116) as the waveguide layers 110, 210, or 112, 212 of the polarization converter. In other examples, the cross-sectional structure of the connecting waveguide 50 is similar to either the first cross-sectional structure of the polarization converter 100 or the second cross-sectional structure of the polarization converter 200. The connecting waveguide 50 has a cross-sectional width 55a, b that tapers from a first cross-sectional width 150, 55a to a second cross-sectional width 250, 55b. This can reduce losses due to mode mismatch when coupling light from the first polarization converter 100 to the second polarization converter 200. In an embodiment, the tapering of the connecting waveguide 50 can take the form of an adiabatic taper, which, as will be appreciated by those skilled in the art, can ensure a gradual and appropriately shaped transition of the propagation mode from the first waveguide width to the second waveguide width, thereby reducing propagation losses through the connecting waveguide 50. Of course, those skilled in the art will understand that the first polarization converter 100 and the second polarization converter 200 can, in an embodiment, be butt-coupled to each other without the connecting waveguide 50.

[0040] The function of the polarization converters 100, 200 will now be described with reference to FIGS. 4a and 4b. The following description will be given in the context of the first polarization converter 100, but will also describe the function of the second polarization converter 200. FIG. 4a relates to input light, and FIG. 4b relates to output light. This is in the context of linearly polarized light entering the polarization converter 100, but it should be understood that similar principles apply to different polarizations. FIGS. 4a and 4b show a transverse electric (TE) polarization axis 402 and a transverse magnetic (TM) polarization axis 404. The light propagation axis 118 is into the page in FIGS. 4a and 4b (perpendicular to both the TE polarization axis 402 and the TM polarization axis 404). With respect to FIGS. 2 or 3, the TM polarization axis 404 is parallel to the first axis 116, and the TE polarization axis 402 is parallel to the second axis 114.

[0041] For linearly polarized light, the direction of the electric field of the propagating light, as indicated by the first light propagation axis 118, can be shown relative to the TE polarization axis 402 and the TM polarization axis 404. Arrow 406 indicates TE-polarized linearly polarized light. Due to the cross-sectional structure of the polarization converter 100, the mode supported by the polarization converter 100 is tilted relative to the polarization axes 402 and 404, which means that the mode supported by the polarization converter 100 is a hybrid mode. In the polarization converter 100 having the configuration shown in Figures 2 and 3, the polarization converter 100 with the offset portions 110 and 112 and the tilted surfaces 120a and 120b provides boundary conditions for the light propagating in the waveguide that result in a tilted mode.

[0042] Due to its cross-sectional structure and resulting boundary conditions, the polarization converter 100 supports a first hybrid mode having an electric field tilted with respect to the TE axis. The terms "tilt," "angle," and "tilt" are used interchangeably herein to refer to the angle of the first hybrid mode with respect to the TE axis. The polarization converter 100 also supports a second hybrid mode that is orthogonal to the first hybrid mode. In other words, the polarization orientation of the first hybrid mode is different from that of the second hybrid mode. A hybrid mode, as referred to herein, is a mode of light having an electric field that has a non-zero component along the TE polarization axis and a non-zero component along the TM polarization axis.

[0043] 4A and 4B show a first hybrid mode 408 and a second hybrid mode 410. The first hybrid mode 408 and the second hybrid mode 410 illustrate an example of hybrid modes that may exist in a polarization converter as light propagates through the polarization converter. In this example, the first hybrid mode 408 and the second hybrid mode 410 result from light having TE polarization (having an electric field along the TE polarization axis 402) entering the polarization converter for propagation through the polarization converter, as indicated by arrow 406.

[0044] The tilt angle (relative to the TE axis 402) for the first hybrid mode 408 is assumed to be 45 degrees. Such a tilt angle for the first hybrid mode 408 may result, for example, from the geometry of the polarization converter 100. In these examples, the angle of the second hybrid mode 410 relative to the TE axis 402 is also 45 degrees, and the first mode 408 and the second mode 410 have equal electric fields. Those skilled in the art will understand that the first mode 408 and the second mode 410 have a 45-degree tilt angle and the phase relationship shown in FIG. 4a, and have equal and opposite components along the TM polarization axis 404, which combine to correspond to TE polarization.

[0045] The second polarization converter 200 also supports a hybrid mode, a third hybrid mode, and a fourth hybrid mode, where the third hybrid mode has a different polarization orientation than the fourth hybrid mode. Note that, as used herein, the second polarization converter 200's support of the third and fourth modes does not necessarily mean that the polarization converter 200 also supports the first and second modes; rather, "third" and "fourth" are labels to distinguish between the modes of the first and second polarization converters, rather than implying, for example, specific support of some higher-order modes.

[0046] The second polarization converter 200 having a second cross-sectional structure different from the first cross-sectional structure of the first polarization converter 100 means that the third and fourth hybrid modes have different tilt angles, or in other words, different polarization orientations, than the first and second hybrid modes of the first polarization converter 100. The tilt angles for the third and fourth hybrid modes supported by the second polarization converter 200 are determined by the geometry of the second cross-sectional structure. For example, the angle of the third mode may be 22.5 degrees relative to the TE mode, and the angle of the fourth mode may be -67.5 degrees.

[0047] The tilt angle of the mode supported by a polarization converter depends on the cross-sectional width of the polarization converter's cross-sectional structure. Thus, two substantially identical (within acceptable tolerances) polarization converters can be initially fabricated, and the cross-sectional width of the second polarization converter in a pair can be reduced relative to the cross-sectional width of the first polarization converter in the pair, e.g., by lithography and / or etching, so that the mode supported by the second polarization converter has a different tilt angle than the mode supported by the first polarization converter. This principle is easily extended to initially fabricating three or more polarization converters and etching their respective widths. Having multiple converters, each supporting a different tilt angle, can help the polarization control device 10 access a larger region of the Poincaré sphere, or even the entirety, as described in more detail below.

[0048] Furthermore, the described arrangement of the cross-sectional structure of the polarization converters 100, 200 has different propagation constants for the first hybrid mode 408 and the second hybrid mode 410 of the first polarization converter. This arrangement results in birefringence, such that the first hybrid mode 408 and the second hybrid mode 410 experience different effective refractive indices as they propagate through the polarization converter. The effective refractive index of a waveguide is a dimensionless number that describes how fast light of a particular mode travels through the waveguide and how much light is attenuated through the waveguide. The refractive index of a material is a dimensionless number that describes the phase velocity of light waves in the material and how light is attenuated through the material. While the effective refractive index and refractive index are commonly expressed as complex numbers, only the real component of the refractive index is considered herein. The real component of the refractive index is the speed of light in a vacuum divided by the phase velocity of the light wave in the material. In some examples, the effective refractive index and / or refractive index depend on the wavelength of light being considered. Here, when a comparison is made between two effective refractive indices or between two refractive indices, the comparison is between the real components for the same wavelength of light.

[0049] This means that as the first hybrid mode 408 and the second hybrid mode 410 propagate, the phase difference between the first hybrid mode 408 and the second hybrid mode 410 changes. In other words, the phases of the first mode 408 and the second mode 410 evolve differently as the first mode 408 and the second mode 410 propagate through the polarization converter, with the phase of the light in one mode changing more quickly than the phase of the light in the other mode. As will be appreciated by those skilled in the art, this description also applies to the third and fourth modes of the second polarization converter 200.

[0050] 5a and 5b show schematic side cross-sections taken in a plane parallel to the first LPA 118 and the second LPA 218 of the polarization converters 100 and 200 of Fig. 3. The first polarization converter 100 has a length 302 parallel to the first optical propagation axis 118. Similarly, the second polarization converter 200 has a length 322 parallel to the second optical propagation axis 218.

[0051] As light in the first and second hybrid modes propagates along the first polarization converter 100 with their respective propagation constants, a phase difference develops between the first and second hybrid modes. Thus, the length of the first polarization converter 100 determines the phase difference between the first and second modes after propagation through the first polarization converter 100 for a fixed respective propagation constant. More generally, the optical path length of the first polarization converter 100 determines the phase difference developed between the first and second modes after propagation through the first polarization converter 100.

[0052] Consider an input waveguide providing TE light 406 to the first polarization converter 100. A first hybrid mode 408 and a second hybrid mode 410 are excited with a relative phase difference of zero. Light in these modes propagates along the length 302 of the first polarization converter 100, resulting in a relative phase difference of π, as seen in the location of the second hybrid mode 410 in FIG. 4b. In FIG. 4b, the first hybrid mode 408 and the second hybrid mode 410 excite TM light 412 in the output waveguide. This phase difference between the first hybrid mode 408 and the second hybrid mode 410 rotates the input TE mode 406 into the output TM mode 404. For example, at a phase difference of π radians (and integer multiples thereof), the modes are out of phase. At a phase difference of 2π radians (and integer multiples thereof), the modes are in phase. The length 302 of the first polarization converter 100 for restoring the phase of the modes of light propagating therethrough is called the beat length. For example, if a first mode and a second mode start their propagation in phase within the first polarization converter 100, these modes will return in phase after propagating an integer multiple of the beat length within the first polarization converter 100.

[0053] As discussed above, the described configuration of the first polarization converter 100 results in different propagation constants for the first and second hybrid modes. The described configuration of the first polarization converter 100 (with the described cross-sectional structure) results in birefringence therein, in that the first and second hybrid modes follow different effective refractive indices. The propagation constant of the first hybrid mode in the first polarization converter 100 can be represented by β1, and the propagation constant of the second hybrid mode can be represented by β2. The difference between these propagation constants can be expressed as Δβ=β1-β2. Those skilled in the art will understand that β represents the phase propagation. That is,

[0054]

number

[0055] The following equation (1) shows the beat length Lλ of the first polarization converter 100 for the first hybrid mode and the second hybrid mode. In the following equation (2), λ is a given wavelength, and Δn represents the difference in effective refractive index between the first hybrid mode and the second hybrid mode. Δn=n1-n2

[0056]

number

[0057] In some examples, the beat length is affected by the thickness and / or cladding of the first polarization converter. In some examples, the first polarization converter is curved, resulting in a curved light propagation axis and a beat length along the curve. In these examples, if the first hybrid mode 408 is at a 45-degree angle with respect to the TE axis 402, the linear polarization of light of a given wavelength can be rotated as described above by selecting the length of the first polarization converter 100 to be half the beat length multiplied by an odd integer. For example, a first linear polarization (TE polarization in the above example) can be converted to a second linear polarization (TM polarization in the above example). Phase control is the basis of a polarization converter, or rotator, as will be understood by those skilled in the art. As will be understood, the second polarization converter 200 shown in FIG. 5b can control the phase of light in a manner similar to the first polarization converter 100 of FIG. 5a.

[0058] Figure 6 is a sketch of the Poincaré sphere. Those skilled in the art will understand that all polarization states can be mapped onto the surface of the so-called Poincaré sphere. Points located at the equator of the Poincaré sphere represent all linear polarization angles. The poles of the Poincaré sphere represent clockwise and counterclockwise circular polarizations. The points corresponding to TE and TM polarizations can be seen in Figure 6a.

[0059] The points corresponding to the first hybrid mode (HM1) and the second hybrid mode (HM2) lie at the equator of the Poincaré sphere and are labeled HM1 and HM2 in Figure 6a. The location of the hybrid modes at the equator depends on the tilt, or angle, of the first hybrid mode with respect to the TE axis.

[0060] For the first hybrid mode, which is at a 45-degree angle with respect to the TE axis, the first hybrid mode corresponds to point M1, and the second hybrid mode corresponds to point M2. The axis intersecting HM1 and HM2 is perpendicular to the axis intersecting the TE and TM polarization points at the equator of the Poincaré sphere. Propagation of the hybrid modes through the polarization converter corresponds to a rotation around the axis intersecting HM1 and HM2 of the point representing the polarization when the hybrid modes recombine, such that their phases evolve differently. A 180-degree rotation around the axis intersecting HM1 and HM2 results in a polarization conversion, for example, from TE polarization to TM polarization, and half the circumference of the Poincaré sphere is traversed. This corresponds, for example, to the optical path length of the polarization converter being half a beat length.

[0061] 6b illustrates the effect of active control of the effective refractive index of a mode on the polarization states accessible by the polarization converter, where active control is achieved using at least one control element, such as an electrical contact layer (e.g., the electrical contact layer 132 of the first polarization converter 100). The electrical contact layer 132 is configured to receive a control signal that determines whether a positive voltage is applied to the polarization converter, for example, by carrier injection, or a negative voltage, for example, by carrier depletion, where the choice of carrier injection or carrier depletion depends on the material composition and structure of the polarization converter. A voltage, which can be referred to as a potential difference, is applied between the electrical contact layer 132 and the substrate of the photonic integrated circuit.

[0062] Application of a voltage and carrier injection / depletion to a polarization converter, such as any of the polarization converters described herein, can result in a change in the effective refractive index of the mode supported by the polarization converter. The effective refractive index of the first mode can change by the same amount as the effective refractive index of the second mode. In such a case, the birefringence remains unchanged, and the optical path length of the polarization converter changes by the same amount in both modes. Alternatively, the effective refractive index of the first mode can change by a different amount than the effective refractive index of the second mode. In this case, the birefringence of the polarization converter is modified by a control element. This changes the optical path length of the polarization converter in the first mode by a different amount than the optical path length in the second mode. In either case, the phase difference between the two modes can be controlled. The specific manner in which the phase difference is created depends on the system, including the material composition and cross-sectional structure of the polarization converter.

[0063] In the example of Figure 6b, with no voltage (V0) applied to the polarization converter by the electrical contact layer, the input state of light I2 rotates around an axis intersecting HM1 and HM2 to become the output polarization state of light O2. When the applied voltage is changed to voltage V1, the effective refractive index of the first and second modes changes, resulting in a faster phase difference between the first and second modes. This means that the polarization of the input light changes to output state O3, traversing a larger portion of the Poincaré sphere from input I2 than was traversed to reach the polarization state of output O2. When voltage V2, V2 > V1, is applied, the polarization state of input state I2 is changed to output polarization O4, so a larger portion of the Poincaré sphere is traversed. Thus, active control of the polarization converter allows the change in polarization state to be controlled and selected using a single polarization converter as a rotation around the rotation axis defined by the hybrid mode tilt angles HM1 and HM2.

[0064] A polarization converter in a polarization control device having a length corresponding to 1 / 4 of the beat length means that the polarization converter passively produces a change to the polarization state spanning 1 / 4 of the circumference of the Poincaré sphere, i.e., the polarization state is changed by this amount without actively applying an electric field across the polarization converter.

[0065] The passive effect of the polarization converter generates a first phase difference, and the active control effect due to the application of an electric field generates a second phase difference that can increase or decrease the first phase difference. Polarization converters with sufficiently long propagation lengths can be useful so that active control of the polarization converter can change the optical path length of the polarization converter by, for example, half a beat length, thereby changing the polarization state by half the Poincaré sphere. As a result, the polarization converter of a polarization control device can be multiple beat lengths to enable this degree of active polarization control.

[0066] Increasing the birefringence of the polarization converter, and the change to birefringence induced by an applied voltage, can reduce the physical length of the polarization converter required to obtain a desired phase difference (as understood by a reduced beat length according to Equation 2), thereby reducing the overall spatial footprint of the device. Increasing the birefringence can be achieved, for example, by providing a multiple quantum well structure in the waveguide layer of the polarization converter, which in some instances can be sensitive to, for example, the quantum confined Stark effect, which is highly polarization sensitive.

[0067] 7a and 7b illustrate the control provided by two polarization converters, such as the first polarization converter 100 and the second polarization converter 200 shown in FIGS. 2 and 3, arranged in series as part of the polarization control device 10 described herein.

[0068] As described above, in the embodiment, the first polarization converter 100 supports a first mode with a tilt angle of 45 degrees and a second mode orthogonal to the first mode, thereby defining a first axis of rotation through the Poincaré sphere defined by HM1 and HM2. In the embodiment, the second polarization converter 200 supports a third mode HM3 and a fourth mode HM4 with tilt angles different from the first and second modes. The third mode HM3 and the fourth mode HM4 define a second axis of rotation through the Poincaré sphere defined by HM3 and HM4. By providing two axes of rotation, the input state can access a wider region of the Poincaré sphere compared to only one axis of rotation.

[0069] The input state I2 has an initial state of polarization. When the input state I2 is input to the first polarization converter 100, and then the phase difference between the first mode and the second mode is obtained, it rotates around the first rotation axis HM1, HM2 to generate an intermediate state polarization IS1, which is the light at the output of the first polarization converter 100. A voltage V1 is applied to the first polarization converter 100, i.e., the condition

[0070]

number

[0071]

number

[0072] 7b, a voltage V2 is applied across the first polarization converter 100, and a voltage V3 is applied across the second polarization converter 200. The input state I2 is coupled into the first and second modes of the first polarization converter 100, propagates through the first polarization converter 100, and, after experiencing the phase difference as described above, is changed to an intermediate polarization state IS2, where IS2 is the condition that indicates the voltage V2 applied to the first polarization converter 100.

[0073]

number

[0074]

number

[0075] Thus, different polarization states of the output state relative to the input state can be achieved by varying the voltages applied across the first polarization converter 100 and the second polarization converter of the polarization control device 10. The exact values ​​of the voltages applied to the first and second polarization converters, including whether the voltages are positive or negative, and the resulting changes to the refractive index and birefringence that determine the rotation around the Poincaré sphere, depend on the exact electro-optical properties of the polarization converters used.

[0076] By randomly varying the voltages applied to each of the first polarization converter 100 and the second polarization converter 200 so that the induced phase difference between the first and second modes and the third and fourth modes varies randomly over time, the output polarization can be made to vary randomly over time, or in other words, scrambled, thereby enabling the polarization control device 10 to function as a polarization scrambler. For example, if a portion of the output light is measured, e.g., by weakly coupling a portion of the output mode into a separate measurement arm of the PIC, the output polarization state can be monitored. Such a measurement arm may include the use of a polarization-dependent photodetector or polarization filtering to establish the output polarization state. Control of the polarization control device 10 can thereby be configured to allow any input polarization state to be changed to a known output polarization, allowing the polarization control device 10 to function as a polarization controller. By receiving an appropriate control signal at at least one control element, the polarization control device 10 can be used as a polarization scrambler or a polarization controller. In other words, by varying the control signals provided to the polarization control device, the same combination of components can be used to achieve multiple different functions, such as the functions of a polarization controller and / or a polarization scrambler. Polarization control device 10 may be provided with control signals from an external control system, or polarization control device 10 may include appropriate control circuitry for providing control signals to at least one control element.

[0077] FIG. 8 schematically illustrates a polarization control device 11 according to one embodiment, showing a Poincaré sphere indicating the polarization rotation axes of the polarization control device 11. The polarization control device 11 includes an input waveguide 20. In addition, the polarization control device 11 includes a first polarization converter 100, a first connecting waveguide 50, and a second polarization converter 200 arranged in series, which are the same as the first polarization converter 100, the first connecting waveguide 50, and the second polarization converter 200 of the polarization control device 10 described above with reference to FIGS. 2 and 3. The polarization control device 11 of FIG. 8 further includes a second connecting waveguide 51 having the above-described characteristics, a third polarization converter 300, and an output waveguide 40, also arranged in series. The second connecting waveguide 51 is configured to receive light from the second polarization converter 200 and provide light to the third polarization converter 300. The optical polarization converter 300 is configured to provide light received from the second connecting waveguide 51 to the output waveguide 40 .

[0078] The function and form of the third polarization converter 300 in this embodiment is substantially similar or the same as the first polarization converter 100 and the second polarization converter 200. In this context, substantially similar means that the structure differs only in the manner described below, and those skilled in the art will understand that the function within the context of polarization control is the same as the first polarization converter and the second polarization converter. The third polarization converter 300 supports a fifth mode and a sixth mode and includes a third cross-sectional structure (not shown) in a plane perpendicular to the third optical propagation axis 318, which cross-sectional structure determines the tilt angle or orientation of the fifth mode and the sixth mode, and the tilt angle of the fifth mode is different from the tilt angle of the sixth mode.

[0079] The third cross-sectional structure is different from the first cross-sectional structure and the second cross-sectional structure, such that the fifth and sixth modes have a different polarization orientation from the first and second modes, as well as a different polarization orientation from the third and fourth modes. In the example of Figure 8, the third cross-sectional structure is similar in that the third cross-sectional structure includes a third intermediate portion that is sloped in the same direction as the second intermediate portion of the second cross-sectional structure (i.e., sloped upward from left to right), but the third cross-sectional width of the third polarization converter 300 is smaller than the second cross-sectional width of the second polarization converter 200, such that the fifth and sixth modes have a different polarization orientation from the third and fourth modes, as well as the first and second modes. In other examples, the third cross-sectional structure may include an intermediate portion that is sloped in the same direction as the first intermediate portion of the first cross-sectional structure, such that the fifth and sixth modes have a different polarization orientation from the third and fourth modes, and the first and second modes. In a further example, all three polarization converters include intermediate portions that are tilted in the same direction, with only the different respective cross-sectional widths providing different tilt angles for the supported modes.

[0080] The third polarization converter 300 is birefringent, so that the fifth mode has a higher effective refractive index than the sixth mode. The third polarization converter 300 has a control element, in this example an electrical contact layer, configured to change the effective refractive index of the fifth and sixth modes by applying a voltage across the third polarization converter 300 in response to a signal.

[0081] Considering the Poincaré sphere, the third polarization converter 300 defines a third axis of rotation through the fifth hybrid mode HM5 and the sixth hybrid mode HM6. This further expands the range of positions on the sphere, and therefore the range of polarization states accessible by the polarization control device 11. Also, by providing another degree of control freedom, the flexibility of the device in achieving a given output polarization state can be improved. This additional degree of freedom can be used to trace different paths on the Poincaré sphere to a given output state.

[0082] In this example, the third polarization converter 300 supports a mode with a different polarization orientation than the modes supported by the first polarization converter 100 and the second polarization converter 200. However, in other examples, the polarization conversion device may instead include a third polarization converter identical to the first polarization converter, allowing rotation about the same rotation axis. For example, a polarization control device including a combination of polarization converters with tilt angles of 30 degrees, 60 degrees, and 30 degrees relative to the TE mode may allow polarization conversion from any input polarization to any output polarization. In other examples, a polarization control device including a first polarization converter with a tilt angle of 20-40 degrees, e.g., 30 degrees, a second polarization converter with a tilt angle of 0 degrees (i.e., a mode aligned with the TE / TM modes of the photonic integrated circuit), and a third polarization converter with a tilt angle of 50-70 degrees, e.g., 60 degrees, may be suitable.

[0083] In yet a further example, a 0°, 45°, 0° arrangement may be suitable: in such a scheme, the first polarization converter (0°) acts as a phase shifter to the TE / TM input state (so a TE / TM input state may not be needed), the second polarization converter (45°) rotates the polarization away from the TE / TM, and the third polarization converter (0°) accesses the remainder of the sphere.

[0084] Additionally, while providing multiple polarization converters has the effect of increasing the overall optical path length of the polarization converter, having multiple polarization converters with redundancy between the rotations provided by the polarization converters may allow lower voltages to be applied to any individual section.

[0085] In yet a further example, the polarization control device may include a polarization converter, or multiple polarization converters, as part of a series configuration, that lack active control and that are positioned before, between, or after the polarization converters with active control.

[0086] 9 schematically illustrates a polarization control device 12 including a first polarization converter 100-B, a second polarization converter 200-B, and a third polarization converter 300B. The first polarization converter 100-B has a first cross-sectional width (which is the overall width of the first cross-sectional structure, as previously described) that varies along the length of the first polarization converter, and this length is parallel to the first optical propagation axis 118 of the first polarization converter. Similarly, the second polarization converter 200-B has a cross-sectional width that varies along the length of the second polarization converter, and the third polarization control device 300-B has a third cross-sectional width that varies along the length of the third polarization converter, each of which is parallel to the respective optical propagation axis 218, 318 of the respective polarization converter.

[0087] The birefringence of a polarization converter can depend on the voltage applied across the converter. In such cases, changing the voltage applied to the converter does not cause the polarization state to rotate equatorially around the axis of rotation because the phase difference is not linear with the voltage. Instead, the polarization state experiences a shift in curvature. This can lead to complex control dynamics. Varying the cross-sectional width of each polarization converter can result in birefringence that depends on the propagation length within the polarization converter. That is, for example, at a first propagation distance within a first polarization converter, there is a first birefringence between the first and second modes, and at a second propagation distance within the first polarization converter, there is a second birefringence between the first and second modes that is different from the first birefringence. This can be used to reduce or attenuate the effect of the nonlinear voltage-dependent birefringence of the polarization converter, simplifying the control dynamics and / or improving the precision of polarization control.

[0088] 10-12 show further examples of cross-sectional structures of polarization converters, the shapes of which can achieve tilted mode angles. The polarization converters in other examples of polarization control devices described herein may feature the cross-sectional structures of any of the described examples of polarization converters in any combination. For example, the first cross-sectional structure of a first polarization converter of a polarization control device may be, for example, according to FIG. 2, and the second cross-sectional structure of a second polarization converter of a polarization control device may be, for example, according to FIG. 10, or FIG. 11, or FIG. 12, or some variation thereof.

[0089] Additionally, the orientation of the cross-sectional features of each polarization converter that provide the tilted boundary conditions, and therefore the tilted mode angles, e.g., the tilted intermediate portions in the examples of Figures 1-3, can be selected according to the desired polarization control of the polarization control device. In the examples of Figures 1-3, the first polarization converter 100 has a first intermediate portion 128 that slopes upward from right to left, and the second polarization converter 200 has an intermediate portion 228 that slopes upward from left to right. In other examples, the first intermediate portion 128 may slope in the same direction as the second intermediate portion 228 (i.e., both slope upward from left to right or both slope upward from right to left relative to their respective horizontal axes), and the mode tilt angle of the first polarization converter may be different from that of the second polarization converter if the first polarization converter has a different cross-sectional width than the second polarization converter.

[0090] Figure 10 schematically illustrates a cross-sectional side view of an optical polarization converter 400, according to an embodiment. In Figure 10, features corresponding to those shown in Figure 2 are labeled with similar reference numbers with an additional suffix "-4" (except for the optical polarization converter itself, which is labeled with reference number 400).

[0091] The polarization converter 400 has a cross-sectional structure described herein with respect to a vertical axis 116-4 and a horizontal axis 114-4, each perpendicular to the direction of light propagation 118-4 through the polarization converter 400, and the vertical axis 116-4 is perpendicular to the horizontal axis 114-4.

[0092] The light polarization converter 400 of FIG. 10 corresponds to the first light polarization converter 100 (and may include any combination of the features described above with respect to the first light polarization converter 100), except for the following differences: The first bonding surface 120a-4 of the light polarization converter 400 of FIG. 10 is at a substantially 90-degree angle (within acceptable tolerances) relative to the first surface 104a-4 and the third surface 106a-4 of FIG. 10. In these examples, the first bonding surface 120a-4 corresponds to the sidewall of the second substrate layer 136-4 of FIG. 10. Similarly, the second bonding surface 120b-4 is at a substantially 90-degree angle relative to the second surface 104b-4 and the fourth surface 106b-4 of FIG. 10.

[0093] In these examples, as a result of the angle of the first and second bonding surfaces 120a, 120b-4, the optical polarization converter 400 of Figure 10 does not include an intermediate portion. In the example of Figure 10, the first portion 110-4 is continuous with the second portion 112-4 instead of having an intermediate sloped portion therebetween. As in the examples of Figures 2 and 3, the waveguide layer 108-4 of the optical polarization converter 400 is a single waveguide layer for the propagation of light.

[0094] The waveguide layer 108-4 of FIG. 10 includes a second portion 112-4 that is offset relative to the first portion 110-4 in a direction parallel to the second axis 116-4 and is continuous with the first portion 110-4, as shown in FIG.

[0095] The cross-sectional structure of the polarization converter 400 of FIG. 10 supports hybrid modes. Similar to the cross-sectional structures of the polarization converter examples of FIGS. 2 and 3, this arrangement of the cross-sectional structure of the polarization converter 400 of FIG. 10 provides a "tilted" or tilted boundary condition for light propagating within the waveguide layer 108-4, providing hybrid modes. This is because light propagating within the polarization converter 400 occupies successive waveguide layer regions that are offset in a direction parallel to the second axis 116-4. Additionally, the depicted cross-sectional structure of the polarization converter 400 results in different propagation constants for different hybrid modes. The depicted cross-sectional structure, including the waveguide layer 108-4, provides birefringence such that the first and second hybrid modes follow different refractive indices within the polarization converter 400 of FIG. 10. For example, for light having TE polarization, indicated by arrow 206 in FIG. 4a, entering optical polarization converter 400 in FIG. 10 for propagation through waveguide layer 108-4, first hybrid mode 408 and second hybrid mode 410 arise with different propagation constants (and consequently different phase evolutions).

[0096] The cross-sectional structures of both the polarization converter 400 of FIG. 10 and the first polarization converter 100 create hybrid modes and different propagation constants for the hybrid modes. As discussed above, the presence of hybrid modes and their different propagation constants results in a conversion of the polarization of light. Therefore, similar to the first optical polarization converter 100 of the example of FIG. 2, the optical polarization converter 400 of the example of FIG. 10 can be used to convert the polarization of incident light. The electrical contact layer 132-4, as described in the previous example, is used to apply a voltage, thereby achieving active control of the polarization rotation performed by the polarization converter 400 of FIG. 10.

[0097] 2 and 3 or the example of FIG. 10 may be used depending on various factors, such as the complexity and cost of fabrication and the desired level of control over the hybrid mode and its propagation constant. For example, the angled (slanted) intermediate waveguide section 128 in the examples of FIGS. 2 and 3 allows for finer tuning of the hybrid mode characteristics (e.g., tilt). However, the example of FIG. 10 may be easier to fabricate because the stepped structure of polarization converter 400 eliminates the need to fabricate a polarization converter with an inclined surface.

[0098] FIG. 11 schematically illustrates a polarization converter 500 according to an embodiment. The polarization converter 500 has a cross-sectional structure described herein with respect to a vertical axis 116-5 and a horizontal axis 114-5, each perpendicular to a direction of light propagation 118-5 through the polarization converter 500, and the vertical axis 116-5 is perpendicular to the horizontal axis 114-5. The polarization converter 500 of FIG. 11 includes a first waveguide layer 108-5 between a substrate 102-5 (which may be considered a first cladding layer) and a second cladding layer 130-5. In the cross-sectional structure of the polarization converter 500 of FIG. 11, the orientation of the first waveguide layer 108-5 can be set according to the desired polarization conversion characteristics. The waveguide layer 108-5 can be considered to be tilted or slanted, for example, by an interior angle α relative to the substrate 102-5 or (180°-α) relative to the second cladding layer 130-5. Thus, because the waveguide layer 108-5 is angled in this manner, unlike the first cladding layer (which corresponds to the substrate 102-5 in this example) or the second cladding layer 130-5, the waveguide layer 108-5 is not, for example, parallel to the substrate 102-5. For example, the inclined surface 190 of the waveguide layer may be angled with respect to the substrate 102-5 by an interior angle α of 30-65 degrees, e.g., 30-40 degrees, such as approximately 35 degrees (within acceptable manufacturing tolerances), 50-65 degrees, 50-55 degrees, 55-60 degrees, or 60-65 degrees, such as approximately 55 degrees or 60 degrees (within acceptable manufacturing tolerances), depending on the plane of the crystalline material; and / or the second surface may be angled with respect to the third surface by an interior angle β of approximately 90 degrees, within acceptable manufacturing tolerances. The third surface may be, for example, substantially parallel to the fourth surface (e.g., parallel within acceptable manufacturing tolerances).

[0099] The material from which the waveguide layer 108-5 is formed may be, for example, a crystalline material, and the angle of the first angled surface corresponds to the {111} of the crystalline material. This {111} notation follows the Miller index system for designating planes or homologous planes within a crystal, as known to those skilled in the art. Such planes may also be referred to as crystal planes. The angle of the first angled surface may be easily obtained using appropriate fabrication methods, for example, by using specific etchants that are selective to certain crystal planes.

[0100] Such an etching technique is selective to etch the material of the waveguide layer 108-5 without etching (or particularly etching more slowly) the material forming the first and second cladding layers. Thus, during etching, the waveguide layer is removed from between the first and second cladding layers, e.g., from below the second cladding layer, to form the first angled surface. Thus, in an embodiment, there is a region 520 between the first and second cladding layers where the waveguide layer 108-5 is not present.

[0101] The first angled surface 190 of the waveguide layer 108-5, angled in this manner, causes the polarization converter 500 of FIG. 1 to support tilted modes. The internal angle α can be selected to determine the polarization orientation of the supported modes. The polarization converter 500 is birefringent such that a first of the supported hybrid modes has a higher effective refractive index than a second of the supported hybrid modes. Thus, the cross-sectional structure displayed in FIG. 11 , formed, for example, by the use of underetching techniques, can be configured to provide a polarization converter for use in the polarization control devices described herein. The electrical contact layer 132-5 enables active control over the polarization rotation achieved by the polarization converter 500.

[0102] FIG. 12 features a polarization converter 600 according to an embodiment. In FIG. 12, features corresponding to those shown in FIG. 2 are labeled with similar reference numbers with an additional suffix "-6" (except for the optical polarization converter itself, which is labeled with reference number 600). The polarization converter 600 has a cross-sectional structure described herein with respect to a vertical axis 116-6 and a horizontal axis 114-6, each perpendicular to the direction of light propagation 118-6 through the polarization converter 600, and the vertical axis 116-6 is perpendicular to the horizontal axis 114-6. The cross-sectional structure of the polarization converter 600 in FIG. 12 includes a waveguide layer 108-6 between and in contact with a first cladding layer, which in this example is a substrate layer 102-6, and a second cladding layer 130-6. The waveguide layer includes a first offset portion 112-6 partially bounded by the first surface 106a-6 and the second surface 106b-6, a second offset portion 110-6 partially bounded by the third surface 104a-6 and the fourth surface 104b-6, and an intermediate portion 128-6 between the first and second bonding surfaces 120a-6 and 120b-6. The first bonding surface 120b-6 bonds the first surface 104b-6 and the third surface 106b-6, and the second bonding surface 120a-6 bonds the second surface 104a-6 and the fourth surface 106a-6. The intermediate portions thereby bond the offset portions to each other. The portion 110-6 is lower than the portion 112-6 relative to the vertical axis 116. 2 and 3. Additionally, the cross-sectional structure of the polarization converter 600 of FIG. 12 features a sloped outer surface 190 of the waveguide layer 108-6, which forms a non-orthogonal (i.e., acute or obtuse) interior angle α with the substrate layer 102-6 and the second cladding layer 130-6. As described with reference to FIG. 11, this may be based on the crystal planes of the waveguide layer 108-6 and may be formed using under-etching techniques.

[0103] 2, 3, 10, 11, and 12, the cross-sectional structure of the polarization converter 600 of FIG. 12 enables tilt modes and birefringence of these tilt modes. Electrical contact layer 132-5 can be used to actively control the polarization rotation achieved by polarization converter 600. By combining offset sections and outer surfaces 190 that are non-orthogonal to adjacent surfaces, the cross-sectional structure of polarization converter 600 has numerous design parameters that can be selected to achieve the desired tilt modes and birefringence in a simple manner.

[0104] FIG. 13 is a schematic diagram of a polarization converter 700 according to an embodiment. The polarization converter 700 has a cross-sectional structure described herein with respect to a vertical axis 116-7 and a horizontal axis 114-7, each perpendicular to the direction of light propagation 118-7 through the polarization converter 700, and the vertical axis 116-7 is perpendicular to the horizontal axis 114-7. The cross-sectional structure of the polarization converter 700 of FIG. 13 includes a waveguide layer 108-7 between and in contact with a first cladding layer, which in this example is a substrate layer 102-7, and a second cladding layer 130-7. An electrical contact layer 132-7, as described in the previous embodiment, is used to apply a voltage, thereby achieving active control of the polarization rotation performed by the polarization converter 700 of FIG. 13.

[0105] The first cladding layer, substrate layer 102-7, includes a first surface 170a-7 at a first location 124a-7 relative to the horizontal axis 114-7 and a second surface 170b-7 at a second location 124b-7 relative to the horizontal axis 114-7. Thus, the first surface 170a-7 (left side relative to the orientation of FIG. 13) and the second surface 170b-7 (right side relative to the orientation of FIG. 13) define a cross-sectional width of the substrate layer 102-7 that extends from the first location 124a-7 to the second location 124b-7.

[0106] The second cladding layer 130-7 includes a first surface 174-7 at a third location 124c-7 relative to the horizontal axis 114-7 and a second surface 174b-7 at a second location 124b-7 relative to the horizontal axis 114-7. Thus, the first surface 174a-7 (left side in the orientation of FIG. 13 ) and the second surface 174b-7 (right side in the orientation of FIG. 13 ) define a cross-sectional width of the second cladding layer 130-7 that extends from the third location 124c-7 to the second location 124b-7. Because the third location 124c-7 is between the first location 124a-7 and the second location 124b-7, the cross-sectional width of the first cladding layer, the substrate layer 102-7, is greater than the cross-sectional width of the second cladding layer 130-7.

[0107] The waveguide layer 108-7 includes a first surface 171-7 at a first location 124a-7 and a second surface 173-7 at a third location 124c-7. The first surface 171-7 of the waveguide layer 108-7 is bonded to the second surface 173-7 of the waveguide layer 108-7 by a bonding surface 172-7, which are substantially perpendicular (within acceptable manufacturing tolerances) to each other. The waveguide layer 108-7 includes a third surface 171b-7 at a second location 124b-7 relative to the horizontal axis. Thus, the waveguide layer 108-7 has a first cross-sectional width defined by the distance relative to the horizontal axis 114-7 between the first surface 171-7 of the waveguide layer 108-7 and the third surface 171b-7 of the waveguide layer, and a second cross-sectional width also defined by the horizontal distance between the second surface 173-7 of the waveguide layer 108-7 and the third surface 171b-7 of the waveguide layer 108-7.

[0108] The first surface 171-7 of the waveguide layer 108-7 can be described as being offset from the second surface 173-7. For example, in the examples of Figures 2 and 10, the offset surfaces extend parallel (or substantially parallel within acceptable manufacturing tolerances) to the horizontal axis 114, while in the example of Figure 13, the offset surfaces 171-7, 173-7 extend parallel (or substantially parallel within acceptable manufacturing tolerances) to the vertical axis 116-7.

[0109] In the example of FIG. 13, the first surface 171-7 of the cladding layer 108-7 is five times longer (in the direction of the vertical axis 116-7) than the second surface 173-7 of the cladding layer 108-7. The relative lengths of the surfaces 171-7, 173-7, and the bonding surface 172-7, and therefore the first and second cross-sectional widths, can be selected to achieve a desired mode tilt angle of the waveguide layer 108-7. In the example of FIG. 13, the mode angle is primarily determined by (or, in other words, is most sensitive to) the first cross-sectional width. That is, in the example of FIG. 13, the lateral confinement of the mode is determined by the cross-sectional structure of the second cladding layer and the first cross-sectional width of the waveguide layer 108-7 extending to position 124c-7.

[0110] In other examples, the location of the offset surface may be different, such that either or both of the first cladding layer or the second cladding layer have a step in their cross-sectional structure (and therefore have two cross-sectional widths) corresponding to the offset surface, similar to or instead of the waveguide layer.

[0111] The polarization converter 700, having offset surfaces 171-7, 173-7 and therefore a waveguide layer 108-7 with two cross-sectional widths, introduces asymmetric boundary conditions that rotate the mode angles of the supported modes relative to a uniform planar waveguide layer.

[0112] The provision of tilted mode angles through the geometric features of the cross-sectional structure profiles shown in Figures 2, 3, and 10-13 provides a polarization converter that is easily fabricated. Active control, allowing a series of polarization converters to be used as polarization scramblers or controllers, can be more easily achieved by providing control elements, such as electrical contact layers that may be or include electrodes, that can be used to change the effective refractive index of the modes supported by the constituent polarization converters. Such polarization control devices can be easily fabricated on a common platform, for example, by forming only a single waveguide layer, or limited cladding and waveguide layers, without the need to form complex or disparate sublayers.

[0113] FIG. 14 illustrates schematically an example of a system 1500 that includes the polarization control device 10 of FIG.

[0114] The photonic integrated circuit 1001 includes a light source 80, which in this example is an integrated laser light source. Light from the light source 80 is received at the input waveguide 20 of the polarization control device 10. In other examples, light generated by the system's light source may be provided to other devices in the photonic integrated circuit before being received by the polarization control device 10. The system 1500 includes a control system 90 configured to provide signals to the electrical contact layers 132, 232 of the polarization converters 100, 200 of the polarization control device 10. The control system 90 can provide these signals to the electrical contact layers 132, 232 through the provision of electrical circuits (not shown) within the photonic integrated circuit. In this example, the signals provided to the electrical contact layers determine the potential difference applied to the respective polarization converters. In other examples, the signals can determine the temperature of a heating element in contact with the polarization converters.

[0115] The control system 90 implements a control scheme for controlling the function of the polarization control device 10. For example, the control system 90 may implement a control scheme that enables the polarization control device 10 to function as a polarization scrambler or a control scheme that enables the polarization control device 10 to function as a polarization controller. The output waveguide 40 of the polarization control device 10 outputs light, which has a polarization state that may differ from the polarization state of the input light, depending on the polarization control implemented by the control system 90 interfacing with the polarization control device 10.

[0116] The photonic integrated circuit includes an optical splitter 82 following the output waveguide 40, which provides a portion of the output light to a photodetector 85. The photodetector 85 is polarization-dependent, such that it is sensitive to a first polarization of light and insensitive to a second polarization of light. In this example, the polarization dependence of the photodetector 85 is an inherent property of the photodetector, but in other examples, the photodetector may be polarization-insensitive, and providing a polarizing filter in front of the photodetector may enable a similar measurement of the polarization of the output light. The photodetector 85 may be used, for example, for initial characterization of the polarization control device 10 and thus interface with the control system 90 to provide information to the control scheme implemented by the control system 90. The photodetector may also be used during active use of the device to provide feedback to the control system 90 by measuring the output state of the light. The remaining portion of the light provided by the splitter is output from the output waveguide 40b.

[0117] Figure 15 is a flowchart illustrating a method for controlling the polarization of light in a polarization control device, according to an embodiment. In the example of Figure 15, the method includes controlling a polarization control device, such as polarization control device 10 of Figure 1, having two polarization converters. However, after reading this description, one skilled in the art will readily understand how the method of Figure 15 can be extended to polarization control devices that include more than two polarization converters, such as polarization control devices 11, 12 of Figures 8 and 9.

[0118] In item S101 of FIG. 15 , light is received at a first polarization converter. Receiving light at a converter means, for example, that the light is coupled into a mode supported by the converter. In other words, in item S101, the light is coupled into a first mode and a second mode, e.g., a tilted mode or a hybrid mode, as described above. Receiving light may mean propagating through free space, e.g., by directly coupling an off-chip light source to a polarization control device, or may mean propagating through a waveguide such as input waveguide 10 and then being received. Those skilled in the art will appreciate that there are various ways in which light can be coupled into an integrated optical circuit device.

[0119] In item S103, the required phase shift between the first and second modes is determined. This involves, for example, determining the phase shift required to change the polarization state from a first input state of a first polarization to an intermediate output state of a first polarization according to a desired traversal path of the Poincaré sphere. Such calculations use knowledge of the tilt angles of the first and second modes and, therefore, the rotation axis across the Poincaré sphere when the phase shift occurs. Generally, this involves taking into account the polarization transformation to be achieved by a subsequent polarization converter, e.g., a second polarization converter, to determine the overall traversal path of the Poincaré sphere achieved by polarization control device 10. In this sense, although items S103 and the subsequent items S109 are presented separately, they can be considered to act in conjunction in determining how the polarization state is changed by the polarization control device and may not be performed separately in time.

[0120] In an embodiment, item S103 may include first determining the transfer function of the device (within manufacturing tolerances). This may include characterizing the polarization control device by providing TE-mode light to the polarization control device and measuring the output polarization of the device as the control voltage is swept over its respective range. This may allow the transfer function per section (e.g., for each constituent polarization converter of the polarization control device) to be calculated, which in turn may inform which polarization conversion step should be performed at which polarization converter, and therefore determine the phase shift required for each polarization converter.

[0121] In some examples, item S103 may include an in-line measurement of the polarization state at the first polarization converter, for example, by splitting a portion of the light and performing a polarization measurement. In other examples, item S103 may be performed without requiring a measurement of the polarization state of the light, for example, if the polarization state received by the first polarization converter is known or can be estimated with adequate accuracy. Thus, the calculation of item S103 may occur before item S101 in some examples.

[0122] In item S105, at least one first control element is controlled to shift the phase of the first mode relative to the second mode according to the target phase shift calculated in item S103. For example, in the polarization converter described in FIGS. 2 and 3, the control element includes an electrical contact layer, which includes applying an electric field through the electrical contact layer to change the effective refractive indices of the first and second modes. When propagating through the first polarization converter, the first mode having the first effective refractive index and the second mode having the second effective refractive index acquire a phase difference. It will be understood that in other examples, the control element may be, for example, a heating element. In such a case, the control element is instead controlled to generate a temperature that results in the desired phase shift determined by step S103.

[0123] In item S107, the light is received at a second polarization converter, which includes coupling the light into the third and fourth modes. This may include directly coupling the light from the first and second modes of the first polarization converter into the third and fourth modes of the second converter, for example, by butt coupling, or may include coupling the light using a connecting waveguide, which may have a tapered shape, to receive the light from the first and second modes of the first polarization converter and, in some examples, adiabatically couple the light into the third and fourth modes.

[0124] In item S109, the required phase shift between the third and fourth modes is determined. As in item S103, this may involve, for example, determining the phase shift required to rotate an input polarization state (e.g., the output state of the first polarization converter) to the desired output state of the polarization control device. Such calculations may use knowledge of the tilt angles of the third and fourth modes and, therefore, the rotation axes that traverse the Poincaré sphere when the phase shift occurs. As discussed for item S103, although items S103 and S109 are shown separately in FIG. 15, in embodiments, they may be performed simultaneously according to a control scheme, for example, before the device receives light.

[0125] In item S111, at least one second control element is controlled to shift the phase of the third mode relative to the phase of the fourth mode. In the case of the second polarization converter 200 of Figure 3, this involves controlling the electric field across the polarization converter via the electrical contact layers.

[0126] In item S113, the light is output from the second polarization converter. Through the provision of items S105 and S111, the polarization state of the light has been changed to the desired output state output from the second polarization converter. As in item S107, this may include first coupling the light from the third and fourth modes into a connecting waveguide that, for example, adiabatically tapers and transitions to be mode-matched to a receiving waveguide of the photonic integrated circuit.

[0127] In the exemplary flowchart of Figure 15, only two polarization converters are used. Items S101, S103, and S105 may be repeated for subsequent polarization converters in a polarization control device having more than two polarization converters, as will be understood by those skilled in the art.

[0128] FIG. 16 is a flow diagram illustrating a method of manufacturing a polarization control device according to a further embodiment.

[0129] In item S201, a first polarization converter is formed. In forming the first polarization converter, a first cross-sectional structure is formed, the first cross-sectional structure being in a plane perpendicular to a first optical propagation axis of the first polarization converter. The first cross-sectional structure is configured to support a first mode and a second mode, whereby the polarization orientation of the first mode is different from the polarization orientation of the second mode. In other words, the first mode has a first tilt angle, and the second mode has a second tilt angle, where the first tilt angle is different from the second tilt angle. The first mode has a higher effective refractive index than the second mode.

[0130] The polarization converter and its cross-sectional structural details may be formed by techniques known to those skilled in the art for fabricating integrated optical circuit elements, such as deposition, etching, and lithography.

[0131] As will be appreciated by those skilled in the art, various techniques can be used to deposit layers of semiconductor material in accordance with the embodiments described herein. Such techniques may be known as regrowth techniques, and for example, metalorganic vapor phase epitaxy (MOVPE) or molecular beam epitaxy (MBE) processes may be used.

[0132] In some examples, the substrate material of the polarization converter and / or photonic integrated circuit is InP. In some such examples, a wet etching technique may be used in forming the polarization control device. In some such examples, the wet etching technique is performed using HCl:H3PO4:H2O. In some examples, a mixture of HCl, H3PO4, and HO is selected to etch the desired material (InP in these examples). In other examples, only a mixture of HCl and HO is used as the etchant. As described, performing the wet etching technique provides intermediate substrate surfaces (such as bonding surfaces 120a and 120b shown in FIG. 2) that are angled less than 90 degrees relative to adjacent surfaces, e.g., 104a and 104b shown in FIG. 2. Those skilled in the art will understand that the angle of bonding surfaces 120a and 120b will depend on a combination of the crystalline structure of the substrate material, the etching chemistry, and the details of the technique used.

[0133] In an embodiment, forming the polarization converter includes, for example, dry etching techniques to remove material from either side of the structure to a particular depth desired according to the intended application.

[0134] In item S203, a first control element is formed. This can include, for example, depositing an electrical contact layer on top of the first polarization converter. In another example, it can include forming a heating element on the first polarization converter.

[0135] In item S205, a second polarization converter is formed. In forming the second polarization converter, a second cross-sectional structure is formed, the second cross-sectional structure being in a plane perpendicular to a second optical propagation axis of the second polarization converter. The second cross-sectional structure is configured to support a third mode and a fourth mode, whereby the polarization orientation of the third mode is different from the polarization orientation of the fourth mode, and the polarization orientations of the third mode and the fourth mode are different from the polarization orientations of the first mode and the second mode. The third mode has a higher effective refractive index than the fourth mode.

[0136] The second polarization converter is formed such that the input of the second polarization converter can receive light from the output of the first polarization converter, i.e., they are arranged in series. The second polarization converter can be formed using similar or the same techniques as the first polarization converter.

[0137] In item S205, a second control element is formed. This can include, for example, depositing an electrical contact layer on top of the second polarization converter. In another example, it can include forming a heating element on the second polarization converter.

[0138] The above embodiments are to be understood as illustrative examples of the invention. Further examples of the invention are envisioned.

[0139] For example, the goal of obtaining wide-area polarization control, potentially encompassing the entire Poincaré sphere, has been described above. However, in some instances, control encompassing the entire Poincaré sphere may not be required. For example, an application of the described polarization control devices may be to make small polarization changes to an unknown input polarization state, where the input state is unknown but roughly at a particular polarization, and is not unknown and drawn from all possible polarizations on the Poincaré sphere. In one such instance, it may be sufficient to use multiple closely aligned rotation axes to perform the required polarization control. Similarly, when operating in a polarization scrambler configuration, the polarization control device need not generate polarization states from the entire Poincaré sphere, but rather may only be required to generate polarization states from a portion of the Poincaré sphere.

[0140] In various embodiments herein, the polarization control device includes a waveguide layer between and in contact with a first cladding layer and a second cladding layer, where the first cladding layer is a substrate layer. In other examples, there may be no second cladding layer, as will be understood by those skilled in the art. In such examples, the waveguide layer may be in contact only with the first cladding layer and may otherwise be surrounded by, for example, air. In other examples, the first cladding layer is an additional layer disposed on a substrate of a photonic integrated circuit. The cladding layers may comprise the same material or may have different compositions from one another, for example, determined by the optical performance of the polarization converter.

[0141] The polarization converter may be in series with the light source and may be used to control the polarization output by the light source. The light source may form part of the photonic integrated circuit in which the polarization control device is fabricated, or alternatively may be external to the PIC and coupled into the polarization control device.

[0142] The above examples include control elements in the form of electrical contact layers for carrier injection or carrier depletion of the polarization converter such that a voltage is applied across the polarization converter. However, in other examples, carrier injection / depletion may not occur while a voltage is applied. Also, in further examples, a different control element, e.g., a heating element, may instead be used to change the effective refractive index of the supported mode, or a mechanical element such as an acousto-optic modulator may instead be used as at least one control element of the polarization control device.

[0143] In the above example, different orientations of polarization of modes supported by the polarization converters are achieved by varying the cross-sectional width between each polarization converter. In other examples, variations in other aspects of the cross-sectional structure shape may instead or additionally be changed. For example, the internal angle of the tilted surface may be different between each polarization converter to change the tilt angle. In other examples, the relative amount of offset between portions of the waveguide layers may be varied to achieve different tilt angles. In still further examples, each polarization converter may have a completely different cross-sectional structure from the others to change the mode angle between the constituent polarization converters, for example, according to the embodiments presented herein or variations thereof. As a result, the cross-sectional width may remain the same between each polarization converter, since varying aspects other than the cross-sectional width of the cross-sectional structure to change the polarization orientation of the supported modes can reduce or eliminate the requirement for tapered connection sections, thereby, for example, reducing optical loss in the polarization control device.

[0144] It is to be understood that any feature described in connection with any one embodiment may be used alone or in combination with other features described, and may also be used in combination with one or more features of any other embodiment, or in any combination of any other embodiment. Furthermore, equivalents and modifications not described above may be employed without departing from the scope of the appended claims.

Claims

1. 1. A polarization control device for a photonic integrated circuit, comprising: a first polarization converter configured to support a first mode and a second mode; a second polarization converter configured to support a third mode and a fourth mode; at least one control element; The first polarization converter comprises: a first cross-sectional structure in a plane perpendicular to a first optical propagation axis of the first polarization converter; the first cross-sectional structure is configured to at least partially determine the polarization orientation of the first mode and the polarization orientation of the second mode, respectively, and the polarization orientation of the first mode is different from the polarization orientation of the second mode; the first mode has a higher effective refractive index than the second mode; The second polarization converter comprises: a second cross-sectional structure in a plane perpendicular to a second optical propagation axis of the second polarization converter; the second cross-sectional structure is configured to at least partially determine a polarization orientation of the third mode and a polarization orientation of the fourth mode, respectively, the polarization orientation of the third mode being different from the polarization orientation of the fourth mode; the third mode has a higher effective refractive index than the fourth mode; the first polarization converter is connected in series with the second polarization converter; The at least one control element is responsive to at least one signal to: modifying the effective refractive index of the first mode and the second mode; configured to modify the effective refractive index of the third mode and the fourth mode; a polarization control device, wherein the first cross-sectional structure differs from the second cross-sectional structure such that the first mode and the second mode have a different polarization orientation than the third mode and the fourth mode.

2. the first cross-sectional structure has a first cross-sectional width, the first cross-sectional width being defined by a distance between opposing surfaces of the first polarization converter in a direction perpendicular to the first optical propagation axis; the second cross-sectional structure has a second cross-sectional width, the second cross-sectional width being defined by a distance between opposing surfaces of the second polarization converter in a direction perpendicular to the second optical propagation axis; 2. The polarization control device of claim 1, wherein the first cross-sectional width is different from the second cross-sectional width such that the first mode and the second mode have a different polarization orientation than the third mode and the fourth mode.

3. 3. The polarization control device of claim 2, wherein at least one of the first cross-sectional width or the second cross-sectional width is in a direction parallel to a surface of the polarization control device for placement on a substrate of the photonic integrated circuit.

4. the first cross-sectional structure includes a first inclined surface, the first inclined surface having an acute or obtuse interior angle with a surface of the first cross-sectional structure adjacent to the first inclined surface; or the second cross-sectional structure includes a second inclined surface, the second inclined surface having an acute or obtuse interior angle with a surface of the second cross-sectional structure adjacent to the second inclined surface; The polarization control device according to any one of claims 1 to 3, wherein the polarization control device is at least one of:

5. The first cross-sectional structure is a first portion at least partially bounded by a first surface and a second surface; a second portion at least partially bounded by a third surface and a fourth surface; the first surface is offset from the third surface in a direction perpendicular to the first optical propagation axis, and the first surface is connected to the third surface by a first bonding surface; the second surface is offset from the fourth surface in a direction perpendicular to the first optical propagation axis, and the second surface is connected to the fourth surface by a second interface; or The second cross-sectional structure is a third portion partially bounded by a fifth surface and a sixth surface; a fourth portion partially bounded by a seventh surface and an eighth surface; the fifth surface is offset from the seventh surface in a direction perpendicular to the second optical propagation axis, and the fifth surface is connected to the seventh surface by a third bonding surface; the sixth surface is offset from the eighth surface in a direction perpendicular to the second optical propagation axis, and the sixth surface is connected to the eighth surface by a fourth bonding surface. The polarization control device according to any one of claims 1 to 4, wherein the polarization control device is at least one of:

6. the first surface, the second surface, the third surface, and the fourth surface are parallel to one another; or the fifth surface, the sixth surface, the seventh surface, and the eighth surface are parallel to one another; 6. The polarization control device of claim 5, wherein the polarization control device is at least one of:

7. A polarization control device described in any one of claims 1 to 6, wherein the first cross-sectional structure includes a first intermediate portion, the second cross-sectional structure includes a second intermediate portion, and the structure of the second intermediate portion is a mirror image of the structure of the first intermediate portion in a plane perpendicular to the first light propagation axis and / or the second light propagation axis.

8. the first polarization converter has a length parallel to the first optical propagation axis that is substantially equal to one-quarter the beat length of the wavelength of the input light multiplied by an odd integer; or the second polarization converter has a length parallel to the second optical propagation axis that is substantially equal to one-quarter the beat length of the wavelength of the input light multiplied by an odd integer. The polarization control device according to any one of claims 1 to 7, wherein the polarization control device is at least one of:

9. the first cross-sectional structure includes a first waveguide layer between and in contact with a first cladding layer and a second cladding layer, the first waveguide layer having a higher refractive index than the first cladding layer and the second cladding layer; or the second cross-sectional structure includes a second waveguide layer between and in contact with a third cladding layer and a fourth cladding layer, the second waveguide layer having a higher refractive index than the third cladding layer and the fourth cladding layer; The polarization control device according to any one of claims 1 to 8, wherein the polarization control device is at least one of:

10. the first waveguide layer is indium gallium arsenide phosphide or indium aluminum gallium arsenide, and the first cladding layer and the second cladding layer are each indium phosphide; or the second waveguide layer is indium gallium arsenide phosphide or indium aluminum gallium arsenide, and the third cladding layer and the fourth cladding layer are each indium phosphide; 10. The polarization control device of claim 9, wherein the polarization control device is at least one of:

11. the first cross-sectional structure has a first cross-sectional width, the first cross-sectional width being defined by a distance between opposing surfaces of the first polarization converter in a direction perpendicular to the first optical propagation axis, and the first cross-sectional width varying along a length parallel to the first optical propagation axis; or the second cross-sectional structure has a second cross-sectional width, the second cross-sectional width being defined by the distance between opposing surfaces of the second polarization converter in a direction perpendicular to the second optical propagation axis, and the second cross-sectional width varying along a length parallel to the second optical propagation axis. The polarization control device according to any one of claims 1 to 10, wherein the polarization control device is at least one of:

12. the first cross-sectional structure has a first cross-sectional width, the first cross-sectional width being defined by a distance between opposing surfaces of the first polarization converter in a direction perpendicular to the first optical propagation axis; the second cross-sectional structure has a second cross-sectional width, the second cross-sectional width being defined by a distance between opposing surfaces of the second polarization converter in a direction perpendicular to the second optical propagation axis; the polarization control device includes a connecting waveguide; the first polarization converter and the second polarization converter are joined by the connecting waveguide; the connecting waveguide has a cross-sectional width in a direction perpendicular to the optical propagation axis of the first connecting waveguide, A polarization control device according to any one of claims 1 to 11, wherein the cross-sectional width of the connecting waveguide tapers from the first cross-sectional width to the second cross-sectional width along the length of the connecting waveguide.

13. The polarization control device of claim 12 , wherein the tapering of the connecting waveguide comprises an adiabatic taper.

14. a third polarization converter configured to support a fifth mode and a sixth mode; The third polarization converter comprises: a third cross-sectional structure in a plane perpendicular to a third optical propagation axis of the third polarization converter; the third cross-sectional structure is configured to at least partially determine a polarization orientation of the fifth mode and the sixth mode, the polarization orientation of the fifth mode being different from the polarization orientation of the sixth mode; the fifth mode has a higher effective refractive index than the sixth mode; the at least one control element is configured to modify the effective refractive index of the fifth mode and the sixth mode in response to the at least one signal and / or at least one further signal; The third cross-sectional structure is the first cross-sectional structure such that the fifth mode and the sixth mode each have a different polarization orientation than the first mode and the second mode; or the second cross-sectional structure such that the fifth mode and the sixth mode each have a different polarization orientation than the third mode and the fourth mode; The polarization control device according to any one of claims 1 to 13, which is different from at least one of

15. The at least one control element is responsive to the at least one signal to: applying an electric field across the first polarization converter to change the effective refractive index of the first mode and the second mode; and / or The polarization control device according to any one of claims 1 to 14, which is an electrode configured to change the effective refractive index of the third mode and the fourth mode by applying an electric field across the second polarization converter.

16. 16. A polarization control device according to any one of claims 1 to 15, wherein the polarization control device is operable to perform at least two different polarization control functions depending on the at least one signal received by the at least one control element.

17. The polarization control device of claim 16 , wherein the polarization control device is operable as at least one of a polarization scrambler or a polarization controller.

18. A method for manufacturing a polarization control device according to any one of claims 1 to 17.

19. 1. A photonic integrated circuit comprising a polarization control device, a first polarization converter configured to support a first mode and a second mode; a second polarization converter configured to support a third mode and a fourth mode; at least one control element; The first polarization converter comprises: a first cross-sectional structure in a plane perpendicular to a first optical propagation axis of the first polarization converter; the first cross-sectional structure is configured to at least partially determine the polarization orientation of the first mode and the polarization orientation of the second mode, respectively, and the polarization orientation of the first mode is different from the polarization orientation of the second mode; the first mode has a higher effective refractive index than the second mode; The second polarization converter comprises: a second cross-sectional structure in a plane perpendicular to a second optical propagation axis of the second polarization converter; the second cross-sectional structure is configured to at least partially determine a polarization orientation of the third mode and a polarization orientation of the fourth mode, respectively, the polarization orientation of the third mode being different from the polarization orientation of the fourth mode; the third mode has a higher effective refractive index than the fourth mode; the first polarization converter is connected in series with the second polarization converter; The at least one control element is responsive to at least one signal to: modifying the effective refractive index of the first mode and the second mode; configured to modify the effective refractive index of the third mode and the fourth mode; The photonic integrated circuit, wherein the first cross-sectional structure differs from the second cross-sectional structure such that the first mode and the second mode have a different polarization orientation than the third mode and the fourth mode.

20. a light source for inputting light into the polarization control device; 20. The photonic integrated circuit of claim 19, further comprising: an output waveguide for receiving light from the polarization control device.

21. a third polarization converter configured to support a fifth mode and a sixth mode; The third polarization converter comprises: a third cross-sectional structure in a plane perpendicular to a third optical propagation axis of the third polarization converter; the third cross-sectional structure is configured to at least partially determine a polarization orientation of the fifth mode and the sixth mode, the polarization orientation of the fifth mode being different from the polarization orientation of the sixth mode; the fifth mode has a higher effective refractive index than the sixth mode; the at least one control element is configured to modify the effective refractive index of the fifth mode and the sixth mode in response to the at least one signal and / or at least one further signal; The third cross-sectional structure is the first cross-sectional structure such that the fifth mode and the sixth mode each have a different polarization orientation than the first mode and the second mode; or the second cross-sectional structure such that the fifth mode and the sixth mode each have a different polarization orientation than the third mode and the fourth mode; 21. A photonic integrated circuit according to claim 19 or 20, wherein said integrated circuit is different from at least one of

22. A system comprising the photonic integrated circuit according to any one of claims 19 to 21, a control system configured to provide the at least one signal to the at least one control element to at least partially determine a polarization of light in the output waveguide.

23. 1. A method of manufacturing a polarization control device, comprising: forming a first polarization changer, the first polarization changer including a first cross-sectional structure in a plane perpendicular to a first optical propagation axis of the first polarization changer, the first cross-sectional structure configured to support a first mode and a second mode, the polarization orientation of the first mode being different from the polarization orientation of the second mode, and the first mode having a higher effective refractive index than the second mode; forming a second polarization converter, the second polarization converter including a second cross-sectional structure in a plane perpendicular to a second optical propagation axis of the second polarization converter, the second cross-sectional structure configured to support a third mode and a fourth mode, the polarization orientation of the third mode being different from the polarization orientation of the fourth mode, and the third mode having a higher effective refractive index than the fourth mode; the first polarization converter is connected in series with the second polarization converter; the first cross-sectional structure is different from the second cross-sectional structure such that the polarization orientations of the first and second modes are different from the polarization orientations of the third and fourth modes; forming at least one control element, the at least one control element configured to change an effective refractive index of at least the first mode and the second mode, or the third mode and the fourth mode, in response to at least one signal.

24. the first cross-sectional structure has a first cross-sectional width, the first cross-sectional width being defined by a distance between opposing surfaces of the first polarization converter in a direction perpendicular to the first optical propagation axis; the second cross-sectional structure has a second cross-sectional width, the second cross-sectional width being defined by a distance between opposing surfaces of the second polarization converter in a direction perpendicular to the second optical propagation axis; 24. The method of claim 23, wherein the first cross-sectional width is different from the second cross-sectional width such that the first mode and the second mode have a different polarization orientation than the third mode and the fourth mode.

25. forming at least the first polarization converter or the second polarization converter includes forming at least the first cross-sectional structure or the second cross-sectional structure having an inclined surface; 25. The method of claim 23 or 24, wherein the angled surface has an acute or obtuse interior angle with respect to a surface of the first cross-sectional structure or the second cross-sectional structure adjacent the angled surface.

26. 26. The method of any one of claims 23 to 25, wherein the at least one control element is an electrode configured to apply an electric field across at least one of the first polarization converter or the second polarization converter in response to the at least one signal.

27. 1. A method for controlling light polarization in a photonic integrated circuit, comprising: receiving light at a polarization control device of the photonic integrated circuit; The polarization control device a first polarization converter configured to support a first mode and a second mode; a second polarization converter configured to support a third mode and a fourth mode; at least one control element; The first polarization converter comprises: a first cross-sectional structure in a plane perpendicular to a first optical propagation axis of the first polarization converter; the first cross-sectional structure is configured to at least partially determine a polarization orientation of the first mode and the second mode, the polarization orientation of the first mode being different from the polarization orientation of the second mode; the first mode has a higher effective refractive index than the second mode; The second polarization converter comprises: a second cross-sectional structure in a plane perpendicular to a second optical propagation axis of the second polarization converter; the second cross-sectional structure is configured to at least partially determine a polarization orientation of the third mode and the fourth mode, the polarization orientation of the third mode being different from the polarization orientation of the fourth mode; the third mode has a higher effective refractive index than the fourth mode; the first polarization converter is connected in series with the second polarization converter; The at least one control element is responsive to at least one signal to: modifying the effective refractive index of the first mode and the second mode; configured to modify the effective refractive index of the third mode and the fourth mode; the first cross-sectional structure differs from the second cross-sectional structure such that the first mode and the second mode have a different polarization orientation than the third mode and the fourth mode; The method comprises: receiving light at the first polarization converter of the polarization control device; determining a required phase shift between the first mode and the second mode; controlling the at least one control element to effect the required phase shift between the first mode and the second mode; receiving light at the second polarization converter of the polarization control device; determining a required phase shift between the third mode and the fourth mode; and controlling the at least one control element to effect the required phase shift between the third mode and the fourth mode.

28. the at least one control element comprises an electrode in electrical contact with the first polarization converter, and applies a voltage across the first polarization converter to modify the effective refractive index of the first mode and the second mode to produce the required phase shift between the first mode and the second mode; or the at least one control element comprises an electrode in electrical contact with the second polarization converter, and applies a voltage across the second polarization converter to modify the effective refractive index of the third mode and the fourth mode to produce the required phase shift between the third mode and the fourth mode.

28. The method of claim 27, wherein the at least one of

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