Polarization control

Abstract

The invention discloses a polarization control device for a photonic integrated circuit. The polarization control device comprises a first polarization converter and a second polarization converter, the first polarization converter has a first cross-sectional structure and supports a first mode and a second mode with different effective refractive indexes and different polarization orientations. The second polarization converter has a second cross-sectional structure and supports a third mode and a fourth mode having different effective refractive indices from each other and different polarization orientations from each other. A control element modifies the effective refractive indices in response to a signal.

Description

Background Art

[0001] Polarization control systems can be used in photonic integrated circuits (PICs) to perform polarization control of light. A polarization control system can be designed and manufactured for use in a specific application within a PIC. For example, a PIC can include a polarization control system for changing light of a first polarization to light of a second polarization. BRIEF DESCRIPTION OF THE DRAWINGS

[0002] Figure 1 A top view of a first light polarization control device according to the first example is schematically shown.

[0003] Figure 2 A cross-sectional structure of a first polarization converter of the optical polarization control device according to the first example is schematically illustrated.

[0004] Figure 3 A cross-sectional structure of a first and a second polarization converter of a light polarization control device according to a first example is schematically illustrated.

[0005] Figure 4a and Figure 4b A first polarization of light and a second polarization of light directed through a polarization converter are schematically depicted according to an example.

[0006] Figure 5a and Figure 5b A side cross section of a first and a second optical polarization converter according to a first example is schematically shown.

[0007] Figure 6a , Figure 6b , Figure 7a and Figure 7b The control of a polarization state of light using a polarization control device according to an example is schematically illustrated.

[0008] Figure 8 A top view of a polarization control device according to the second example is schematically shown, and a diagram of a Poincare sphere for polarization control of the polarization control device according to the second example is schematically shown.

[0009] Fig. 9 A top view of a light polarization control device according to a further example is schematically shown.

[0010] Fig.10 A cross-sectional structure of a polarization converter of a light polarization control device according to a further example is schematically illustrated.

[0011] Fig.11 A cross-sectional structure of a polarization converter of a light polarization control device according to yet a further example is schematically illustrated.

[0012] Fig.12 A cross-sectional structure of a polarization converter of a light polarization control device according to yet a further example is schematically illustrated.

[0013] Fig.13 A cross-sectional structure of a polarization converter of a light polarization control device according to yet a further example is schematically illustrated.

[0014] Fig.14 Schematically depicting an example including Figure 1 A system of photonic integrated circuits featuring a polarization control device.

[0015] Fig.15 is a flow chart illustrating a method of controlling the polarization state of light in a photonic integrated circuit using a polarization control device described herein, according to an example.

[0016] Fig.16 is a flow chart illustrating a method of manufacturing a polarization control device described herein according to further examples. DETAILED DESCRIPTION

[0017] The examples described herein are related to a semiconductor structure for a PIC. More specifically, the examples described herein are related to an optical polarization control device for a PIC. Such an optical polarization control device may be, for example, a polarization scrambler or a polarization controller, and may be simply referred to as a polarization control device.

[0018] In some examples, a PIC is composed of basic building blocks intended for constructing a PIC. Basic building blocks include various components that each have a specific function. An example of a basic building block is a waveguide structure. Basic building blocks can have a specific effect on light incident on them. The examples described herein are about a light polarization control device that can be used as a basic building block of a PIC.

[0019] An optical polarization control device is a device that can be used, for example, to actively control the polarization of light. Active control refers to the state of the polarization of light that can be dynamically modified during use of the device. In this way, the functionality of the device itself is dynamically modifiable during use, in contrast to a passive effect in which the functionality of a device is fixed at the time of manufacture of the device. For example, a properly configured optical polarization control device can be used to modify the polarization of any input polarization to a specific output polarization. For example, an arbitrary input polarization (e.g., linear, elliptical, or circular) is changed to vertically polarized light by a properly configured optical polarization control device. In this example, the polarization control device acts as a polarization controller. In another example, an input polarization can be modified to a random output polarization. In a first example, a vertically polarized input can be changed to an elliptical polarization, and in a second example, a vertically polarized input can be changed to a circular polarization, and in a third example, a vertically polarized input can be changed to a linear polarization. In this example, the polarization control device acts as a polarization scrambler.

[0020] Optical polarization control devices can be fabricated from a combination of polarization converters, which are typically passive devices that produce a fixed change in polarization state, arranged in series with an interferometer and a phase shifter. For example, the sequence of components (polarization converter, interferometer, and phase shifter) required to implement a polarization controller is typically different from the sequence of components required to implement a polarization scrambler. In such instances, the accuracy of the polarization scrambler and controller may depend on the performance of the constituent polarization converters.

[0021] In the examples described herein, a polarization control device herein includes a combination of actively controlled polarization converters, such that the same combination of components can be used to implement a plurality of different functions, such as the functionality of a polarization controller and / or a polarization scrambler.

[0022] In examples described herein, a polarization control device has: a first polarization converter having a first cross-sectional structure, the first polarization converter supporting a first mode and a second mode with different polarization orientations and different effective refractive indices, and a second polarization converter having a second cross-sectional structure, the second polarization converter supporting a third mode and a fourth mode with different polarization orientations and different effective refractive indices. At least one control element is configured to change the effective refractive index of the first, second, third and fourth modes. In examples, this combination of features gives a polarization control device that is more compact than known polarization control devices, which reduces a footprint (occupied surface area) of the polarization control device on a PIC. The polarization control device can additionally implement the functions of both a polarization controller and a polarization scrambler with appropriate control signals.

[0023] Figure 1A top view of a light polarization control device 10 of a first embodiment is schematically shown. The light polarization control device 10 is used in a PIC. Figure 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 appreciate that the inverse relationship in which light propagates in a direction opposite to that described above also applies to the described components in series. More generally, an optical polarization control device according to an example herein includes at least two polarization converters arranged in series, and in an example may include an input waveguide, an output waveguide and a plurality of connecting waveguides for joining the input waveguide, the output waveguide and a plurality of polarization converters. In use, light may be considered to be substantially parallel to a light propagation axis (which is in Figure 1 The light propagates through the light polarization control device 10 in one direction (indicated by arrow 18).

[0024] Figure 2 Schematically depicted Figure 1 The side cross section of the first polarization converter 100 of the polarization control device 10 is shown in FIG. 1 ; the side cross section is perpendicular to a first light propagation axis LPA (which is Figure 2 The cross-sectional structure of the first polarization converter 100 is viewed in a cross-sectional plane (into the page, as indicated by symbol 118). The optical polarization converter 100 is used for a PIC, and a substrate plane of the PIC 1001 is in Figure 2 The optical polarization converter 100 is located below the optical polarization converter 100. 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 (e.g., silicon carbide).

[0025] The cross-sectional structure of a polarization converter refers to the composition of the polarization converter in a cross-sectional plane perpendicular to a light propagation axis of the polarization converter. That is, it refers to the geometry and material composition of the constituent layers forming the polarization converter, the geometry being described in a plane perpendicular to the light propagation axis. More generally, the cross-sectional structure of a component configured to guide light refers herein to the geometry and material composition of the constituent layers forming the component in a cross-sectional plane perpendicular to a light 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 (in this example, it 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 will modify the refractive index of the first and second cladding layers and the first waveguide layer 108 by using an electro-optic effect (such as the Pockels effect, the Kerr effect, the plasmon and / or the band filling effect) which will be described later.

[0026] The first waveguide layer 108 includes a first portion 110 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 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 each other.

[0027] The first surface 104a is offset from the third surface 106a along a first axis 114 and a second axis 116 each perpendicular to a light propagation axis for converting the polarization of light. The first surface 104a is connected to the third surface 106a via 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 via a second bonding surface 120b of the first cladding layer 130. The second portion 112 is thereby offset 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.

[0028] The first axis 114 is relative to Figure 2 The horizontal axis of the orientation shown in FIG. 1 and the second axis 116 is relative to Figure 2104a 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 length of each portion described herein is along the first light propagation axis as indicated by the symbol 118. As referred to herein, the terms height, top, and bottom are relative to the second axis 116. Figure 2 , the first surface 104a is a lower surface of the substrate 102, and the third surface 106a is an upper surface of the substrate 102. The upper surface 106a provides a step upward (in a direction parallel to the vertical axis) from the lower surface 104a. Therefore, in this example, the first surface 104a and the third surface 106a are offset in position along the vertical axis 116, for example, such that the first surface 104a and the third surface 106a are displaced and / or vertically spaced from each other, and the second surface 104b and the fourth surface 106b are displaced and / or vertically spaced from each other.

[0029] 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 a first waveguide layer 108 is provided in which light can propagate along a first light propagation axis indicated by symbol 118, and the first waveguide layer 108 has different portions offset from each other in a direction parallel to the vertical axis 116.

[0030] exist Figure 2 In the example of , the first engagement surface 120a and the second engagement surface 120b are inclined surfaces. As referred to herein, one surface being inclined relative to another surface means that there is a non-zero angle between the two surfaces in question, and the angle between the two surfaces is either an obtuse angle or an acute angle. In this context, reference may be made to a surface being tilted relative to another surface. The terms "tilted," "angled," and "tilted" are used interchangeably herein to refer to the angle of one entity relative to another entity or relative to a given axis. For example, in Figure 2, the first bonding surface 120a has an obtuse internal angle relative to the first surface 104a and the third surface 106a. In other words, the first bonding surface 120a is non-orthogonal to the first surface 104a and the third surface 106a. The second bonding surface 120b has an obtuse internal angle relative to 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 mentioned herein, the first bonding surface 120a between the first surface 104a and the third surface 106a means that (for example) the first bonding surface 120a is inserted between the first surface 104a and the third surface 106a and is (for example) directly adjacent to each of the first surface 104a and the third surface 106a. Therefore, in an example, the first bonding surface 120a, the first surface 104a and the third surface 106a can be considered together as constituting the top surface of the substrate 102 together.

[0031] exist Figure 2 In an example of , 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 this example, the first engagement surface 120a is non-orthogonal to the first surface 104a. In this example, the third surface 106a is offset from the first surface 104a along the first axis 114 by an amount greater than a 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 a position along the axis relative to the edge of the entity in question (the edge that is located at the earliest position along the axis). For example, in Figure 2 , the edge of the first surface 104a is located at a 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 would mean that the edges of each of the first and third surfaces 104a, 106a are located at the same position (position 124a along the first axis 114). Figure 2 In the example of the first axis 114, the offset is such that the third surface 106a does not overlap with the first surface 104a and the first joint surface 120a is in a vertical direction (relative to Figure 2 In this example, the edge of the third surface 106a is located at a position 126 along the first axis 114 away from the position 124a by a distance greater than the width 122 of the first surface 104a.

[0032] A 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 to 124b relative to the horizontal axis 114 and has a first outer surface 144a having a horizontal position 124a and a second outer surface 144b having a horizontal position 124b, which are opposing surfaces of the first waveguide layer 108. The first cladding layer 130 also spans horizontal positions 124a to 124b relative to the horizontal axis 114. In other words, the first waveguide layer 108 is located between horizontal positions 124a and 124b relative to the first axis 114, or all structures positioned vertically above the substrate 102 are located between horizontal positions 124a and 124b. In this sense, the horizontal positions 124a and 124b define a first cross-sectional width 150 of the first cross-sectional structure and define a distance relative to the horizontal axis 114 parallel to the substrate 102.

[0033] exist Figure 2 In an example of the present invention, the first waveguide layer 108 includes a first intermediate waveguide portion 128 in contact with the first and second bonding surfaces 120a, 120b and between the first portion 110 and the second portion 112. In some examples, as described, the substrate 102 having surfaces 104a, 106a offset in a direction parallel to the second axis 116 and an inclined bonding surface 120a is at least partially formed before the first waveguide layer 108 on top of the substrate 102. In this way, due to the inclined bonding surface 120a, an intermediate portion of the waveguide is formed at an angle corresponding to the angle of the bonding surface 120a of the substrate. Therefore, in examples, the first portion 110, the second portion 112, and the first intermediate waveguide portion 128 can be considered to constitute the first waveguide layer 108 together.

[0034] As described, in Figure 2 In the example of , the first portion 110 is in contact with the first surface 104a of the substrate 102. On the other hand, the second portion 112 is in contact with the third surface 106a that is offset from the first surface 104a in a direction parallel to the second axis 116. This means that the first waveguide layer 108 includes two portions that are offset from each other in a direction parallel to the second axis 116. As described, in this example, the first waveguide layer 108 also includes a first intermediate waveguide portion 128 that is angled relative to the second portion 112 and the first portion 110, as shown in FIG. Figure 2 As shown in .

[0035] Figure 2A polarization converter 100 is shown having a cross-sectional structure including a portion offset in a direction parallel to a vertical axis 116 and an inclined surface relative to a surface of the cross-sectional structure. In other examples described later, the polarization converter may include a cross-sectional structure characterized by an offset portion in a direction parallel to the vertical axis 116 but without an inclined surface, or the polarization converter may include a cross-sectional structure characterized by an inclined surface but without an offset portion. Providing a polarization converter having a cross-sectional structure including at least one of an offset portion and / or an inclined surface provides conversion of polarization of light, as described in further detail below.

[0036] because Figure 2 With a structure of a substrate 102 having a first surface and a second surface 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 (e.g., to define an angled sidewall) on the first waveguide layer 108. For example, one or more materials for the first waveguide layer 108 can be epitaxially grown on the substrate 102, and due to the offset arrangement of the substrate 102, the first waveguide layer 108 having a portion offset in a direction parallel to the vertical axis 116 is formed.

[0037] 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 have indium gallium arsenide phosphide (InGaAsP). However, in other examples, the first waveguide layer 108 includes or has, for example, indium aluminum gallium arsenide (InAlGaAs) having efficient electro-refractive properties. More generally, in some examples, the first waveguide layer 108 includes (Al)InGaAs(P). The elements indicated in brackets are interchangeable and the composition of different elements is selected depending on the desired function. For example, the composition of Ga and As in InGaAs may be selected according to the desired band gap. In some examples, the first waveguide layer 108 is a layer of (Al)InGaAs(P). In other examples, the first waveguide layer 108 includes a plurality of sublayers. In some such examples, the first waveguide layer 108 includes a (Al)InGaAs(P) / (Al)InGaAs(P) multiple quantum well structure in contact with the substrate 102. In some examples, the thickness of the sub-layers is between 5 nanometers and 30 nanometers. The sub-layer stack of the first waveguide layer 108 has a band gap selected according to the desired application of the optical polarization converter 100 .

[0038] As will be appreciated by those skilled in the art, for example, the bandgap, and therefore the refractive index, of InGaAsP can be tuned. 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 propagating light having a wavelength of 1550 nanometers) or 1100 nanometers (e.g., for propagating light having a wavelength of 1310 nanometers). In other examples, the wavelength to which the bandgap is tuned is different.

[0039] The first waveguide layer 108 is used to guide light. The properties of a waveguide layer, including, for example, its 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) in which light can propagate. The first waveguide layer 108 acts as a core layer and has a refractive index that is higher than the refractive index of the surrounding cladding material (in this example, the first cladding layer 130 and the substrate 102). In this case, the core-cladding boundary formed by the surfaces 106a, 106b, 120a, 120b, 104a, 104b in contact with the first waveguide layer 108 can be considered to cause constructive interference of light (which limits the propagation of light within the first waveguide layer 108).

[0040] For example, depending on the desired application of the optical polarization converter 100, a particular optical mode of light is desired to propagate through the first waveguide layer 108. 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 of light propagating in the waveguide and the negative of the Poynting vector. The optical propagation axis is the general direction in which the energy of the optical mode travels through the waveguide 108. The term "mode" as used herein, for example, refers to an optical mode, which can be considered an electromagnetic propagation mode. A mode of a particular waveguide is described herein as being "supported" by the waveguide.

[0041] This arrangement of the first waveguide layer 108 provides 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 indicating the first light propagation axis. However, it should be understood that similar principles apply to light having a different polarization.

[0042] Figure 3 The light polarization control device 10 of the first example is schematically shown. Figure 2 A top view of the cross-sectional structure of the first polarization converter 100 described in FIG. 1 and the cross-sectional structure of the second polarization converter 200 to be described now.

[0043] The second polarization converter 200 has a second cross-sectional structure in a plane perpendicular to a second light propagation axis 218 (enter the page, see symbol 218) of the second polarization converter, and the cross section of the second polarization converter 200 is obtained in a plane parallel to the indicator line 2X. The cross section of the first polarization converter 100 is obtained in a plane parallel to the indicator line 1X. The cross-sectional structure of the first polarization converter is 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 a third axis 214 and a fourth axis 216. Figure 3 , the first axis 114 and the third axis 214 are substantially parallel (within an acceptable tolerance), wherein 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 remain parallel to each other, and thus 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 a cross section of, for example, the first polarization converter, the first axis is still perpendicular to the first light propagation axis 118, and within a cross section of, for example, the second polarization converter, the third axis 214 is still perpendicular to the second light propagation axis 218.

[0044] 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 (in this example, the substrate 202). As for the first polarization converter 100, the second waveguide layer 208 here acts as a core layer and has a refractive index higher than the refractive index of the surrounding cladding layer materials (in this example, the third cladding layer 230 and the substrate 202 as a fourth cladding layer). The second waveguide layer 208 may include a material composition similar to or the same as the first waveguide layer 108.

[0045] The second waveguide layer 208 includes a third portion 210 partially bounded by and in contact with the fifth surface 204a of the substrate 202 and the sixth surface 204b of the third cladding layer 230, and a fourth portion 212 partially bounded by and in contact with a seventh surface 206a of the substrate and an eighth surface 206b of the third cladding layer 230. The fifth surface 204a, the sixth surface 204b, the seventh surface 206a, and the eighth surface 206b are parallel to each other.

[0046] 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 through 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 through a fourth bonding surface 220b of the third cladding layer 230. The fourth portion 212 is thereby offset 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 (such as within acceptable manufacturing tolerances) to the fourth bonding surface 220b. In the example, the third bonding surface 220a, the fifth surface 204a and the seventh surface 206a can be considered together to constitute the top surface of the substrate 202.

[0047] Similar to the first bonding surface 120a and the second bonding surface 120b of the first polarization converter 100, the third bonding surface 220a and the fourth bonding surface 220b are inclined surfaces. The third bonding surface 220a has an obtuse internal angle relative to the fifth surface 204a and the seventh surface 206a. In other words, it is not orthogonal to the fifth surface 204a or the seventh surface 206a. The fourth bonding surface 220b has an obtuse internal angle relative to the sixth surface 204b and the eighth surface 206b. In other words, it is not orthogonal to the sixth surface 204b and the eighth surface 206b.

[0048] exist Figure 3 , the second waveguide layer 208 includes a second intermediate waveguide portion 228 in contact with the third and fourth bonding surfaces 220a, 220b and between the third portion 210 and the fourth portion 212. In some examples, as described, the substrate 202 having the surfaces 204a, 206a offset in a direction parallel to the fourth axis 216 and an inclined bonding surface 220a is at least partially formed before the second waveguide layer 208 on top of the substrate 202. In this way, due to the inclined bonding surface 220a, a second intermediate portion 228 of the second waveguide layer 208 is formed at an angle corresponding to the angle of the bonding surface 220a of the substrate. Therefore, in examples, the third portion 210, the fourth portion 212, and the second intermediate waveguide portion 228 can be considered to constitute the second waveguide layer 208 together.

[0049] A distance between the opposing surfaces of the second cross-sectional structure is defined by horizontal positions 224a and 224b, which are points along the horizontal axis 214. The second waveguide layer 208 spans the horizontal positions 224a to 224b relative to the horizontal axis 214 and has a first outer surface 244a having a horizontal position 224a and a second outer surface 244b having a horizontal position 224b, which are opposing surfaces of the second waveguide layer 208. The third cladding layer 230 also spans the horizontal positions 224a to 224b relative to the horizontal axis 214. In other words, the second waveguide layer 208 is located between the horizontal positions 224a and 224b relative to the third axis 214, or all structures positioned vertically above the substrate 202 are located between the horizontal positions 224a and 224b. In this sense, the horizontal positions 224a and 224b define a second cross-sectional width 250 of the second cross-sectional structure and define a distance relative to the horizontal axis 214 parallel to the substrate 202.

[0050] Although the first cross-sectional structure is topologically similar to the second cross-sectional structure, including similar overall structures, the specific dimensions of constituent surfaces and features of the second cross-sectional structure differ from the first cross-sectional structure of the first polarization converter 100, as described below.

[0051] exist Figure 3 , the first cross-sectional structure has a first portion 110 that can be considered a lower portion of the first cross-sectional structure and a second portion 112 that can be considered an upper 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. The lower portion 110 is farther along the horizontal axis 114 than the upper portion 112. In the second cross-sectional structure, the upper portion 212 is located farther along the horizontal axis 214 than the lower portion 210. In this way, the second middle portion 228 of the second waveguide layer 208 is angled in a direction opposite to the first middle portion 128 of the first waveguide layer 108. In other words, the first middle portion 128 can be considered to be tilted upward (along the vertical axis 116) in the right-to-left direction (opposite to the horizontal axis 114), while the second middle portion 228 can be considered to be tilted upward (along the vertical axis 216) in the left-to-right direction (along the horizontal axis 214), as shown in FIG. Figure 3 As shown in .

[0052] A width 222 of the fifth surface 204a and the sixth surface 204b along the third axis 214 is shorter than a width 122 of the first surface 104a along the first axis 114. Similarly, a width (not labeled) of the seventh surface 206a and the eighth surface 206b is shorter than an equivalent width of the third surface 106a and the fourth surface 106b. A width of the first intermediate portion 128 along the first axis 114 is the same as a width of the second intermediate portion 228 along the third axis 214. The intermediate portions 128, 228 are the same width, but are angled in opposite directions as described above, and thus the second intermediate portion 228 can be considered a mirror image of the first intermediate portion 128. Therefore, 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 merely an example.

[0053] 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 having a waveguide layer at the same height (relative to the vertical axis 116) as the waveguide layer 110, 210 or 112, 212 of the polarization converter. In other examples, the cross-sectional structure of the connecting waveguide 50 is similar to 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, 55b that tapers from the first cross-sectional width 150, 55a to the second cross-sectional width 250, 55b. This can reduce the loss caused by modal mismatch when coupling light from the first polarization converter 100 to the second polarization converter 200. In an example, the taper of the connecting waveguide 50 may take the form of an adiabatic taper, which may reduce propagation losses through the connecting waveguide 50 by ensuring a properly gradual and properly shaped transition of the propagation mode from a first waveguide width to a second waveguide width, as will be understood by those skilled in the art. Of course, those skilled in the art will appreciate that in an example, the first and second polarization converters 100, 200 may be butt-coupled together without a connecting waveguide 50.

[0054] Reference Figure 4a and Figure 4b Now, a function of the polarization converters 100 and 200 will be described. The following description will be in the context of the first polarization converter 100, but a function of the second polarization converter 200 will also be described. Figure 4a is about the input light and Figure 4b It is with respect to the output light. It should be noted that this is in the context of linearly polarized light incident on the polarization converter 100, but it will be appreciated that similar principles apply to light of different polarizations. Figure 4a and Figure 4b A transverse electric (TE) polarization axis 402 and a transverse magnetic (TM) polarization axis 404 are shown. Figure 4a and Figure 4b , the light propagation axis 118 enters the page (perpendicular to both the TE polarization axis 402 and the TM polarization axis 404). Figure 2 or Figure 3 , the TM polarization axis 404 is parallel to the second axis 116 , and the TE polarization axis 402 is parallel to the first axis 114 .

[0055] For linearly polarized light, the direction of the electric field of the light propagating as indicated by the first light propagation axis 118 can be indicated relative to the TE polarization axis 402 and the TM polarization axis 404. Arrow 406 indicates linearly polarized light that is TE polarized. The cross-sectional structure of the polarization converter 100 causes the modes supported by the polarization converter 100 to be tilted relative to the polarization axes 402, 404, which means that the modes supported by the polarization converter 100 are mixed modes. Figure 2 and Figure 3 In the polarization converter 100 of the form shown in FIG. 1 , the polarization converter 100 having the offset portions 110 , 112 and the inclined surfaces 120 a , 120 b provides boundary conditions for light propagating within the waveguide that result in a tilted mode.

[0056] The cross-sectional structure and resulting boundary conditions cause the polarization converter 100 to support a first hybrid mode having an electric field tilted relative to the TE axis. The terms "tilted," "angled," and "tilted" are used interchangeably herein to refer to the angle of the first hybrid mode relative 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 the polarization orientation of the second hybrid mode. As referred to herein, a hybrid mode is an optical mode having an electric field having a non-zero component along the TE polarization axis and a non-zero component along the TM polarization axis.

[0057] Figure 4a and Figure 4b A first mixed mode 408 and a second mixed mode 410 are shown. The first and second mixed modes 408, 410 illustrate an example of mixed modes that may exist within a polarization converter as light propagates through the polarization converter. In this example, the first and second mixed modes 408, 410 are generated by light having TE polarization (having an electric field along the TE polarization axis 402) incident on the optical polarization converter for propagation through the polarization converter as shown by arrows 406.

[0058] The tilt angle of the first hybrid mode 408 (relative to the TE axis 402) is assumed to be 45 degrees. This tilt angle of the first hybrid mode 408 is generated, for example, due to the arrangement of the polarization converter 100. In this example, the angle relative to the TE axis 402 of the second hybrid mode 410 is also 45 degrees and the first and second modes 408, 410 have electric fields of equal magnitude. Those skilled in the art will appreciate that having Figure 4a The first and second modes 408, 410, with a tilt angle of 45 degrees and phase relationship shown in FIG. 4, have equal and opposite components along the TM polarization axis 404, and in combination correspond to TE polarized light.

[0059] The second polarization converter 200 also supports hybrid modes, supporting a third hybrid mode and a fourth hybrid mode, the third hybrid mode having a polarization orientation different from the fourth hybrid mode. It should be noted that, as used herein, the second polarization converter 200 supporting a third mode and a fourth mode does not necessarily mean that the polarization converter 200 also supports a first and second hybrid mode, instead, "third" and "fourth" are labels used to distinguish the modes of the first polarization converter and the second polarization converter rather than implying (for example) specific support for several higher-order modes.

[0060] 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 of the third and fourth hybrid modes supported by the second polarization converter 200 are determined by the arrangement 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 fourth mode may be -67.5 degrees.

[0061] The tilt angle of a mode supported by a polarization converter has a dependency on the cross-sectional width of the cross-sectional structure of the polarization converter. In this way, two substantially identical (within acceptable tolerances) polarization converters may be initially manufactured, and a cross-sectional width of a second polarization converter of the pair may be reduced, for example by photolithography and / or etching, relative to a cross-sectional width of a first polarization converter of the pair, to thereby cause the mode supported by the second polarization converter to have a different tilt angle than the mode supported by the first polarization converter. This principle is easily extended to initially manufacturing more than two polarization converters and etching respective widths. Having a plurality of converters each supporting different tilt angles may help the polarization control device 10 access a larger region of the Poincare sphere or the entire Poincare sphere, as explained in more detail later.

[0062] Furthermore, the described arrangement of the cross-sectional structure of the polarization converters 100, 200 has a different propagation constant for the first and second mixed modes 408, 410 of the first polarization converter. The arrangement results in birefringence, so that the first and second mixed modes 408, 410 experience different effective refractive indices from each other when propagating within the polarization converter. An effective refractive index of a waveguide is a dimensionless number that describes the speed at which a particular mode of light travels through a waveguide and the way in which the light is attenuated through the waveguide. A refractive index of a material is a dimensionless number that describes a phase velocity of a light wave in the material and the way in which the light is attenuated through the material. The effective refractive index and the refractive index are usually expressed as a complex number; however, only the real component of the refractive index is considered herein. The real component of a 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 the refractive index depends on the wavelength of the light being considered. In this context, when a comparison is made between two effective refractive indices or between two refractive indices, the comparison is between real components of light of the same wavelength.

[0063] This means that the phase difference between the first and second hybrid modes 408, 410 changes as the first and second hybrid modes 408, 410 propagate. In other words, as the first and second modes 408, 410 propagate within the polarization converter, the phases of the first and second modes 408, 410 evolve differently, with the phase of light in one mode changing faster than the phase of light in the other mode. Those skilled in the art will appreciate that this description also applies to the third and fourth modes of the second polarization converter 200.

[0064] Figure 5a and Figure 5b Schematically drawn in parallel to Figure 3 1 and 2. The first polarization converter 100 has a length 302 parallel to the first light propagation axis 118. Similarly, the second polarization converter 200 has a length 322 parallel to the second light propagation axis 218.

[0065] When light in the first and second mixed modes propagates along the first polarization converter 100 with respective propagation constants, there is a resulting phase difference between the first and second mixed modes. Therefore, the length of the first polarization converter 100 determines the phase difference between the first and second modes after propagating through the first polarization converter 100 for fixed respective propagation constants. More generally, the optical path length of the first polarization converter 100 determines the phase difference generated between the first and second modes after propagating through the first polarization converter 100.

[0066] Consider an input waveguide that provides TE light 406 to the first polarization converter 100. The first hybrid mode 408 and the second hybrid mode 410 are excited with a relative phase difference of zero. The light in these modes propagates along the first polarization converter 100 for a length 302, which results in a relative phase difference of π, as shown by Figure 4b The position of the second mixed mode 410 in FIG. Figure 4b , the first hybrid mode 408 and the second hybrid mode 410 will excite TM light 412 in an output waveguide. In this way, the increase of a phase difference between the first and second hybrid modes 408, 410 has rotated an input TE mode 406 to an output TM mode 412. 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. A length 302 of the first polarization converter 100 used to restore the phase of the modes of light propagating in the first polarization converter 100 is called the beat length. For example, if the first and second modes start those propagations in phase within the first polarization converter 100, then after propagating an integer multiple of the beat length within the first polarization converter 100, the modes will be restored to be in phase.

[0067] As discussed above, the described arrangement of the first polarization converter 100 causes a different propagation constant for the first and second hybrid modes. The described arrangement of the first polarization converter 100 (having the described cross-sectional structure) causes the presence of birefringence because the first and second hybrid modes experience different effective refractive indices from each other. 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 in such propagation constants can be represented as Δβ=β1-β2. Those skilled in the art will appreciate that β represents phase propagation, i.e.:

[0068]

[0069] where λ is a given wavelength and n eff is the effective refractive index of a light mode at a given wavelength.

[0070] The following equation 1 shows the beat length L of the first polarization converter 100 for the first and second mixed modes: λ In the following equation (2), λ is a given wavelength and Δn represents the difference in effective refractive index of the first and second hybrid modes: Δn=n1-n2.

[0071]

[0072] In some instances, the beat length is affected by the thickness and / or cladding of the first polarization converter. In some instances, the first polarization converter is curved, and therefore the light propagation axis is curved, and the beat length is obtained along a curve. In such an instance, where the first hybrid mode 408 has a 45 degree angle relative to the TE axis 402, by selecting the length of the first polarization converter 100 to be an odd number multiplied by half of the beat length, the linear polarization of light of a given wavelength can be rotated as described above. For example, a first linear polarization (TE polarization in the above example) can be converted into a second linear polarization (TM polarization in the above example). As will be appreciated by those skilled in the art, phase control is the basis of a polarization converter or rotator. As will be appreciated, Figure 5b The second polarization converter 200 shown in FIG. Figure 5a The first polarization converter 100 controls the phase of light in one manner.

[0073] Figure 6a is a sketch of the Poincare sphere. Those skilled in the art will appreciate that all polarization states can be mapped onto the surface of the so-called Poincare sphere. Points located on the equator of the Poincare sphere represent all angles of linear polarization. The poles of the Poincare sphere represent clockwise and counterclockwise circular polarization. Figure 6a Points corresponding to TE polarization and TM polarization can be seen in .

[0074] The points corresponding to the first (HM1) and second (HM2) mixed modes are located on the equator of the Poincare sphere. Figure 6a The positions of the hybrid modes on the equator depend on the tilt (in other words, the angle) of the first hybrid mode relative to the TE axis.

[0075] In the case where the first hybrid mode has a 45 degree angle relative to the TE axis, the first hybrid mode corresponds to point HM1 and the second hybrid mode corresponds to point HM2. An axis passing through HM1 and HM2 is perpendicular to an axis passing through the TE and TM polarization points on the equator of the Poincare sphere. The propagation of the hybrid modes through the polarization converter, where their phases evolve differently from each other, corresponds to a rotation of a point representing the polarization around the axis passing through HM1 and HM2 when the hybrid modes are recombined. A 180 degree rotation around an axis passing through HM1 and HM2 results in a polarization conversion, for example, from TE polarization to TM polarization, where half of the circumference of the Poincare sphere is traversed. This corresponds to, for example, an optical length of the polarization converter that is half of a beat frequency length.

[0076] Figure 6bThe effect of active control of the effective refractive index of a mode of a polarization converter on the polarization states accessible through the polarization converter is shown, the active control being achieved using at least one control element such as an electrical contact layer (e.g., electrical contact layer 132 in the first polarization converter 100). The electrical contact layer 132 is configured to receive a control signal that determines the value of a positive voltage applied to the polarization converter, e.g., by carrier injection, or a negative voltage, e.g., by carrier depletion, wherein the choice of carrier injection or depletion depends on the material composition and structure of the polarization converter. A voltage, which may be referred to as a potential difference, is applied between the electrical contact layer 132 and the substrate of the photonic integrated circuit.

[0077] The application of a voltage and carrier injection / depletion to a polarization converter (such as any of the polarization converters of the examples herein) can produce 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 this case, the birefringence is constant and the optical length of the polarization converter changes by the same amount for both modes. Alternatively, the effective refractive index of the first mode can change by an amount different from the effective refractive index of the second mode. In this case, the birefringence of the polarization converter is modified by a control element. This causes the optical length of the polarization converter to change by an amount different from the optical length of the second mode for the first mode. In either case, the phase difference generated between the two modes can be controlled. The specific manner in which the phase difference is generated will depend on the system, such as the material composition and cross-sectional structure of the polarization converter.

[0078] exist Figure 6b In an example, with no voltage (V0) applied to the polarization converter through an electrical contact layer, an input state of light I2 is rotated about an axis passing through HM1 and HM2 to an output polarization state of light O2. When the applied voltage is changed to a voltage V1, the effective refractive index of the first and second modes changes so that a phase difference is generated more quickly between the first and second modes, meaning that the polarization of the input light changes to an output state O3, with a greater portion of the Poincare sphere traversed from the input I2 than the portion traversed by the polarization state to the output O2. When a voltage V2 is applied, V2>V1, an even larger portion of the Poincare sphere is traversed because the polarization state of the input state I2 is modified to output a polarization state O4. Active control of the polarization converter in this manner allows the use of a single polarization converter to control and select the change in polarization state as a rotation about the axis of rotation defined by the hybrid mode tilt angles HM1, HM2.

[0079] A polarization converter of a polarization control device having a length corresponding to one quarter of a beat frequency length means that the polarization converter passively produces a change in a polarization state across one quarter of the circumference of the Poincare sphere, i.e., the polarization state is modified by this amount without actively applying an electric field across the polarization converter. The passive effect of the polarization converter will produce a first phase difference, and the active control effect caused by the application of an electric field will produce a second phase difference that can be added to the first phase difference or reduce the first phase difference. It may be useful for a polarization converter to have a propagation length that is long enough so that active control of the polarization converter can change the optical length of the polarization converter by, for example, half of a beat frequency length to thereby change the polarization state by half of the Poincare sphere. Therefore, the length of a polarization converter of a polarization control device can be a multiple of the beat frequency length to achieve this degree of active polarization control.

[0080] Increasing the birefringence of a polarization converter and the change in 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 the reduction in the beat length according to Equation 2), which can reduce the overall spatial footprint of the device. For example, increased birefringence can be achieved by providing, for example, a multi-quantum well structure in the waveguide layer of the polarization converter, which in some examples can be sensitive to the highly polarization-sensitive quantum confined Stark effect.

[0081] Figure 7a and Figure 7b A diagram showing a polarization control device 10 described herein with two polarization converters (eg, Figure 2 and Figure 3 The control provided by the first and second polarization converters 100, 200 shown in FIG.

[0082] As described above, in the example, 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 to define a first rotation axis defined by HM1 and HM2 through the Poincare sphere. In the example, the second polarization converter 200 supports a third mode HM3 and a fourth mode HM4 having a tilt angle different from the first and second modes. The third and fourth modes HM3 and HM4 define a second rotation axis defined by HM3 and HM4 through the Poincare sphere. By providing two rotation axes, an input state can access a larger area of ​​the Poincare sphere than if only one rotation axis is provided.

[0083] The input state I2 has an initial polarization state. The input state I2 is input into a polarization converter 100, so that the acquisition of a phase difference between the first and second modes results in a rotation around the first rotation axis HM1, HM2 to produce an intermediate polarization state IS1 (the state of the light at the output of the first polarization converter 100). A voltage V1 is applied to the first polarization converter 100, i.e., under the condition (voltage V1 applied to the first polarization converter 100) to reach the intermediate state IS1. The intermediate state IS1 is then coupled into the third and fourth modes of the second polarization converter 200, so that the acquisition of a phase difference between the third and fourth modes results in a rotation around the second rotation axis HM3, HM4 to reach an output state O5 (in this case, it is a linear polarization). A voltage V1 is applied to the second polarization converter 200, i.e., under the condition The output state O5 is reached under (the voltage V1 applied to the first polarization converter 100 and the voltage V2 applied to the second polarization converter 200). The voltage may be applied to the first and second polarization converters 100, 200 using at least one control element which may include an electrical contact layer. The first and second polarization converters 100, 200 may be controlled using (several) separate control elements or both may be controlled using a common control element or a plurality of control elements.

[0084] exist Figure 7b In the embodiment, 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 and is modified to an intermediate polarization state IS2 after propagating through the first polarization converter 100 and a phase difference is added as previously described, wherein under the condition Down to IS2, Indicates a voltage V2 applied to the first polarization converter 100. The intermediate state IS2 is then coupled into the third and fourth modes of the second polarization converter 200 and is modified to the condition IS2 after propagating through the second polarization converter 200. The output state O6 is reached in which a voltage V2 has been applied across the first polarization converter 100 and a voltage V3 has been applied across the second polarization converter 200, wherein V3>V1>V2.

[0085] In this way, different polarization states for an output state can be achieved for an input state by modifying the voltage applied across the first and second polarization converters 100, 200 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 in refractive index and birefringence (which determine the rotation around the Poincare sphere) will depend on the exact electro-optical properties of the polarization converters used.

[0086] By randomly varying the voltage applied to each of the first and second polarization converters 100, 200, such that the phase difference induced between the first and second modes and the third and fourth modes varies randomly over time, the polarization output can be randomly varied (or in other words, scrambled) over time, 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 measuring arm of the PIC), the polarization state of the output can be monitored. Such a measuring arm may include a polarization-dependent photodetector, or use polarization filtering, in order to establish the polarization state of the output. The control of the polarization control device 10 can thereby be configured so that the polarization state of any input can be modified to a known output polarization, thereby enabling the polarization control device 10 to function as a polarization controller. By receiving appropriate control signals at at least one control element, the polarization control device 10 can function as a polarization scrambler or a polarization controller. In other words, by modifying the control signal provided to the polarization control device, the same combination of components can be used to implement a plurality of different functions, such as the functionality of a polarization controller and / or a polarization scrambler. The polarization control device 10 may be provided with a control signal from an external control system, or the polarization control device 10 may include appropriate control circuitry to provide a control signal to at least one control element.

[0087] Figure 8 A polarization control device 11 according to an example is schematically shown, and a Poincare sphere showing the polarization rotation axis of the polarization control device 11 is shown. 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 similar to those previously described with reference to FIG. Figure 2 and Figure 3 The first polarization converter 100 , the first connecting waveguide 50 , and the second polarization converter 200 of the described polarization control device 10 are identical. Figure 8The polarization control device 11 further includes a second connecting waveguide 51, a third polarization converter 300 and an output waveguide 40 having the above features, which are also arranged in series. The second connecting waveguide 51 is configured to receive light from the second polarization converter 200 and provide the light to the third polarization converter 300. The optical polarization converter 300 is configured to provide the light received from the second connecting waveguide 51 to the output waveguide 40.

[0088] In the example, the function and form of the third polarization converter 300 are substantially similar or identical to the first and second polarization converters 100, 200. In this context, substantially similar means that the structure is different only in the manner described below, and the function in the context of polarization control will be understood by those skilled in the art to be the same as the first and second polarization converters. The third polarization converter 300 supports a fifth mode and a sixth mode and includes a third cross-sectional structure (not drawn) in a plane perpendicular to a third light propagation axis 318, the third cross-sectional structure determining the tilt angle or polarization orientation of the fifth mode and the sixth mode, the tilt angle of the fifth mode being different from the tilt angle of the sixth mode.

[0089] The third cross-sectional structure is different from the first cross-sectional structure and the second cross-sectional structure, so that the fifth mode and the sixth mode have polarization orientations different from the first mode and the second mode, and different from the polarization orientations of the third mode and the fourth mode. Figure 8 In the example of , the third cross-sectional structure is similar to the second cross-sectional structure in that the third cross-sectional structure includes a third middle portion tilted in the same direction as the second middle portion of the second cross-sectional structure (i.e., tilted upward from left to right), but a third cross-sectional width of the third polarization converter 300 is smaller than the second cross-sectional width of the second polarization converter 200, which causes the fifth and sixth modes to have polarization orientations different from the third and fourth modes and also different from the first and second modes. In other examples, the third cross-sectional structure may include a middle portion tilted in the same direction as the first middle portion of the first cross-sectional structure, so that the fifth and sixth modes have polarization orientations different from the third and fourth modes and the first and second modes. In further examples, all three polarization converters include middle portions tilted in the same direction, and different respective cross-sectional widths individually provide different tilt angles for the supported modes.

[0090] The third polarization converter 300 is birefringent so that the fifth mode has an effective refractive index higher than the sixth mode. The third polarization converter 300 has a control element, which in this example is an electrical contact layer configured to modify the effective refractive index of the fifth mode and the sixth mode by applying a voltage across the third polarization converter 300 in response to a signal.

[0091] Taking into account the Poincare sphere, the third polarization converter 300 defines a third rotation axis through the fifth and sixth hybrid modes HM5, HM6. This can further enhance the range of positions on the sphere, i.e., the range of polarization states that can be accessed by the polarization control device 11. It can also improve the flexibility of the device in achieving a given output polarization state by providing another control degree of freedom. This additional degree of freedom can be used to obtain different paths across the Poincare sphere to a given output state.

[0092] In this example, the third polarization converter 300 supports modes with polarization orientations different from the modes supported by the first and second polarization converters 100, 200, but in other examples, a polarization converter device may instead include a third polarization converter that is identical to a first polarization converter and allows a 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 (for example) the TE mode may allow polarization conversion from any input to any output polarization. In other examples, a polarization control device including a first polarization converter with a tilt angle between 20 degrees and 40 degrees (for example, 30 degrees), a second polarization converter with a tilt angle of 0 degrees (i.e., a mode aligned to the TE / TM mode of the photonic integrated circuit), and a third polarization converter with a tilt angle between 50 degrees and 70 degrees (for example, 60 degrees) may be suitable.

[0093] In yet a further example, a 0 degree, 45 degree, 0 degree arrangement may be suitable. In such a scheme, the first polarization converter (0 degree) acts as a phase shifter for the TE / TM input state (and thus may not be needed for the TE / TM input state), the second polarization converter (45 degree) rotates the polarization away from TE / TM, and the third polarization converter (0 degree) accesses the remainder of the sphere.

[0094] Furthermore, if the effect of providing multiple polarization converters is to increase the overall optical length of the polarization converters, having multiple polarization converters with redundancy between the rotations provided by the polarization converters may enable a lower voltage to be applied to any individual segment.

[0095] In yet further examples, a polarization control device may include a polarization converter or multiple polarization converters as part of a series that lacks active control before a polarization converter with active control, inserted between polarization converters with active control, or after a polarization converter with active control.

[0096] Fig. 9A polarization control device 12 including first, second and third polarization converters 100-B, 200-B, 300-B is schematically illustrated. The first polarization converter 100-B has a first cross-sectional width (which is the total width of the first cross-sectional structure, as previously described) that varies along a length of the first polarization converter, the length being parallel to a first light propagation axis 118 of the first polarization converter. Similarly, the second polarization converter 200-B has a cross-sectional width that varies along a length of the second polarization converter and the third polarization converter 300-B has a third cross-sectional width that varies along a length of the third polarization converter, these respective lengths remaining parallel to the respective light propagation axes 218, 318 of the respective polarization converters.

[0097] The birefringence of a polarization converter can depend on the voltage applied across the converter. In this case, the modification of the voltage applied to a converter does not cause the polarization state to undergo an equatorial rotation about an axis of rotation because the increase in phase difference is not linear with the voltage. Instead, the curvature tracked by the polarization state will drift. This can lead to non-trivial control dynamics. Changing the cross-sectional width of each polarization converter can introduce a birefringence that depends on the propagation length within the polarization converter. That is, at a first propagation distance within (for example) the 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 offset the effects of the nonlinear voltage-dependent birefringence of the polarization converter, thereby simplifying the control dynamics and / or increasing the accuracy of polarization control.

[0098] Figures 10 to 12 Further examples of cross-sectional structures of polarization converters are shown, whose cross-sectional geometry can achieve a tilted modal angle. Polarization converters in other examples of polarization control devices described herein can feature cross-sectional structures of any described examples of polarization converters in any combination. For example, a first cross-sectional structure of a first polarization converter of a polarization control device can be based on, for example, Figure 2 , and a second cross-sectional structure of a second polarization converter of the polarization control device can be based on (for example) Fig.10 or Fig.11 or Fig.12 , or a variation thereof.

[0099] The characteristics of the cross-sectional geometry of each polarization converter (which provides the tilting boundary conditions and thus the tilting modal angles, e.g., Figures 1 to 3 The orientation of the tilted middle part of the example. Figures 1 to 3In the example of the first polarization converter 100, the first middle portion 128 is tilted upward from left to right, and the second polarization converter 200 is tilted upward from left to right. In other examples, the first middle portion 128 can be tilted in the same direction as the second middle portion 228 (i.e., both are tilted upward from left to right, or both are tilted upward from right to left, relative to their respective horizontal axes), and if the first polarization converter has a cross-sectional width different from that of the second polarization converter, the modal tilt angle of the first polarization converter can be made different from that of the second polarization converter.

[0100] Fig.10 A side cross section of a light polarization converter 400 according to an example is schematically shown. Fig.10 In, corresponding to Figure 2 Features shown in are labeled with similar reference numerals with an additional numeral "-4" added at the end (except for the optical polarization converter itself, which is labeled with reference numeral 400).

[0101] The polarization converter 400 has a cross-sectional structure, which is described herein with respect to a vertical axis 116 - 4 and a horizontal axis 114 - 4 each perpendicular to a light propagation direction 118 - 4 through the polarization converter 400 , the vertical axis 116 - 4 being perpendicular to the horizontal axis 114 - 4 .

[0102] Fig.10 The optical polarization converter 400 corresponds to the first optical polarization converter 100 (and may include any combination of features described above with respect to the first optical polarization converter 100 ), except for the following differences. Fig.10 The first bonding surface 120a-4 of the optical polarization converter 400 is substantially opposite to Fig.10 The first surface 104a-4 and the third surface 106a-4 are at a 90 degree angle (within an acceptable tolerance). In this example, the first joint surface 120a-4 corresponds to Fig.10 Similarly, the second bonding surface 120b-4 is substantially opposite to the side wall of the second substrate layer 136-4. Fig.10 The second surface 104b-4 and the fourth surface 106b-4 form an angle of 90 degrees.

[0103] In this example, due to the angles of the first and second engagement surfaces 120a-4, 120b-4, Fig.10 The optical polarization converter 400 does not include a middle portion. Fig.10 In the example of , the first portion 110-4 and the second portion 112-4 are continuous, rather than having an intermediate, inclined portion therebetween. Figure 2 and Figure 3In the case of the example of FIG. 4 , the waveguide layer 108 - 4 of the optical polarization converter 400 is a single waveguide layer for light propagation.

[0104] Fig.10 The waveguide layer 108-4 includes a second portion 112-4, which 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, such as Fig.10 As shown in .

[0105] Fig.10 The cross-sectional structure of the polarization converter 400 causes it to support mixed modes. Figure 2 and Figure 3 Cross-sectional structure of an example of a polarization converter, Fig.10 This arrangement of the cross-sectional structure of the polarization converter 400 provides a "tilted" or tilted boundary condition for light propagating within the waveguide layer 108-4, thereby providing a hybrid mode. This is because the light propagating within the polarization converter 400 occupies mutually consecutive waveguide layer regions that are offset in a direction parallel to the second axis 116-4. In addition, the described cross-sectional structure of the polarization converter 400 induces a different propagation constant for different hybrid modes. The described cross-sectional structure including the waveguide layer 108-4 provides birefringence, so that the first and second hybrid modes are in Fig.10 The polarization converter 400 has different refractive indexes. Figure 4a The arrow 406 shows the incident Fig.10 For light with TE polarization propagating through the waveguide layer 108 - 4 , the first and second hybrid modes 408 , 410 emerge with different propagation constants (and thus different phase evolutions) on the optical polarization converter 400 .

[0106] Fig.10 The cross-sectional structures of the polarization converter 400 and the first polarization converter 100 both generate mixed modes and different propagation constants of the mixed modes. As described above, the existence of mixed modes and their different propagation constants provide conversion of light polarization. Therefore, similar to Figure 2 The first optical polarization converter 100 of the example, Fig.10 The optical polarization converter 400 of the embodiment can be used to convert the polarization of incident light. As described in the previous embodiment, an electrical contact layer 132-4 is used to apply a voltage and thereby achieve the polarization of incident light. Fig.10 The polarization converter 400 performs active control of the polarization rotation.

[0107] Depending on various factors such as the complexity and cost of manufacturing and the desired level of control over the mixed modes and their propagation constants, the Figure 2 and Figure 3 Instances or Fig.10 For example, an angled (inclined) Figure 2 and Figure 3 The presence of the intermediate waveguide section 128 of the example provides for finer tuning of the properties of the hybrid mode, such as tilt. Fig.10 Examples of the invention may be easier to manufacture because the stepped structure of the polarization converter 400 eliminates the need to manufacture a polarization converter having a tilted surface.

[0108] Fig.11 A polarization converter 500 according to an example is schematically depicted. 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 light propagation direction 118-5 through the polarization converter 500, the vertical axis 116-5 being perpendicular to the horizontal axis 114-5. Fig.11 The polarization converter 500 includes a first waveguide layer 108-5 between a substrate 102-5 (which can be regarded as a first cladding layer) and a second cladding layer 130-5. Fig.11 In the cross-sectional structure of the polarization converter 500 of FIG. 1 , a certain orientation of the first waveguide layer 108-5 can be set according to the desired polarization conversion properties. The waveguide layer 108-5 can be considered to be tilted or inclined, for example, by an internal angle α obtained relative to the substrate 102-5, or (180°-α) obtained relative to the second cladding layer 130-5. Therefore, when the waveguide layer 108-5 is angled in this way, unlike the first cladding layer (in this example, which corresponds to the substrate 102-5) or the second cladding layer 130-5, the waveguide layer 108-5 is (for example) non-parallel to the substrate 102-5. For example, an inclined surface 190 of the waveguide layer is angled at an internal angle α of, for example, 30 to 65 degrees (and depending on a plane of a crystalline material, 30 to 40 degrees, such as approximately 35 degrees (within acceptable manufacturing tolerances), or 50 to 65 degrees, such as approximately 55 or 60 degrees (within acceptable manufacturing tolerances), 50 to 55 degrees, 55 to 60 degrees, or 60 to 65 degrees) relative to the substrate 102-5, and / or the second surface is angled at an internal angle β of approximately 90 degrees within acceptable manufacturing tolerances relative to the third surface. The third surface is, for example, substantially parallel to the fourth surface (e.g., parallel within acceptable manufacturing tolerances).

[0109] The material forming the waveguide layer 108-5 is, for example, a crystalline material, wherein the angle of the first angled surface corresponds to a {111} plane of the crystalline material. The {111} notation is based on the Miller index system used to designate a plane or family of planes in a crystal, as will be known to those skilled in the art. Such a plane may also be referred to as a crystal plane. The angle of the first angled surface may be obtained simply by using an appropriate fabrication method (e.g., using a particular etchant that is selective for a particular crystal plane).

[0110] This etching method is selective so as to etch the material of the waveguide layer 108-5 without etching (or etching significantly slower) the material forming the first and second cladding layers. Thus, to form the first angled surface 190, the waveguide layer material is removed from between the first and second cladding layers (e.g., from under the second cladding layer) during etching. In the example, there is therefore a region 520 between the first and second cladding layers 102-5, 130-5 where the waveguide layer 108-5 is not present.

[0111] With the first angled surface 190 of the waveguide layer 108-5 angled in this manner, Figure 1 The polarization converter 500 supports tilted modes. The internal angle α can be selected to determine the polarization orientation of the supported mode. The polarization converter 500 is birefringent such that a first mode of the supported hybrid mode has a higher effective refractive index than a second mode of the supported hybrid mode. Thus, for example, by using an under-etching technique, Fig.11 The cross-sectional structure shown in FIG. 500 can be arranged to provide a polarization converter for use in the polarization control devices described herein. An electrical contact layer 132 - 5 enables active control of the polarization rotation achieved by the polarization converter 500 .

[0112] Fig.12 A polarization converter 600 according to an example is featured. Fig.12 In, corresponding to Figure 2 Features shown in FIG. 6 are labeled with similar reference numerals with an additional numeral "-6" added at the end (except for the optical polarization converter itself, which is labeled with reference numeral 600). The polarization converter 600 has a cross-sectional structure, described herein with respect to a vertical axis 116-6, each perpendicular to a light propagation direction 118-6 through the polarization converter 600, and a horizontal axis 114-6, the vertical axis 116-6 being perpendicular to the horizontal axis 114-6. Fig.12The cross-sectional structure of the polarization converter 600 includes a waveguide layer 108-6 between and in contact with a first cladding layer (in this example, it is the 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 and second surfaces 106a-6, 106b-6, and a second offset portion 110-6 partially bounded by the third and fourth surfaces 104a-6, 104b-6, and an intermediate portion 128-6 between the first and second bonding surfaces 120a-6, 120b-6. The first bonding surface 120a-6 bonds the first and third surfaces 106a-6, 104a-6, and the second bonding surface 120b-6 bonds the second and fourth surfaces 106b-6, 104b-6. The intermediate portion thereby bonds the offset portions to each other. The portion 110-6 is lower than the portion 112-6 relative to the vertical axis 116. The middle portion 128-6 of the joint offset portions 112-6, 110-6 is inclined, similar to the reference Figure 2 and Figure 3 The middle part described. In addition, Fig.12 The cross-sectional structure of the polarization converter 600 is characterized by an inclined outer surface 190 of the waveguide layer 108-6, and the inclined outer surface 190 forms a non-orthogonal (in other words, an acute angle or an obtuse angle) internal angle α with the substrate layer 102-6 and the second cladding layer 130-6. Fig.11 As described, this can be formed based on a crystal plane of the waveguide layer 108-6 and by using an under-etching technique.

[0113] for Figure 2 , Figure 3 , Fig.10 , Fig.11 and Fig.12 The polarization converters 100, 200, 400, 500 shown in FIG. Fig.12 The cross-sectional structure of the polarization converter 600 allows for a tilt mode and birefringence of such a tilt mode. An electrical contact layer 132-5 can be used to provide active control of the polarization rotation achieved by the polarization converter 600. By combining offset portions and outer surfaces 190 that are not orthogonal to adjacent surfaces, the cross-sectional structure of the polarization converter 600 has numerous design parameters that can be selected in a straightforward manner to achieve the desired tilt mode and birefringence.

[0114] Fig.13 1 is a schematic diagram of a polarization converter 700 according to an example. The polarization converter 700 has a cross-sectional structure, which is described herein with respect to a vertical axis 116-7 and a horizontal axis 114-7 each perpendicular to a light propagation direction 118-7 through the polarization converter 700, wherein the vertical axis 116-7 is perpendicular to the horizontal axis 114-7. Fig.13The cross-sectional structure of the polarization converter 700 includes a waveguide layer 108-7 between and in contact with a first cladding layer (in this example, the substrate layer 102-7) and a second cladding layer 130-7. As described in the previous example, an electrical contact layer 132-7 is used to apply a voltage and thereby achieve the desired polarization. Fig.13 The polarization converter 700 performs active control of the polarization rotation.

[0115] The first cladding layer, substrate layer 102-7 includes a first surface 170a-7 at a first position 124a-7 relative to the horizontal axis 114-7 and a second surface 170b-7 at a second position 124b-7 relative to the horizontal axis 114-7. Fig.13 oriented left-hand side) and the second surface 170b-7 (relative to Fig.13 The right-hand side of the orientation of ) defines a cross-sectional width of the substrate layer 102-7 that spans from the first position 124a-7 to the second position 124b-7.

[0116] The second coating layer 130-7 includes a first surface 174a-7 at a third position 124c-7 relative to the horizontal axis 114-7 and a second surface 174b-7 at a second position 124b-7 relative to the horizontal axis 114-7. Fig.13 oriented left-hand side) and the second surface 174b-7 (relative to Fig.13 The right-hand side of the orientation of the second cladding layer 130-7 defines a cross-sectional width of the second cladding layer 130-7, which crosses from the third position 124c-7 to the second position 124b-7. The third position 124c-7 is located between the first position 124a-7 and the second position 124b-7, and therefore 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.

[0117] 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 that is substantially (within acceptable manufacturing tolerances) perpendicular to each of the first surface 171-7 and the second surface 173-7. 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 between the first surface 171-7 of the waveguide layer 108-7 and the third surface 171b-7 of the waveguide layer relative to the horizontal axis 114-7, and a second cross-sectional width again 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.

[0118] The first surface 171-7 of the waveguide layer 108-7 can be described as being offset from the second surface 173-7. Figure 2 and Fig.10 In the example of FIG. 1 , the offset surface extends parallel (or substantially parallel, within acceptable manufacturing tolerances) to the horizontal axis 114, and Fig.13 In the example of FIG. 1 , the offset surfaces 171 - 7 , 173 - 7 extend parallel (or substantially parallel, within acceptable manufacturing tolerances) to the vertical axis 116 - 7 .

[0119] exist Fig.13 In the example of , the first surface 171-7 of the waveguide layer 108-7 is five times longer (in a direction perpendicular to the axis 116-7) than the second surface 173-7 of the waveguide layer 108-7. The relative lengths of the surfaces 171-7, 173-7, and the bonding surface 172-7, and thus the first and second cross-sectional widths, may be selected to achieve a desired modal tilt angle of the waveguide layer 108-7. Fig.13 In the example of , the modal angle is mainly determined by the first section width (or in other words, is most sensitive to the first section width). That is, in Fig.13 In the example of , 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.

[0120] In other examples, the position of the offset surface may be different, such as either or both of the first or second cladding layers having a step in its cross-sectional structure corresponding to an offset surface (thereby having two cross-sectional widths), as well as the waveguide layer or replacing the waveguide layer.

[0121] The polarization converter 700 having a waveguide layer 108-7 with offset surfaces 171-7, 173-7 and thus two cross-sectional widths introduces asymmetric boundary conditions that rotate the modal angles of the supported modes relative to a uniform planar waveguide layer.

[0122] According to the Figure 2 , Figure 3 , Fig.10 , Fig.11 , Fig.12 and Fig.13 The illustrated example provides a polarization converter that is straightforward to manufacture by providing tilted mode angles through geometric features of the cross-sectional structural profile. Active control that allows a series of polarization converters to be used as a polarization scrambler or controller can be further directly achieved by providing a control element (such as an electrical contact layer) that can be an electrode or include an electrode, which can be used to modify the effective refractive index of the mode supported by the polarization converter. Such a polarization control device can be easily manufactured in a common platform by forming only a single waveguide layer or limited cladding layers and a waveguide layer, without the need to form complex or heterogeneous sublayers, for example.

[0123] Fig.14 Schematically depicting the Figure 1 An example of a system 1500 of a polarization control device 10 as part of a photonic integrated circuit 1001 .

[0124] The photonic integrated circuit 1001 includes a light source 80, which in this example is an integrated laser 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 light source of the system may be provided to other devices of the photonic integrated circuit before being received at the polarization control device 10. The system 1500 includes a control system 90, which is 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 may provide such signals to the electrical contact layers 132, 232 by providing circuitry (not shown) within the photonic integrated circuit. In this example, the signal provided to the electrical contact layer determines the potential difference applied to the respective polarization converter. In other examples, the signal may determine a temperature of a heating element that is in contact with the polarization converter.

[0125] The control system 90 implements a control scheme to control a functionality 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 having a polarization state that may be different from the polarization state of the input light, depending on the polarization control implemented by the control system 90 interfaced with the polarization control device 10.

[0126] The photonic integrated circuit includes an optical splitter 82 after an output waveguide 40 that 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 polarization filter before 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 inform 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 an output waveguide 40b.

[0127] Fig.15 FIG. 1 is a flow chart illustrating a method of controlling a polarization of light in a polarization control device according to an example. Fig.15 In an example of the invention, a polarization control device (such as Figure 1 However, after reading the description, it will be easy for a person skilled in the art to understand how to Fig.15 The method is extended to a polarization control device including more than two polarization converters (such as Figure 8 and Fig. 9 Polarization control devices 11, 12).

[0128] exist Fig.15At item S101 of , light is received at a first polarization converter. Light is received at a converter means, for example, that the light is coupled into a mode supported by the converter. In other words, at item S101, the light is coupled into a first mode and a second mode, which are, for example, tilted or mixed modes, as previously described. The received light may have, for example, propagated through free space, such as coupling an off-chip light source directly into the polarization control device, or may have been received after propagating through a waveguide, such as input waveguide 20. Those skilled in the art will appreciate that there are various ways in which light can be coupled into an integrated photonic circuit device.

[0129] At item S103, a required phase shift between the first and second modes is determined. For example, this involves determining the phase shift required to modify the polarization state from a first, input state to an intermediate output state of the first polarization based on a required traversal path of the Poincare sphere. This calculation uses the tilt angles of the first and second modes and therefore the knowledge of the rotation axis about which the Poincare sphere will be traversed when a phase shift is produced. In general, this will involve taking into account the polarization conversion that will be achieved by subsequent polarization converters (e.g., the second polarization converter) in order to determine a total traversal path of the Poincare sphere achieved by the polarization control device 10. In this sense, although items S103 and later item S109 are presented separately, they can be regarded as working together in determining how the polarization state will be modified by the polarization control device and may not be performed in a time-separated manner.

[0130] In an example, item S103 may involve first determining the transfer function of the device (within manufacturing tolerances). This may involve characterizing the polarization control device by providing TE mode light into the polarization control device, and measuring the output polarization of the device as the control voltages are swept through their respective ranges. This may enable calculation of the transfer function for each section (e.g., for each constituent polarization converter of the polarization control device), which in turn may inform what polarization conversion step should be performed by which polarization converter, and therefore determine the phase shift required for each polarization converter.

[0131] In an example, item S103 may involve an online measurement of the polarization state at the first polarization converter, for example, by coupling off a portion of the light to make a polarization measurement. In other examples, item S103 may be performed without measuring the polarization state of the light, for example, where the polarization state received by the first polarization converter is known or can be estimated to a suitable accuracy. Thus, in some examples, the calculation of item S103 may be performed before item S101.

[0132] 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. Figure 2 , Figure 3 In the case of the polarization converter described in , where the control element comprises an electrical contact layer, this involves applying an electric field via the electrical contact layer so as to modify the effective refractive index of the first and second modes. When propagating through the first polarization converter, the first mode with a first effective refractive index and the second mode with a second effective refractive index acquire a phase difference. It will be appreciated that in other examples, the control element may be, for example, a heating element. In this case, the control element is instead controlled to produce a temperature that results in the desired phase shift determined by step S103.

[0133] In item S107, light is received at the second polarization converter. This involves coupling the light into the third mode and the fourth mode. This may involve coupling the light of the first mode and the second mode from the first polarization converter directly into the third mode and the fourth mode of the second converter by, for example, butt coupling, or may involve coupling the light using a connecting waveguide, which may, for example, have a taper and receive the light of the first and second modes from the first polarization converter and, in some examples, adiabatically transform to couple the light into the third and fourth modes.

[0134] In item S109, the required phase shift between the third and fourth modes is determined. According to item S103, this (for example) involves determining the phase shift required to rotate the input polarization state (e.g., the output state of the first polarization converter) to the desired output state of the polarization control device. This calculation uses the knowledge of the tilt angles of the third and fourth modes and therefore the rotation axis about which the Poincare sphere will be traversed when a phase shift is generated. As described for item S103, although the items of S103 and S109 are in Fig.15 , but in an example, according to a control scheme, they can be executed simultaneously, for example, before the device receives light.

[0135] 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. Figure 3 In the case of the second polarization converter 200, this involves controlling the electric field across the polarization converter via an electrical contact layer.

[0136] In item S113, light is output from the second polarization converter. By providing items S105 and S111, the polarization state of the light has been modified to a desired output state output from the second polarization converter. Similar to item S107, this may involve first coupling light from the third and fourth modes into a connecting waveguide that is (for example) adiabatically tapered to transition to mode matching with a receiving waveguide of the photonic integrated circuit.

[0137] exist Fig.15In the example flow chart of , only two polarization converters are used. As will be appreciated by those skilled in the art, items S101 , S103 , and S105 may be repeated for subsequent polarization converters in a polarization control device having more than two polarization converters.

[0138] Fig.16 is a flow chart illustrating a method of manufacturing a polarization control device according to a further example.

[0139] 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 light 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, the first tilt angle being different from the second tilt angle. The first mode has an effective refractive index higher than the second mode.

[0140] A polarization converter and the details of its cross-sectional structure may be formed by techniques known to those skilled in the art for fabricating integrated photonic circuit elements, such as deposition, etching, photolithography.

[0141] As will be appreciated by those skilled in the art, various techniques may be used to deposit a semiconductor material layer according to the examples described herein. Such a technique may be referred to as a regrowth technique, for example, a metal organic vapor phase epitaxy (MOVPE) or a molecular beam epitaxy (MBE) process may be used.

[0142] In some examples, the substrate material for a polarization converter and / or photonic integrated circuit is InP. In some such examples, a wet etching process can be used to form the polarization control device. In some such examples, a wet etching process is performed using HCl:H3PO4:H2O. In some examples, a mixture of HCl, H3PO4, and H2O is selected to etch the desired material (in this example, InP). In other examples, only a mixture of HCl and H2O is used as an etchant. As described, performing a wet etching process provides an intermediate substrate surface (such as Figure 2 The intermediate substrate surface is relative to an adjacent surface (eg, Figure 2 104a, 104b) are at an angle less than 90 degrees. Those skilled in the art will appreciate that the angle of the bonding surfaces 120a, 120b will depend on a combination of the crystal structure of the substrate material, the etching chemistry, and the details of the process used.

[0143] In an example, forming a polarization converter involves, for example, a dry etching process to remove material from either side of the structure, as needed to a particular depth depending on the intended application.

[0144] In item S203, a first control element is formed. This may involve, for example, depositing an electrical contact layer on top of the first polarization converter. In other examples, this may involve forming a heating element on the first polarization converter.

[0145] 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 light 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 an effective refractive index higher than the fourth mode.

[0146] The second polarization converter is formed so that an input of the second polarization converter can receive light from an output of the first polarization converter, ie, so that the first polarization converter and the second polarization converter can be arranged in series. The second polarization converter can be formed using similar or the same technology as the first polarization converter.

[0147] In item S207, a second control element is formed. This may involve, for example, depositing an electrical contact layer on top of the second polarization converter. In other examples, it may involve forming a heating element on the second polarization converter.

[0148] The above examples are to be understood as illustrative examples of the present invention. Further examples of the present invention are envisaged.

[0149] For example, thus far, a goal of obtaining a wide range of polarization control (potentially covering the entire Poincare sphere) has been described. However, in some instances, control covering the entire Poincare sphere may not be required. For example, one application of the described polarization control device may be to perform a small polarization modification on an unknown input polarization state, where the input state is unknown but approximately at a specific polarization, rather than unknown and derived from any possible polarization on the Poincare sphere. In such an example, it may only be necessary to use a plurality of closely aligned rotation axes to perform the necessary polarization control. Similarly, when operating in a polarization scrambler configuration, it may not be necessary for the polarization control device to generate polarization states from the entire Poincare sphere, but rather it may only be necessary for the polarization control device to generate polarization states from a portion of the Poincare sphere.

[0150] In various examples herein, a polarization control device includes a waveguide layer between and in contact with a first cladding layer and a second cladding layer, the first cladding layer being a substrate layer. In other examples, as will be appreciated by those skilled in the art, there may not be a second cladding layer. In such examples, the waveguide layer may only be in contact with a first cladding layer, and otherwise be surrounded by, for example, air. In other examples, the first cladding layer is an additional layer disposed on the substrate of the photonic integrated circuit. The cladding layers may comprise the same material, or have different compositions from one another, for example as determined by the optical performance of the polarization converter.

[0151] The polarisation converter may be connected in series with a light source and used to control the polarisation output by the light source.The light source may form part of a photonic integrated circuit on which the polarisation control means is fabricated, or may alternatively be external to the PIC and coupled into the polarisation control means.

[0152] The above examples include a control element in the form of an electrical contact layer for carrier injection or carrier depletion of a polarization converter, such that a voltage is applied across a polarization converter. However, in other examples, carrier injection / depletion may not occur when a voltage is applied. In still 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 a control element of at least one control element of a polarization control device.

[0153] In the above examples, different polarization orientations of modes supported by a polarization converter are achieved by variations in cross-sectional width between polarization converters. In other examples, variations in other forms of the geometry of the cross-sectional structure may be varied alternatively or additionally. For example, the internal angle of an inclined surface may be varied between polarization converters to modify the tilt angle. In other examples, the relative offset between portions of the waveguide layer may be varied to achieve different tilt angles. In yet further examples, each polarization converter may have a cross-sectional structure that is completely different from other polarization converters, for example, according to the examples presented herein or variations thereof to vary the modal angles between constituent polarization converters. Thereby, the cross-sectional width may remain the same between polarization converters, because varying forms other than the cross-sectional width of the cross-sectional structure to vary the polarization orientation of the supported modes may reduce or remove the requirement for a tapered connection section and thereby reduce, for example, optical losses of a polarization control device.

[0154] It should be understood that any feature described for any 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 any other example in any combination. In addition, equivalents and modifications not described above may also be adopted without departing from the scope of the attached invention claims.

Claims

1. A polarization control device for a photonic integrated circuit, the polarization control device comprising: A first polarization converter, the first polarization converter is configured to support a first mode and a second mode, the first polarization converter comprising: a first cross-sectional structure in a plane perpendicular to a first light 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, respectively, the polarization orientation of the first mode is different from the polarization orientation of the second mode, and The first mode has an effective refractive index higher than that of the second mode; a second polarization converter, the second polarization converter being configured to support a third mode and a fourth mode, the second polarization converter comprising: a second cross-sectional structure in a plane perpendicular to a second light 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, respectively, the polarization orientation of the third mode being different from the polarization orientation of the fourth mode, and The third mode has an effective refractive index higher than that of the fourth mode; The first polarization converter is connected in series with the second polarization converter; and At least one control element, the at least one control element being configured to respond to at least one signal: modifying the effective refractive index of the first mode and the second mode; and modifying the effective refractive index of the third mode and the fourth mode, The first cross-sectional structure is different from the second cross-sectional structure, so that the first mode and the second mode have polarization orientations different from the third mode and the fourth mode.

2. The polarization control device according to claim 1, wherein: The first cross-sectional structure has a first cross-sectional width defined by a distance between opposite surfaces of the first polarization converter in a direction perpendicular to the first light propagation axis, and The second cross-sectional structure has a second cross-sectional width defined by a distance between opposite surfaces of the second polarization converter in a direction perpendicular to the second light propagation axis, The first cross-sectional width is different from the second cross-sectional width, so that the first mode and the second mode have different polarization orientations from the third mode and the fourth mode.

3. The polarization control device according to 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 being arranged on a substrate of the photonic integrated circuit.

4. A polarization control device according to any one of the preceding claims, wherein: At least one of the following: The first cross-sectional structure includes a first inclined surface having an acute or obtuse internal angle relative to 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 having an acute or obtuse internal angle relative to a surface of the second cross-sectional structure adjacent to the second inclined surface.

5. A polarization control device according to any one of the preceding claims, wherein: At least one of the following: The first cross-sectional structure comprises a layer, the layer comprising: a first portion, the first portion being at least partially bounded by a first surface and a second surface, and a second portion, the second portion being at least partially delimited by a third surface and a fourth surface, The first surface is offset from the third surface in a direction perpendicular to the first light propagation axis, the first surface is connected to the third surface via a first joint surface, and the second surface is offset from the fourth surface in a direction perpendicular to the first light propagation axis, the second surface is connected to the fourth surface via a second joint surface; or The second cross-sectional structure comprises a layer, the layer comprising: a third portion, the third portion being partially delimited by a fifth surface and a sixth surface, and a fourth portion, the fourth portion being partially delimited by a seventh surface and an eighth surface, The fifth surface is offset from the seventh surface in a direction perpendicular to the second light propagation axis, and the fifth surface is connected to the seventh surface via a third joining surface, and the sixth surface is offset from the eighth surface in a direction perpendicular to the second light propagation axis, and the sixth surface is connected to the eighth surface via a fourth joining surface.

6. The polarization control device according to claim 5, wherein: At least one of the following: The first surface, the second surface, the third surface and the fourth surface are parallel to each other; or The fifth surface, the sixth surface, the seventh surface and the eighth surface are parallel to each other.

7. A polarization control device according to any one of the preceding claims, wherein: The first cross-sectional structure includes a first middle part, and the second cross-sectional structure includes a second middle part. In a plane perpendicular to the first light propagation axis and / or the second light propagation axis, a structure of the second middle part is a symmetrical mirror image of a structure of the first middle part.

8. A polarization control device according to any one of the preceding claims, wherein: At least one of the following: The first polarization converter has a length parallel to the first light propagation axis, the length being substantially equal to an odd number multiplied by one quarter of a beat length of a wavelength of the input light; or The second polarization converter has a length parallel to the second light propagation axis, the length being substantially equal to an odd number multiplied by one fourth of the beat frequency length of the wavelength of the input light.

9. A polarization control device according to any one of the preceding claims, wherein: At least one of the following: The first cross-sectional structure includes a first waveguide layer between a first cladding layer and a second cladding layer and in contact with the first cladding layer and the 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 a third cladding layer and a fourth cladding layer and in contact with the third cladding layer and the fourth cladding layer, wherein the second waveguide layer has a refractive index higher than that of the third cladding layer and the fourth cladding layer.

10. The polarization control device according to claim 9, wherein: At least one of the following: The first waveguide layer comprises indium gallium arsenic phosphide or indium aluminum gallium arsenide and the first cladding layer and the second cladding layer each comprise indium phosphide; or The second waveguide layer has indium gallium arsenic phosphide or indium aluminum gallium arsenide and the third cladding layer and the fourth cladding layer each have indium phosphide.

11. A polarization control device according to any one of the preceding claims, wherein: At least one of the following: The first cross-sectional structure has a first cross-sectional width defined by a distance between opposite surfaces of the first polarization converter in a direction perpendicular to the first light propagation axis, The first cross-sectional width varies along a length parallel to the first light propagation axis; or The second cross-sectional structure has a second cross-sectional width defined by a distance between opposite surfaces of the second polarization converter in a direction perpendicular to the second light propagation axis, The second cross-sectional width varies along a length parallel to the second light propagation axis.

12. A polarisation control device according to any preceding claim, wherein: The first cross-sectional structure has a first cross-sectional width defined by a distance between opposite surfaces of the first polarization converter in a direction perpendicular to the first light propagation axis, and The second cross-sectional structure has a second cross-sectional width defined by a distance between opposite surfaces of the second polarization converter in a direction perpendicular to the second light propagation axis; The polarization control device comprises: A connecting waveguide, the first polarization converter and the second polarization converter are connected by the connecting waveguide, the connecting waveguide has a cross-sectional width in a direction perpendicular to a light propagation axis of the connecting waveguide, and the cross-sectional width of the connecting waveguide gradually decreases from the first cross-sectional width to the second cross-sectional width along a length of the connecting waveguide.

13. The polarization control device according to claim 12, wherein: The taper of the connecting waveguide comprises an adiabatic cone.

14. The polarization control device according to any one of the preceding claims, further comprising a third polarization converter configured to support a fifth mode and a sixth mode, the third polarization converter comprising: a third cross-sectional structure in a plane perpendicular to a third light 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, and The fifth mode has an effective refractive index higher than that of the sixth mode, wherein 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 a further at least one signal, and Wherein, the third cross-sectional structure is different from at least one of the following: The first cross-sectional structure enables the fifth mode and the sixth mode to each have a polarization orientation different from that of the first mode and the second mode; or The second cross-sectional structure enables the fifth mode and the sixth mode to each have a polarization orientation different from that of the third mode and the fourth mode.

15. A polarization control device according to any one of the preceding claims, wherein: The at least one control element is an electrode configured to respond to the at least one signal: modifying the effective refractive indices of the first mode and the second mode by applying an electric field across the first polarization converter; and / or The effective refractive indices of the third mode and the fourth mode are modified by applying an electric field across the second polarization converter.

16. A polarization control device according to any one of the preceding claims, wherein: The polarization control arrangement is operable to perform at least two different polarization control functions depending on the at least one signal received via the at least one control element.

17. The polarization control device according to claim 16, wherein: The polarization control device can operate as at least one of: a polarization scrambler or a polarization controller.

18. A method of manufacturing a polarisation control device according to any preceding claim.

19. A photonic integrated circuit, the photonic integrated circuit comprising a polarization control device, the polarization control device comprising: A first polarization converter, the first polarization converter is configured to support a first mode and a second mode, the first polarization converter comprising: a first cross-sectional structure in a plane perpendicular to a first light 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, respectively, the polarization orientation of the first mode is different from the polarization orientation of the second mode, and The first mode has an effective refractive index higher than that of the second mode; a second polarization converter, the second polarization converter being configured to support a third mode and a fourth mode, the second polarization converter comprising: a second cross-sectional structure in a plane perpendicular to a second light 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, respectively, the polarization orientation of the third mode being different from the polarization orientation of the fourth mode, and The third mode has an effective refractive index higher than that of the fourth mode; The first polarization converter is connected in series with the second polarization converter; and At least one control element, the at least one control element being configured to respond to at least one signal: modifying the effective refractive index of the first mode and the second mode; and modifying the effective refractive index of the third mode and the fourth mode, The first cross-sectional structure is different from the second cross-sectional structure, so that the first mode and the second mode have polarization orientations different from the third mode and the fourth mode.

20. The photonic integrated circuit of claim 19, further comprising: a light source, the light source being used to input light into the polarization control device; An output waveguide is used to receive the light from the polarization control device.

21. The photonic integrated circuit of claim 19 or claim 20, further comprising: A third polarization converter configured to support a fifth mode and a sixth mode, the third polarization converter comprising: a third cross-sectional structure in a plane perpendicular to a third light 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, and The fifth mode has an effective refractive index higher than that of the sixth mode, wherein 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 a further at least one signal, and Wherein, the third cross-sectional structure is different from at least one of the following: The first cross-sectional structure enables the fifth mode and the sixth mode to each have a polarization orientation different from that of the first mode and the second mode; or The second cross-sectional structure enables the fifth mode and the sixth mode to each have a polarization orientation different from that of the third mode and the fourth mode.

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

23. A method for manufacturing a polarization control device, the method comprising: forming a first polarization converter, the first polarization converter comprising a first cross-sectional structure in a plane perpendicular to a first light propagation axis of the first polarization converter, the first cross-sectional structure being configured to support a first mode and a second mode, a polarization orientation of the first mode being different from a polarization orientation of the second mode, the first mode having an effective refractive index higher than that of the second mode; forming a second polarization converter, the second polarization converter comprising a second cross-sectional structure in a plane perpendicular to a second light propagation axis of the second polarization converter, the second cross-sectional structure being configured to support a third mode and a fourth mode, a polarization orientation of the third mode being different from a polarization orientation of the fourth mode, the third mode having an effective refractive index higher than that of 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, so that the polarization orientations of the first mode and the second mode are different from the polarization orientations of the third mode and the fourth mode; and At least one control element is formed, the at least one control element being configured to modify the 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 method of claim 23, wherein: The first cross-sectional structure has a first cross-sectional width defined by a distance between opposite surfaces of the first polarization converter in a direction perpendicular to the first light propagation axis, The second cross-sectional structure has a second cross-sectional width defined by a distance between opposite surfaces of the second polarization converter in a direction perpendicular to the second light propagation axis, The first cross-sectional width is different from the second cross-sectional width, so that the first mode and the second mode have different polarization orientations from the third mode and the fourth mode.

25. A method according to claim 23 or claim 24, wherein: 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, wherein the inclined surface has an acute or obtuse internal angle relative to a surface of the first cross-sectional structure or the second cross-sectional structure adjacent to the inclined surface.

26. The method according to 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. A method for controlling polarization of light in a photonic integrated circuit, the method comprising: Light is received at a polarization control device of the photonic integrated circuit, the polarization control device comprising: A first polarization converter, the first polarization converter is configured to support a first mode and a second mode, the first polarization converter comprising: a first cross-sectional structure in a plane perpendicular to a first light 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, respectively, the polarization orientation of the first mode is different from the polarization orientation of the second mode, and The first mode has an effective refractive index higher than that of the second mode; a second polarization converter, the second polarization converter being configured to support a third mode and a fourth mode, the second polarization converter comprising: a second cross-sectional structure in a plane perpendicular to a second light 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, and The third mode has an effective refractive index higher than that of the fourth mode; The first polarization converter is connected in series with the second polarization converter; At least one control element, the at least one control element being configured to respond to at least one signal: modifying the effective refractive index of the first mode and the second mode, and modifying the effective refractive index of the third mode and the fourth mode; The first cross-sectional structure is different from the second cross-sectional structure, so that the first mode and the second mode have polarization orientations different from the third mode and the fourth mode; The method includes: receiving light at the first polarization converter of the polarization control device; determining a desired phase shift between the first mode and the second mode; controlling the at least one control element to produce the desired phase shift between the first mode and the second mode; receiving light at the second polarization converter of the polarization control device; determining a desired phase shift between the third mode and the fourth mode; The at least one control element is controlled to produce the desired phase shift between the third mode and the fourth mode.

28. The method according to claim 27, wherein: At least one of the following The at least one control element comprises an electrode in electrical contact with the first polarization converter, and producing the desired phase shift between the first mode and the second mode comprises applying a voltage across the first polarization converter to modify the effective refractive indices of the first mode and the second mode; or The at least one control element includes an electrode in electrical contact with the second polarization converter, and producing the desired phase shift between the third mode and the fourth mode includes applying a voltage across the second polarization converter to modify the effective refractive indices of the third mode and the fourth mode.

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