Polarization rotators

Abstract

To provide polarization rotators which allow polarization rotation in any direction and which support simultaneous realization of polarization rotation for multiple waveguides on a single waveguide chip, without the need to assemble necessary parts separately for each waveguide.SOLUTION: The invention concerns a polarization rotator, comprising: a first waveguide layer containing at least a first waveguide, the first waveguide having an input end and an output end; a second waveguide layer having at least a second waveguide, the second waveguide having an input end and an output end; and at least one first vertical mirror element arranged at the end of at least one of the waveguides to couple light between the output end of the first waveguide and the input end of the second waveguide. The optical axis of the first or second waveguide which has the vertical mirror element at its end is rotated in its waveguide layer at a first angle to thereby induce rotation of polarization of light coupled between the first and second waveguides with a polarization rotation amount that corresponds to the first angle.SELECTED DRAWING: Figure 7D

Description

Technical Field

[0001] The present invention relates to optical components such as photonic integrated circuits and systems using a polarization rotator.

Background Art

[0002] The present invention relates to a polarization rotator, and more specifically, to a photonic integrated circuit (PIC), that is, a planar waveguide circuit. The waveguide forming the photonic integrated circuit is generally formed on the surface of a chip such as a silicon chip. In such a planar waveguide, light propagates along the surface of the chip in the direction of the local z-axis.

[0003] A polarization rotator is an optical device that rotates the polarization axis of a linearly polarized light beam by a selected angle and is an important component in optical isolators, polarization splitter-rotators, and circulators. Although the rotation of linearly polarized light is the subject of the present disclosure, the same or similar concepts can also be applied to circular or elliptical polarization.

Summary of the Invention

Problems to be Solved by the Invention

[0004] In free-space optics, these polarization rotation devices can be based on the Faraday effect, birefringence, or reflection. A Faraday rotator requires a waveguide material having a magneto-optical effect. In a birefringent polarization rotator, linearly polarized light is decomposed into two components, and the phase difference between these components is caused by birefringence and also changes the polarization. Birefringence is a material ​material properties, and in the case of waveguides, generally induced by waveguide geometry, stress within the waveguide material, or both. Light passing through one or more reflections may experience a phase shift between polarization components and, consequently, its polarization may be rotated.

[0005] Integrated polarization manipulators often rely on the use of several types of birefringent waveplates, which are made either from an asymmetric waveguide region made of a birefringent material or from individual waveplates and inserted into etched holes across the waveguide. Waveplate-based polarizers generally rely on interference between polarization modes, which makes these polarization modes wavelength-dependent and not suitable for very broadband applications.

[0006] Cross-polarization coupling or mode evolution has also been proposed in waveguide circuits. Rotators based on mode evolution function on the principle of adiabatically changing the cross-section of the waveguide, whereby the orientation of the polarization eigenmodes gradually rotates by the required amount. The transition needs to be adiabatic so that no power coupling occurs between different modes, which makes the device extremely long and difficult to fabricate using conventional waveguide processes. Furthermore, it is difficult to achieve non-orthogonal polarization rotation, i.e., polarization rotation other than 90°, with these types of polarizers.

[0007] The optical power of light can be split into two polarization eigenmodes, each of which propagates along the waveguide at its characteristic speed. The polarization modes in a planar waveguide can often be approximated as pure TE and pure TM modes, in which the light ​​​​​​​​​​​​​​​ When propagating along the optical axis of the waveguide, i.e., the local z-axis, the electric field is along the surface of the waveguide chip oriented (TE, x-axis), or oriented along the normal of the waveguide chip (TM, y axis). The velocity is defined by the effective refractive index (n eff ) of the mode. In a birefringent waveguide , the two polarization modes have different n eff . Therefore, the phase difference between the two modes, and thus the polarization of light, varies along the waveguide. The total electric field of light can thus be regarded as consisting of two components, i.e., one component (E x ) points along the x-axis , and the other component (E y ) points along the y-axis. This approximation is used to explain the present invention, but the present invention is not limited to the use of pure TE modes and pure TM modes.

[0008] In a micron-scale silicon-on-insulator (SOI) waveguide, it is difficult to rotate the polarization by an angle other than a multiple of 9 0°. Currently, a 45° polarization rotation in an SOI waveguide is necessary to realize a fully integrated optical isolator based on Faraday rotation .

[0009] Known integrated polarization rotators are extremely sensitive to fabrication errors and process variations because both the orientation of the polarization eigenmodes and the amount of birefringence need to be accurately controlled . This limits their application to commercial devices. Some techniques such as inserting a thin waveplate into an etched trench hole are highly desired from the perspective of assembly. In particular, many known methods relying on the interference effect between waveguide modes are also inherently wavelength-dependent.

[0010] ​​​​The object of the present invention is to provide a method for the detection of 100% chromatic aberrations for any desired rotation angle and for a wide wavelength range. By using one or more planar waveguide chips to achieve polarization rotation, The goal is to develop a waveguide chip using a micron-scale SOI platform. However, the solution presented below is based on a need that emerged during the development of The present invention can be applied to many other planar waveguide platforms.

[0011] An important advantage of the present invention is that it allows for polarization rotation in any direction and is a single-conductor Supports simultaneous realization of polarization rotation for multiple waveguides on a waveguide chip, with individual polarization rotation for each waveguide. There is no need to assemble any other necessary parts. [Means for solving the problem]

[0012] According to the present invention there is provided a polarization rotator, the polarization rotator comprising: a first waveguide layer including at least a first waveguide, the first waveguide having an input end and an output end; the first waveguide layer having a power end; a second waveguide layer including at least a second waveguide, said second waveguide having an input end and an output end; the second waveguide layer having a power end; Coupling light between the output end of the first waveguide and the input end of the second waveguide. and a second waveguide disposed at the end of at least one of the first and second waveguides. at least a first vertical mirror element; Equipped with.

[0013] The optical axis of the first or second waveguide having the vertical mirror element at its end is According to this, the waveguide layer is rotated at a first angle, thereby forming a first waveguide and a second waveguide. Induce the polarization rotation of the light coupled with the second waveguide by an amount corresponding to the first angle This can be done.

[0014] The first waveguide layer is at least a third waveguide having an input end portion and an output end portion Between the third waveguide and the output end portion of the second waveguide and the input end portion of the third waveguide At least one of the second and third waveguides is arranged at the end portion to couple light And at least a second vertical mirror element disposed at the end portion. According to the present invention, the optical axis of the waveguide having the second vertical mirror element is rotated by the second angle in the waveguide layer, thereby inducing the polarization rotation of the light coupled from the second waveguide to the third waveguide by an amount corresponding to the second angle. In some embodiments, the second waveguide layer is orthogonal to the first waveguide layer. The optical axis of the first waveguide is rotated by the first angle in the first waveguide layer, and the first vertical mirror element is connected to the output end portion of the first waveguide to couple light from the output end portion of the first waveguide to the input end portion of the second waveguide. The second vertical mirror element is connected to the input end portion of the third waveguide to couple light from the output end portion of the second waveguide to the input end portion of the third waveguide. According to the present invention, the optical axis of the third waveguide is rotated by the second angle in the first waveguide layer, thereby inducing the polarization rotation of the light. In other embodiments, the second waveguide layer is orthogonal to the first waveguide layer. This can induce the polarization rotation of the light coupled from the second waveguide to the third waveguide by an amount corresponding to the second angle.

[0015] In some embodiments, the second waveguide layer is orthogonal to the first waveguide layer. The optical axis of the first waveguide is rotated by the first angle in the first waveguide layer, and the first vertical mirror element is connected to the output end portion of the first waveguide to couple light from the output end portion of the first waveguide to the input end portion of the second waveguide. The second vertical mirror element is connected to the input end portion of the third waveguide to couple light from the output end portion of the second waveguide to the input end portion of the third waveguide. According to the present invention, the optical axis of the third waveguide is rotated by the second angle in the first waveguide layer, thereby inducing the polarization rotation of the light. The optical axis of the first waveguide is rotated by the first angle in the first waveguide layer, and The first vertical mirror element is connected to the output end portion of the first waveguide to couple light from the output end portion of the first waveguide to the input end portion of the second waveguide, and The second vertical mirror element is connected to the input end portion of the third waveguide to couple light from the output end portion of the second waveguide to the input end portion of the third waveguide. Also, according to the present invention, the optical axis of the third waveguide is rotated by the second angle in the first waveguide layer, thereby inducing the polarization rotation of the light. The second vertical mirror element is connected to the input end portion of the third waveguide to couple light from the output end portion of the second waveguide to the input end portion of the third waveguide, and According to the present invention, the optical axis of the third waveguide is rotated by the second angle in the first waveguide layer, thereby inducing the polarization rotation of the light. According to the present invention, the optical axis of the third waveguide is rotated by the second angle in the first waveguide layer, thereby inducing the polarization rotation of the light. This can induce the polarization rotation of the light.

[0016] In other embodiments, the second waveguide layer is orthogonal to the first waveguide layer. The first The optical axis of the input end of the second waveguide is rotated by the first angle in the second waveguide layer. Also, the first vertical mirror element is connected to the input end of the second waveguide, and couples light from the output end of the first waveguide to the input end of the second waveguide. Also, the second vertical mirror element is connected to the output end of the second waveguide, and couples light from the output end to the input end of the third waveguide. Also, the optical axis of the output end of the second waveguide is rotated by the second angle in the second waveguide layer, thereby being able to induce further polarization rotation of light.

[0017] In some embodiments, the second waveguide layer is above or below the first waveguide layer, the second and first waveguide layers are parallel to each other, the first vertical mirror element is connected to the output end of the first waveguide, and a third vertical mirror element is connected to the input end of the second waveguide, thereby being able to couple light from the first waveguide to the second waveguide. In some embodiments, the first waveguide layer can have at least a third waveguide deposited on this first waveguide layer, the third waveguide having an input end and an output end, the second vertical mirror element is connected to the input end of the third waveguide, and a fourth vertical mirror element is connected to the output end of the second waveguide, thereby being able to couple light from the second waveguide to the third waveguide. In some embodiments, the second waveguide is a straight waveguide having a third vertical mirror element connected to its input end for coupling light from the first waveguide to the second waveguide.

[0018] Further, a fourth vertical mirror element is connected to the output end thereof to couple light from the second waveguide to the third waveguide.

[0019] In some embodiments, the second waveguide can include a horizontal light turning element such as a bending section or a horizontal TIR mirror, and the horizontal light turning element turns the light horizontally in the second waveguide layer, enabling the coupling of light from the output of the first waveguide to the input of the third waveguide. The horizontal light turning element can adjust or compensate for the polarization-dependent phase shift in the polarization rotator.

[0020] According to other embodiments, the polarization rotator according to the present invention has the following features, namely: · The third waveguide is oriented in the same direction as the first waveguide on the first substrate. · The third waveguide is oriented in the opposite direction to the first waveguide on the first substrate. · The second waveguide has a straight and parallel input portion and output portion, and a horizontal light turning element such as a bending section or a horizontal TIR mirror that connects the input portion and the output portion. · The straight waveguide region and / or the horizontal light turning element has a polarization-dependent phase shift that compensates or supplements the polarization-dependent phase shift induced by the vertical mirror element, thereby enabling the rotation of linearly polarized input light to linearly polarized output light. · The second waveguide and any mirror elements in the second waveguide layer are fabricated using a layered fabrication such as 3D printing or direct writing. It can have one or several of the above.

Brief Description of the Drawings

[0021]

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Embodiments for Carrying Out the Invention

[0022] Polarization rotation naturally occurs when light propagates from one waveguide to another (or from one birefringent material to another birefringent material) and when the polarization eigenmodes (or the directions of the fast axis and the slow axis) in the two propagation media are directed in different directions. In a planar optical waveguide, it is extremely difficult to rotate the eigenmode with respect to the substrate of the waveguide. with respect to the substrate of the waveguide. are directed in different directions. is extremely difficult.

[0023] However, the rotation of the waveguide's eigenmode with respect to the waveguide substrate is a common method for realizing polarization rotation on a planar waveguide circuit. The gradual introduction of asymmetry in the waveguide causes the birefringence in the asymmetric waveguide to gradually rotate the polarization eigenmode while reflecting the polarization with respect to one direction of the polarization eigenmode. This corresponds to using a waveplate in free-space optics. Another example of an asymmetric waveguide with a tilted eigenmode is a periodic grating where the cross-section of the asymmetric waveguide alternates along the device, or a waveguide whose shape changes gradually. Figure 1 shows how the rotation of one waveguide chip 2 with respect to another waveguide chip 1 around the common optical axis in the waveguide pair enables polarization rotation for one waveguide pair on the same two waveguide chips, but not for any other arbitrary waveguide pair. In Figure 1, the two waveguide chips 1 and 2 are rotated by an angle β around the rotation axis R defined as the common axis of the primary waveguides 1A and 2A on the two chips. The other waveguides 1B, 2B, 1C, and 2C on chips 1 and 2 do not align after rotation. The present invention is based on the idea that it is possible to rotate the polarization by turning the light from the input waveguide up or down to rotate the output waveguide on a waveguide chip or waveguide layer different from the input waveguide. This is demonstrated in Figure 2, which schematically shows a plan view and a side view of a polarization rotation element composed of two waveguides 1 and 2. These waveguides are optically coupled to each other by a vertical mirror element 4. The mirror 4 couples the two waveguides 1

[0024]

[0025] ​​​​​​​​​​​​​​​, as indicated by the two thick arrows, incoming light is coupled by reflecting the light into the upper waveguide 2. Of course, the direction of the light can also be reversed. When the light is coupled from the input waveguide 1 to the upper output waveguide 2 and physically rotated relative to each other by an arbitrary angle β, polarization rotation occurs between the two waveguides. In FIG. 2 showing the basic concept of the present invention, by rotating the output waveguide 2 by an angle β around the vertical optical axis between the two waveguides with respect to the input waveguide 1, polarization rotates between the two straight waveguides 1 and 2. The light is coupled between the two waveguides by the mirror element 4, and the polarization is rotated between the mirrors when the light propagates in the vertical direction. More specifically, the waveguides 1 and 2 are in two separate waveguide layers arranged on top of each other. The waveguide layers are on a common substrate and in the same plane and include a photonic integrated circuit or at least two separate waveguides. The waveguide layers of waveguides 1 and 2 can be deposited on two separate substrates, whereby they are on two separate waveguide chips arranged on top of each other and parallel to each other (see, for example, FIG. 3). As an alternative, the two separate waveguide layers of waveguides 1 and 2 can be deposited on a common substrate, whereby they are two waveguide layers on a single waveguide chip. The first or input waveguide 1 has an input end where light enters as indicated by the thick arrow and an output end at the mirror element 4 as shown in the side view of the figure. The opposite is true for the second or output waveguide 2. The first mirror element 4 is connected to the output end of the first or input waveguide 1, and the second mirror element is connected to the input end of the second waveguide 2. The first and

[0026]

[0027] ​​​​​​​​​​​​​​​The waveguide layers in the second waveguide are aligned with each other, whereby the first mirror element couples light to the second mi rror element.

[0028] As shown in the figure, the second waveguide and its optical axis are rotated by an angle β with respect to the optical axis of the first waveguide, thereby inducing a polarization rotation corresponding to the first angle β in the light coupled from the first waveguide to the second waveguide. In this embodiment, light travels vertically between the two waveguide layers, and polarization rotation occurs when the optical axes of the two waveguides can rotate relative to each other within a small gap between the two vertical mirrors. According to an important aspect of the present invention, a large number of input waveguides and output waveguides can be integrated within two parallel waveguide layers, and light from all waveguides can be simultaneously coupled to all output waveguides. Preferably, the vertical mirror element 4 reflects light 90° upward or downward. In this case, the axis of rotation of the waveguide to be turned is perpendicular to the plane of the waveguide layer. In the following description, the vertical mirror element is defined as mirror-reflected light that exits the waveguide layer, i.e., upward or downward with respect to the light propagation direction in the waveguide before reflection. The horizontal mirror is defined as mirror-reflected light that reflects left or right within the waveguide layer, i.e., with respect to the light propagation direction in the waveguide before reflection.

[0029] Next, referring to FIG. 3, which schematically shows a plan view and a side view of a polarization rotation element formed by three waveguides. These waveguides are optically coupled to each other by vertical mirror elements. In the following description, the vertical mirror element is defined as mirror-reflected light that exits the waveguide layer, i.e., upward or downward with respect to the light propagation direction in the waveguide before reflection. The horizontal mirror is defined as mirror-reflected light that reflects left or right within the waveguide layer, i.e., with respect to the light propagation direction in the waveguide before reflection. Preferably, the vertical mirror element 4 reflects light 90° upward or downward. In this case, the axis of rotation of the waveguide to be turned is perpendicular to the plane of the waveguide layer.

[0030] In the following description, the vertical mirror element is defined as mirror-reflected light that exits the waveguide layer, i.e., upward or downward with respect to the light propagation direction in the waveguide before reflection. The horizontal mirror is defined as mirror-reflected light that reflects left or right within the waveguide layer, i.e., with respect to the light propagation direction in the waveguide before reflection. In the following description, the vertical mirror element is defined as mirror-reflected light that exits the waveguide layer, i.e., upward or downward with respect to the light propagation direction in the waveguide before reflection. The horizontal mirror is defined as mirror-reflected light that reflects left or right within the waveguide layer, i.e., with respect to the light propagation direction in the waveguide before reflection. The horizontal mirror is defined as mirror-reflected light that reflects left or right within the waveguide layer, i.e., with respect to the light propagation direction in the waveguide before reflection.

[0031] Next, referring to FIG. 3, which schematically shows a plan view and a side view of a polarization rotation element formed by three waveguides. These waveguides are optically coupled to each other by vertical mirror elements And are shown. These waveguides are optically coupled to each other by vertical mirror elements. ​​are combined. In this embodiment of the present invention, the polarization rotation is respectively the turning angles β1 and β2 achieved as a combination of two polarization rotations between waveguides 1, 2, and 3 due to . In this embodiment, two waveguides 1 and 3 are deposited on the first substrate 5, and the first waveguide is part of the layer. In the illustration of FIG. 3, waveguide 2 can be deposited on a second substrate (not shown), and it is part of the second waveguide layer on an individual waveguide chip. However, this second waveguide layer having waveguide 2 can also be deposited above or below the first waveguide layer on the same substrate 5.

[0032] To realize the second waveguide layer in the polarizer according to the present invention, additive manufacturing, ion beam etching, or 3D manufacturing methods can also be used. For example, the waveguides and mirrors in the second waveguide layer can be formed by 3D printing or laser direct writing technology. According to these methods, a curved mirror can be manufactured that avoids the need to add a conventional straight waveguide between two mirrors in the second waveguide layer. Other advantages of additive manufacturing, ion beam etching, or 3D manufacturing methods are that the different waveguides and mirrors in the second waveguide layer can generally have different orientations, unlike wet-etched mirrors that are aligned by the crystal orientation of the material. The option of using mirrors manufactured by additive manufacturing, ion beam etching, or 3D manufacturing methods can be used for the second waveguide layer in all embodiments of the present invention.

[0033] Connect the first mirror element 4 to the output end of the first waveguide 1, and connect the second mirror element 4 to the second waveguide It is connected to the input end of the second waveguide 2. Further, the third mirror element 4 is connected to the output end of the second waveguide 2, and the fourth mirror element 4 is connected to the input end of the third waveguide 3. The first and second waveguide layers are aligned such that the first mirror element couples light to the second mirror element and the third mirror element couples light to the fourth mirror element. When coupling light from the first vertical mirror to the second vertical mirror, the optical axis of the second waveguide 2 is rotated by an angle β1 with respect to the optical axis of the first waveguide 1. In this embodiment, the second waveguide layer is disposed on top of the first waveguide layer, and the two waveguide layers are parallel to each other. It is connected to the input end of the third waveguide 3. The first and second waveguide layers are aligned such that the first mirror element couples light to the second mirror element and the third mirror element couples light to the fourth mirror element. When coupling light from the first vertical mirror to the second vertical mirror, the optical axis of the second waveguide 2 is rotated by an angle β1 with respect to the optical axis of the first waveguide 1. In this embodiment, the second waveguide layer is disposed on top of the first waveguide layer, and the two waveguide layers are parallel to each other. The first and second waveguide layers are aligned such that the first mirror element couples light to the second mirror element and the third mirror element couples light to the fourth mirror element. When coupling light from the first vertical mirror to the second vertical mirror, the optical axis of the second waveguide 2 is rotated by an angle β1 with respect to the optical axis of the first waveguide 1. In this embodiment, the second waveguide layer is disposed on top of the first waveguide layer, and the two waveguide layers are parallel to each other. The first and second waveguide layers are aligned such that the first mirror element couples light to the second mirror element and the third mirror element couples light to the fourth mirror element. When coupling light from the first vertical mirror to the second vertical mirror, the optical axis of the second waveguide 2 is rotated by an angle β1 with respect to the optical axis of the first waveguide 1. In this embodiment, the second waveguide layer is disposed on top of the first waveguide layer, and the two waveguide layers are parallel to each other. The first and second waveguide layers are aligned such that the first mirror element couples light to the second mirror element and the third mirror element couples light to the fourth mirror element. When coupling light from the first vertical mirror to the second vertical mirror, the optical axis of the second waveguide 2 is rotated by an angle β1 with respect to the optical axis of the first waveguide 1. In this embodiment, the second waveguide layer is disposed on top of the first waveguide layer, and the two waveguide layers are parallel to each other. The first and second waveguide layers are aligned such that the first mirror element couples light to the second mirror element and the third mirror element couples light to the fourth mirror element. When coupling light from the first vertical mirror to the second vertical mirror, the optical axis of the second waveguide 2 is rotated by an angle β1 with respect to the optical axis of the first waveguide 1. In this embodiment, the second waveguide layer is disposed on top of the first waveguide layer, and the two waveguide layers are parallel to each other. The first and second waveguide layers are aligned such that the first mirror element couples light to the second mirror element and the third mirror element couples light to the fourth mirror element. When coupling light from the first vertical mirror to the second vertical mirror, the optical axis of the second waveguide 2 is rotated by an angle β1 with respect to the optical axis of the first waveguide 1. In this embodiment, the second waveguide layer is disposed on top of the first waveguide layer, and the two waveguide layers are parallel to each other.

[0034] In the embodiment of FIG. 3, two polarization rotations occur in the same clockwise direction when viewed along the light propagation direction, i.e., when the light travels upward in the first rotation and when the light travels downward in the first rotation. In the plan view, the positive rotations of the angles β1 and β2 appear to be in opposite directions to each other. In the embodiment of FIG. 3, two polarization rotations occur in the same clockwise direction when viewed along the light propagation direction, i.e., when the light travels upward in the first rotation and when the light travels downward in the first rotation. In the plan view, the positive rotations of the angles β1 and β2 appear to be in opposite directions to each other. In the embodiment of FIG. 3, two polarization rotations occur in the same clockwise direction when viewed along the light propagation direction, i.e., when the light travels upward in the first rotation and when the light travels downward in the first rotation. In the plan view, the positive rotations of the angles β1 and β2 appear to be in opposite directions to each other. In the embodiment of FIG. 3, two polarization rotations occur in the same clockwise direction when viewed along the light propagation direction, i.e., when the light travels upward in the first rotation and when the light travels downward in the first rotation. In the plan view, the positive rotations of the angles β1 and β2 appear to be in opposite directions to each other.

[0035] In the embodiment of FIG. 3, waveguide bends or horizontal mirrors are not required. However, from an assembly perspective, the coupling of light in the mirror element is sensitive to etching errors, so the mirror element needs to be precisely manufactured (etched). This will be described in detail below. In the embodiment of FIG. 3, waveguide bends or horizontal mirrors are not required. However, from an assembly perspective, the coupling of light in the mirror element is sensitive to etching errors, so the mirror element needs to be precisely manufactured (etched). This will be described in detail below. In the embodiment of FIG. 3, waveguide bends or horizontal mirrors are not required. However, from an assembly perspective, the coupling of light in the mirror element is sensitive to etching errors, so the mirror element needs to be precisely manufactured (etched). This will be described in detail below.

[0036] There are multiple options for mounting the mirror element employed in the present invention. Refer to FIGS. 4A and 4B. For example, it can be based on a total internal reflection (TIR) mirror as shown in FIG. 4A or an external metal mirror as shown in FIG. 4B. There are multiple options for mounting the mirror element employed in the present invention. Refer to FIGS. 4A and 4B. For example, it can be based on a total internal reflection (TIR) mirror as shown in FIG. 4A or an external metal mirror as shown in FIG. 4B. There are multiple options for mounting the mirror element employed in the present invention. Refer to FIGS. 4A and 4B. For example, it can be based on a total internal reflection (TIR) mirror as shown in FIG. 4A or an external metal mirror as shown in FIG. 4B.

[0037] FIGS. 4A and 4B present 3D schematic views of the vertical mirror elements used in the present invention.

[0038] In the embodiment of FIG. 4A, the mirror surface 41 reflects light upward (or downward) from the waveguide layer of the waveguide 40 and also has a TIR mirror surface 41 with a negative angle and an output facet 42 and can be configured. Further, a metal coating or other reflective coating can also be applied on the mirror surface 41. The output facet can be anti-reflection coated to reduce unwanted reflections.

[0039] In the embodiment of FIG. 4B, the mirror element 43 can be composed of a transparent waveguide facet 45 at the end of the waveguide 43, followed by an external (indicated by the dashed line D) metal mirror surface 44 can be arranged. The vertical facet 45 can be anti-reflection coated to reduce unwanted reflections. The mirror surface 44 can also have some non-metallic reflective coating. Obviously, using the same technology as FIGS. 4A and 4B, a horizontal mirror element can be manufactured or the light can be reflected downward instead of upward.

[0040] The inclined mirror surface according to FIG. 4A or FIG. 4B can be fabricated using anisotropic dry etching at an inclined angle, for example, with an ion beam etcher, or using anisotropic wet etching to produce a flat surface along a predetermined crystal plane. For SOI waveguides, potassium hydroxide (KOH) or tetramethylammonium hydroxide (TMAH) is commonly used as the wet etching method. A mirror of exactly 45° can be manufactured either by adding a suitable surfactant for etching the substance on the wafer or by using a special crystal orientation. In the most common crystal orientation

[0100] , the Si​​​​​​​​​​ Mirrors with an angle of 55° are most easily manufactured on a reticle, and in some embodiments of the present invention, such mirrors can be used. However, a mirror angle of 45° is a preferred option of the present invention, reflecting light upward or downward in the vertical direction. Any mirror can be fabricated using the additive manufacturing or ion beam etching as described above. By any of these processes, different mirror orientations in the same waveguide layer can be achieved, which enables the realization of different polarization rotations in the same waveguide layer or chip.

[0041]

[0042] A practical problem with the design shown in FIG. 3 (and FIGS. 5 - 6) is that in some cases, the position of the vertical mirror element cannot be precisely controlled. For example, a wet-etched 45° mirror generally causes some variation in the width of the etching region. In this case, for example, the exact distance between any two mirror elements at different angles facing each other as shown in FIG. 3 varies on the wafer or from wafer to wafer. This causes misalignment between the optical axes of the waveguides. For example, if a manufacturing defect causes a shift closer to the waveguide corresponding to each of the mirrors shown in FIG. 3, the first and fourth mirrors in waveguide layer 1 move further apart from each other, while the second and third mirrors in waveguide layer 2 move closer to each other. This makes it impossible to perfectly align both mirror pairs by aligning the waveguide layers with each other. Some embodiments of the present invention can avoid this potential problem when based on using the same mirror (exactly the same orientation) in each waveguide layer (see FIGS. 7 and 8). ​

[0043] When determining the polarization rotation of the waveguides according to the present invention in general, the mirrors and TIR mirrors in particular The phase shift introduced by the dielectric mirror to the s and p polarizations must be taken into account. For the case, these phase shifts can be calculated using the Fresnel equations, In this case, s and p polarization refer to polarization states with orthogonal electric and magnetic fields in the plane of incidence. For a directional reflecting TIR mirror, the s-polarized light corresponds to the TE mode of the waveguide, and the electric field corresponds to the TE mode of the waveguide. It lies in the plane of the waveguide layer and is oriented along the x-axis of the waveguide's local coordinate system. The p-polarized light corresponds to the TM mode of the waveguide, and the electric field is perpendicular to the plane of the waveguide layer and perpendicular to the waveguide. In some embodiments of the present invention, the vertical Polarization-dependent phase shifts in mirror elements can be achieved, for example, by using metallic mirrors. Can be minimized.

[0044] Polarization-dependent phase shifts occur at horizontal mirrors, waveguide bends, and in straight waveguides. This can occur even when the effective indices of the TE and TM modes are different. The modes propagate at different velocities, and the phase shift between them accumulates as a function of propagation distance. This in turn creates a desired polarization dependent phase shift in a portion of the polarization rotator. , opening up the possibility of compensating for unwanted phase shifts in other parts of the same device.

[0045] In the ideal case, the polarization dependent phase shift in each section of the polarization rotator is zero or π, and any linearly polarized input light becomes linearly polarized when propagating through the polarization rotator. Polarization-dependent phase shifts that are not integer multiples of π usually convert linearly polarized light into elliptically polarized light. This is done. For ease of understanding, a detailed analysis of the polarization rotation with respect to elliptical polarization is extremely complex and difficult to explain, so linearly polarized light is assumed as follows.

[0046] The polarization-dependent phase shift in the TIR mirror depends on at least the angle of incidence of light (α), the waveguide material (silicon) and the cladding material (e.g., air, silicon dioxide or silicon nitride). It is a function of

[0047] For the case where the polarization-dependent phase shift Δφ in the vertical mirror element is not zero, there are several solutions. For example, one solution is to use one or more compensating optical elements having a non-zero polarization-dependent phase shift of -Δφ adjacent to the vertical mirror element. For example, a birefringent waveguide region or a horizontal TIR mirror can be integrated into the same waveguide in which the vertical mirror element is integrated. This technique is applied to any arbitrary linearly polarized input light. One skilled in the art can calculate the effect of the polarization-dependent phase shift on the polarization in any polarization configuration described herein and for designing a polarization rotator, so that any unwanted polarization-dependent phase shift can be compensated by other similar phase shifts to maintain linearly polarized light when necessary. An example of this is to add one horizontal TIR mirror adjacent to each vertical TIR mirror element, whereby they compensate for each other's polarization-dependent phase shifts (see, for example, FIG. 7C). Referring again to FIG. 3, four vertical mirror elements 4 are shown, i.e., the entire vertical mirror elements at the ends of each of the waveguides 1, 2, 3 at two polarization rotation interfaces. However, these vertical mirrors Referring again to FIG. 3, four vertical mirror elements 4 are shown, i.e., the entire vertical mirror elements at the ends of each of the waveguides 1, 2, 3 at two polarization rotation interfaces. However, these vertical mirrors Referring again to FIG. 3, four vertical mirror elements 4 are shown, i.e., the entire vertical mirror elements at the ends of each of the waveguides 1, 2, 3 at two polarization rotation interfaces. However, these vertical mirrors Referring again to FIG. 3, four vertical mirror elements 4 are shown, i.e., the entire vertical mirror elements at the ends of each of the waveguides 1, 2, 3 at two polarization rotation interfaces. However, these vertical mirrors

[0048] Referring again to FIG. 3, four vertical mirror elements 4 are shown, i.e., the entire vertical mirror elements at the ends of each of the waveguides 1, 2, 3 at two polarization rotation interfaces. However, these vertical mirrors Referring again to FIG. 3, four vertical mirror elements 4 are shown, i.e., the entire vertical mirror elements at the ends of each of the waveguides 1, 2, 3 at two polarization rotation interfaces. However, these vertical mirrors Some of the elements can be replaced by a U-shaped second waveguide layer when the second waveguide layer turns to an upright position with respect to the first waveguide layer.

[0049] FIG. 5A is a schematic side view of a polarization rotator according to the present invention on two substrates (or waveguide chips) 5 and 7 where two waveguide layers are orthogonal to each other. Three waveguides 1, 2, and 3 are optically coupled to each other in the same manner as in FIG. 3, but in this case only two vertical mirror elements 4 are required. In this embodiment, the second waveguide 2 has a straight input portion 2a, a straight output portion 2c, and an intermediate curved waveguide path portion 2b connecting these input and output portions.

[0050] More specifically, the intermediate waveguide 2b forms the second waveguide layer into a horizontal U-bend portion and turns the upward-propagating light downward toward the waveguide 3. The U-shaped loop based on the waveguide bend portion generally causes a polarization-dependent phase shift that needs to be considered when designing the polarization rotator. It is extremely difficult to directly deposit the waveguide 2 on the same (first) substrate 5 as the waveguides 1 and 3, and thus two separate waveguide chips are combined with each other in a suitable method to realize the structure shown in FIG. 5A.

[0051] FIG. 5A further shows the possibility of providing studs 8 and alignment receivers 8a for passive mechanical alignment between the two waveguide layers, which is particularly useful in embodiments where the substrates (or waveguide chips) 5, 7 are orthogonal to each other. However, this mechanical alignment concept can be applied to any polarization rotator using two separate waveguide chips.

[0052] FIG. 5B shows a plan view of the embodiment of FIG. 5A and shows various arrangement options in waveguides 1 and 3 on the substrate 5 as indicated by the double-headed arrows. Therefore, it goes without saying that the position of the substrate (or waveguide chip) 7 needs to be adjusted accordingly. The waveguides 1 and 3 in FIG. 5A can be regarded as being inclined with respect to the plane shown, but this can only be understood from the projection view of FIG. 5B. In the two examples shown, the waveguides 1 and 3 are parallel to each other, but this is not essential.

[0053] In FIG. 6, another embodiment is shown in which four horizontal TIR mirrors 10 are used for the waveguide 2 which is a part of the waveguide layer 2 on the substrate (or waveguide chip) 7. Here, the second waveguide 2 has a straight input portion, a straight output portion, and an intermediate waveguide portion, and this intermediate waveguide portion further has the TIR mirrors 10 and a straight waveguide region connecting these mirrors. The intermediate waveguide portion effectively forms an equivalent of a U-shaped loop connecting the input portion and the output portion to each other. Here, the mirrors 10 turn light horizontally in the waveguide 2. In order to reduce the problem of polarization phase shift occurring due to the TIR mirrors, an appropriate number of mirrors 10 with appropriate mirror angles are used to effectively form a U-shaped loop and to cause a phase shift of about 2π between the TE polarization and the TM polarization. Therefore, the horizontal TIR mirrors virtually do not change the polarization state. The number of mirrors (four) and their angles in FIG. 6 (and FIG. 7B) are selected only with respect to the schematic diagram and can be optimized to achieve any desired phase shift between the two polarization modes in the waveguide 2. Such optimization

[0054] do not substantially change the polarization state. The number of mirrors (four) and their angles in FIG. 6 (and FIG. 7B) are selected only with respect to the schematic diagram and can be optimized to achieve any desired phase shift between the two polarization modes in the waveguide 2. Such optimization ​​​​​​​​In one example, a U-shaped loop is created that reflects linearly polarized light with respect to the polarization axis of the waveguide, which is to achieve a π phase shift between TE polarization and TM polarization.

[0055] Needless to say, as long as waveguide 2 optically couples waveguides 1 and 3, any number of mirrors with any reflection angle can be used. Regarding the arrangement of waveguides on substrates (or waveguide chips) 5 and 7, refer to FIG. 5B. A common advantage of the embodiments shown in FIGS. 5 and 6 is the ability to reduce the number of vertical mirror elements.

[0056] The choice between horizontal bends, horizontal mirrors, and vertical mirror elements can be affected by optical losses and polarization dependent phase shifts. As previously explained, TIR mirrors generally have a finite phase shift Δφ, but straight waveguides and waveguide bends can also produce a finite Δφ. Sometimes, to compensate for the finite Δφ in the vertical mirror element, or to reflect linearly polarized light with respect to one polarization axis, the finite Δφ can be used in a straight waveguide bend, or horizontal mirror.

[0057] In one embodiment of the present invention, to reflect polarization with respect to one polarization eigenstate, a number of bends or TIR mirrors are used to produce a π phase shift between two polarization modes in the second waveguide. In some other embodiments, to avoid the net effect of Δ φ that cannot be reduced to zero, a number of bends or TIR mirrors are used to produce a 2π phase shift between two polarization modes in the second waveguide. This is schematically shown in FIG. 6 with four TIR mirrors 10, but the number of horizontal mirrors (or bends) and The angles should be optimized for the waveguides, mirrors, and bending structures used.

[0058] To reduce any misalignment between the optical axes of the waveguides, some embodiments of the present invention provide that all mirrors in the same waveguide layer (or waveguide chip) have the same orientation and are arranged such that the waveguides are antiparallel. This facilitates alignment between the waveguide layers, especially when the positions of the vertical mirror elements have finite fabrication tolerances.

[0059] One such embodiment is shown in FIG. 7A, which is a schematic plan view of a polarization rotator having three waveguides 1, 2, and 3 that are optically coupled to each other and each have a vertical mirror element 4 perpendicular to all three waveguides. In this embodiment, two polarization rotations occur in the same direction, and waveguide 2 includes a horizontal U-shaped loop bend 2b. Waveguides 1 and 3 are antiparallel (180°) when angle β1 = β2 and the U-shaped loop is rotated 180°. Also, it is important to pay attention to the polarization-dependent phase shift that generally occurs when the waveguide makes a U-turn. One approach is to intentionally increase the finite phase shift Δφ to 2π (i.e., 360°) or an integer multiple thereof, so that waveguide 2 and its vertical mirror element do not produce any polarization-dependent phase shift.

[0060]

[0061] FIG. 7B is a schematic plan view of a polarization rotator having three waveguides 1, 2, and 3 that are optically coupled to each other and each have a vertical mirror element 4 perpendicular to all three waveguides. In this example, two polarization rotations occur in the same direction, and waveguide 2 makes waveguides 1 and 3 antiparallel. It has two straight waveguides connected by a horizontal TIR mirror 10 that turns the U-shaped light. Here To illustrate the idea of β1 = β2, four 90° mirrors are used and schematically only shown. If the TIR mirror 10 produces a phase shift of about 360° between TE polarization and TM polarization, it does not substantially change the polarization state in waveguide 2. Such a horizontal TIR mirror can also compensate for the undesirable phase shift caused by the vertical mirror element. For example, the polarization dependence of the two vertical TIR mirrors (90° turning angle) at the end of waveguide 2 can be compensated by the two horizontal TIR mirrors (90° turning angle) in waveguide 2. The same compensation can be applied to the TIR mirror in waveguide 1 by adding horizontal TIR mirrors in waveguides 1 and 3. This is shown in FIGS. 7C and 7D. In the embodiment of FIG. 5A, it was previously shown how the vertical mirror element of waveguide 2 could be eliminated by using a U-shaped bend. In some embodiments, instead, the mirror elements can be omitted from waveguides 1 and 3. This is shown in FIG. 8. In this case, the chip 12 of waveguide 2 is turned to an upright position, and all the vertical mirror elements 4 connected to this waveguide chip are aligned and located on the same chip. The substrate or chip 12 of the second waveguide 2 is arranged at the edge of the substrate or chip 5 on which waveguides 1 and 3 are fabricated,

[0062] or arranged in a cavity etched in the substrate or chip 5. The two coordinate system rotations (β1 and β2) can be freely selected, but according to an embodiment of the present invention, the two mirrors 4 can have the same orientation, whereby waveguide 2 also forms a U-shaped loop 2b. In this case, the chip 12 of waveguide 2 is turned to an upright position, and all the vertical mirror elements 4 connected to this waveguide chip are aligned and located on the same chip. The substrate or chip 12 of the second waveguide 2 is arranged at the edge of the substrate or chip 5 on which waveguides 1 and 3 are fabricated, or arranged in a cavity etched in the substrate or chip 5. The two coordinate system rotations (β1 and β2) can be freely selected, but according to an embodiment of the present invention, the two mirrors 4 can have the same orientation, whereby waveguide 2 also forms a U-shaped loop 2b. rotations (β1 and β2) can be freely selected, but according to an embodiment of the present invention, the two mirrors 4 can have the same orientation, whereby waveguide 2 also forms a U-shaped loop 2b.

[0063] In this embodiment, waveguides 1 and 3 are antiparallel, and waveguide 2 forms a horizontal bending section 2 b. Therefore, the two polarization rotations are performed in the same direction. The chip 12 of waveguide 2 rotates with respect to waveguides 1 and 2 on the same chip 5. The front view is along the light propagation direction of waveguide 1 and the side view is the side view of waveguides 1 and 3 (waveguide 1 is behind waveguide 3). .

[0064] In this configuration, when Δφ = 0, the polarization rotation is β1 + β2. However, when the vertical mirror element 4 or waveguide 2 and its U-shaped bending section 2b cause some finite phase shift Δφ, a preferred approach is to adjust the polarization-dependent phase shift in the individual elements such that, for example , as described with reference to FIGS. 7B, 7C, and 7D, its net effect is adjusted to be zero or a multiple of 2π .

[0065] The advantages of these embodiments include a small number of upward reflection mirrors and the need for insensitivity to mirror etching .

[0066] Various waveguide embodiments and the local coordinate systems used for the vertical mirror elements are illustrated in FIG. 9 . The thick arrows indicate the coupling of light between waveguides, and the z-axis (z1 - z3) represents the main direction of light propagation , i.e., the optical axes of waveguides 1 - 3. The x-axis and y-axis represent the main electric field directions of the polarization eigenmodes in the waveguides and their corresponding mirror elements. Waveguide 2 is displaced away from the target location for clarity .

[0067] FIG. 10 shows the basic building block on a micron-scale SOI platform. Metallized upward reflection waveguide mirror 13, single-mode rib waveguide 14, and simulate​​ The resulting mode field distribution 14a, the horizontal TIR mirror 15, between the rib waveguide and the strip waveguide the rib - strip converter 16 for adiabatic coupling, and the vertical taper 17 between two waveguide thicknesses are shown.

[0068] FIG. 11 shows a schematic diagram of an embodiment of an integrated isolator or circulator according to the present invention, which consists of two polarization beam splitters (PBSs) 18, two 45° Faraday rotators (FRs) 19, and two 45° (reversible) polarization rotators 20.

[0069] Various coordinate system rotations and the resulting polarization rotations are shown in FIGS. 12 - 20. All the figures are presented as seen in the cross - section of the optical propagation and in the main direction, with the coordinate axes, (linear) polarization states, and their rotations. Light can, of course, propagate in the reverse direction as long as the system is reversible.

[0070] FIG. 12 shows the basic principle of the coordinate system rotation (β) between the input waveguide and the output waveguide (e.g., waveguides 1 and 2 in FIGS. 2 or 9). When a vertical mirror element is integrated at the end of the waveguide, the rotation occurs with respect to the coordinate system of the mirror element.

[0071] FIG. 13 shows an example of linear polarization and its direction (α) in both the original coordinate system (x in , y in ) and the new coordinate system ( x out , y out ) after the coordinate system rotation (β). The polarization eigenmodes are generally aligned along the x - axis and y - axis of the waveguide. The polarization angle is α in the coordinate system of the first (input) waveguide and α in the coordinate system of the second (output) waveguide. in out ​​

[0072] The sequential coordinate system rotations (β1 + β2) among waveguides 1, 2, and 3 in the same direction are shown in Fig. 14 where the indices 1, 2, and 3 refer to waveguides 1, 2, and 3. The rotation resulting from the polarization angle is shown in Fig. 15, where the linearly polarized light after two sequential coordinate system rotations (β1 + β2) is shown, and the polarization rotation angles are α1 and α3 before and after the two rotations, respectively

[0073] Two sequential and opposite direction coordinate system rotations β1>0 and β2<0 in the reverse direction are shown in Fig. 16. For clarity, the two polarization rotations are shown to have different magnitudes, but in many cases, it is desirable for the two to be of equal magnitude but opposite in direction

[0074] Fig. 17 shows a method of summing two polarization rotations even when the two coordinate system rotations occur in the reverse direction. This is based on the reflection of polarization with respect to the y (or x) axis, and this reflection can be achieved by inducing a phase difference of Δφ = π between two polarization eigenmodes in waveguide 2. The original polarization (OP) is shown by the thick solid line at angle α1, and the reflected final polarization (FP) is shown by the double line at angle α3, respectively. α2 and α'2 represent the polarization before and after polarization reflection in waveguide 2

[0075] In Fig. 18, a special case of Fig. 15 is shown where the two coordinate system rotations are compensatory, i.e., the case where β1 + β2 = π. In this case, the effective polarization rotation is zero, but when the polarization rotator is part of a phase-sensitive waveguide circuit, the absolute phase of the light still needs to be considered. If there is no phase change between the two polarizations, the total polarization rotation is π > ​​​​​​​​​​​​​ is substantially zero.

[0076] Figures 19 and 20 show the polarization rotation in the case of compensatory rotation when polarization is reflected between two rotations. These figures show the two rotations and one polarization step by step, which are difficult to see in a single figure.

[0077] In Figure 19, the linearly polarized light after the first coordinate system rotation of the angle β1 between waveguide 1 and waveguide 2 and the polarization reflection with respect to the y2 axis following it are shown. The angles of the initial polarization OP and the final polarization FP are α1 and α'2, respectively.

[0078] In Figure 20, the linearly polarized light after two successive coordinate system rotations (β1 and β2) between waveguides 1, 2, and 3 and the polarization reflection with respect to the y2 axis between these rotations are shown. The initial polarization angle is α1, and the final polarization angle is α3. In this embodiment, the two rotations are compensatory (β2 = π - β1).

[0079] It should be understood that the disclosed embodiments of the present invention are not limited to the disclosed specific structures, process steps, or materials, but can be extended to their equivalents recognized by those skilled in the art. Furthermore, the terms used in this specification are used only for the purpose of describing specific embodiments and are not intended to be limiting. If there is any reference to any technology, it is not an approval that this technology forms part of the common general knowledge in any country, nor should it be received as such in any suggestive form.

[0080] Throughout this specification, "one embodiment" or "an embodiment" ​​​​​​​​​​A reference to an “embodiment” means that a particular feature, structure, or characteristic described in connection with that embodiment is included in at least one embodiment of the invention. Thus, the phrases “in one embodiment” or “in an embodiment” in various places throughout this specification are not necessarily all referring to the same embodiment.

[0081] Various embodiments and examples of the invention may be referred to herein, along with alternatives for their various components. Such embodiments, examples, and variations should not be construed as being substantially equivalent to one another,

[0082] but rather should be regarded as separate and autonomous expressions of the invention. Furthermore, the described features, structures, or characteristics may be combined in any suitable manner in one or more embodiments. In this specification, for example, numerous specific details such as length, width, shape, etc. have been presented, but this is for the purpose of fully understanding the embodiments of the invention. However, those skilled in the art will recognize that the invention can be

[0083] practiced without one or more of these specific details, or with other methods, components, materials, etc. In other instances, well-known structures, materials, or operations are not shown in detail or described in order to avoid obscuring the It will be apparent. Therefore, except as recited in the claims, the present invention is not intended to be limited thereby.

Claims

1. In a polarization rotator, ・ a first waveguide layer including at least a first waveguide, the first waveguide having an input end and an output end, the first waveguide layer; ・ a second waveguide layer including at least a second waveguide, the second waveguide having an input end and an output end, the second waveguide layer; ・ at least a first vertical mirror element disposed at the end of at least one of the first and second waveguides so as to couple light between the output end of the first waveguide and the input end of the second waveguide; and comprising The optical axis of the first or second waveguide having the vertical mirror element at its end is rotated by a first angle in the waveguide layer, thereby inducing polarization rotation of the light coupled between the first waveguide and the second waveguide by an amount corresponding to the first angle. A polarization rotator.

2. In the polarization rotator according to claim 1, the first waveguide layer is at least a third waveguide having an input end and an output end, the third waveguide, and between the output end of the second waveguide and the input end of the third waveguide so as to couple light, at least a second vertical mirror element disposed at the end of at least one of the second and third waveguides; and the optical axis of the waveguide having the second vertical mirror element is rotated by a second angle in the waveguide layer, thereby inducing polarization rotation of the light coupled from the second waveguide to the third waveguide by an amount corresponding to the second angle. A polarization rotator.

3. In the polarization rotator according to claim 1 or 2, the second waveguide layer is above or below the first waveguide layer, the second and first waveguide layers are parallel to each other, the first vertical mirror element is connected to the output end of the first waveguide, and a third vertical mirror element is connected to the input end of the second waveguide, thereby coupling light from the first waveguide to the second waveguide. A polarization rotator.

4. In the polarization rotator according to claim 2, at least a third waveguide deposited on the first waveguide has an input end and an output end, the second vertical mirror element is connected to the input end of the third waveguide, and a fourth vertical mirror element is connected to the output end of the second waveguide, thereby coupling light from the second waveguide to the third waveguide. A polarization rotator.

5. In the polarization rotator according to claim 2, the second waveguide layer is perpendicular to the first waveguide layer, the optical axis of the first waveguide is rotated by the first angle in the first waveguide layer, and the first vertical mirror element is connected to the output end of the first waveguide to couple light from the output end of the first waveguide to the input end of the second waveguide. Also, the second vertical mirror element is connected to the input end of the third waveguide to couple light from the output end of the second waveguide to the input end of the third waveguide. Further, the optical axis of the third waveguide is rotated by the second angle in the first waveguide layer, thereby inducing polarization rotation of light. A polarization rotator.

6. In the polarization rotator according to claim 2, the second waveguide layer is perpendicular to the first waveguide layer, the optical axis of the input end of the second waveguide is rotated by the first angle in the second waveguide layer, and the first vertical mirror element is connected to the input end of the second waveguide to couple light from the output end of the first waveguide to the input end of the second waveguide. Also, the second vertical mirror element is connected to the output end of the second waveguide to couple light from the output end of the second waveguide to the input end of the third waveguide. Further, the optical axis of the output end of the second waveguide is rotated by the second angle in the second waveguide layer, thereby inducing further polarization rotation of light. A polarization rotator.

7. In the polarization rotator according to any one of claims 2 to 4, the second waveguide has a third vertical mirror element connected to its input end to couple light from the first waveguide to the second waveguide, and is a straight waveguide. Also, a fourth vertical mirror element is connected to its output end to couple light from the second waveguide to the third waveguide. A polarization rotator.

8. In the polarization rotator according to any one of claims 2 to 6, the second waveguide includes a horizontal light turning element such as a bending portion or a horizontal TIR mirror. The horizontal light turning element turns the light horizontally in the second waveguide layer to enable coupling of light from the output of the first waveguide to the input of the third waveguide. A polarization rotator.

9. In the polarization rotator according to claim 8, the horizontal light turning element adjusts or compensates for polarization-dependent phase shift in the polarization rotator. A polarization rotator.

10. ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ In the polarization rotator according to any one of claims 2 to 9, the third waveguide is common A polarization rotator that is oriented in the same direction as the first waveguide on a substrate.

11. In the polarization rotator according to any one of claims 2 to 9, the third waveguide is common A polarization rotator that is oriented in a direction opposite to that of the first waveguide on a substrate.

12. In the polarization rotator according to claim 8, the second waveguide has a straight and parallel input portion and An output portion, and a horizontal light turning element such as a bending portion or a horizontal TIR mirror, A polarization rotator having the horizontal light turning element that connects the input portion and the output portion.

13. In the polarization rotator according to any one of claims 1 to 12, a polarization-dependent phase shift that compensates or supplements the polarization-dependent phase shift induced by the vertical mirror element A straight waveguide region and / or a horizontal light turning element such as a bending portion or a horizontal TIR mirror is included Thereby enabling rotation of linearly polarized input light to linearly polarized output light. A polarization rotator.

14. In the polarization rotator according to any one of claims 1 to 13, the second waveguide and any mirror element in the second waveguide layer Are fabricated using a layered fabrication such as 3D printing or direct writing A polarization rotator.

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