polarization rotator

Abstract

The invention relates to a polarization rotator comprising a first waveguide layer comprising at least a first waveguide having an input end and an output end, a second waveguide layer having at least a second waveguide having an input end and an output end, and at least a first vertical mirror element arranged at an 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 waveguide or the second waveguide having a vertical mirror element at its end is rotated in its waveguide layer by a first angle to cause a polarization rotation of light coupled between the first waveguide and the second waveguide by an amount corresponding to the first angle.

Description

Technical Field

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

[0002] The present invention relates to polarization rotators, and more particularly, to polarization rotators for photonic integrated circuits (PICs), i.e., planar waveguide circuits. The waveguides forming a PIC are typically formed on the surface of a chip (e.g., a silicon chip). In such planar waveguides, light propagates along the chip's surface in the direction of the local z-axis.

[0003] Polarization rotators are optical devices that rotate the polarization axis of a linearly polarized light beam by a selected angle. They are key components in optical isolators, polarization splitters, rotators, and circulators. While linear polarization rotation is the primary subject of this disclosure, the same or similar concepts can also be applied to circular or elliptical polarization.

[0004] In free-space optics, these polarization rotation devices can be based on the Faraday effect, birefringence, or reflection. Faraday rotators require waveguide materials that exhibit a magneto-optical effect. In birefringent polarization rotators, linearly polarized light is decomposed into two components, and a phase difference between these components occurs due to birefringence, changing the polarization. Birefringence can be a material property, and in the case of waveguides, it is typically caused by the waveguide geometry, stress in the waveguide material, or both. Light that passes through one or more reflections can also experience a phase shift between its polarization components, thus also rotating its polarization.

[0005] Integrated polarization manipulators are typically based on the use of some type of birefringent waveplate, made of either an asymmetric waveguide segment or a separate waveplate made of birefringent material and inserted into a slot etched through the waveguide. Waveplate-based polarization rotators are typically based on interference between polarization modes, which makes them wavelength-dependent and unsuitable for very broadband applications.

[0006] In waveguide circuits, cross-polarization coupling or mode evolution has also been proposed. Mode evolution-based rotators operate by adiabatically modifying the waveguide cross-section, gradually rotating the orientation of the polarization eigenmode by a desired amount. This transformation requires adiabatic heating to prevent power coupling between the different modes, making the devices very long and difficult to fabricate using conventional waveguide processing. These types of polarization rotators are also difficult to achieve with non-orthogonal polarization rotations, i.e., rotations other than 90°.

[0007] The optical power of light can be split into two polarization eigenmodes, each propagating along the waveguide with its characteristic velocity. The polarization modes in a planar waveguide can usually be approximated as pure TE and TM modes, where the electric field is oriented along the surface of the waveguide chip (TE, x-axis) or along the normal of the waveguide chip (TM, y-axis) when the light propagates along the optical axis of the waveguide (i.e., the local z-axis). The velocity is determined by the effective refractive index (n) of the mode. eff ) definition. In a birefringent waveguide, the two polarization modes have different n eff Therefore, the phase difference between the two modes, and hence the polarization of the light, will vary along the waveguide. Therefore, the total electric field of the light can be considered to consist of two components: one component (E x ), which is oriented along the x-axis; one component (E y ), which is oriented 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 and TM modes.

[0008] Polarization rotation in micrometer-scale silicon-on-insulator (SOI) waveguides is difficult to achieve by angles other than multiples of 90 degrees. Currently, a 45-degree polarization rotation is required to realize a fully integrated optical isolator based on Faraday rotation in SOI waveguides.

[0009] Known integrated polarization rotators are very sensitive to manufacturing errors and process variations because both the orientation of the polarization eigenmodes and the amount of birefringence need to be precisely controlled. This limits their application in commercial devices. Some techniques, such as those using thin waveplates inserted into etched grooves, are very demanding from an assembly perspective. Many known methods, particularly those relying on interference effects between waveguide modes, are also inherently wavelength-dependent.

[0010] Purpose of the Invention

[0011] The present invention aims to achieve polarization rotation of any desired angle and over a wide wavelength range using one or more planar waveguide chips. This goal stems from a need that arose when developing waveguide chips using micron-scale SOI platforms. However, the solutions presented below can be applied to many other planar waveguide platforms.

[0012] An important advantage of the present invention is that it allows polarization rotation to any direction and it supports polarization rotation of multiple waveguides simultaneously on a single waveguide chip without the need to assemble the necessary components for each waveguide separately. Summary of the Invention

[0013] According to the present invention, there is provided a polarization rotator comprising:

[0014] - a first waveguide layer comprising at least a first waveguide having an input end and an output end,

[0015] - a second waveguide layer having at least a second waveguide having an input end and an output end, and

[0016] - At least a first vertical mirror element arranged at the end of at least one of said waveguides to couple light between the output end of the first waveguide and the input end of the second waveguide.

[0017] According to the present invention, the optical axis of a first waveguide or a second waveguide having a vertical mirror element at its end can be rotated by a first angle in its waveguide layer to cause a polarization rotation of light coupled between the first waveguide and the second waveguide, the rotation amount corresponding to the first angle.

[0018] The first waveguide layer may include at least a third waveguide having an input end and an output end, and at least a second vertical mirror element disposed at an end of at least one of the second and third waveguides to couple light between the output end of the second waveguide and the input end of the third waveguide. According to the present invention, the optical axis of the waveguide having the second vertical mirror element may be rotated by a second angle within the waveguide layer to cause polarization rotation of light coupled from the second waveguide to the third waveguide by an amount corresponding to the second angle.

[0019] In some embodiments, the second waveguide layer is perpendicular to the first waveguide layer, whereby the optical axis of the first waveguide is rotated by the first angle in the first waveguide layer, and whereby the first vertical mirror element is coupled 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, and the second vertical mirror element is coupled 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, and whereby according to the present invention, the optical axis of the third waveguide can be rotated by the second angle in the first waveguide layer to cause polarization rotation of the light.

[0020] In other embodiments, the second waveguide layer is perpendicular to the first waveguide layer, whereby the optical axis in the input end of the second waveguide is rotated by the first angle in the second waveguide layer, and whereby the first vertical mirror element is coupled 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, and the second vertical mirror element is coupled 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, and whereby the optical axis in the output end of the second waveguide is rotated by the second angle in the second waveguide layer to further induce polarization rotation of the light.

[0021] In some embodiments, a second waveguide layer is located above or below the first waveguide layer, and the second waveguide layer and the first waveguide layer are parallel to each other, whereby the first vertical mirror element is coupled to the output end of the first waveguide, and the third vertical mirror element is coupled to the input end of the second waveguide to couple light from the first waveguide to the second waveguide. In some embodiments, the first waveguide layer may have at least a third waveguide deposited thereon, the third waveguide having an input end and an output end, whereby the second vertical mirror element is coupled to the input end of the third waveguide, and the fourth vertical mirror element is coupled to the output end of the second waveguide to couple light from the second waveguide to the third waveguide.

[0022] In some embodiments, wherein the second waveguide is a straight waveguide, a third vertical mirror element and a fourth vertical mirror element are provided, wherein the third vertical mirror element is coupled to an input end of the second waveguide to couple light from the first waveguide to the second waveguide, and the fourth vertical mirror element is coupled to an output end of the second waveguide to couple light from the second waveguide to the third waveguide.

[0023] In some embodiments, the second waveguide can include a horizontal light rotation element, such as a bend or horizontal TIR mirror, that horizontally rotates light in the second waveguide layer to allow coupling of light from the output of the first waveguide to the input of the third waveguide. The horizontal light rotation element can adjust or compensate for polarization-dependent phase shifts in the polarization rotator.

[0024] According to other embodiments, the polarization rotator of the present invention may include one or more of the following features:

[0025] - A third waveguide is oriented on the first substrate in the same direction as the first waveguide.

[0026] - A third waveguide is oriented on the first substrate in an opposite direction to the first waveguide.

[0027] - The second waveguide comprises straight and parallel input and output sections and a horizontal light turning element, such as a bend or a horizontal TIR mirror, to couple said input and output sections together.

[0028] - the straight waveguide segments and / or the horizontal light rotation elements are chosen to have a polarization dependent phase shift that compensates or complements the polarization dependent phase shift caused by the vertical mirror elements, thereby allowing a rotation of a linear input polarization into a linear output polarization.

[0029] - The second waveguide and any mirror elements in the second waveguide layer are manufactured by additive manufacturing (e.g. 3D printing or direct writing). BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 The concept of rotating one waveguide relative to another around their common optical axis is shown, which does not support simultaneous rotation between multiple waveguide pairs;

[0031] Figure 2 shows how to achieve polarization rotation between two straight waveguides according to an embodiment of the present invention;

[0032] Figure 3 shows how to achieve polarization rotation between three straight waveguides according to an embodiment of the present invention;

[0033] Figure 4A and Figure 4B shows examples of mirror elements employed in at least some embodiments of the present invention;

[0034] Figure 5A and Figure 5B Schematic side and top views of the polarization rotator of the present invention are shown;

[0035] Figure 6 shows a schematic side view of another embodiment of the photonic integrated circuit of the present invention;

[0036] Figure 7A 、 Figure 7B 、 Figure 7C and Figure 7D shows how to implement polarization rotation between two parallel waveguides using a U-shaped ring waveguide therebetween according to other embodiments of the present invention;

[0037] Figure 8 shows a front view and a side view of another exemplary photonic integrated circuit of the present invention;

[0038] Figure 9 The coordinate systems used in various waveguides are listed;

[0039] Figure 10 Some basic building blocks on the micron-scale SOI platform are shown;

[0040] Figure 11 A schematic diagram of an embodiment of an integrated isolator of the present invention is shown;

[0041] Figure 12 The basic principle of coordinate system rotation between two waveguides is shown;

[0042] Figure 13 shows the linear polarization rotation caused by a coordinate system rotation between two waveguides;

[0043] Figure 14 The basic principle of coordinate system rotation between three waveguides is shown;

[0044] Figure 15 shows the linear polarization rotation caused by the rotation of the two coordinate systems between the three waveguides;

[0045] Figure 16 The basic principle of opposite coordinate system rotation between two waveguides is shown;

[0046] Figure 17 shows the linear polarization rotation caused by two opposite coordinate system rotations between the three waveguides and reflection of one polarization in the second waveguide;

[0047] Figure 18 shows the linear polarization rotation caused by the rotation of two auxiliary coordinate systems between the three waveguides;

[0048] Figure 19 shows linear polarization rotation with one coordinate system rotation between two waveguides and one polarization reflection in the second waveguide; and

[0049] Figure 20 The linear polarization rotation is shown with two coordinate systems rotated and one polarization reflected in the second waveguide. DETAILED DESCRIPTION

[0050] When light propagates from one waveguide to another (or from one birefringent material to another), polarization rotation naturally occurs, and the orientations of the polarization eigenmodes (or the fast and slow axes) in the two propagation media are different. In planar optical waveguides, it is quite difficult to rotate the eigenmodes relative to the waveguide substrate.

[0051] However, the rotation of a waveguide's eigenmodes relative to the substrate of the same waveguide is a common method for implementing polarization rotators in planar waveguide circuits. Gradually introducing asymmetry into a waveguide gradually rotates the polarization eigenmodes, while birefringence in an asymmetric waveguide reflects polarization relative to the direction of one of the polarization eigenmodes. This corresponds to the use of waveplates in free-space optics. Other examples of asymmetric waveguides with tilted eigenmodes are periodic gratings with alternating asymmetric waveguide cross-sections along the device, or waveguides with gradually changing shapes.

[0052] Figure 1 It shows how the rotation of one waveguide chip 2 relative to the other waveguide chip 1 about the common optical axis of the waveguide pair allows the polarization rotation of that one waveguide pair, but not of any other waveguide pair on the same two waveguide chips. Figure 1 In FIG, two waveguide chips 1 and 2 are rotated by an angle β around a rotation axis R, which is defined as the common optical axis of the main waveguides 1A and 2A on the two chips. After the rotation, the other waveguides 1B, 2B, 1C, 2C on the chips 1 and 2 are misaligned.

[0053] The invention is based on the idea that by turning the light up or down from the input waveguide, the polarization can be rotated to any direction by rotating the output waveguide, which is located on a different waveguide chip or layer as the input waveguide. This can be done in any direction. Figure 2 This is demonstrated in Figure 1, which schematically shows top and side views of a polarization rotation element with two waveguides 1 and 2. The waveguides are optically coupled to each other using a vertical mirror element 4. Mirror 4 couples incident light between the two waveguides 1 and 2 by reflecting light upward into waveguide 2, as indicated by the bold arrows. The direction of light can naturally also be reversed. When light is coupled from input waveguide 1 to output waveguide 2, polarization rotation occurs between the two waveguides when the two waveguides are physically rotated by an arbitrary angle β relative to each other.

[0054] exist Figure 2 In FIG, the basic concept of the invention is shown, whereby the polarization is rotated between two straight waveguides 1 and 2 by rotating the output waveguide 2 by an angle β relative to the input waveguide 1 about the vertical optical axis between the two waveguides. Light is coupled between the two waveguides by a mirror element 4, and the polarization is rotated between the mirrors as the light propagates vertically.

[0055] More specifically, waveguides 1 and 2 are located in two separate waveguide layers that have been placed on top of each other. The waveguide layer comprises a photonic integrated circuit or at least two separate waveguides on a common substrate and in the same plane. The waveguide layers of waveguides 1 and 2 can be deposited on two separate substrates so that they are on two separate waveguide chips that are placed on top of each other and parallel to each other (see, for example, Figure 3 ). Alternatively, two separate waveguide layers of waveguides 1 and 2 can be deposited on a common substrate so that they are two waveguide layers on a single waveguide chip. The first or input waveguide 1 has an input end where light enters (as shown by the bold arrow) and an output end at its mirror element 4, as clearly seen in the side view. The second or output waveguide 2 is the opposite. The first mirror element 4 is coupled to the output end of the first or input waveguide 1, and the second mirror element is coupled to the input end of the second waveguide 2. The waveguide layers of the first and second waveguides are aligned so that the first mirror element couples light to the second mirror element.

[0056] As shown, the second waveguide and its optical axis are rotated by an angle β relative to the optical axis of the first waveguide to cause a polarization rotation of the light coupled from the first waveguide into the second waveguide by an amount corresponding to the first angle β. In this example, the polarization rotation occurs as the light travels vertically between the two waveguide layers, and the optical axes of the two waveguides can be rotated relative to each other in the small gap between the two vertical mirrors.

[0057] According to an important aspect of the present invention, it is possible to integrate multiple input and output waveguides in two parallel waveguide layers, while coupling light from all input waveguides to all output waveguides. Preferably, the vertical mirror element 4 reflects the light 90° upward or downward, in which case the axis of rotation of the waveguide to be rotated is perpendicular to the plane of the waveguide layers.

[0058] In the following, a vertical mirror element is defined as a mirror that reflects light out of the waveguide layer (i.e., upward or downward relative to the direction of propagation of the light in the waveguide before reflection). A horizontal mirror is defined as a mirror that reflects light within its waveguide layer (i.e., left or right relative to the direction of propagation of the light in the waveguide before reflection).

[0059] Now go to Figure 3 , which is a schematic diagram of a top view and a side view of a polarization rotation element with three waveguides that are optically coupled to each other via vertical mirror elements. In this embodiment of the invention, polarization rotation is achieved as a combination of two polarization rotations between waveguides 1, 2 and 3 due to the rotation angles β1 and β2, respectively. In this embodiment, the two waveguides 1 and 3 are deposited on a first substrate 5 and are part of a first waveguide layer. Figure 3 In the illustration of , the waveguide 2 can be deposited on a second substrate (not shown) and is part of a second waveguide layer on a separate waveguide chip. However, the second waveguide layer with the waveguide 2 can also be deposited above or below the first waveguide layer on the same substrate 5.

[0060] Additive manufacturing, ion beam etching, or other 3D manufacturing methods can also be used to implement the second waveguide layer in the polarization rotator of the present invention. For example, the waveguides and mirrors in the second waveguide layer can be formed using 3D printing or laser direct writing techniques. These methods can produce curved mirrors, thus avoiding the need to add a traditional straight waveguide between two mirrors in the second waveguide layer. Another advantage of additive manufacturing, ion beam etching, and other 3D manufacturing methods is that different waveguides and mirrors in the second waveguide layer can have different orientations, unlike wet-etched mirrors, which are typically aligned with a common crystal orientation of the material. The option of using mirrors produced by additive manufacturing, ion beam etching, or other 3D manufacturing methods in the second waveguide layer can be used in all embodiments of the present invention.

[0061] A first mirror element 4 is coupled to the output of the first waveguide 1, and a second mirror element 4 is coupled to the input of the second waveguide 2. Furthermore, a third mirror element 4 is coupled to the output of the second waveguide 2, and a fourth mirror element 4 is coupled to the input of the third waveguide 3. The first and second waveguide layers are aligned so 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 light is coupled from the first vertical mirror to the second vertical mirror, the optical axis of the second waveguide 2 is rotated by an angle β1 relative to the optical axis of the first waveguide 1. In this example, the second waveguide layer is positioned on top of the first waveguide layer, and the two waveguide layers are parallel to each other.

[0062] exist Figure 3 In the embodiment of FIG, when viewed along the direction of light propagation, i.e., when the light travels upward in the first rotation and downward in the second rotation, the two polarization rotations occur in the same clockwise direction. In a top view, the positive rotations of angles β1 and β2 appear in opposite directions.

[0063] exist Figure 3 In the embodiment of the invention, no waveguide bends or horizontal mirrors are required. However, from an assembly perspective, the mirror elements need to be precisely manufactured (etched) because the optical coupling at the mirror elements is sensitive to etching errors. This will be discussed in detail later.

[0064] There are several options for implementing the mirror element used in the present invention, see Figure 4A and Figure 4B For example, it can be based on Figure 4A Total internal reflection (TIR) ​​mirrors as shown or Figure 4B External metal mirror shown.

[0065] Figure 4A and Figure 4B A schematic 3D illustration of a vertical mirror element used in the present invention is provided.

[0066] exist Figure 4A In the example shown, the mirror 41 reflects light upward (or downward) from the waveguide layer of the waveguide 40 and may include a TIR mirror 41 having a negative angle and an output face 42. The mirror 41 may also have a metal coating or other reflective coating. The output face may be anti-reflective coated to reduce unwanted reflections.

[0067] exist Figure 4B In the example of FIG, the mirror element 43 may consist of a vertical transparent waveguide face 45 at the end of the waveguide 43, followed by an external (as shown by the dashed line D) metal mirror face 44. The vertical face 45 may be anti-reflective coated to reduce unwanted reflections. The mirror face 44 may also have some non-metallic reflective coating. Obviously, the reflection from Figure 4A and Figure 4BThe same technology can be used to create horizontal mirror elements, or to reflect light downwards instead of upwards.

[0068] according to Figure 4A or Figure 4B Tilted mirrors can be produced by using anisotropic dry etching at an inclined angle (e.g. with an ion beam etcher) or by using anisotropic wet etching that produces a flat surface along a specific crystal plane. For SOI waveguides, potassium hydroxide (KOH) or tetramethylammonium hydroxide (TMAH) are commonly used wet etching methods. By adding suitable surfactants to the etching chemicals or using special crystal orientations on the wafer, 45° mirrors can be produced accurately. Using the most common crystal orientation

[100] , mirrors with a 55° angle on silicon are the easiest to produce, and in some embodiments of the present invention, such mirrors can be used. However, a 45° mirror angle reflects light vertically upward or downward, which is the preferred option for the present invention.

[0069] As mentioned above, any mirror can also be fabricated using additive manufacturing or ion beam etching. With either process, it is possible to have different mirror orientations in the same waveguide layer, which allows different polarization rotations to be achieved in the same waveguide layer or chip.

[0070] Figure 3 (and Figures 5 to Figure 6 A practical challenge of the design shown in FIG 1 is that in some cases, the position of the vertical mirror element cannot be precisely controlled. For example, wet etching a 45° mirror typically has some variation in the width of the etched area. Then, at different angles (e.g., Figure 3 The exact distance between any two mirror elements (facing each other in the wafer) varies across the wafer or from wafer to wafer. This results in misalignment between the optical axes of the waveguides. For example, if a manufacturing defect causes Figure 3 If all the mirrors in the waveguide layer are moved closer to their corresponding waveguides, the first and fourth mirrors in waveguide layer 1 are moved away from each other, while the second and third mirrors in waveguide layer 2 are moved closer to each other. This makes it impossible to perfectly align the two mirror pairs by aligning the waveguide layers to each other. If some embodiments of the present invention are based on using the same mirrors (in exactly the same orientation) in each waveguide layer (see Figures 7 and Figure 8 ), they can avoid this potential challenge.

[0071] When determining the polarization rotation of the waveguide of the present invention as a whole, the phase shifts introduced by the mirrors (and in particular the TIR mirrors) for s and p polarizations must be taken into account. For dielectric mirrors, these phase shifts can be calculated using the Fresnel equations, where s-polarization and p-polarization refer to polarization states with perpendicular electric and magnetic fields at the plane of incidence. For an upward-reflecting TIR mirror, s-polarization corresponds to the TE mode of the waveguide, and the electric field lies in the plane of the waveguide layer and is oriented along the x-axis of the local coordinate system of the waveguide. Correspondingly, p-polarization corresponds to the TM mode of the waveguide, and the electric field is perpendicular to the plane of the waveguide layer and is oriented along the y-axis of the local coordinate system of the waveguide. In some embodiments of the present invention, the polarization-dependent phase shifts in the vertical mirror elements can be minimized, for example, by using metallic mirrors.

[0072] Polarization-dependent phase shifts can also occur at horizontal mirrors, waveguide bends, and even in straight waveguides. If the effective refractive indices of the TE and TM modes are different, these modes propagate at different speeds and accumulate a phase shift between them as the propagation distance changes. This also opens up the possibility of generating a desired polarization-dependent phase shift in one part of a polarization rotator to compensate for an undesired phase shift in another part of the same device.

[0073] Ideally, the polarization-dependent phase shift of each section of the polarization rotator would be zero or a multiple of π, and any linearly polarized input light would remain linearly polarized as it propagates through the polarization rotator. Polarization-dependent phase shifts that are not integer multiples of π typically convert linear polarization into elliptical polarization. For simplicity, linear polarization is assumed below, as detailed analysis of polarization rotation of elliptical polarization is very complex and difficult to illustrate.

[0074] The polarization-dependent phase shift in a TIR mirror is a function of at least the angle of incidence of the light (α), the waveguide material (silicon), and the cladding material (e.g., air, silicon dioxide, or silicon nitride).

[0075] For the case where the polarization dependent phase shift Δφ in the vertical mirror elements is not zero, there are several solutions. For example, one solution could be to use one or more compensating optical elements with a non-zero polarization dependent phase shift –Δφ in the vicinity of the vertical mirror elements. For example, a birefringent waveguide segment or a horizontal TIR mirror could be integrated into the same waveguide into which the vertical mirror elements are integrated. This approach works for any linear input polarization. One skilled in the art can calculate the effect of the polarization dependent phase shift on the polarization in any of the polarization configurations described herein and design the polarization rotator such that any unwanted polarization dependent phase shift is compensated with another similar phase shift to maintain linear polarization of the light when desired. An example of this is to add a horizontal TIR mirror in the vicinity of each vertical TIR mirror element such that they compensate for each other's polarization dependent phase shift (see e.g. Figure 7C ).

[0076] See again Figure 3 , a total of four vertical mirror elements 4 are shown, namely one at the end of each waveguide 1, 2, 3 at the two polarization rotation interfaces. However, some of these vertical mirror elements can be replaced by U-turns in the second waveguide layer if the second waveguide layer is rotated into an upright position relative to the first waveguide layer.

[0077] Figure 5A is a schematic side view of the polarization rotator of the present invention, in which two waveguide layers are located on two substrates (or waveguide chips) 5 and 7 that are orthogonal to each other. The three waveguides 1, 2 and 3 are optically coupled to each other, as in Figure 3 In this case, however, only two vertical mirror elements 4 are required. In this embodiment, the second waveguide 2 comprises a straight input portion 2a and a straight output portion 2c and an intermediate curved waveguide portion 2b connecting the input and output portions together.

[0078] More specifically, the intermediate waveguide 2b forms a horizontal U-shaped bend in the second waveguide layer to deflect the upwardly propagating light downwardly towards waveguide 3. U-shaped loops based on waveguide bends usually result in polarization-dependent phase shifts, which need to be considered when designing a polarization rotator. It would be very difficult to deposit waveguide 2 directly on the same (first) substrate 5 as waveguides 1 and 3, so Figure 5A The preferred approach for the structure shown in is to assemble two separate waveguide chips together.

[0079] Figure 5A Also illustrated is the possibility of having passive mechanical alignment studs 8 and mating sockets 8a between the two waveguide layers, which is particularly useful in embodiments where the substrates (or waveguide chips) 5, 7 are perpendicular to each other. However, this mechanical alignment concept can be applied to any polarization rotator using two separate waveguide chips.

[0080] Figure 5B Shown Figure 5A , illustrating various placement options of the waveguides 1 and 3 on the substrate 5, as indicated by the double arrows. Of course, the position of the substrate (or waveguide chip) 7 then needs to be adjusted accordingly. Figure 5A Waveguides 1 and 3 in the figure are tilted outwards to the viewing plane, but only Figure 5B This is shown in the projections in . In the two examples shown here, waveguides 1 and 3 are parallel to each other, but this is not necessary.

[0081] exist Figure 6, another embodiment is shown using four horizontal TIR mirrors 10 in a waveguide 2 that is part of a waveguide layer 2 on a substrate (or waveguide chip) 7. Here, the second waveguide 2 comprises a straight input section and a straight output section, as well as an intermediate waveguide section that further comprises the TIR mirrors 10 and a straight waveguide segment connecting these mirrors. The intermediate waveguide section effectively forms the equivalent of a U-shaped loop connecting the input and output sections together.

[0082] Here, the mirror 10 turns the light horizontally in the waveguide 2. To alleviate the polarization phase shift caused by the TIR mirror, an appropriate number of mirrors 10 with appropriate mirror angles can be used to effectively form a U-shaped loop while producing a phase shift of approximately 2π between TE and TM polarizations. Therefore, the horizontal TIR mirror cannot effectively change the polarization state. Figure 6 (and Figure 7B ) are for schematic purposes only and can be optimized to achieve any desired phase shift between the two polarization modes in waveguide 2. An example of such an optimization is to achieve a π phase shift between TE and TM polarizations to form a U-shaped ring that reflects linear polarization relative to the polarization axis of the waveguide.

[0083] Obviously, any number of mirrors with any reflection angles can be used as long as waveguide 2 can be optically coupled to waveguides 1 and 3. For the placement of the waveguides on the substrate (or waveguide chip) 5 and 7, see Figure 5B Figure 5 and Figure 6 A common advantage of the illustrated embodiments is that the number of vertical mirror elements can be reduced.

[0084] The choice between horizontal bends, horizontal mirrors, and vertical mirror elements can be influenced by their optical losses and polarization-dependent phase shifts. As explained previously, TIR mirrors typically have a finite phase shift Δφ, but straight waveguides and waveguide bends can also produce finite element Δφ. Sometimes, finite element Δφ can be used in straight waveguides, bends, or horizontal mirrors to compensate for the finite element Δφ in vertical mirror elements or to reflect linear polarization about one of the polarization axes.

[0085] In one embodiment of the present invention, multiple bends or TIR mirrors may be used to create a π phase shift between the two polarization modes in the second waveguide to reflect the polarization relative to one of the polarization eigenstates. In some other embodiments, multiple bends or TIR mirrors may be used to create a 2π phase shift between the two polarization modes in the second waveguide to avoid the net effect of Δφ which cannot be reduced to zero. This is in Figure 6 Four TIR mirrors 10 are schematically illustrated in FIG, but the number and angles of the horizontal mirrors (or bends) should be optimized for the waveguide, mirror and bend structure used.

[0086] To reduce any misalignment between the optical axes of the waveguides, according to some embodiments of the present invention, all mirrors in the same waveguide layer (or waveguide chip) can be arranged to have the same orientation, so that the waveguides are antiparallel. This simplifies alignment between waveguide layers, especially when the positions of the vertical mirror elements have limited manufacturing tolerances.

[0087] exist Figure 7A One such embodiment is illustrated in FIG, which is a schematic top view of a polarization rotator having three waveguides 1, 2, and 3 optically coupled to one another using vertical mirror elements 4 in all three waveguides (four mirrors total). In this embodiment, both polarization rotations occur in the same direction, and waveguide 2 includes a horizontal U-shaped bend 2b. If angle β1 = β2 and the U-shaped bend is rotated 180°, waveguides 1 and 3 are antiparallel (180°).

[0088] Likewise, it is important to be aware of the polarization-dependent phase shift that typically occurs when a waveguide makes a U-turn. One approach is to intentionally increase the finite phase shift Δφ to 2π (i.e., 360°) or a multiple thereof, effectively rendering the waveguide 2 and its vertical mirror element free of any polarization-dependent phase shift.

[0089] Figure 7B is a schematic top view of a polarization rotator with three waveguides, which are optically coupled to each other with vertical mirror elements 4 in all three waveguides 1, 2, 3. In this example, the two polarization rotations occur in the same direction and waveguide 2 has two straight waveguides connected to horizontal TIR mirrors 10, which rotate the light so that waveguides 1 and 3 are antiparallel. The four 90° mirrors 4 are used here only schematically to illustrate the idea of ​​realizing β1=β2. If the TIR mirrors 10 produce a phase shift of about 360° between TE and TM polarization, they do not effectively change the polarization state in waveguide 2. Such horizontal TIR mirrors can also compensate for unwanted phase shifts caused by vertical mirror elements. For example, the polarization dependence of the two vertical TIR mirrors 4 at the ends of waveguide 2 (90° rotation angle) can be compensated by two horizontal TIR mirrors in waveguide 2 (90° rotation angle). By adding horizontal TIR mirrors in waveguides 1 and 3, the same compensation can be performed for the vertical TIR mirrors in waveguides 1 and 3. This is in Figure 7C and Figure 7D Shown in.

[0090] exist Figure 5A In the embodiment of FIG. 1 , it was previously shown how to eliminate the vertical mirror element in waveguide 2 by using a U-shaped bend. In some embodiments, the mirror elements can be omitted from waveguides 1 and 3. Figure 8. In this case, the chip 12 of waveguide 2 is rotated to an upright position, and all vertical mirror elements 4 coupled to this waveguide chip are aligned with and located on the same chip. The substrate or chip 12 of the second waveguide 2 is placed onto the edge of the substrate or chip 5 used to fabricate waveguides 1 and 3, or into a cavity etched into the substrate or chip 5. The two coordinate rotations (β1 and β2) can be freely selected, but according to one embodiment of the present invention, the two mirrors 4 can have the same orientation, so that the waveguide 2 once again forms a U-shaped loop 2b.

[0091] In this embodiment, both polarization rotations occur in the same direction, antiparallel to waveguides 1 and 3, and waveguide 2 forms a horizontal bend 2b. The chip 12 of waveguide 2 is rotated relative to waveguides 1 and 2, which are located on the same chip 5. The front view is along the direction of light propagation in waveguide 1, and the side view is a side view of waveguides 1 and 3 (waveguide 1 is located behind waveguide 3).

[0092] In this configuration, if Δφ = 0, the polarization rotation is β1 + β2. However, if the vertical mirror element 4 or the waveguide 2 and its U-bend 2b result in any finite phase shift Δφ, the preferred approach is to adjust the polarization-dependent phase shifts of the individual elements so that their net effect is zero or a multiple of 2π, e.g. Figure 7B 、 Figure 7C and Figure 7D shown.

[0093] Advantages of these embodiments include requiring fewer upper mirrors and being insensitive to mirror etching.

[0094] Figure 9 The local coordinate systems used in various waveguide examples and vertical mirror elements are explained. Bold arrows indicate optical coupling between waveguides, and the z-axis (z1–z3) represents the primary direction of light propagation, i.e., the optical axis of waveguides 1–3. The x- and y-axes represent the dominant electric field directions of the polarization eigenmodes in the waveguides and their corresponding mirror elements. For clarity, waveguide 2 is moved away from its target position.

[0095] Figure 10 Some basic building blocks on a micron-scale SOI platform are shown, including a metallized upper reflective waveguide mirror 13, a single-mode ridge waveguide 14 and its simulated mode field distribution 14a, a horizontal TIR mirror 15, a ridge-stripe converter 16 for adiabatic coupling between the ridge waveguide and the stripe waveguide, and a vertical taper 17 between the two waveguide thicknesses.

[0096] Figure 11 A schematic diagram of one embodiment of an integrated isolator or circulator of the present invention is shown, comprising two polarization beam splitters (PBS) 18 , two 45° Faraday rotators (FR) 19 and two 45° (mutual) polarization rotators 20 .

[0097] Various coordinate system rotations and resulting polarization rotations such as Figures 12 to 20 All figures show the coordinate axes, the (linear) polarization states and their rotations observed in the cross section and the main direction of light propagation. As long as the systems are reciprocal, light can naturally also propagate in the opposite direction.

[0098] Figure 12 The input and output waveguides (e.g. Figure 2 or Figure 9 The basic principle of the coordinate system rotation (β) between the waveguides 1 and 2 in FIG. If a vertical mirror element is integrated into the end of the waveguide, a rotation of the coordinate system relative to the mirror element occurs.

[0099] Figure 13 The original coordinate system (x in ,y in ) and the new coordinate system (x out ,y out ) and their orientation (α). The polarization eigenmodes are typically aligned along the x- and y-axes of the waveguide. The polarization angle is α in the coordinate system of the first (input) waveguide. in , and in the coordinate system of the second (output) waveguide is α out .

[0100] Figure 14 The continuous coordinate rotation (β1+β2) between waveguides 1, 2 and 3 in the same direction is shown in FIG. The indices 1, 2 and 3 refer to waveguides 1, 2 and 3. The resulting polarization angle rotation is Figure 15 , which shows the linear polarization after two consecutive coordinate system rotations β1+β2, and the polarization rotation angles are: α1 before the two rotations and α3 after the two rotations.

[0101] Figure 16 , two consecutive and opposite coordinate system rotations occurring in opposite directions, β1 > 0 and β2 < 0, are illustrated. For clarity, the two polarization rotations are shown to have different magnitudes, although in many cases it is possible to design two equally large but opposite rotations.

[0102] Figure 17 A method for adding two polarization rotations is shown, even if the two coordinate system rotations occur in opposite directions. This is based on the reflection of the polarization about the y (or x) axis, which can be achieved by inducing a Δφ = π phase difference between the two polarization eigenmodes in waveguide 2. The original polarization (OP) is illustrated by a bold solid line at an angle α1, and the reflected final polarization (FP) is illustrated by a double line at an angle α3. Angles α2 and α'2 represent the polarization before and after the polarization reflection in waveguide 2.

[0103] exist Figure 18 Shown in Figure 15 This is a special case where the two coordinate system rotations are complementary, i.e., β1 + β2 = π. In this case, the effective polarization rotation is zero, but if the polarization rotator is part of a phase-sensitive waveguide circuit, the absolute phase of the light must be taken into account. In the absence of any phase change between the two polarizations, the total polarization rotation is π, i.e., effectively zero.

[0104] Figure 19 and Figure 20 Illustrated is the polarization rotation in the case of an auxiliary rotation when the polarization is reflected between the two rotations. These figures provide a step-by-step illustration of two rotations and one reflection, which is difficult to see in a single figure.

[0105] exist Figure 19 In FIG, the linear polarization after the first coordinate rotation of the waveguide 1 and the waveguide 2 by an angle β1 and the subsequent reflection of the polarization about the y2 axis are shown. The original polarization OP angle and the final polarization FP angle are α1 and α'2, respectively.

[0106] exist Figure 20 Figure 3 shows the linear polarization after two consecutive coordinate rotations (β1 and β2) between waveguides 1, 2, and 3, as well as the polarization reflection about the y2 axis between the rotations. The initial polarization angle is α1, and the final polarization angle is α3. In this example, the two rotations are complementary (β2 = π – β1).

[0107] It should be understood that the embodiments of the present invention disclosed herein are not limited to the specific structures, process steps or materials disclosed herein, but extend to their equivalents that will be recognized by those skilled in the relevant art. It should also be understood that the technical terms used herein are used only for the purpose of describing specific embodiments and are not intended to be limiting. Reference to any prior art (if any) is not and should not be taken as an admission or any form of suggestion that the prior art constitutes part of the common general knowledge in any country.

[0108] Reference throughout this specification to "one embodiment" or "an embodiment" means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, the appearances of the phrases "in one embodiment" or "in an embodiment" in various places throughout this specification are not necessarily all referring to the same embodiment.

[0109] Various embodiments and examples of the present invention, along with alternatives to its various components, may be mentioned herein. It should be understood that such embodiments, examples, and alternatives should not be considered as de facto equivalents of each other, but rather as independent and autonomous representations of the present invention.

[0110] In addition, in one or more embodiments, the described features, structures, or characteristics may be combined in any suitable manner. In the description, many specific details, such as examples of lengths, widths, shapes, etc., are provided to provide a thorough understanding of the embodiments of the present invention. However, those skilled in the relevant art will recognize that the present invention can be implemented without one or more of the specific details, or with other methods, components, materials, etc. In other cases, well-known structures, materials, or operations are not shown or described in detail to avoid obscuring aspects of the present invention.

[0111] Although the above examples illustrate the principles of the present invention in one or more specific applications, it will be apparent to those skilled in the art that numerous modifications may be made in form, usage, and implementation details without exercising inventiveness and without departing from the principles and concepts of the present invention. Therefore, the present invention is not intended to be limited except by the following claims.

Claims

1. A polarization rotator, comprising: a first waveguide layer comprising at least a first waveguide having an input end and an output end; a second waveguide layer having at least a second waveguide having an input end and an output end; at least a first vertical mirror element arranged at an end of at least one of the first waveguide and the second waveguide to couple light between an output end of the first waveguide and an input end of the second waveguide; wherein the optical axis of the first waveguide or the second waveguide having the first vertical mirror element at its end is rotated at a first angle in its waveguide layer to cause polarization rotation of light coupled between the first waveguide and the second waveguide, the rotation amount corresponding to the first angle to allow polarization rotation to any direction, wherein the first waveguide layer and the second waveguide layer are independent of each other, and the first vertical mirror element is a total internal reflection mirror.

2. The polarization rotator according to claim 1, wherein The first waveguide layer has at least a third waveguide having an input end and an output end; and at least a second vertical mirror element arranged at an end of at least one of the second waveguide and the third waveguide to couple light between the output end of the second waveguide and the input end of the third waveguide, and wherein an optical axis of the waveguide having the second vertical mirror element is rotated by a second angle in its waveguide layer to cause polarization rotation of light coupled from the second waveguide to the third waveguide by an amount corresponding to the second angle.

3. The polarization rotator according to claim 2, wherein: The second waveguide layer is located above or below the first waveguide layer, and the second waveguide layer and the first waveguide layer are parallel to each other, whereby the first vertical mirror element is coupled to the output end of the first waveguide and the third vertical mirror element is coupled to the input end of the second waveguide to couple light from the first waveguide to the second waveguide.

4. The polarization rotator according to claim 2, wherein: The at least third waveguide deposited on the first waveguide layer has an input end and an output end, whereby the second vertical mirror element is coupled to the input end of the third waveguide and a fourth vertical mirror element is coupled to the output end of the second waveguide to couple light from the second waveguide to the third waveguide.

5. The polarization rotator according to claim 2, wherein: The second waveguide layer is perpendicular to the first waveguide layer, whereby the optical axis of the first waveguide is rotated by the first angle in the first waveguide layer, and whereby the first vertical mirror element is coupled 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, and the second vertical mirror element is coupled 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, and whereby the optical axis of the third waveguide is rotated by the second angle in the first waveguide layer to cause polarization rotation of the light.

6. The polarization rotator according to claim 2, wherein: The second waveguide layer is perpendicular to the first waveguide layer, whereby the optical axis in the input end of the second waveguide is rotated by the first angle in the second waveguide layer, and whereby the first vertical mirror element is coupled 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, and the second vertical mirror element is coupled 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, and whereby the optical axis in the output end of the second waveguide is rotated by the second angle in the second waveguide layer to further induce polarization rotation of the light.

7. The polarization rotator according to any one of claims 2 to 4, wherein: The second waveguide is a straight waveguide having a third vertical mirror element and a fourth vertical mirror element, wherein the third vertical mirror element is coupled to the input end of the second waveguide to couple light from the first waveguide to the second waveguide, and the fourth vertical mirror element is coupled to the output end of the second waveguide to couple light from the second waveguide to the third waveguide.

8. The polarization rotator according to any one of claims 2 to 6, wherein: The second waveguide includes a horizontal light rotation element that horizontally rotates light in the second waveguide layer to allow coupling of light from the output of the first waveguide to the input of the third waveguide.

9. The polarization rotator according to claim 8, wherein The horizontal light rotating element adjusts or compensates for the polarization-dependent phase shift in the polarization rotator.

10. The polarization rotator according to any one of claims 2 to 6, wherein: The third waveguide is oriented on a common substrate in the same direction as the first waveguide.

11. The polarization rotator according to any one of claims 2 to 6, wherein: The third waveguide is oriented on a common substrate in an opposite direction to the first waveguide.

12. The polarization rotator according to claim 8, wherein The second waveguide includes straight and parallel input and output portions and the horizontal light rotation element to couple the input and output portions together.

13. The polarization rotator according to claim 1 , comprising a straight waveguide segment and / or a horizontal light rotating element having a polarization-dependent phase shift that compensates or supplements the polarization-dependent phase shift caused by the corresponding vertical mirror element, thereby allowing a linear input polarization to be rotated into a linear output polarization.

14. The polarization rotator according to any one of claims 1 to 6, wherein: The second waveguide and any mirror elements in the second waveguide layer are manufactured using additive manufacturing.

Images