Polarization rotation beam splitter and preparation method thereof

Abstract

The invention relates to the technical field of optical devices, and discloses a polarization rotation beam splitter and a preparation method thereof. A waveguide core layer of the polarization rotation beam splitter comprises an input waveguide, a first gradual change waveguide, a second gradual change waveguide, a first output waveguide and a second output waveguide. The first tapered waveguide comprises a first branch and a second branch, and the first branch and the second branch are different in height; the first branch is provided with a first boundary, the second branch is provided with a second boundary, and the first boundary and the second boundary are staggered in the first direction; in the second direction, the first boundary and the side, relatively away from the second gradient waveguide, of the input waveguide are staggered, and the second boundary and the side, relatively close to the second gradient waveguide, of the input waveguide are staggered. According to the invention, by defining the first boundary and the second boundary, the first gradient waveguide with the section changing continuously along the light propagation direction is obtained, the composite waveguide in the coupling region is optimized, and the device has the advantages of compact structure, large bandwidth, low insertion loss, high extinction ratio and the like.

Description

Technical Field

[0001] This invention relates to the field of optical device technology, specifically to a polarization rotating beam splitter and its fabrication method. Background Technology

[0002] With the rapid development of optical communication technology, silicon photonics has attracted widespread attention in optical interconnects, sensing, and signal processing due to its advantages such as high integration, low power consumption, and compatibility with CMOS processes. However, the high refractive index difference between silicon and silicon dioxide in the SOI platform makes the nanowaveguides highly polarization sensitive, especially in systems connected to optical fibers, where the uncertainty of the input light polarization state can seriously affect device performance. Based on this, polarization rotating beam splitters (PSRs) have been proposed, and their mainstream designs are based on mode coupling or mode hybridization mechanisms, resulting in various structures such as directional coupling, Y-branching, bent waveguide, and dual-etched types. Although these devices are relatively compact, they still generally suffer from limited process tolerance and significant wavelength dependence, and their performance in terms of insertion loss, crosstalk, and bandwidth remains unsatisfactory.

[0003] Current polarization rotating beam splitters mainly include structures based on subwavelength gratings and structures based on mode evolution and adiabatic waveguides. Subwavelength grating-based structures introduce a subwavelength grating structure with a period much smaller than the wavelength into the waveguide. By designing the grating's duty cycle, period, and shape, it exhibits drastically different equivalent refractive indices for TE and TM polarized light, offering high structural compactness and flexible design freedom, achieving small device sizes while maintaining excellent performance. However, this high degree of freedom also implies greater design complexity, and the small feature size in the grating structure places high demands on fabrication precision, often making it difficult to apply to practical finished product designs, and limiting fabrication processes. In contrast, structures based on mode evolution and adiabatic waveguides, through careful design of the waveguide's cross-sectional shape and size, utilize waveguide asymmetry to guide the smooth evolution of one polarization mode into another. They offer larger bandwidth and higher process tolerance, showing significant advantages over subwavelength grating-based structures in terms of performance, bandwidth, and process tolerance. However, since the device's high performance depends on smooth mode transitions, a gradual design of the waveguide width is required, easily leading to increased size, typically reaching hundreds of micrometers. Summary of the Invention

[0004] This invention provides a polarization rotating beam splitter to solve the problem that existing polarization rotating beam splitters cannot simultaneously achieve low loss, compact structure and large bandwidth.

[0005] In a first aspect, the present invention provides a polarization rotating beam splitter, comprising: a substrate layer, a waveguide core layer, and a cladding layer stacked together, wherein the waveguide core layer is disposed on one side surface of the substrate layer, and the cladding layer covers the waveguide core layer; the waveguide core layer comprises: an input region, a coupling region, and an output region arranged sequentially along a first direction, wherein the input region comprises an input waveguide, the coupling region comprises a first tapered waveguide and a second tapered waveguide, and the output region comprises a first output waveguide and a second output waveguide; one end of the first tapered waveguide is connected to the input waveguide, and the other end is connected to the first output waveguide; the second tapered waveguide and the first tapered waveguide are disposed at intervals along a second direction on the substrate layer, and the second tapered waveguide is connected to the second output waveguide, wherein the first direction and the second direction are parallel to the surface of the substrate layer and form a predetermined angle; The first and second graded waveguides are asymmetrically arranged in a second direction. The first graded waveguide includes a first portion and a second portion connected together. The first and second portions have different average heights in a third direction, which is perpendicular to the surface of the substrate. The edges of the first and second portions on adjacent sides in the second direction completely overlap on the substrate. In the second direction, the first portion has a first boundary line on the side away from the second graded waveguide, and the second portion has a second boundary line on the side away from the second graded waveguide. Both the first and second boundary lines are longitudinal boundaries extending along the third direction. The first portion, on the side away from the second graded waveguide, is formed by extending the edges of the input waveguide and the first output waveguide on the side away from the second graded waveguide to the first dividing line; the second portion, on the side away from the second graded waveguide, is formed by extending the edges of the input waveguide and the first output waveguide on the side closer to the second graded waveguide to the second dividing line; in the first direction, the first dividing line and the second dividing line are offset; in the second direction, the first dividing line is offset from the side of the input waveguide away from the second graded waveguide, and the second dividing line is offset from the side of the input waveguide closer to the second graded waveguide.

[0006] Beneficial Effects: The polarization rotation beamsplitter in this embodiment defines a first graded waveguide with a first boundary line and a second boundary line P2 at a top-view angle. The cross-section of the first graded waveguide continuously changes along the light propagation direction. Combined with the asymmetrical design of the second graded waveguide, the combination of the two achieves structural optimization of the composite waveguide in the coupling region. First, through this unique composite waveguide design, the two core functions of polarization rotation and mode separation are integrated and performed in parallel within a single coupling region, fundamentally avoiding the problem of excessive size caused by traditional cascading of multiple functional devices. This integrated design not only significantly reduces the device size, but more importantly, it simultaneously achieves low insertion loss and high extinction ratio within a compact size. Secondly, the coupling region features an asymmetric, thermally graded design where the waveguide cross-section, or waveguide width, continuously varies along the light propagation direction. This allows the device to meet phase-matching conditions for different wavelengths at different locations. This mechanism overcomes the bandwidth limitations of traditional devices, achieving ultra-flat, large-bandwidth characteristics within commonly used communication bands with low additional loss. This enables the device to stably support high-speed, multi-wavelength optical communication systems, significantly enhancing application tolerance and capabilities. Finally, this method of optimizing composite waveguides by defining a few "key points" simplifies complex design problems, greatly reducing optimization variables and computational costs. This makes rapid optimization of high-performance, complex devices possible, contributing to research and development as well as mass production.

[0007] In one alternative implementation, in the second direction, the first dividing line protrudes from the side of the input waveguide that is relatively far away from the second tapered waveguide, and the second dividing line is located between the side of the input waveguide that is relatively close to the second tapered waveguide and the first dividing line, and is offset from both sides of the input waveguide in the second direction.

[0008] Beneficial effects: The first dividing line protrudes from the rear side of the output waveguide in the second direction, and the second dividing line is located between the front and rear sides of the input waveguide in the second direction, and does not correspond to the two sides. This setting makes the area of ​​the first part in the first gradient waveguide as large as possible, while the area of ​​the second part is as small as possible, thereby improving the efficiency of polarization rotation and mode separation.

[0009] In one alternative implementation, in the third direction, the height of the first portion is equal at all points, the height of the second portion is equal at all points, and the height of the first portion is greater than the height of the second portion.

[0010] Beneficial effects: In the third direction, the upper surfaces of the first and second sections are horizontal planes, and the cross-sections are both rectangular. This structure helps to simplify the fabrication process, and the waveguide has a relatively regular shape, which facilitates quantitative analysis and mass production.

[0011] In one alternative implementation, the first portion has the same height in all directions in the third direction; in the second direction, from the side closer to the second graded waveguide to the end of the second boundary line away from the substrate, the height of the second portion gradually increases in the third direction, and the height of the second boundary line is equal to the height of the first portion.

[0012] Beneficial effects: In the third direction, the upper surface of the first part is a plane, and the upper surface of the second part is an inclined plane. The inclined plane converges from the front side near the second direction to the upper end of the second boundary line. In this way, the cross section of the second part perpendicular to the first direction is a right trapezoid, while the cross section of the first part is still rectangular. This makes the cross section size change of the first gradient waveguide in the direction of light propagation more gradual, expands the phase matching situation, and further improves the polarization rotation and mode separation efficiency.

[0013] In one alternative implementation, in the first direction, the distance L3 between the first boundary line and the input waveguide is less than the distance L1 between the second boundary line and the input waveguide, and the distance L1 between the second boundary line and the input waveguide is greater than or equal to the distance L2 between the second boundary line and the output waveguide; in the second direction, the distance W4 between the first boundary line and the side of the input waveguide away from the second tapered waveguide is less than or equal to the width W1 of the input waveguide, and the distance W2 between the second boundary line and the side of the input waveguide closer to the second tapered waveguide is less than the width W1 of the input waveguide.

[0014] Beneficial effects: The above-mentioned limitation that L3 is less than L1 and L1 is greater than or equal to L2 helps the first part to have a larger spatial extension near the input area in terms of length dimension. This helps to provide more ample mode evolution space in the initial stage of the optical signal entering the coupling region, ensuring that higher-order modes can be efficiently excited and coupled, and ensuring optical coupling characteristics; W4 is less than or equal to W1 and W2 is less than W1, which helps to ensure the coupling performance of the optical signal in terms of width dimension.

[0015] In one alternative implementation, the distance W2 between the second boundary line and the side of the input waveguide closer to the second tapered waveguide satisfies the following relationship: W2 > 0.2 μm, and W1 - W2 ≥ 0.2 μm between W1 and W2.

[0016] Beneficial effects: Under the above-mentioned constraints, it can ensure that the second part of the first graded waveguide has a sufficiently large gradient space in the second direction, while also ensuring that its proportion in the first graded waveguide is still relatively small compared to the first part, thus maximizing the waveguide area for optical transmission and improving the optical signal transmission efficiency.

[0017] In one alternative implementation, the edges of the first and second graded waveguides that are close to each other are parallel to each other along a first direction; along the first direction, the distance between the second graded waveguide and the first graded waveguide gradually increases on the side of the second graded waveguide that is relatively far away from the first graded waveguide.

[0018] Beneficial effects: In the second direction, the front edge of the first graded waveguide and the rear edge of the second graded waveguide are parallel to each other; simultaneously, the front edge of the second graded waveguide in the second direction is set as a slope, and the distance between this slope and the rear edge of the second graded waveguide in the second direction gradually increases along the light propagation direction, that is, the width of the second graded waveguide changes from narrow to wide, and the width of the right waveguide is consistent with the width of the second output waveguide. In other words, the front edge of the second graded waveguide in the second direction remains parallel to the light propagation direction, its lateral dimension is consistent with the first graded waveguide, and the spacing in the longitudinal direction is maintained, thereby improving coupling efficiency while meeting the requirements of fabrication precision.

[0019] In one alternative implementation, in the third direction, the heights of the input waveguide, the first portion of the first tapered waveguide, the second tapered waveguide, the first output waveguide, and the second output waveguide are equal. The output end surface of the input waveguide is consistent with the input end surface of the first tapered waveguide, the output end surface of the first tapered waveguide is consistent with the input end surface of the first output waveguide, and the output end surface of the second tapered waveguide is consistent with the input end surface of the second output waveguide.

[0020] Beneficial effects: Except for the second part in the first graded waveguide, the height of the other parts of the waveguide core layer is equal; moreover, the area and shape of the surfaces where the parts are connected are completely consistent, ensuring the efficiency of optical signal transmission and coupling, reducing optical mode loss, and improving the quality of optical signal transmission.

[0021] In one optional implementation, the input waveguide and the first output waveguide are strip-shaped straight waveguides, and the second output waveguide includes a first part and a second part. The first part is an S-shaped curved waveguide, and the ratio of its length along the first direction to the maximum offset along the second direction is greater than or equal to 5. The second part is a strip-shaped straight waveguide.

[0022] Beneficial effects: The input waveguide, the first output waveguide, and the second part of the second output waveguide are all strip-shaped straight waveguides extending along the first direction; the first part of the second output waveguide is an S-shaped curved waveguide, which can be composed of two arcs of the same or different linear shapes. The closer the S-shaped curved waveguide is to a straight waveguide, the more beneficial it is to reduce additional losses and suppress mode hybridization. Therefore, in this embodiment, the ratio of the lateral dimension of the S-shaped curved waveguide in the first direction to the longitudinal dimension in the second direction is set to be greater than or equal to 5. While reducing additional losses, it effectively suppresses the hybridization of optical modes in the curved waveguide, thereby improving the overall extinction ratio of the device.

[0023] Secondly, the present invention also provides a method for fabricating a polarization rotating beam splitter, for fabricating the above-mentioned polarization rotating beam splitter, the method comprising: Provide a substrate layer; A waveguide core layer is formed on one side surface of a substrate layer. The waveguide core layer includes an input region, a coupling region, and an output region arranged sequentially along a first direction. The waveguide core layer includes: forming an input waveguide, an initial first graded waveguide, a second graded waveguide, a first output waveguide, and a second output waveguide on one side surface of the substrate layer. The initial first graded waveguide and the second graded waveguide are spaced apart in a second direction. The initial first graded waveguide has a first boundary line on the side away from the second graded waveguide. A portion of the thickness of a local area of ​​the initial first graded waveguide is removed to form a first graded waveguide with a first portion and a second portion of unequal height. The second portion has a second boundary line on the side relatively away from the second graded waveguide. A cladding layer is formed on the side of the waveguide core layer away from the substrate layer, and the cladding layer covers the waveguide core layer.

[0024] Beneficial Effects: The fabrication method of the above-described polarization rotation beamsplitter reduces the requirements for fabrication processes by defining key points and forming an asymmetric thermally adiabatic composite waveguide in the coupling region through two etching processes. The asymmetric thermally adiabatic composite waveguide in the coupling region has a waveguide cross-section that continuously varies along the light propagation direction, allowing for phase matching of different wavelengths at different positions, increasing the operating bandwidth, and effectively suppressing the coupling of other undesired optical signals, improving the polarization purity of the output optical signal, and maintaining low insertion loss and a high extinction ratio. Furthermore, optimizing the composite waveguide by defining "key points" in the coupling region helps to achieve smooth control of waveguide width variations, effectively enhancing the thermal insulation characteristics of the optical field conversion, reducing scattering loss caused by mode abrupt changes, and enabling the optical field to smoothly complete polarization rotation and mode separation. This structure integrates the parallel processing of polarization rotation and mode separation into a single coupling region, eliminating the need for cascading multiple functional devices and significantly reducing the beamsplitter size. Therefore, the polarization rotation beamsplitter of this embodiment possesses excellent characteristics such as compact structure, large bandwidth, low insertion loss, and high extinction ratio, making it suitable for various applications such as optical communication and optical sensing. Attached Figure Description

[0025] To more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0026] Figure 1 This is a schematic diagram of the polarization rotating beam splitter according to an embodiment of the present invention; Figure 2 This is a top view schematic diagram of the polarization rotating beam splitter according to an embodiment of the present invention; Figure 3 This is a cross-sectional schematic diagram of a polarization rotating beam splitter perpendicular to a first direction according to an embodiment of the present invention; Figure 4 This is another cross-sectional view of the polarization rotating beam splitter of this invention, perpendicular to the first direction. Figure 5 The polarization rotating beam splitter of this invention has an output optical transmission spectrum in the 1500-1700 nm band when inputting TEO mode. Figure 6 This is a diagram showing the optical field transmission distribution of the polarization rotating beam splitter in TE0 mode at a wavelength of 1570 nm, according to an embodiment of the present invention. Figure 7 The polarization rotating beam splitter of this invention has an output transmission spectrum in the 1500-1700 nm band when inputting TM0 mode. Figure 8 This is a diagram showing the optical field transmission distribution of the polarization rotating beam splitter in TM0 mode at a wavelength of 1570 nm, according to an embodiment of the present invention.

[0027] Explanation of reference numerals in the attached figures: 1. Substrate layer; 11. Bottom silicon layer; 12. Buried oxide layer; 2. Waveguide core layer; 10. Input region; 101. Input waveguide; 20. Coupling region; 201. First graded waveguide; 2011. First section; a. First boundary line; b. Second boundary line; 2012. Second section; 202. Second graded waveguide; 30. Output region; 301. First output waveguide; 302. Second output waveguide; 3021. First part; 3022. Second part; 3. Cladding. Detailed Implementation

[0028] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0029] refer to Figures 1 to 8This embodiment provides a polarization rotating beam splitter, including a substrate layer 1, a waveguide core layer 2, and a cladding layer 3 stacked together. The waveguide core layer 2 is disposed on one side surface of the substrate layer 1, and the cladding layer 3 covers the waveguide core layer 2. The waveguide core layer 2 includes an input region 10, a coupling region 20, and an output region 30 arranged sequentially along a first direction. The input region 10 includes an input waveguide 101, the coupling region 20 includes a first gradient waveguide 201 and a second gradient waveguide 202, and the output region 30 includes a first output waveguide 301 and a second output waveguide 302. One end of the first gradient waveguide 201 is connected to the input waveguide 101, and the other end is connected to the first output waveguide 301. The second gradient waveguide 202 and the first gradient waveguide 201 are disposed at intervals along a second direction on the substrate layer 1, and the second gradient waveguide 202 is connected to the second output waveguide 302. The first direction and the second direction are parallel to the surface of the substrate layer 1 and form a preset angle.

[0030] Specifically, the aforementioned substrate layer 1 and waveguide core layer 2 can be obtained using a silicon-on-insulator (SiI) substrate. The upper silicon layer of the SiI substrate is etched to obtain the waveguide core layer 2. The bottom silicon layer 11 and the middle buried silicon dioxide layer 12 serve as the substrate layer 1 of the device. The thin layer covering the waveguide core layer 2 can be made of silicon dioxide material with a thickness of 8 μm. Figure 3 and Figure 4 As shown. For example, the thickness of the underlying silicon 11 can be set to 1 mm, the thickness of the buried oxide layer 12 to 3 μm, and the thickness of the waveguide core layer 2 can be designed according to the process requirements. The waveguide core layer 2 is divided into an input region 10, a coupling region 20, and an output region 30. The light propagation direction is along the first direction from the input region 10 to the output region 30. The input waveguide 101 of the input region 10 is used to receive different mode polarized light input from the external structure, such as TE0 mode and TM0 mode. The coupling region 20 includes a first gradient waveguide 201 and a second gradient waveguide 202 spaced apart along the second direction. It receives different mode polarized light input from the input waveguide 101 and simultaneously realizes polarization rotation and mode separation. Then, the optical signals of different modes are transmitted to the first output waveguide 301 and the second output waveguide 302 of the output region 30 for final output.

[0031] refer to Figure 1 and Figure 2The first gradient waveguide 201 and the second gradient waveguide 202 are asymmetrically arranged in a second direction. The first gradient waveguide 201 includes a first portion 2011 and a second portion 2012 connected to each other. The average heights of the first portion 2011 and the second portion 2012 are different in a third direction, which is perpendicular to the surface of the substrate layer 1. The projections of the edges of the first portion 2011 and the second portion 2012 on the substrate layer 1 on the side closest to each other in the second direction overlap. In the second direction, the side of the first portion 2011 away from the second gradient waveguide 202 has a first boundary line a, and the side of the second portion 2012 away from the second gradient waveguide 202 has a second boundary line b. Both the first boundary line a and the second boundary line b are longitudinal boundaries extending along the third direction. The first portion 2011, on the side of the contour away from the second gradient waveguide 202, is formed by extending the edge line of the input waveguide 101 and the first output waveguide 301 away from the second gradient waveguide 202 to the first dividing line a; the second portion 2012, on the side of the contour away from the second gradient waveguide 202, is formed by extending the edge line of the input waveguide 101 and the first output waveguide 301 closer to the second gradient waveguide 202 to the second dividing line b; in the first direction, the first dividing line a and the second dividing line b are offset; in the second direction, the first dividing line a is offset from the side of the input waveguide 101 away from the second gradient waveguide 202, and the second dividing line b is offset from the side of the input waveguide 101 closer to the second gradient waveguide 202.

[0032] For ease of understanding, the first direction, second direction, and third direction mentioned above can also be understood as the x-axis, y-axis, and z-axis of a three-dimensional coordinate system; Figure 2 Taking the shown perspective as an example, the first direction is described in terms of left and right, with the left pointing to the right; the second direction is described in terms of front and back, with the back pointing to the front. The front side is... Figure 2 The lower side of the middle; then the third party describes the position above and below, and from top to bottom.

[0033] Specifically, the first gradient waveguide 201 includes a first portion 2011 and a second portion 2012. The projections of the front side of the first portion 2011 and the rear side of the second portion 2012 in the second direction onto the substrate layer 1 are completely overlapping. The first portion 2011 has a first dividing line a on its rear side along the second direction. The rear sides of the first portion 2011 located on the left and right sides of the first dividing line a in the first direction extend gently. That is, the cross-section of the first portion 2011 at various positions along the first direction has a gently changing width in the second direction. The second portion 2012 has a second dividing line b on its rear side along the second direction. The rear sides of the second portion 2012 located on the left and right sides of the second dividing line b in the first direction also extend gently. That is, the cross-section of the second portion 2012 at various positions along the first direction also has a gently changing width in the second direction. Meanwhile, the first portion 2011 and the second portion 2012 have different heights in the third direction. For example, the thickness corresponding to the region of the second portion 2012 can be removed by etching on the initial first gradient waveguide 201 to obtain a first gradient waveguide 201 with different thicknesses for the first portion 2011 and the second portion 2012. Therefore, the first portion 2011 and the second portion 2012 have cross-sections whose area changes continuously along the light propagation distance, and the change of the cross-section is also a smooth transition. Based on this, this embodiment also designs the first dividing line a and the second dividing line b to be staggered in the first direction, and also staggered with the side of the input waveguide 101 in the second direction. This means that even if the first part 2011 has the same size and shape of cross-section at different positions in the first direction, the first gradient waveguide 201 will have a cross-section that changes continuously with the light propagation distance due to the different cross-sections of the corresponding second part 2012. This breaks the symmetry of the light field, improves the polarization rotation efficiency, and the sufficiently slow geometric gradient allows the light field to smoothly adapt to the waveguide width change, avoiding unnecessary energy transfer during mode conversion, and achieving high efficiency, low loss, and wide bandwidth optical power and mode conversion. At the same time, the above design of the first gradient waveguide 201 makes it asymmetrically set with the second gradient waveguide 202 in the second direction, which helps to efficiently integrate polarization rotation and mode separation in the same coupling region 20, and greatly reduces the lateral length of the coupling region 20.

[0034] As can be seen, in this embodiment, the first boundary line a and the second boundary line b are vertical straight lines perpendicular to the surface of the substrate layer 1. Therefore, they are in... Figure 2The top view shown indicates the dividing points. Therefore, in this embodiment, the first gradient waveguide 201, or the composite waveguide of the coupling region 20, is optimized through the first dividing line a and the second dividing line b. Alternatively, the optimization of the composite waveguide of the coupling region 20 can be achieved by optimizing the first dividing point P1 and the second dividing point P2, which also helps simplify the fabrication process. Of course, it is not excluded that the first dividing line a and the second dividing line b may have a certain angle of inclination with the surface of the substrate layer 1.

[0035] The first graded waveguide 201 described above has a first waveguide cross-section that changes continuously along the direction of light propagation, and the second graded waveguide 202 has a second waveguide cross-section that changes continuously along the direction of light propagation. In this embodiment, the first waveguide cross-section is an asymmetrical shape. Here, asymmetry includes non-axisymmetric or non-centrosymmetric. For example, the asymmetrical shape can be... Figure 3 The stepped shape shown, or Figure 4 The figures shown include right-angled trapezoids, but are not limited to these.

[0036] like Figure 2 As shown, when a TE0 mode optical signal is input to port A of input waveguide 101, the optical signal enters the coupling region 20 from input waveguide 101 and propagates to the right to the first tapered waveguide 201. At this time, the widths of the first tapered waveguide 201 and the second tapered waveguide 202 do not meet the phase matching condition, and the optical field cannot couple into the second tapered waveguide 202. Instead, it directly enters the first output waveguide 301 and outputs TE0 mode optical power from port B. When a TM0 mode optical signal is input to port A of input waveguide 101, the optical signal also enters the coupling region 20 from input waveguide 101 and propagates to the right into the first tapered waveguide 201. At this time, because the phase matching condition is met, the input TM0 mode optical signal is first converted to TE1 mode, then to TE0 mode, and enters the second tapered waveguide 202. Finally, it outputs TE0 mode optical power from port C of the second output waveguide 302.

[0037] Phase matching is a crucial physical requirement for efficient coupling and transmission of optical energy between different waveguides. It means that the propagation constants of light in the two waveguide structures must be equal. The propagation constant is closely related to the waveguide width and the light mode. When the input light is in TE0 mode, there is no position in the coupling region 20 where the propagation constant of the TE0 mode in the first tapered waveguide 201 is equal to that in the second tapered waveguide 202. Therefore, their propagation constants are different, leading to phase mismatch and preventing effective coupling of energy to the target waveguide. Consequently, the light field continues to propagate along its original path. When the input light is in TM0 mode, because the waveguide width of the coupling region 20 gradually increases, the light mode rotates with the increasing propagation distance, changing to TE1 mode. Then, in the subsequent coupling region 20, there is a position where the propagation constant of the TE1 mode in the first tapered waveguide 201 is equal to that of the TE0 mode in the second tapered waveguide 202. Therefore, phase matching is satisfied, and the optical energy is effectively coupled to the second tapered waveguide 202.

[0038] The TE0 mode mentioned above is the fundamental mode of the transverse electric (TE) mode propagating in a waveguide. Specifically, it refers to an electromagnetic wave mode in which the electric field vibrates only in a plane perpendicular to the propagation direction, with no electric field component along the propagation direction, and the field strength has no fluctuations in the cross-section. The subscript "0" indicates zero-order fluctuation. The TM0 mode refers to the fundamental mode of the transverse magnetic (TM) mode. Its core characteristics are that the longitudinal component of the magnetic field is zero, the longitudinal component of the electric field is non-zero, and there are no field strength fluctuations in the cross-section. The subscript "0" indicates zero-order fluctuation. TE1 refers to the first-order mode of the transverse electric mode, that is, the electric field has one-order field strength fluctuation in the cross-section perpendicular to the propagation direction. The subscript "1" indicates first-order.

[0039] The additional loss of the polarization rotating beam splitter is calculated from the ratio of the output optical power at the corresponding output port to that at the input port. Figure 5 The output optical transmission spectrum in the 1500-1700 nm band in TEO mode in this embodiment is shown. The additional loss of port B corresponds to the additional loss of the TEO mode light source, and its optimal additional loss is -0.012 dB. In the entire 1500-1700 nm band, the additional loss of the TEO mode light source is better than -0.041 dB. The additional loss of port C corresponds to the crosstalk of the TEO mode light source. The crosstalk is better than -31.43 dB in the entire band. The extinction ratio of TEO mode is calculated by the difference between the additional losses of port B and port C. The extinction ratio is better than -30.91 dB in the entire band. Figure 6 This is the optical field transmission distribution diagram of the TE0 mode at a wavelength of 1570 nm in this embodiment. Since the phase matching condition is not met, the TE0 mode optical field input from port A is directly output from port B in TE0 form.

[0040] Figure 7 The image shows the input light transmission spectrum in the 1500-1700 nm band in TM0 mode in this embodiment. The additional loss at port C corresponds to the additional loss of the TM0 mode light source, with an optimal additional loss of -0.169 dB. Furthermore, the additional loss of the TM0 mode light source is better than 0.458 dB in the 1500-1700 nm band, exhibiting a large bandwidth characteristic of 185 nm. The additional loss at port B corresponds to the crosstalk level of the TM0 mode light source, with crosstalk better than -30.41 dB in the entire 1500-1700 nm band. The extinction ratio of the TM0 mode light source is calculated from the loss difference between port B and port C, and the extinction ratio is better than -29.72 dB in the entire 1500-1700 nm band. Figure 8 This is the optical field transmission distribution diagram of TM0 mode at a wavelength of 1570 nm in this embodiment. Due to the phase matching condition, the TM0 mode optical signal input from port A is successively converted to TE1 mode and TE0 mode, and finally output from port C in TE0 form.

[0041] Therefore, the polarization rotating beamsplitter in this embodiment can achieve an operating bandwidth of 185 nm, covering the communication band of 1515-1700 nm. Simulation results show that this polarization rotating beamsplitter based on an adiabatic conical structure effectively solves polarization sensitivity while significantly improving key performance indicators such as insertion loss, crosstalk, and bandwidth. Specifically, its lowest additional loss is 0.169 dB in TM0 mode and 0.012 dB in TE0 mode, with an extinction ratio better than 29.72 dB across the entire operating band. Compared with traditional solutions, this design maintains low loss, low crosstalk, and large bandwidth characteristics while keeping the overall size within 65 μm, achieving a good balance between performance and compactness.

[0042] In summary, the polarization rotation beamsplitter of this embodiment defines a first tapered waveguide 201 with a first boundary line a and a second boundary line b by defining a first tapered waveguide 201 at a first boundary point P1 and a second boundary point P2 in a top-view perspective. The cross-section of the first tapered waveguide 201 changes continuously along the light propagation direction. Combined with the asymmetrical design of the second tapered waveguide 202, the two work together to achieve structural optimization of the composite waveguide in the coupling region 20. First, through this unique composite waveguide design, the two core functions of polarization rotation and mode separation are integrated and performed in parallel within a single coupling region 20, fundamentally avoiding the problem of excessive size caused by traditional cascading of multiple functional devices. The device length can even be shortened to 65 μm. This integrated design not only significantly reduces the device volume, but more importantly, it simultaneously achieves low insertion loss and high extinction ratio within a compact size. For example, the insertion loss of the TE0 mode is less than 0.041 dB, the insertion loss of the TM0 mode is less than 0.458 dB, and the extinction ratio is greater than 29.7 dB. Secondly, the coupling region 20 features an asymmetric, thermally graded waveguide cross-section, or waveguide width, that continuously varies along the light propagation direction. This allows the device to meet phase-matching conditions for different wavelengths at different locations. This mechanism overcomes the bandwidth limitations of traditional devices, achieving an ultra-flat broadband characteristic of up to 185nm in the 1515-1700 nm band with an additional loss of less than 0.5 dB. This enables the device to stably support high-speed, multi-wavelength optical communication systems, significantly enhancing application tolerance and capabilities. Finally, this method of constructing composite waveguides by defining and optimizing a few "key points" simplifies complex design problems, greatly reducing optimization variables and computational costs. This makes rapid optimization of high-performance, complex devices possible, contributing to research and development as well as mass production.

[0043] In other words, the polarization rotation beamsplitter of this embodiment, through a simplified "key point" optimization design, reduces the requirements for fabrication processes, resulting in an asymmetric, thermally adiabatic composite waveguide within the coupling region 20. The waveguide cross-section continuously varies along the light propagation direction, allowing phase matching of different wavelengths at different positions and increasing the operating bandwidth. For example, it enables the input TM0 mode optical signal to be converted into a TE1 mode optical signal, and then further converted back into a TE0 mode output. Simultaneously, it effectively suppresses the coupling of other undesirable mode optical signals, improves the polarization purity of the output optical signal, and maintains low insertion loss and high extinction ratio. Moreover, by optimizing the composite waveguide through "key points" within the coupling region 20, smooth control of waveguide width variations can be achieved, effectively enhancing the thermal insulation characteristics of the optical field conversion and reducing scattering loss caused by mode abrupt changes. This allows the optical field to smoothly complete polarization rotation and mode separation. This structure integrates the parallel processing of polarization rotation and mode separation within a single coupling region 20, eliminating the need for cascading multiple functional devices and significantly reducing the beamsplitter size. Therefore, the polarization rotating beam splitter of this embodiment has excellent characteristics such as compact structure, large bandwidth, low insertion loss and high extinction ratio, and is suitable for various application scenarios such as optical communication and optical sensing.

[0044] In one embodiment, in the second direction, the first dividing line a protrudes from the side of the input waveguide 101 that is relatively far away from the second tapered waveguide 202, and the second dividing line b is located between the side of the input waveguide 101 that is relatively close to the second tapered waveguide 202 and the first dividing line a, and is offset from both sides of the input waveguide 101 in the second direction.

[0045] like Figure 1 and Figure 2 As shown, the first dividing line a protrudes from the rear side of the input waveguide 101 in the second direction, and the second dividing line b is located between the front and rear sides of the input waveguide 101 in the second direction, and does not correspond to the two sides. This arrangement makes the area of ​​the first part 2011 in the first gradient waveguide 201 as large as possible, while the area of ​​the second part 2012 is as small as possible, thereby improving the efficiency of polarization rotation and mode separation.

[0046] In one alternative implementation, such as Figure 3 As shown, in the third direction, the height of the first part 2011 is equal at all points, the height of the second part 2012 is equal at all points, and the height of the first part 2011 is greater than the height of the second part 2012.

[0047] That is, in the third direction, the upper surfaces of the first part 2011 and the second part 2012 are horizontal planes, and the cross-sections are both rectangular. This structure helps to simplify the manufacturing process, and the waveguide has a relatively regular shape, which is convenient for quantitative analysis and mass production.

[0048] In another alternative implementation, such as Figure 4 As shown, in the third direction, the height of the first section 2011 is equal at all points; from the side near the second gradient waveguide 202 to the end of the second boundary line b away from the substrate layer 1, the height of the second section 2012 gradually increases in the third direction, and the height of the second boundary line b is equal to the height of the first section 2011.

[0049] That is, in the third direction, the upper surface of the first part 2011 is a plane, and the upper surface of the second part 2012 is an inclined plane. The inclined plane converges from the front side near the second direction to the upper end of the second boundary line b. Thus, the cross section of the second part 2012 perpendicular to the first direction is a right trapezoid, while the cross section of the first part 2011 is still rectangular. This makes the cross section size change of the first gradient waveguide 201 in the direction of light propagation more gradual, expands the phase matching situation, and further improves the polarization rotation and mode separation efficiency.

[0050] In one embodiment, reference Figure 2 In the first direction, the distance L3 between the first dividing line a and the input waveguide 101 is less than the distance L1 between the second dividing line b and the input waveguide 101, and the distance L1 between the second dividing line b and the input waveguide 101 is greater than or equal to the distance L2 between the second dividing line b and the first output waveguide 301; in the second direction, the distance W4 between the first dividing line a and the side of the input waveguide 101 away from the second tapered waveguide 202 is less than or equal to the width W1 of the input waveguide 101, and the distance W2 between the second dividing line b and the side of the input waveguide 101 closer to the second tapered waveguide 202 is less than the width W1 of the input waveguide 101.

[0051] For example, the first gradient waveguide 201 is generally an asymmetric pentagon, wherein the first part 2011 is an irregular hexagon and the second part 2012 is a triangle. The initial first gradient waveguide 201 with an asymmetric pentagon is first formed by etching, and then the thickness of the region corresponding to the second part 2012 in the asymmetric pentagon is removed by secondary etching. The thickness of the first part 2011 of the asymmetric pentagon is 0.22 μm and the thickness of the second part 2012 is 0.07 μm.

[0052] The second section 2012 is triangular in shape. Along the direction of light propagation, the width of the second section 2012 starts from 0 μm, increases to W2 after an adiabatic gradient of length L1, and then decreases back to 0 μm after an adiabatic gradient of length L2. Preferably, L1 is greater than L2. For example, the size of L1 can be 40 μm, the size of W2 can be 0.22 μm, and the size of L1 can be 15 μm.

[0053] The first section 2011 is an irregular hexagonal shape. Along the light propagation direction, the widths of its left and right ends are consistent with the widths of the input waveguide 101 and the first output waveguide 301, respectively, both being 0.5 μm. That is, the size of W1 can be 0.5 μm. The front edge of the first section 2011 is continuous with the rear contour of the second section 2012. Along the light propagation direction, the rear edge of the first section 2011 starts from the right rear vertex of the input waveguide 101, extends for a distance of L3 while the waveguide width increases upward by W4, and then gradually converges from the first dividing point P1, finally smoothly connecting with the left rear vertex of the first output waveguide 301. The size of W4 is usually no larger than the size of W1 to ensure the coupling performance of the optical signal. The size of L3 can be 3 μm, the size of W4 can be 0.27 μm, and the subsequent convergence length can be 50 μm.

[0054] In one embodiment, reference Figure 2 The distance W2 between the second boundary line b and the side of the input waveguide 101 closer to the second tapered waveguide 202 satisfies the following relationship: W2 > 0.2 μm, and W1 - W2 ≥ 0.2 μm between W1 and W2.

[0055] Under the above-mentioned constraints, it can be ensured that the second part 2012 of the first gradient waveguide 201 has a sufficiently large gradient space in the second direction, while also ensuring that it still accounts for a relatively small proportion in the first gradient waveguide 201 compared with the first part 2011, thereby maximizing the waveguide area for optical transmission and improving the optical signal transmission efficiency.

[0056] like Figure 1 and Figure 2 As shown, the edges of the first tapered waveguide 201 and the second tapered waveguide 202 that are close to each other are parallel to each other; along the first direction, the distance between the side of the second tapered waveguide 202 that is relatively far away from the first tapered waveguide 201 and the first tapered waveguide 201 gradually increases.

[0057] That is, the front side of the first gradient waveguide 201 and the rear side of the second gradient waveguide 202 are parallel to each other. The front side of the second gradient waveguide 202 is set as a slope, and the distance between this slope and the rear side of the second gradient waveguide 202 gradually increases along the light propagation direction. In other words, the shape of the second gradient waveguide 202 from a top view is a right trapezoid. Along the light propagation direction, the width of the second gradient waveguide 202 gradually increases. The width of the left waveguide tip is 0.2 μm (W5), and the width of the right waveguide is consistent with the width W3 of the second output waveguide 302, which can be 0.37 μm. The front side of the second gradient waveguide 202 remains parallel to the light propagation direction, its lateral length is consistent with the first gradient waveguide 201, and it maintains a 0.2 μm gap in the longitudinal direction. That is, the gap size can be 0.2 μm, improving coupling efficiency while meeting the requirements of the fabrication process.

[0058] In one embodiment, in the third direction, the heights of the input waveguide 101, the portion of the first tapered waveguide 201 connected to the input waveguide 101 and the first output waveguide 301, the second tapered waveguide 202, and the output waveguide are equal. The output end surface of the input waveguide 101 is consistent with the input end surface of the first tapered waveguide 201, the output end surface of the first tapered waveguide 201 is consistent with the input end surface of the first output waveguide 301, and the output end surface of the second tapered waveguide 202 is consistent with the input end surface of the second output waveguide 302.

[0059] That is, except for the second part 2012 in the first gradient waveguide 201, the height of the other parts of the waveguide core layer 2 is equal; and the area and shape of the surfaces where the parts are connected are completely consistent, which ensures the efficiency of optical signal transmission and coupling, reduces optical mode loss, and improves the quality of optical signal transmission.

[0060] In one embodiment, such as Figure 1 and Figure 2 As shown, the input waveguide 101 and the first output waveguide 301 are strip-shaped straight waveguides. The second output waveguide 302 includes a first part 3021 and a second part 3022. The first part 3021 is an S-shaped curved waveguide, and the ratio of its length along the first direction to the maximum offset along the second direction is greater than or equal to 5. The second part 3022 is a strip-shaped straight waveguide.

[0061] For example, the input waveguide 101, the first output waveguide 301, and the second part 3022 of the second output waveguide 302 are all strip-shaped straight waveguides extending along the first direction; the first part 3021 of the second output waveguide 302 is an S-shaped curved waveguide, which can be composed of two arcs of the same or different linear shapes. In this embodiment, the S-shaped curved waveguide is composed of two arcs with the same radius and the same central angle. The larger the radius of the arc, the closer the S-shaped curved waveguide is to a straight waveguide, which is more conducive to reducing additional losses and suppressing hybridization. Therefore, in this embodiment, the ratio of the lateral dimension S1 of the S-shaped curved waveguide in the first direction to the longitudinal dimension S2 in the second direction is set to be greater than or equal to 5. For example, the lateral dimension S1 can be 10 μm and the longitudinal dimension S2 can be 1 μm. While reducing additional losses, it effectively suppresses the hybridization of optical modes in the curved waveguide, thereby improving the overall extinction ratio of the device.

[0062] In summary, this embodiment provides a solution for a high-bandwidth polarization rotation beamsplitter based on an adiabatic conical structure. This device is implemented on a standard silicon photonics platform. Through innovative waveguide structure design, it ensures low insertion loss while allowing for relatively flexible feature sizes, reducing dependence on nanoscale fabrication precision and improving fabrication yield. It can be fabricated using standard wafer fabrication processes without special steps, exhibiting good scalability and industrialization prospects. The core function of the device is to achieve efficient polarization state management and separation, and it can be widely applied to integrated photonic systems requiring precise polarization control, such as polarization diversity receiver modules in coherent optical communication systems, polarization multiplexing / demultiplexing modules, and polarization coding loops in quantum information processing, providing a reliable device solution for high-performance on-chip polarization processing.

[0063] This embodiment also provides a method for fabricating a polarization rotating beam splitter, which includes the following steps: Step S101: Provide substrate layer 1.

[0064] For example, a substrate layer 1 and a waveguide core layer 2 can be obtained using a silicon-on-insulator (SiI) substrate. The waveguide core layer 2 is obtained by etching the upper silicon layer of the SiI substrate. The bottom silicon layer 11 and the middle silicon dioxide buried oxide layer 12 serve as the substrate layer 1 of the device. Figure 3 and Figure 4 As shown. The thickness of the bottom silicon 11 can be 1 mm, and the thickness of the buried oxide layer 12 is 3 μm.

[0065] Step S102: A waveguide core layer 2 is patterned on one side surface of the substrate layer 1. The waveguide core layer 2 includes an input region 10, a coupling region 20, and an output region 30 arranged sequentially along a first direction. This includes: forming an input waveguide 101, an initial first gradient waveguide 201, a second gradient waveguide 202, a first output waveguide 301, and a second output waveguide 302 on one side surface of the substrate layer 1. The initial first gradient waveguide 201 and the second gradient waveguide 202 are spaced apart in a second direction. The initial first gradient waveguide 201 has a first boundary line a on the side away from the initial second gradient waveguide 202. Partial thickness is removed from a local area of ​​the initial first gradient waveguide 201 to form a first gradient waveguide 201 with a first portion 2011 and a second portion 2012 of unequal height. The second portion 2012 has a second boundary line b on the side relatively away from the second gradient waveguide 202.

[0066] Specifically, the steps for forming the waveguide core layer 2 include: firstly, etching the upper silicon layer of the silicon-on-insulator (SOI) substrate to form an input region 10 including an input waveguide 101, a coupling region 20 including an initial first tapered waveguide 201 and a second tapered waveguide 202, and an output region 30 including a first output waveguide 301 and a second output waveguide 302, each waveguide having equal thickness; then, secondly etching the initial first tapered waveguide 201 to remove a portion of its thickness near the local area of ​​the second tapered waveguide 202, forming a first tapered waveguide 201 including a first portion 2011 and a second portion 2012, the first portion 2011 and the second portion 2012 having different thicknesses. The initial first gradient waveguide 201 forms a first dividing line a on the side relatively far from the second gradient waveguide 202. The first dividing line a is offset from the rear side of the input waveguide 101. For example, from a top view, the initial first gradient waveguide 201 is an asymmetrical pentagon. The vertex located on the rear side, which is the upper end of the first dividing line a, protrudes from the rear side of the input waveguide 101. The two adjacent sides connected to this vertex are connected to the right rear vertex of the input waveguide 101 and the left rear vertex of the first output waveguide 301, respectively. The second portion 2012, formed by the subsequent secondary etching, forms a second boundary line b on the side relatively far from the second gradient waveguide 202. The second boundary line b is offset from the front and rear sides of the first output waveguide 301. For example, from a top view, the second portion 2012 can be a triangle. The base of the triangle coincides with the side of the asymmetric pentagon closest to the second gradient waveguide 202. The vertex in the second portion 2012 opposite to the base of the triangle is also the upper endpoint corresponding to the second boundary line b. In the second direction, it is located between the front and rear sides of the input waveguide 101. The two adjacent sides connected to this vertex are respectively connected to the right front vertex of the input waveguide 101 and the left front vertex of the first output waveguide 301.

[0067] Of course, there can be two or more first boundary lines a protruding from the rear side of the input waveguide 101. That is, the initial first gradient waveguide 201 can be hexagonal, heptagonal, etc., as long as it is an asymmetrical shape and the second boundary line b is offset from all the first boundary lines a in the first direction. In addition, there can be more than one second boundary line b, which is also offset from the first boundary line a in the first direction, so that the cross-section of the first gradient waveguide 201 changes continuously in the direction of light propagation.

[0068] In step S103, a cladding layer 3 is formed on the side of the waveguide core layer 2 away from the substrate layer 1, and the cladding layer 3 covers the waveguide core layer 2.

[0069] For example, the cladding 3 can be an 8 μm thick silicon dioxide layer to ensure total reflection of the optical signal in the waveguide core layer 2, thus guaranteeing the quality and efficiency of optical signal transmission.

[0070] The fabrication method of the aforementioned polarization rotation beamsplitter reduces the requirements for fabrication processes by defining key points and forming an asymmetric thermally adiabatic composite waveguide in the coupling region 20 through two etching processes. The asymmetric thermally adiabatic composite waveguide within the coupling region 20 has a waveguide cross-section that continuously varies along the light propagation direction, allowing for phase matching of different wavelengths at different locations, increasing the operating bandwidth, and effectively suppressing the coupling of other undesired optical signals. This improves the polarization purity of the output optical signal, maintaining low insertion loss and a large extinction ratio. Furthermore, optimizing the composite waveguide by defining "key points" within the coupling region 20 helps achieve smooth control over waveguide width variations, effectively enhancing the thermal insulation characteristics of the optical field conversion and reducing scattering loss caused by mode abrupt changes. This allows the optical field to smoothly complete polarization rotation and mode separation. This structure integrates the parallel processing of polarization rotation and mode separation within a single coupling region 20, eliminating the need for cascading multiple functional devices and significantly reducing the beamsplitter size. Therefore, the polarization rotating beam splitter of this embodiment has excellent characteristics such as compact structure, large bandwidth, low insertion loss and high extinction ratio, and is suitable for various application scenarios such as optical communication and optical sensing.

[0071] Further functional descriptions of the above structures are the same as those of the corresponding embodiments described above, and will not be repeated here.

[0072] Although embodiments of the invention have been described in conjunction with the accompanying drawings, those skilled in the art can make various modifications and variations without departing from the spirit and scope of the invention, and such modifications and variations all fall within the scope defined by the appended claims.

Claims

1. A polarization rotating beam splitter, characterized in that, include: A substrate layer, a waveguide core layer, and a cladding layer are stacked together, wherein the waveguide core layer is disposed on one side surface of the substrate layer, and the cladding layer covers the waveguide core layer; The waveguide core layer includes an input region, a coupling region, and an output region arranged sequentially along a first direction. The input region includes an input waveguide, the coupling region includes a first tapered waveguide and a second tapered waveguide, and the output region includes a first output waveguide and a second output waveguide. One end of the first tapered waveguide is connected to the input waveguide, and the other end is connected to the first output waveguide. The second tapered waveguide and the first tapered waveguide are spaced apart on the substrate layer along a second direction, and the second tapered waveguide is connected to the second output waveguide. The first direction and the second direction are parallel to the surface of the substrate layer and form a preset angle. The first and second graded waveguides are asymmetrically arranged in the second direction. The first graded waveguide includes a first portion and a second portion connected together. The first portion and the second portion have different average heights in a third direction, which is perpendicular to the surface of the substrate. The edges of the first portion and the second portion that are close to each other in the second direction completely overlap on the substrate. In the second direction, the first portion has a first boundary line on the side away from the second graded waveguide, and the second portion has a second boundary line on the side away from the second graded waveguide. Both the first and second boundary lines are longitudinal boundaries extending along the third direction. A portion of the contour away from the second graded waveguide is formed by extending the edges of the input waveguide and the first output waveguide away from the second graded waveguide to the first dividing line; a second portion of the contour away from the second graded waveguide is formed by extending the edges of the input waveguide and the first output waveguide closer to the second graded waveguide to the second dividing line; in the first direction, the first dividing line and the second dividing line are offset; in the second direction, the first dividing line is offset from the side of the input waveguide away from the second graded waveguide, and the second dividing line is offset from the side of the input waveguide closer to the second graded waveguide.

2. The polarization rotating beam splitter according to claim 1, characterized in that, In the second direction, the first dividing line protrudes from the side of the input waveguide that is relatively far away from the second graded waveguide, and the second dividing line is located between the side of the input waveguide that is relatively close to the second graded waveguide and the first dividing line, and is offset from both sides of the input waveguide in the second direction.

3. The polarization rotating beam splitter according to claim 1, characterized in that, In the third direction, the height of the first portion is equal at all points, the height of the second portion is equal at all points, and the height of the first portion is greater than the height of the second portion.

4. The polarization rotating beam splitter according to claim 1, characterized in that, In the third direction, the height of the first portion is equal at all points; in the second direction, from the side near the second graded waveguide to the end of the second boundary line away from the substrate, the height of the second portion gradually increases in the third direction, and the height of the second boundary line is equal to the height of the first portion.

5. The polarization rotating beam splitter according to any one of claims 1-4, characterized in that, In the first direction, the distance L3 between the first dividing line and the input waveguide is less than the distance L1 between the second dividing line and the input waveguide, and the distance L1 between the second dividing line and the input waveguide is greater than or equal to the distance L2 between the second dividing line and the output waveguide; In the second direction, the distance W4 between the first dividing line and the side of the input waveguide away from the second tapered waveguide is less than or equal to the width W1 of the input waveguide, and the distance W2 between the second dividing line and the side of the input waveguide closer to the second tapered waveguide is less than the width W1 of the input waveguide.

6. The polarization rotating beam splitter according to claim 5, characterized in that, The distance W2 between the second boundary line and the side of the input waveguide closer to the second gradient waveguide satisfies the following relationship: W2 > 0.2 μm, and W1 - W2 ≥ 0.2 μm between W1 and W2.

7. The polarization rotating beam splitter according to claim 1, characterized in that, The edges of the first and second tapered waveguides that are close to each other are parallel to each other along the first direction; along the first direction, the distance between the second tapered waveguide and the first tapered waveguide gradually increases on the side of the second tapered waveguide that is relatively far away from the first tapered waveguide.

8. The polarization rotating beam splitter according to claim 1, characterized in that, In the third direction, the heights of the input waveguide, the first portion of the first tapered waveguide, the second tapered waveguide, the first output waveguide, and the second output waveguide are equal. The output end surface of the input waveguide is consistent with the input end surface of the first tapered waveguide, the output end surface of the first tapered waveguide is consistent with the input end surface of the first output waveguide, and the output end surface of the second tapered waveguide is consistent with the input end surface of the second output waveguide.

9. The polarization rotating beam splitter according to claim 1, characterized in that, The input waveguide and the first output waveguide are strip-shaped straight waveguides. The second output waveguide includes a first part and a second part. The first part is an S-shaped curved waveguide, and the ratio of its length along the first direction to the maximum offset along the second direction is greater than or equal to 5. The second part is a strip-shaped straight waveguide.

10. A method for fabricating a polarization rotating beam splitter, used to fabricate the polarization rotating beam splitter according to any one of claims 1-9, characterized in that, The preparation method includes: Provide a substrate layer; A waveguide core layer is patterned on one side surface of the substrate layer. The waveguide core layer includes an input region, a coupling region, and an output region arranged sequentially along a first direction. The process includes: forming an input waveguide, an initial first graded waveguide, a second graded waveguide, a first output waveguide, and a second output waveguide on one side surface of the substrate layer. The initial first graded waveguide and the second graded waveguide are spaced apart in a second direction. The initial first graded waveguide has a first boundary line on the side away from the second graded waveguide. A portion of the thickness of a local area of ​​the initial first graded waveguide is removed to form a first graded waveguide with a first portion and a second portion of unequal height. The second portion has a second boundary line on the side relatively away from the second graded waveguide. A cladding layer is formed on the side of the waveguide core layer opposite to the substrate layer, and the cladding layer covers the waveguide core layer.

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