An asymmetric, anisotropic lithium niobate thin-film Y-branch polarization beam splitter and optical device

Abstract

This application discloses an asymmetric anisotropic lithium niobate thin-film Y-branch polarization beamsplitter and optical device, belonging to the field of optical device technology. The lithium niobate thin-film Y-waveguide includes: an input segment, which is a strip waveguide for single-mode transmission; an asymmetric Y-branch region, whose Y-branch includes two transition regions, which are curved; a first output arm, a gradient-etched ridge waveguide for transmitting TE-mode optical signals, whose extension direction forms an angle of less than 10° with one optical principal axis of the lithium niobate thin-film crystal; and a second output arm, a shallowly etched ridge waveguide of equal width for transmitting TM-mode optical signals, whose extension direction forms an angle of less than 5° with the other optical principal axis of the lithium niobate thin-film crystal. The width of the second output arm is greater than the width of the first output arm, and the height of the second output arm is less than the height of the first output arm. This solution achieves efficient and low-loss separation of two polarization states, with a compact device structure and compatibility with standard nanofabrication processes.

Description

Technical Field

[0001] This application relates to the field of integrated photonic device technology, specifically to an asymmetric, anisotropic lithium niobate thin-film Y-branched polarization beam splitter and optical device. Background Technology

[0002] In integrated photonic circuits, the management and control of the polarization state of light is a crucial and challenging issue. Many photonic devices, such as modulators, detectors, and filters, are highly sensitive to the polarization state of incident light. Therefore, achieving polarization-independent operation or separating input light with arbitrary polarization states into pure TE (transverse electric) and TM (transverse magnetic) modes is a prerequisite for constructing complex and stable photonic systems. Traditional polarization beam splitting schemes are mainly based on the following technical approaches:

[0003] 1. Multilayer dielectric film type PBS (Polarizing Beam Splitter): This method utilizes the difference in reflection and transmission characteristics of different polarized light at the interface of a dielectric film to split the beam. Although this method is mature, it is bulky and difficult to integrate monolithically with planar optical waveguide circuits. It is typically used in free-space optical systems.

[0004] 2. Silicon-based photonics PBS: In silicon photonics, multimode interferometers (MMIs), directional couplers (DCs), or photonic crystal structures are commonly used to achieve polarization beam splitting. However, directly transplanting these structures onto lithium niobate thin films encounters many problems: MMIs and DCs are usually large in size, which is not conducive to high integration; and the isotropic nature of silicon makes it impossible for their design methods to fully utilize the anisotropic advantages of lithium niobate.

[0005] 3. Traditional Lithium Niobate Waveguide PBS: Existing lithium niobate PBSs mostly employ simple asymmetric Y-branch or directional coupling structures. Their design approach is often limited to adjusting the waveguide width or gap, falling under the category of "geometric parameter optimization." This type of design has the following limitations:

[0006] Performance trade-offs: In order to obtain a high extinction ratio, a long thermal gradient region is often required, which leads to an excessively large device size, contradicting the high integration advantage of thin-film lithium niobate films.

[0007] Ignoring material properties: The inherent optical anisotropy of lithium niobate crystals (i.e., its refractive index ellipsoidal properties) is not fully utilized. It is only regarded as a high refractive index material, which restricts the degree of design freedom and has a low performance limit.

[0008] Loss and crosstalk: At the branch point, significant scattering loss is easily generated due to the abrupt change in mode. Simultaneously, it is highly sensitive to process variations (such as etching depth and waveguide width fluctuations), leading to increased polarization crosstalk. Summary of the Invention

[0009] This application aims to provide an asymmetric anisotropic lithium niobate thin film Y-branch polarization beam splitter and optical device that can solve the aforementioned technical problems.

[0010] In a first aspect, the technical solution of this application provides an asymmetric anisotropic lithium niobate thin-film Y-branch polarization beam splitter, comprising, from bottom to top, a silicon substrate, a silicon dioxide buried oxide layer, a lithium niobate thin-film Y-waveguide, and an upper cladding layer. The lithium niobate thin-film Y-waveguide includes an input segment, an asymmetric Y-branch region, a first output arm, and a second output arm, wherein:

[0011] The input segment is a strip waveguide and transmits in single mode at a wavelength of 1550nm;

[0012] The asymmetric Y-branch region is connected to the input segment, and its Y-branch includes two transition regions, each of which is a curve.

[0013] The first output arm is a gradient deep-etched ridge waveguide connected to a transition region for transmitting TE mode optical signals. Its extension direction has an angle of less than 10° with one optical principal axis of the lithium niobate thin film crystal.

[0014] The second output arm is a shallowly etched ridge waveguide of equal width, connected to another transition region, for transmitting TM mode optical signals, and its extension direction has an angle of less than 5° with another optical principal axis of the lithium niobate thin film crystal.

[0015] The width of the second output arm is greater than the width of the first output arm, and the height of the second output arm is less than the height of the first output arm.

[0016] Preferably, the asymmetric anisotropic lithium niobate thin film Y-branch polarization beam splitter:

[0017] The width of the first output arm is in the range of 300nm to 500nm, and the width of the second output arm is in the range of 550nm to 750nm.

[0018] The height of the first output arm is (500±a) nm, and the height of the second output arm is (200±b) nm;

[0019] Where a and b are redundant parameters.

[0020] Preferably, the asymmetric anisotropic lithium niobate thin film Y-branch polarization beam splitter:

[0021] The width of the input segment is (450±c)nm, and the height is (500±d)nm;

[0022] Among them, c and d are redundant parameters.

[0023] Preferably, the asymmetric anisotropic lithium niobate thin film Y-branch polarization beam splitter:

[0024] The transition region adopts a curve with a radius of (50±e)μm;

[0025] Where e is a redundant parameter.

[0026] Preferably, the asymmetric anisotropic lithium niobate thin film Y-branch polarization beam splitter:

[0027] The Y-branching angle of the asymmetric Y-branching region is between 1° and 5°.

[0028] Secondly, the present application provides a process for preparing the asymmetric anisotropic lithium niobate thin film Y-branch polarization beam splitter described in any of the first aspects, comprising:

[0029] Substrate selection and preparation: Select a lithium niobate thin film wafer, which consists of a silicon substrate, a silicon dioxide buried oxide layer, and a lithium niobate thin film from bottom to top;

[0030] Electron beam lithography: Electron beam photoresist is spin-coated onto the surface of a lithium niobate film, and the designed asymmetric anisotropic Y waveguide pattern is transferred to the photoresist surface using an electron beam exposure system;

[0031] Dry etching: An inductively coupled plasma reactive ion etching system is used, with photoresist as a mask. First, the entire asymmetric anisotropic Y waveguide pattern is shallowly etched to a depth equal to the height of the second output arm. Then, the second output arm is masked, and the first output arm is fully etched to a depth equal to the height of the first output arm.

[0032] Resin removal and cleaning: Remove residual photoresist to obtain the effective working area;

[0033] Upper cladding deposition: A silicon dioxide thin film is deposited on the surface of the effective region as an upper cladding by plasma-enhanced chemical vapor deposition to obtain an asymmetric anisotropic lithium niobate thin film Y-branch polarization beam splitter.

[0034] Preferably, in the substrate selection and preparation step of the process, the thickness of the lithium niobate film is (500±g) nm; where g is a redundant parameter.

[0035] Preferably, in the dry etching step of the process, the shallow etching depth is (200±b) nm, and the full etching depth is (500±a) nm, where a<g.

[0036] Preferably, in the upper cladding deposition step of the process, a silicon dioxide thin film with a thickness of 1.0~2.0μm is deposited on the surface of the effective working area as the upper cladding.

[0037] Thirdly, this application provides an optical device, which includes the asymmetric anisotropic lithium niobate thin film Y-branch polarization beam splitter described in any of the first aspects; wherein:

[0038] The optical device is a Mach-Zehnder interferometer, an optical switch, or a polarization diversity receiver.

[0039] The technical solution provided in this application has the following technical effects compared with the prior art:

[0040] The asymmetric anisotropic lithium niobate thin-film Y-branch polarization beamsplitter and optical device provided in this application include a lithium niobate thin-film Y-waveguide. This Y-waveguide comprises an input segment, an asymmetric Y-branch region, a first output arm, and a second output arm. The asymmetric Y-branch region includes two transition regions, each of which is curved to minimize scattering loss. The first and second output arms, connected to the asymmetric Y-branch region, have different widths and heights; one is a gradient deep-etched ridge waveguide, and the other is a uniform-width shallow-etched waveguide. The ridge waveguide design allows for differences in the cross-sectional geometry of the two output arms. Furthermore, the angle between the extension direction of the first output arm and one optical principal axis of the lithium niobate thin-film crystal is less than 10°, and the angle between the extension direction of the second output arm and the other optical principal axis is less than 5°. This achieves an asymmetric orientation of the waveguide extension directions of the two output arms relative to the optical principal axes of the lithium niobate crystal, creating distinctly different equivalent refractive index transmission paths for TE-mode and TM-mode optical signals. This enables efficient and low-loss separation of the two polarization states without the need for adiabatic gradients. The design scheme described in this application provides the device with advantages such as compact structure and compatibility with standard nanofabrication processes. Attached Figure Description

[0041] Figure 1 This is a schematic diagram of the lithium niobate thin film Y-waveguide in an asymmetric anisotropic lithium niobate thin film Y-branch polarization beam splitter according to one embodiment of this application;

[0042] Figure 2 This is a schematic cross-sectional view of the first output arm of the asymmetric anisotropic lithium niobate thin film Y-branch polarization beam splitter according to an embodiment of this application;

[0043] Figure 3 This is a schematic cross-sectional view of the second output arm of the asymmetric anisotropic lithium niobate thin film Y-branch polarization beam splitter according to one embodiment of this application;

[0044] Figure 4 This is a schematic diagram of the structure of the transition region according to one embodiment of this application;

[0045] Figure 5 This is a schematic diagram of the finite element simulation results of the optical field energy of the TE mode in the first output arm in the embodiment of this application.

[0046] Figure 6 This is a schematic diagram of the finite element simulation results of the optical field energy of the TM mode in the second output arm in this embodiment of the application;

[0047] Figure 7 A process flow diagram for preparing the asymmetric anisotropic lithium niobate thin film Y-branch polarization beam splitter described in the embodiments of this application. Detailed Implementation

[0048] The specific embodiments of this application will be further described below with reference to the accompanying drawings.

[0049] It is readily understood that, based on the technical solution of this application, various structural and implementation methods can be interchanged by those skilled in the art without altering the essential spirit of this application. Therefore, the following detailed embodiments and accompanying drawings are merely illustrative examples of the technical solution of this application and should not be considered as the entirety of this application or as limitations or restrictions on the technical solution of the application.

[0050] This application provides an asymmetric anisotropic lithium niobate thin-film Y-branch polarization beamsplitter and optical device, which is a high-performance, compact polarization beamsplitter. This device achieves efficient and low-loss separation of transverse electric mode (TE mode) and transverse magnetic mode (TM mode) by synergistically optimizing the geometric asymmetry of the waveguide and the optical anisotropy of the lithium niobate crystal. The solution in this application is based on a lithium niobate thin-film wafer, which has a structure comprising, from bottom to top, a silicon substrate, a silicon dioxide buried oxide layer, and a lithium niobate thin film. The following detailed description, in conjunction with specific embodiments, illustrates the solution.

[0051] This embodiment provides an asymmetric, anisotropic lithium niobate thin-film Y-branch polarization beam splitter, comprising, from bottom to top, a silicon substrate, a silicon dioxide buried oxide layer, a lithium niobate thin-film Y-waveguide, and an upper cladding layer, as shown below. Figure 1As shown, the lithium niobate thin film Y waveguide includes an input segment 101, an asymmetric Y-branch region 102, a first output arm 103, and a second output arm 104. The input segment 101 is a strip waveguide that transmits in single mode at a wavelength of 1550nm. The asymmetric Y-branch region 102 is connected to the input segment 101, and its Y-branch includes two transition regions 1021, each of which is a curve. The first output arm 103 is a gradient-etched ridge waveguide connected to one of the transition regions 1021 and is used to transmit TE mode optical signals. The angle between its extension direction and one optical principal axis of the lithium niobate thin film crystal (the X-axis direction for Z-cut wafers) is less than 10°. The gradient-etched ridge waveguide is characterized by a deep etching depth and a gradually changing waveguide width. Deep etching can provide stronger optical field confinement, while the gradient design helps to achieve smooth mode conversion. This design provides extremely strong mode confinement and a high effective refractive index for the TE mode (whose principal electric field direction is parallel to the plane of the lithium niobate thin film crystal and perpendicular to the direction of light propagation), while also introducing a large phase mismatch or cutoff condition for the TM mode. The second output arm 104 is a shallowly etched ridge waveguide of equal width, connected to another transition region 1021, and is used to transmit TM mode optical signals. Its extension direction forms an angle of less than 5° with another optical principal axis of the lithium niobate thin film crystal (the Z-axis direction for a Z-cut wafer). A ridge waveguide is a common waveguide structure characterized by ridge-like protrusions used to alter the propagation characteristics of electromagnetic waves. Equal width shallow etching refers to a process feature where the ridge width remains consistent and the etching depth is shallow. This is commonly used in microwave and photonic devices to optimize performance. This design fully utilizes the relatively high ordinary light refractive index and electro-optic coefficient of the lithium niobate thin film crystal for the TM mode (whose principal electric field direction is perpendicular to the plane of the lithium niobate thin film crystal), providing an optimized transmission channel for the TM mode and giving it a high effective refractive index. The width of the second output arm 104 is greater than the width of the first output arm 103, and the height of the second output arm 104 is less than the height of the first output arm 103.

[0052] The direction of the optical principal axis of the lithium niobate thin film crystal is known, and it is marked when the lithium niobate thin film wafer leaves the factory. Therefore, in this solution, given the known directions of each optical principal axis of the lithium niobate thin film crystal, it is possible to achieve a requirement that the angle between the extension direction of the first output arm 103 and the direction of the corresponding optical principal axis is less than a set degree. Ideally, the extension direction of the first output arm 103 and the second output arm 104 is parallel to the direction of the corresponding optical principal axis.

[0053] In addition, since the 1550nm band is a core band with high usage in the field of optical fiber communication (i.e., C-band 1530nm-1565nm), the output segment of this application realizes single-mode transmission of optical signals in this band, which makes the device applicable to a variety of optical fiber communication scenarios and suitable for integration into optical devices in different fields.

[0054] In the above-described scheme of this embodiment, the asymmetric anisotropic lithium niobate thin-film Y-branch polarization beamsplitter includes a lithium niobate thin-film Y-waveguide. This lithium niobate thin-film Y-waveguide includes an input segment 101, an asymmetric Y-branch region 102, a first output arm 103, and a second output arm 104. The asymmetric Y-branch region includes two transition regions 1021, each of which is a curve to minimize scattering loss. The first output arm 103 and the second output arm 104, connected to the asymmetric Y-branch region, have different widths and heights; one is a gradient deep-etched ridge waveguide, and the other is a uniform-width shallow-etched ridge waveguide. This design results in differences in the cross-sectional geometry of the two output arms. Furthermore, the angle between the extension direction of the first output arm 103 and one optical principal axis of the lithium niobate thin-film crystal is less than 10°, and the angle between the extension direction of the second output arm 104 and the other optical principal axis of the lithium niobate thin-film crystal is less than 5°. This achieves an asymmetric orientation of the waveguide extension directions of the two output arms relative to the optical principal axes of the lithium niobate crystal, creating distinctly different equivalent refractive index transmission paths for TE-mode and TM-mode optical signals. This enables efficient and low-loss separation of the two polarization states without the need for adiabatic gradients. Through this design, the device possesses the advantages of a compact structure and compatibility with standard nanofabrication processes.

[0055] Preferably, in the asymmetric anisotropic lithium niobate thin-film Y-branch polarization beam splitter, the width of the first output arm is in the range of 300 nm to 500 nm, and the width of the second output arm is in the range of 550 nm to 750 nm; the height of the first output arm is (500 ± a) nm, and the height of the second output arm is (200 ± b) nm; where a and b are redundant parameters. Ideally, such as Figure 2 and Figure 3 As shown, the first output arm 103 is a gradient deep etched ridge waveguide, and the second output arm 104 is a uniform width shallow etched ridge waveguide. Preferably, the width of the first output arm 103 is 400nm and the height is 500nm, and the width of the second output arm 104 is 600nm and the height is 200nm. In actual processing, a certain processing error is allowed, that is, a and b are zero in the most ideal state.

[0056] More preferably, the width of the input segment 101 is (450±c) nm and the height is (500±d) nm; where c and d are redundant parameters. In specific implementation, the input segment 101 is designed with a width of 450 nm and a height of 500 nm, thereby ensuring that a 1550 nm wavelength beam can achieve single-mode transmission within the input segment 101.

[0057] In some preferred solutions, such as Figure 1 and Figure 4 As shown, the transition region 1021 uses a curve with a radius of (50±e) μm; where e is a redundancy parameter. The Y-branching angle A of the asymmetric Y-branching region is between 1° and 5°. Ideally, the transition region 1021 is connected using an Euler bend with a radius of 50 μm to smooth the evolution of the optical field. The bifurcation angle A is set to 2°.

[0058] In the above embodiments of this application, when unpolarized light (or arbitrary polarized light) enters the asymmetric Y-branch region 102 from the input segment 101, the TE mode and TM mode begin to evolve. Due to the dual asymmetric design, near the Y-branch point, the TE mode "senses" that the first output arm 103 (narrow and deep, along the X-axis) provides a more suitable equivalent refractive index channel and mode field distribution. Based on the principle that light field energy tends to concentrate in regions with high equivalent refractive index, and the requirement for continuity of mode evolution, the energy of the TE mode is adiabatically coupled into the first output arm 103. Similarly, the TM mode "senses" that the second output arm 104 (wide and shallow, along the Z-axis) is its optimal transmission path, because the design of the second output arm 104 maximizes the lattice anisotropy of lithium niobate, which is friendly to the TM mode. Ultimately, the TE mode is output from the first output arm 103, and the TM mode is output from the second output arm 104, thereby achieving polarization beam splitting.

[0059] To verify the effectiveness of the proposed solution, the following tests were conducted: 1550nm unpolarized light was input to input segment 101 via fiber coupling, and the power at the output ports of the first output arm 103 and the second output arm 104 was measured respectively. Let the TE optical power output from the first output arm 103 be P_TE, and the leaked TM optical power be P'_TM. Then the TE extinction ratio is: 10*log10(P_TE / P'_TM), and the TE mode optical field energy distribution is as follows: Figure 5 As shown. Let the TM optical power output from the second output arm 104 be P_TM, and the leaked TE optical power be P'_TE. Then the TM extinction ratio is 10*log10(P_TM / P'_TE), and the TM mode optical field energy distribution is as follows. Figure 6As shown. In the above embodiments of this application, at a wavelength of 1550nm, the TE mode extinction ratio can reach 28dB and the TM mode extinction ratio can reach 26dB. The calculated sum of the TE mode light field energy output by the first output arm 103 and the TM mode light field energy output by the second output arm 104 is almost the same as the light field energy of the 1550nm unpolarized light received by the input segment 101. This indicates that the first output arm 103 and the second output arm 104 have completed the separation of the TE mode and the TM mode without causing excessive light energy loss.

[0060] Based on the above test results, the proposed solution has the following significant advantages:

[0061] Extremely high polarization extinction ratio: Through the synergistic design of geometric asymmetry and crystal orientation optimization of the first and second output arms in the asymmetric Y-branch region, equivalent refractive index difference and mode mismatch are created for the TE mode and TM mode respectively, which ensures the efficient separation of the TE mode and TM mode from the physical principle, achieving a polarization extinction ratio of more than 25dB in the C-band (1530nm-1565nm).

[0062] Compact structure: The design of the asymmetric Y-branch region itself can achieve mode separation within a short interaction length (typically less than 100 μm), and the overall device size can be reduced by more than 50% compared with the traditional design based on adiabatic evolution, which is conducive to high-density integration.

[0063] Fully utilize material properties: elevate the optical anisotropy of lithium niobate from a background parameter to a core degree of design freedom, realizing the synergistic design of "material-structure-function". This is an essential innovation that distinguishes it from isotropic materials (such as silicon) devices, with obvious performance advantages.

[0064] Excellent process tolerance: Since the separation mechanism relies on the overall characteristic difference between the two arms, rather than a single, sensitive critical dimension, it has better tolerance to process fluctuations such as etching depth and waveguide width.

[0065] Functional scalability: This design concept can be further extended to other functional devices, such as polarization rotators and mode beam splitters, providing a general technical route for achieving complex polarization and mode management on lithium niobate thin films.

[0066] This application also provides a process for preparing the above-described asymmetric anisotropic lithium niobate thin film Y-branch polarization beam splitter, such as... Figure 7 As shown, it includes the following steps:

[0067] Substrate selection and preparation: A lithium niobate thin film wafer is selected, which consists of a silicon substrate, a silicon dioxide buried oxide layer, and a lithium niobate thin film from bottom to top. Preferably, the lithium niobate thin film wafer is Z-cut (i.e., the Z-axis of the crystal is perpendicular to the wafer surface), and the thickness of the lithium niobate thin film is (500±g) nm; where g is a redundant parameter, and ideally g is zero; the thickness of the silicon dioxide buried oxide layer is preferably 2 μm.

[0068] Electron beam lithography: Electron beam photoresist is spin-coated onto the surface of a lithium niobate film. The designed asymmetric anisotropic Y-waveguide pattern is then transferred onto the photoresist surface using an electron beam exposure system. As mentioned earlier, the preferred designed asymmetric anisotropic Y-waveguide pattern is as follows: the input segment is a strip waveguide with a width of 450 nm and a height of 500 nm to ensure single-mode transmission at a wavelength of 1550 nm; the asymmetric Y-branching region has a bifurcation angle A of 2°, and the transition region is connected using Euler bends with a radius of 50 μm to smooth the optical field evolution; the first output arm has a width of 400 nm; and the second output arm has a width of 600 nm.

[0069] Dry etching: An inductively coupled plasma reactive ion etching (ICP-IR) system is used, employing photoresist as a mask. First, the entire asymmetric anisotropic Y-waveguide pattern is shallowly etched to a depth equal to the height of the second output arm. Then, the second output arm is masked, and the first output arm is fully etched to a depth equal to its height. The shallow etching depth is 200 nm, and the full etching depth is 500 nm, reaching the buried oxide layer of silicon dioxide. This ensures that the two output arms have different heights and different cross-sectional shapes: one is a narrow-width, deeply etched gradient deep-etched ridge waveguide, and the other is a wide-width, shallowly etched equal-width shallow-etched ridge waveguide.

[0070] Resin removal and cleaning: Remove residual photoresist to obtain the effective working area;

[0071] Upper cladding deposition: A silicon dioxide thin film is deposited on the surface of the effective region as an upper cladding using plasma-enhanced chemical vapor deposition (PECVD) to obtain an asymmetric anisotropic lithium niobate thin film Y-branched polarization beam splitter. The upper cladding is used to protect the waveguide and isolate it from the environment.

[0072] In the step of depositing the upper cladding layer, a silicon dioxide film with a thickness of 1.0~2.0μm is deposited on the surface of the effective working area as the upper cladding layer, preferably a silicon dioxide film with a thickness of 1.5μm.

[0073] This application also provides an optical device, which includes the asymmetric anisotropic lithium niobate thin film Y-branch polarization beamsplitter described in any of the above embodiments; wherein: the optical device is a Mach-Zehnder interferometer, an optical switch, or a polarization diversity receiver.

[0074] As needed, the above technical solutions can be combined to achieve the best technical effect.

[0075] The above are merely the principles and preferred embodiments of this application. It should be noted that, for those skilled in the art, several other modifications can be made based on the principles of this application, and these modifications should also be considered within the scope of protection of this application.

Claims

1. An asymmetrically inhomogeneous lithium niobate thin film Y-branch polarization splitter, characterized in that, From bottom to top, it comprises a silicon substrate, a buried silicon dioxide layer, a lithium niobate thin film Y waveguide and an upper cladding layer, the lithium niobate thin film Y waveguide comprises an input section, an asymmetric Y branch region, a first output arm and a second output arm, wherein: The input section is a strip waveguide and transmits a single mode at a wavelength of 1550 nm; The asymmetric Y branch region is connected with the input section, and the Y branch thereof comprises two transition regions, each of which is a curve; The first output arm is a gradually deep etched ridge waveguide connected with one of the transition regions, used for transmitting a TE mode optical signal, and the angle between the extension direction of the first output arm and one of the optical principal axes of the lithium niobate thin film crystal is less than 10°; The second output arm is an equal-width shallow etched ridge waveguide connected with the other transition region, used for transmitting a TM mode optical signal, and the angle between the extension direction of the second output arm and the other optical principal axis of the lithium niobate thin film crystal is less than 5°; The width of the second output arm is greater than that of the first output arm, and the height of the second output arm is less than that of the first output arm.

2. The asymmetrically divergent lithium niobate thin film Y branch polarization beam splitter according to claim 1, wherein: The width of the first output arm is in the range of 300 nm to 500 nm, and the width of the second output arm is in the range of 550 nm to 750 nm; The height of the first output arm is (500±a) nm, and the height of the second output arm is (200±b) nm; Wherein, a and b are redundant parameters.

3. The asymmetrically divergent lithium niobate thin film Y branch polarization beam splitter according to claim 2, wherein: The width of the input section is (450±c) nm, and the height is (500±d) nm; Wherein, c and d are redundant parameters.

4. The asymmetrically divergent lithium niobate thin film Y branch polarization beam splitter according to claim 3, wherein: The transition region adopts a curve with a radius of (50±e) μm; Wherein, e is a redundant parameter.

5. The asymmetrically divergent lithium niobate thin film Y branch polarization beam splitter according to any one of claims 1-4, wherein: The Y branch bifurcation angle of the asymmetric Y branch region is between 1° and 5°.

6. A process for preparing the asymmetrically inhomogeneous lithium niobate thin film Y-branch polarization beam splitter according to any one of claims 1 to 5, characterized in that It comprises: Substrate selection and preparation: selecting a lithium niobate thin film wafer, which comprises a silicon substrate, a buried silicon dioxide layer and a lithium niobate thin film from bottom to top; Electron beam lithography: spin coating electron beam resist on the surface of the lithium niobate thin film, and transferring the designed asymmetrically divergent Y waveguide pattern to the resist surface by using an electron beam exposure system; Dry etching: using an inductively coupled plasma reactive ion etching system, using photoresist as a mask, first performing shallow etching on the entire asymmetrically divergent Y waveguide pattern, and the etching depth is the height of the second output arm; Then shield the second output arm, and perform full etching on the first output arm, and the etching depth is the height of the first output arm; Degumming and cleaning: removing the residual photoresist to obtain an effective action area; Upper cladding deposition: depositing a layer of silicon dioxide film as an upper cladding layer on the surface of the effective action area by plasma enhanced chemical vapor deposition method, to obtain an asymmetrically divergent lithium niobate thin film Y branch polarization beam splitter.

7. The process of claim 6, wherein: the thickness of the lithium niobate film is (500±g) nm in the step of substrate selection and preparation; wherein g is a redundancy parameter.

8. The process of claim 6, wherein: the etching depth of the shallow etching is (200±b) nm and the etching depth of the full etching is (500±a) nm in the step of dry etching, wherein a 9. The process of claim 6, wherein: a silicon dioxide film with a thickness of 1.0-2.0 μm is deposited as the upper cladding layer on the surface of the active area in the step of upper cladding layer deposition.

10. An optical device, comprising the asymmetrically inhomogeneous lithium niobate film Y-branch polarization beam splitter of any one of claims 1-5; wherein: the optical device is a Mach-Zehnder interferometer, an optical switch, or a polarization diversity receiver. ​

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