Thin film lithium niobate based compact broadband polarization rotator splitter and method of fabrication
By designing a compact broadband polarization rotating beam splitter based on thin-film lithium niobate, and utilizing a mode converter and mode evolver, combined with stimulated Raman adiabatic path technology, the problems of insufficient bandwidth and small process tolerance in existing technologies are solved, achieving efficient polarization rotation and cost reduction.
Patent Information
- Application Number
- CN202411160068.6
- Authority / Receiving Office
- CN · China
- Patent Type
- Patents(China)
- Current Assignee / Owner
- Filing Date
- 2024-08-22
- Publication Date
- 2025-12-19
- Estimated Expiration
- 2044-08-22
AI Technical Summary
Existing polarization rotating beam splitters have insufficient bandwidth and small process tolerances, making it difficult to integrate devices and reduce costs.
A compact broadband polarization rotating beam splitter based on thin-film lithium niobate was designed. A mode converter and a mode evolver were used to achieve the separation and rotation of the TM0 and TE0 components using stimulated Raman adiabatic path technology. Combined with the mode hybridization phenomenon, the device was fabricated on the substrate by photolithography and etching techniques.
It significantly expands the operating bandwidth of the device, improves the process tolerance, reduces the dependence on polarization-maintaining fiber arrays, reduces system costs, and is simple to fabricate and highly adaptable.
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Figure CN118795602B_ABST
Abstract
Description
TECHNICAL FIELD
[0001] The present application belongs to the technical field of integrated optics, and more particularly, relates to a compact broadband polarization-rotating beam splitter based on thin-film lithium niobate and a preparation method. BACKGROUND
[0002] The polarization degree of freedom can increase the capacity of an optical communication system by using polarization division multiplexing technology, so in this case, the control of polarization is very necessary. The polarization-rotating beam splitter is a key component to complete this function, which divides two orthogonal polarized optical signals into different output ports, and rotates the polarization state of one port by 90°. Such a device can be used in polarization division multiplexing applications. On the other hand, the modulation unit in the optical module is usually a polarization-sensitive device, which generally requires transverse electric mode polarized light to work. A polarization maintaining fiber array is needed to connect between the light source and the modulation unit, which is expensive, resulting in a significant increase in the cost of the entire scheme. When a non-polarization-maintaining fiber array is used, the polarization needs to be controlled to maintain the transverse electric mode polarization.
[0003] Lithium niobate crystal (LiNbO3, LN for short) belongs to a negative uniaxial crystal with non-central symmetry, and has a wide wavelength transmission range of about 350nm to 5500nm. The crystal exhibits excellent physical properties, including excellent piezoelectric, dielectric, ferroelectric, electro-optic, acousto-optic and nonlinear optical properties. Therefore, it is considered to be the best ferroelectric material in terms of comprehensive indicators, and is given the reputation of "optical silicon" due to its excellent performance in the optical field. Lithium niobate crystal can realize passive devices, active devices, nonlinear devices, etc. Traditional lithium niobate based on bulk materials is also often used to prepare polarization-rotating beam splitters, but due to the small waveguide refractive index difference and large waveguide size, it is difficult to realize integration. SUMMARY
[0004] In view of the above defects or improvement needs of the prior art, the purpose of the present application is to provide a compact broadband polarization-rotating beam splitter based on thin-film lithium niobate and a preparation method, aiming to solve the problems of insufficient bandwidth and small process tolerance of the current device. By designing different waveguide transmission sections, using mode evolution, mode hybridization and stimulated Raman adiabatic path technology, the TM0 component and the TE0 component in the input light are separated, and the TM0 component is rotated to obtain pure TE0 light.
[0005] To achieve the above-mentioned purpose, according to one aspect of the present application, a compact broadband polarization-rotating beam splitter based on thin-film lithium niobate is provided, which comprises, from bottom to top, a substrate layer, a buried oxygen layer, a thin-film lithium niobate layer and a dielectric film material layer, wherein,
[0006] The thin film lithium niobate layer is photoetched into a mode spot converter, a mode evolver, a width-tapered taper waveguide, a first curved waveguide and a second curved waveguide.
[0007] The mode evolver comprises a mode evolver first waveguide, a mode evolver second waveguide and a mode evolver third waveguide; the output end of the mode spot converter is connected with the input end of the mode evolver first waveguide.
[0008] The output end of the mode evolver first waveguide, the width-tapered taper waveguide and the first curved waveguide are connected in sequence; the output end of the mode evolver third waveguide is connected with the second curved waveguide; the mode evolver second waveguide and the mode evolver third waveguide are located on the same side of the mode evolver first waveguide, the mode evolver first waveguide is parallel to the mode evolver third waveguide, and the mode evolver second waveguide is located between the mode evolver first waveguide and the mode evolver third waveguide and is inclined at a preset angle.
[0009] The width of the mode evolver first waveguide, the mode evolver second waveguide and the mode evolver third waveguide remains unchanged.
[0010] As a further preferred embodiment of the present application, the thin film lithium niobate layer is X-cut.
[0011] As a further preferred embodiment of the present application, the mode spot converter is composed of three taper waveguides for realizing mutual conversion of TM0 mode at a narrow end and TE1 mode at a wide end while TE0 mode remains unchanged; the width w0 of the waveguide at the narrow end of the mode spot converter is smaller than the width of TM0 mode and TE1 mode hybridization and supports TE0 and TM0 mode transmission; the width w3 of the mode evolver first waveguide is greater than the width of TM0 mode and TE1 mode hybridization; the width and length of the three taper waveguides in the mode spot converter are calculated by using particle swarm algorithm according to mode evolution theory, so that TM0 mode and TE1 mode are efficiently and adiabatically coupled in a relatively short length.
[0012] As a further preferred embodiment of the present application, the width of the mode evolver first waveguide and the widths of the mode evolver second waveguide and the mode evolver third waveguide in the mode evolver satisfy |Neff TE1 |=|Neff TE0 |, |Neff TE1 | is the effective refractive index of TE1 mode in the mode evolver first waveguide, and |Neff TE0 | is the effective refractive index of TE0 mode in the mode evolver second waveguide and the mode evolver third waveguide.
[0013] As a further preferred embodiment of the present application, the mode evolver is determined by stimulated Raman adiabatic path technology, and is composed of three waveguides, the first waveguide a and the third waveguide c of the mode evolver are parallel, have equal length l, and have a spacing large enough to prevent direct coupling, and the second waveguide b of the mode evolver is placed in the middle of ac and is inclined at a specific angle; the spacing between the first waveguide input end of the mode evolver and the second waveguide of the mode evolver is the same as the spacing between the second waveguide of the mode evolver and the left port of the third waveguide of the mode evolver; the spacing between the second waveguide of the mode evolver and the left port of the third waveguide of the mode evolver is the same as the spacing between the first waveguide output end of the mode evolver and the second waveguide of the mode evolver.
[0014] As a further preferred embodiment of the present application, the first waveguide of the mode evolver is converted in width to be consistent with the first curved waveguide by a width-gradual taper waveguide; the second waveguide of the mode evolver, the third waveguide of the mode evolver, the first curved waveguide and the second curved waveguide have consistent width.
[0015] As a further preferred embodiment of the present application, the substrate layer is specifically a silicon substrate layer, a quartz substrate layer or a sapphire substrate layer.
[0016] As a further preferred embodiment of the present application, the refractive index of the dielectric film material layer is less than 2, and the material used in the dielectric film material layer with a refractive index less than 2 is at least one of silicon dioxide, aluminum oxide, silicon oxynitride, SU8 photoresist and SOG spin-coated glass.
[0017] According to another aspect of the present application, the present application provides a preparation method of the above-mentioned compact wideband polarization-rotating beam splitter based on thin-film lithium niobate, characterized by comprising the following steps:
[0018] S1. Preparing a patterned mask on the surface of thin-film lithium niobate on an insulator, wherein the thin-film lithium niobate on the insulator comprises, from bottom to top, a substrate layer, a buried oxygen layer and a thin-film lithium niobate layer;
[0019] S2. Using thin-film etching technology to transfer the pattern to the thin-film lithium niobate layer on the substrate obtained in S1 to prepare a mode spot converter, a mode evolver, a width-gradual taper waveguide, a first curved waveguide and a second curved waveguide;
[0020] S3. Cleaning the substrate obtained in step S2 to remove the mask and redeposition;
[0021] S4. Covering the dielectric film material with a refractive index less than 2 on the substrate obtained in S3 by thin-film deposition or spin coating to obtain the compact wideband polarization-rotating beam splitter based on thin-film lithium niobate.
[0022] As a further preferred embodiment of the present application, in step S1, the photolithography etching technology is based on a stepper photolithography machine, a contact photolithography machine, a projection photolithography machine, electron beam lithography, or laser direct writing;
[0023] In step S2, the thin film etching technology is specifically ion beam etching, reactive ion etching, and inductively coupled plasma etching.
[0024] In step S3, the thin film deposition method is at least one of physical vapor deposition and chemical vapor deposition.
[0025] Compared with the prior art, the present application has the following beneficial effects:
[0026] 1. The present application converts TM0 mode into TE1 mode through a mode spot converter, and realizes wideband polarization rotation splitting by using the stimulated Raman adiabatic path technology in the mode evolutioner. This process effectively separates the TM0 component from the TE0 component in the input light and rotates the polarization of the TM0 component, finally obtaining pure TE0 mode light. Compared with the prior art, the device in the present application not only significantly expands the working bandwidth, but also greatly improves the process tolerance. The reason for using the stimulated Raman adiabatic path technology is that it can achieve efficient and stable mode conversion and polarization rotation in a wide band. As a non-destructive optical process, the stimulated Raman adiabatic path technology enables efficient energy transfer between different energy levels through gradually changing light fields. Its advantage is that it has a large tolerance to the input wavelength and polarization of light, so it can maintain efficient polarization conversion and rotation in a wider frequency spectrum range. This not only improves the bandwidth of the device, but also enhances the stability and reliability of the system, making it more adaptable in complex optical applications.
[0027] 2. The present application innovatively designs the connection between the light source and the modulation module without relying on a polarization maintaining fiber array. This design effectively manipulates the polarization state of the optical signal in the device to ensure stable transmission of a single polarization state, thereby realizing direct transmission of a non-polarization maintaining fiber array. This method not only successfully eliminates the dependence on expensive polarization maintaining fiber arrays, but also significantly reduces the system cost of the optical module.
[0028] 3. The process steps used in the present application have significant cost advantages, and the manufacturing process is relatively simple and easy to prepare. In addition, this process can be well compatible with existing CMOS processes. At the same time, the minimum line width of the device manufactured by the present application is relatively large, so it does not need to rely on too precise photolithography equipment in the production process. This not only reduces production costs, but also improves the feasibility and popularity of the process. BRIEF DESCRIPTION OF DRAWINGS
[0029] Figure 1Cross-sectional view of the thin film lithium niobate platform used in the compact broadband polarization-rotating beam splitter based on thin film lithium niobate in the present application.
[0030] Figure 2 Top view of the compact broadband polarization-rotating beam splitter based on thin film lithium niobate in the present application.
[0031] Figure 3 Partial structural diagram (a) of the mode spot converter, and principle structural diagram (b) of the mode evolutioner.
[0032] Figure 4 Graph of the effective refractive index variation of different modes with the waveguide width.
[0033] Figure 5 Schematic diagram of the mode field transmission of TE0 and TM0 modes in the compact broadband polarization-rotating beam splitter based on thin film lithium niobate in the present application.
[0034] Figure 6 Schematic diagram of the transmission spectrum simulation curve of TE0 and TM0 mode inputs when the transmission spectrum wavelength is 1450nm-2450nm in the compact broadband polarization-rotating beam splitter based on thin film lithium niobate in the present application; wherein (a) is the transmission spectrum simulation curve of TE0 mode input, and (b) is the transmission spectrum simulation curve of TM0 mode input.
[0035] Figure 7 Schematic diagram of the transmission spectrum simulation curve of TE0 and TM0 mode inputs when the wavelength is 1550nm in the compact broadband polarization-rotating beam splitter based on thin film lithium niobate in the present application; wherein (a) is the transmission spectrum simulation curve of TE0 mode input, and (b) is the transmission spectrum simulation curve of TM0 mode input.
[0036] Figure 8 Schematic diagram of the preparation method flow of the compact broadband polarization-rotating beam splitter based on thin film lithium niobate in the present application.
[0037] The drawing mark: 1-substrate layer, 2-buried oxygen layer, 3-thin film lithium niobate layer, 4-thin film lithium niobate waveguide, 5-dielectric film material layer, 6-mode spot converter, 7-mode evolutioner, 8-width gradually changing tapered waveguide, 9-first curved waveguide, 10-second curved waveguide, 11-mode evolutioner first waveguide, 12-mode evolutioner second waveguide, 13-mode evolutioner third waveguide. DETAILED DESCRIPTION
[0038] In order to make the objects, technical solutions and advantages of the present application clearer, the present application will be further described in detail below with reference to the drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present application and not used to limit the present application. In addition, the technical features involved in the various embodiments of the present application described below can be combined with each other as long as they do not conflict with each other.
[0039] Embodiment 1
[0040] As shown in Figure 1 , Figure 2 and Figure 3 , a compact broadband polarization-rotating beam splitter based on thin-film lithium niobate adopts a ridge waveguide structure. Accordingly, the substrate layer 1 is a silicon substrate (the thickness of this layer in this embodiment is 500 μm), the buried oxide layer 2 is silicon dioxide (the thickness of this layer in this embodiment is 3 μm), the thin-film lithium niobate layer 3 adopts X-cut and has a thickness of 400 nm, the thin-film lithium niobate waveguide 4 is formed after etching and has a thickness of 200 nm, and the dielectric film material layer 5 with a refractive index less than 2 is silicon dioxide (the thickness of this layer in this embodiment is 1 μm).
[0041] In addition, the compact broadband polarization-rotating beam splitter based on thin-film lithium niobate includes a mode spot converter 6, a mode evolver 7, a width-tapered tapered waveguide 8, a first curved waveguide 9, and a second curved waveguide 10. The mode evolver 7 includes a mode evolver first waveguide 11, a mode evolver second waveguide 12, and a mode evolver third waveguide 13. The output end of the mode spot converter 6 is connected to the input end of the mode evolver first waveguide 11. The output end of the mode evolver first waveguide 11, the width-tapered tapered waveguide 8, and the first curved waveguide 9 are sequentially connected, and the output end of the mode evolver third waveguide 13 is connected to the second curved waveguide 10.
[0042] In this embodiment, the mode spot converter 6 is composed of three tapered waveguides, as shown in Figure 3 (a). The width length is calculated using the particle swarm algorithm according to the mode evolution theory, so that efficient adiabatic coupling between the TM0 mode and the TE1 mode is achieved in a relatively short length. Specifically, w0=0.8 μm, w1=1.2 μm, w2=1.5 μm, w3=2.16 μm, L0=10 μm, L1=100 μm, and L2=250 μm.
[0043] In this embodiment, the mode evolver 7 is determined by the stimulated Raman adiabatic path technology. As shown in Figure 3 (b), it is composed of three waveguides. The mode evolver first waveguide 11 (a) and the mode evolver third waveguide 13 (c) are parallel, have the same length L3, and have a sufficient spacing to prevent direct coupling. The mode evolver second waveguide 12 (b) is placed in the middle of ac and is inclined at a specific angle. gab (y) and g bc (y) are the edge distance between waveguide a and b and the edge distance between waveguide b and c respectively. The coordinate origin is located at the center of waveguide b. The mode evolver 7 is symmetric at (y,z) = (0,0) and g ab (0) = g bc (0), so
[0044] At the same time, The adiabaticity of the system is determined by the coupling strength between waveguides and the tilt angle of the intermediate waveguide b. The second waveguide 12 of the mode evolver, i.e. b, has a slope of γ needs to be much smaller than 1, which is condition A, to satisfy the condition of slow adiabatic light transmission. Wherein or r is a parameter related to the waveguide confinement, k0= k 12 (0) = k 23 (0), i.e. the coupling coefficients of waveguide a and waveguide b, waveguide b and waveguide c, and waveguide a and waveguide c at y = 0. Condition B is k 13 ~ 0 (coupling coefficient between waveguide a and waveguide c) and According to condition A and condition B, the mode evolver 7 is designed to satisfy the realization of adiabatic light transmission. Specifically, Δg = 1 μm, i.e.
[0045] In this embodiment, the widths of the second waveguide 12 of the mode evolver, the third waveguide 13 of the mode evolver, the first curved waveguide 9 and the second curved waveguide 10 are consistent, i.e. w4 = w5 = w6 = w7. The width of the tapered waveguide 8 gradually changes from the width w3 of the first waveguide 11 of the mode evolver to w6, and the length L4 = 20 μm.
[0046] In this embodiment, Figure 4 The figure shows the change of effective refractive index of different modes with the change of waveguide width. According to Figure 4 , the widths of the mode evolver first waveguide 11, the mode evolver second waveguide 12 and the mode evolver third waveguide 13 in the mode evolver 7 and the mode spot converter 6 are obtained under the consideration of process capability, as shown by the dashed line. When the TM0 mode is input, it is converted to the TE1 mode by the mode spot converter 6, and the TE1 mode is propagated in the mode evolver first waveguide 11. The width w3 = 2.16 μm is selected, and the corresponding TE0 mode width is 0.8 μm, so the widths of the mode evolver second waveguide 12 and the mode evolver third waveguide 13 are w4 = w5 = 0.8 μm. Figure 4
[0047] Figure 5 The schematic diagram of mode field transmission of TE0 and TM0 mode in the compact broadband polarization-rotating beam splitter based on thin film lithium niobate is shown, wherein (a) is the propagation of TE0 input, (b) is the propagation of TM0 input, and it can be known that the polarization-rotating beam splitter is high in efficiency.
[0048] Figure 6 The schematic diagram of transmission spectrum simulation curve of TE0 and TM0 mode input when the transmission spectrum wavelength is 1450-2450 nm in the compact broadband polarization-rotating beam splitter based on thin film lithium niobate in the application is shown. When the wavelength is 1450-2450 nm, the TE0 mode component of the thr port is close to 0 dB when the TE0 mode is incident, which indicates that the transmission is good, the cross talk is small, and the TM0 mode component is lower than -40 dB, which indicates that there is no strong TM0 mode component in the thr port. The TE0 mode component and the TM0 mode component in the cro port are both less than -40 dB, which indicates that the cross talk of the TE0 mode input is small, the TE0 mode is mainly output from the thr port, and the extinction ratio is high. When the TM0 mode is incident, the TE0 mode component of the cro port is close to 0 dB, which indicates that the polarization rotation is excellent, the TM0 mode is converted into the TE0 mode, and the TM0 mode energy in the cro port is lower than -30 dB, which indicates that the cross talk of the unconverted TM0 component is low. The TM0 mode component of the thr port is lower than -18 dB when the wavelength is 1450-2145 nm, and is lower than -10 dB when the wavelength is 2145-2450 nm, which indicates that the conversion effect is excellent when the wavelength is 1450-2145 nm, the extinction ratio is high, and the effect is also good when the wavelength is 2145-2450 nm. In addition, the TE0 mode component of the thr port is lower than -40 dB, which indicates that there is no strong TE0 mode component when the TM0 mode is input.
[0049] Figure 7 The schematic diagram of transmission spectrum simulation curve of TE0 and TM0 mode input when the wavelength is 1550 nm is shown. When L3 is 0-1500 μm, the TE0 mode component of the thr port is kept close to 0 dB when the TE0 mode is input, the TM0 mode component is lower than -40 dB, and the TE0 mode component and the TM0 mode component of the cro port are both lower than -40 dB. When the length is greater than 500 μm, the TM0 mode is efficiently converted into the TE0 mode when the TM0 mode is input. The unconverted TM0 mode component, i.e. the energy propagating in the thr port, tends to be stable when the L3 length is greater than 500 μm, and is lower than -20 dB. In addition, the TE0 mode component in the thr port and the TM0 mode component in the cro port are both lower than -40 dB when the L3 length is greater than 500 μm. The device has high performance and high extinction ratio. Therefore, the length L3 of the mode evolutioner 7 is selected to be 700 μm in the example.
[0050] Embodiment 2
[0051] As Figure 8 shown, it is a preparation method of a compact broadband polarization rotator based on thin film lithium niobate in the embodiment 2 of the present application, comprising the following steps:
[0052] S1. A patterned mask is prepared on the thin film lithium niobate material on the insulator by using photolithography technology;
[0053] S2. The pattern is transferred to the thin film lithium niobate layer by using etching technology on the substrate obtained in step S1, to prepare the mode spot converter 6, the mode evolutioner 7, the width-tapered tapered waveguide 8, the first curved waveguide 9 and the second curved waveguide 10;
[0054] S3. The substrate obtained in step S2 is cleaned to remove the mask and redeposition;
[0055] S4. The silicon dioxide 5 is deposited on the substrate obtained in step S3 by thin film deposition method to obtain a compact broadband polarization rotator based on thin film lithium niobate.
[0056] Among them, the photolithography method includes: step-by-step photolithography, contact photolithography, projection photolithography, electron beam lithography, laser direct writing; the electron beam lithography is used in the embodiment. The thin film etching method includes: ion beam etching, reactive ion etching and inductively coupled plasma etching; the inductively coupled plasma etching is used in the embodiment. The thin film deposition method includes: physical vapor deposition and chemical vapor deposition; the chemical vapor deposition is used in the embodiment. Those skilled in the art can easily understand that the above-mentioned only the preferred embodiment of the present application, and does not limit the present application, any modification, equivalent replacement and improvement made within the spirit and principles of the present application, should be included in the protection scope of the present application.
Claims
1. A compact broadband polarization-rotating beamsplitter based on thin-film lithium niobate, characterized in that, From bottom to top, successively include: substrate layer (1), buried oxygen layer (2), thin film lithium niobate layer (3) and dielectric film material layer (5), wherein, The thin film lithium niobate layer (3) is etched by photolithography to form a mode spot converter (6), a mode evolution device (7), a width gradually changing tapered waveguide (8), a first curved waveguide (9) and a second curved waveguide (10); The mode evolution device (7) is determined by stimulated Raman adiabatic path technology, and includes a mode evolution device first waveguide (11), a mode evolution device second waveguide (12) and a mode evolution device third waveguide (13); an output end of the mode spot converter (6) is connected with an input end of the mode evolution device first waveguide (11); An output end of the mode evolution device first waveguide (11), the width gradually changing tapered waveguide (8) and the first curved waveguide (9) are connected successively; an output end of the mode evolution device third waveguide (13) is connected with the second curved waveguide (10); the mode evolution device second waveguide (12) and the mode evolution device third waveguide (13) are located on the same side of the side of the mode evolution device first waveguide (11), the mode evolution device first waveguide (11) is parallel to the mode evolution device third waveguide (13), the mode evolution device second waveguide (12) is located between the mode evolution device first waveguide (11) and the mode evolution device third waveguide (13) and is inclined at a preset angle; Widths of the mode evolution device first waveguide (11), the mode evolution device second waveguide (12) and the mode evolution device third waveguide (13) remain unchanged.
2. The compact thin film lithium niobate based broadband polarization-rotating beamsplitter of claim 1, wherein, The thin film lithium niobate layer (3) is X-cut.
3. The compact thin film lithium niobate based broadband polarization-rotating beamsplitter of claim 1, wherein, The mode spot converter (6) includes three tapered waveguides, the waveguide widths of the tapered waveguides successively increase, and are used for realizing mutual conversion of TM0 mode at the minimum waveguide width end and TE1 mode at the maximum waveguide width end, while TE0 mode remains unchanged; the minimum waveguide width w0 of the mode spot converter (6) is smaller than the width of TM0 mode and TE1 mode hybridization, and supports TE0 and TM0 mode transmission; the width w3 of the mode evolution device first waveguide (11) is greater than the width of TM0 mode and TE1 mode hybridization.
4. The compact thin film lithium niobate based broadband polarization-rotating beamsplitter of claim 1, wherein, The width of the mode-evolver first waveguide (11) and the width of the mode-evolver second waveguide (12), the mode-evolver third waveguide (13) satisfy , is the effective refractive index of the TE1 mode in the mode-evolver first waveguide (11), is the effective refractive index of the TE0 mode in the mode-evolver second waveguide (12) and the mode-evolver third waveguide (13).
5. The compact thin film lithium niobate based broadband polarization-rotating beamsplitter of claim 1, wherein, Lengths of the mode evolution device first waveguide (11) and the mode evolution device third waveguide (13) are equal; a spacing between the input end of the mode evolution device first waveguide (11) and the mode evolution device second waveguide (12) is the same as a spacing between the output ends of the mode evolution device second waveguide (12) and the mode evolution device third waveguide (13); a spacing between the mode evolution device second waveguide (12) and the left port of the mode evolution device third waveguide (13) is the same as the spacing between the input end of the mode evolution device first waveguide (11) and the mode evolution device second waveguide (12).
6. The compact thin film lithium niobate based broadband polarization-rotating beamsplitter of claim 1, wherein, The width of the mode evolution device first waveguide (11) is the same as the width of the input end of the width gradually changing tapered waveguide (8), the width of the output end of the width gradually changing tapered waveguide (8) is the same as the first curved waveguide (9); the widths of the mode evolution device second waveguide (12), the mode evolution device third waveguide (13), the first curved waveguide (9) and the second curved waveguide (10) are the same.
7. The compact thin film lithium niobate based broadband polarization-rotating beamsplitter of claim 1, wherein, The substrate layer (1) is a silicon substrate layer, a quartz substrate layer or a sapphire substrate layer. The material of the dielectric film material layer (5) is at least one of silicon dioxide, aluminum oxide, silicon oxynitride, SU8 photoresist and SOG spin-on glass.
8. The compact thin film lithium niobate based broadband polarization-rotating beamsplitter of claim 1, wherein, The refractive index of the dielectric film material layer (5) is less than 2.
9. A method of fabricating a thin film lithium niobate based compact broadband polarization-rotating beamsplitter according to any one of claims 1-8, wherein, The method comprises the following steps: S1. Preparing a patterned mask on the surface of the thin-film lithium niobate on insulator, which comprises a substrate layer, a buried oxygen layer and a thin-film lithium niobate layer from bottom to top; S2. Transferring the pattern to the thin-film lithium niobate layer on the substrate obtained in S1 by using a thin-film etching technology to prepare a mode spot converter, a mode evolver, a width-tapered waveguide, a first curved waveguide and a second curved waveguide; S3. Cleaning the substrate obtained in step S2 to remove the mask and redeposition; S4. Covering the dielectric film material on the substrate obtained in S3 by a thin-film deposition method or a spin coating method to obtain a compact broadband polarization rotation beam splitter based on thin-film lithium niobate.
10. The preparation method of claim 9, wherein In step S1, the photoetching technology is based on a step-by-step photoetching machine, a contact photoetching machine, a projection photoetching machine, an electron beam photoetching machine or a laser direct writing machine; In step S2, the thin-film etching technology is specifically ion beam etching, reactive ion etching or inductively coupled plasma etching; In step S3, the thin-film deposition method is at least one of physical vapor deposition and chemical vapor deposition.