A polarization independent electro-optic modulator based on thin film lithium niobate multi-mode waveguide
By designing a thin-film lithium niobate multimode waveguide structure, light waves in TM or TE polarization states are converted into TE1 modes for modulation, solving the problem of polarization state dependence in existing electro-optic modulators, realizing the effective utilization of light with different polarization states, and improving modulation efficiency and signal quality.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Patents(China)
- Current Assignee / Owner
- SOUTH CHINA NORMAL UNIV
- Filing Date
- 2022-12-20
- Publication Date
- 2026-05-12
AI Technical Summary
Existing electro-optic modulators require strict control of the polarization state of light to prevent interference from light waves of other polarization states, which would affect modulation efficiency and signal quality, and would not be able to effectively utilize light waves of other polarization states.
A structure based on thin-film lithium niobate multimode waveguide is adopted. The light wave in TM or TE polarization state is converted into TE1 mode by the mode converter at the input end, and then modulated in the phase shift arm of the multimode waveguide. The original polarization state is restored at the output, so as to achieve simultaneous modulation of TE and TM polarized light.
Simultaneous modulation of TE and TM polarized light was achieved, expanding the application range of electro-optic modulators and improving modulation efficiency and signal quality.
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Figure CN116009292B_ABST
Abstract
Description
Technical Field
[0001] This invention relates to the field of integrated optoelectronic devices, and more specifically to a polarization-independent electro-optic modulator based on a thin-film lithium niobate multimode waveguide. Background Technology
[0002] Electro-optic modulator technology is a modulation technique that superimposes an information-carrying electrical signal onto a carrier light wave. Optical modulation can cause certain parameters of a light wave, such as amplitude, frequency, phase, polarization state, and duration, to change according to certain rules. Electro-optic modulators achieve this through the electro-optic effect of materials, where the change in the refractive index of the material, based on the Pockels effect, is proportional to the electric field, and its coefficient is related to the electro-optic coefficient of the material.
[0003] Lithium niobate (LNiO) materials possess advantages such as high electro-optic coefficients and low C-band loss, making them the preferred material for electro-optic modulators. Furthermore, the emergence of thin-film LNiO and breakthroughs in etching technology have solved the problem of insufficient refractive index difference in waveguides of traditional bulk LNiO materials, enabling stronger mode confinement and representing a solution for next-generation photonic integrated circuits. Modulators based on thin-film LNiO materials hold immense application value in future optical communication fields as next-generation modulators with low modulation voltage, high modulation bandwidth, and low insertion loss.
[0004] Most current electro-optic modulators modulate the fundamental TE mode. From the moment the light wave passes through the grating input device, the polarization state of the light wave needs to be strictly controlled to prevent light of other polarization modes from interfering with the required TE polarized light, affecting the modulation efficiency and the quality of the modulation signal. Therefore, light waves of other polarization states cannot be effectively utilized. Summary of the Invention
[0005] To overcome the shortcomings of existing technologies, this invention discloses a polarization-independent electro-optic modulator based on a thin-film lithium niobate multimode waveguide. Through analysis from the perspective of optical electromagnetic wave theory, the modulator structure is designed and optimized, enabling the effective utilization and modulation of light waves with either TM or TE polarization states. This solves the polarization control problem and expands the application range of electro-optic modulators.
[0006] To achieve the above objectives, the present invention adopts the following structure:
[0007] A polarization-independent electro-optic modulator based on a thin-film lithium niobate multimode waveguide includes a silicon substrate, a buried oxide layer, a lithium niobate layer, and a metal layer stacked sequentially from bottom to top. A thin-film lithium niobate optical waveguide is formed on the X-cut lithium niobate layer using etching technology. The thin-film lithium niobate optical waveguide includes, in sequence, an input straight waveguide, an input mode converter, a multimode beam splitter, a first S-bend waveguide, a multimode waveguide phase shift arm, a second S-bend waveguide, a multimode combiner, an output mode converter, and an output straight waveguide. A metal traveling-wave electrode is disposed at the phase shift arm of the multimode waveguide. The metal traveling-wave electrode includes a metal traveling-wave signal electrode with a set of periodically arranged T-structure metal electrodes connected to each side, and a metal traveling-wave ground electrode with a set of periodically arranged T-structure metal electrodes connected to one side. A cavity is formed by etching a portion of the silicon substrate. Specifically, the traveling wave signal electrodes, each connected to a set of periodically arranged T-structure metal electrodes on both sides, are located between the two phase shift arms of the multimode waveguide, while the two traveling wave ground electrodes, each connected to a set of periodically arranged T-structure metal electrodes on one side, are located outside the two phase shift arms of the multimode waveguide. The entire Mach-Zehnder modulation structure is symmetrically placed.
[0008] In a further embodiment, the input and output mode converters have identical structures, enabling mutual conversion between the narrower TM0 mode and the wider TE1 mode, while the TE0 mode remains unchanged. The narrower waveguide width W1 of the mode converter is smaller than the mode hybridization width of the TM0 and TE1 modes, and supports both TE0 and TM0 modes. The wider waveguide width W2 of the mode converter is larger than the mode hybridization width of the TM0 and TE1 modes. The length L1 of the mode converter is determined by mode evolution theory to achieve complete coupling between the narrower TM0 mode and the wider TE1 mode.
[0009] In a further embodiment, the multimode beamsplitter and multimode beam combiner have the same structure, consisting of a 2×2 multimode interference structure and two sets of input-output tapered cones. The width W4 and length L3 of the multimode interference structure are determined by multimode self-imaging theory. When either the TE0 or TE1 mode is input from one of the ports, 3dB beam splitting of either the TE0 or TE1 mode can be achieved simultaneously. The widths W2 and W5 at the ends of the input-output tapered cones are greater than the hybrid width of the TM0 and TE1 modes, preventing mode conversion when light passes through the tapered cones.
[0010] In a further embodiment, the modulator material is selected as thin-film lithium niobate, comprising, from bottom to top, a silicon substrate layer, a low-refractive-index buried oxide layer, a high-refractive-index ridge-shaped lithium niobate layer, and a metal electrode located above the lithium niobate layer. The upper cladding can be made of a material with a low refractive index, such as air. The T-structure metal electrode, the metal traveling-wave ground electrode, and the metal traveling-wave signal electrode are all made of gold.
[0011] In a further embodiment, the multimode waveguide phase shift arms on both sides of the metal traveling wave signal electrode are ridge-type multimode waveguides. The two ends of the multimode waveguide phase shift arms are connected to the S-bend waveguide via tapered tapers, allowing the TE0 mode and TE1 mode to be transmitted thermally separately. The lithium niobate beneath the electrodes on both sides of the multimode waveguide phase shift arms is not etched, reducing metal absorption loss.
[0012] In a further specific embodiment, in the modulation region, the width W6 of the multimode waveguide phase shift arm needs to be calculated and optimized to ensure that when modulating light in TE0 mode and TE1 mode, the half-wave voltage of the two modes can be equal and the width is as small as possible, so as to achieve simultaneous and efficient modulation of light with different polarizations.
[0013] In a further embodiment, in the modulation region, the structural parameters of the metal traveling-wave electrode containing the T-structure metal electrode and the phase-shifting arm of the multimode waveguide are optimized using full-wave three-dimensional electromagnetic simulation software based on the electromagnetic field finite element method for analyzing microwave engineering problems. This optimizes both the TE0 and TE1 modes to achieve low microwave and optical losses and low half-wave voltage. By partially etching the silicon substrate, the microwave group refractive index is designed to be between the refractive indices of the TE0 and TE1 modes of the multimode waveguide, reducing the refractive index mismatch between microwaves and optical waves, thus enabling both modes to achieve large electro-optic bandwidth modulation simultaneously. The metal traveling-wave electrode is a gold electrode with two thicknesses: the first is a 200nm thick T-structure metal electrode, and the second is a 1.1μm thick metal traveling-wave ground electrode and metal traveling-wave signal main electrode.
[0014] Unlike existing technologies, this invention offers the following advantages: Utilizing the electro-optic effect of thin-film lithium niobate crystals and the principle that multiple transverse electrical modes of a multimode waveguide can be modulated simultaneously, this invention proposes a polarization-independent electro-optic modulator based on a thin-film lithium niobate multimode waveguide. When the TE0 mode is input from any input straight waveguide, its modulation principle is the same as that of a traditional Mach-Zehnder electro-optic modulator on an X-cut thin-film lithium niobate platform. When the TM0 mode is input, it is converted to the TE1 mode by an input-end mode converter, then split by 3dB using a multimode beamsplitter, modulated at the phase-shifting arm of the multimode waveguide, and output via interference from a multimode combiner. Finally, it is converted back to the TM0 mode by an output-end mode converter. Therefore, this invention can achieve simultaneous modulation of TE-polarized and TM-polarized light, providing a new solution for realizing polarization-independent electro-optic modulators, effectively expanding the application range of electro-optic modulators, and solving the modulation problem of different polarization modes. Attached Figure Description
[0015] Figure 1 This is a schematic diagram of the overall structure of the polarization-independent electro-optic modulator based on a thin-film lithium niobate multimode waveguide in this invention.
[0016] Figure 2This is a graph showing the equivalent refractive index variation of different modes as the waveguide width changes.
[0017] Figure 3 The optical field transmission diagram is shown for TE0 light with a wavelength of 1.55 μm when input to the narrow end of the mode converter in this invention.
[0018] Figure 4 The optical field transmission diagram shows the conversion of TM0 light with a wavelength of 1.55 μm into TE1 mode output from the narrower input of the mode converter in this invention.
[0019] Figure 5 The optical field transmission diagram is shown for TE0 light with a wavelength of 1.55 μm when input to the multimode beam splitter in this invention.
[0020] Figure 6 The spectral output of TE0 light with a wavelength of 1.55 μm as input to the multimode beam splitter in this invention.
[0021] Figure 7 The optical field diagram of TM0 light with a wavelength of 1.55 μm, after being converted to TE1 mode by a mode converter and transmitted through a multimode beam splitter in this invention.
[0022] Figure 8 The spectral output of TE1 light with a wavelength of 1.55 μm as input to the multimode beam splitter in this invention.
[0023] Figure 9 This is a cross-sectional schematic diagram of the modulation region in this invention.
[0024] Figure 10 The optical field distribution under static voltage and the RF mode field at a frequency of 100 GHz are obtained from simulations using COMSOL and HFSS.
[0025] Figure 11 The diagram shows the simulation results of the characteristic impedance of the traveling wave electrode in this invention. The dashed line represents the 50-ohm position.
[0026] Figure 12 The diagram shows the group refractive index results of the modulator microwave in the simulation calculation of this invention. The dashed lines represent the group refractive indices of the TE0 and TE1 modes of the multimode waveguide, respectively.
[0027] Figure 13 The diagram shows the electro-optic bandwidth results of the modulator TE0 mode and TE1 mode calculated by simulation in this invention.
[0028] In the figure: 1. Input straight waveguide, 2. Input mode converter, 3. Input tapered cone of multimode beamsplitter, 4. Output tapered cone of multimode beamsplitter, 5. Multimode beamsplitter, 6. First S-bend waveguide, 7. Input tapered cone of multimode waveguide phase shift arm, 8. Multimode waveguide phase shift arm, 9. Metal traveling wave ground electrode, 10. Metal traveling wave signal electrode, 11. Output tapered cone of multimode waveguide phase shift arm, 12. Second S-bend waveguide, 13. Input tapered cone of multimode combiner, 14. Multimode combiner, 15. Output tapered cone of multimode combiner, 16. Output mode converter, 17. Output straight waveguide, 18. T-structure metal electrode, 19. Orifice, 20. Lithium niobate layer, 21. Buried oxide layer, 22. Silicon substrate layer, 23. Cavity. Detailed Implementation
[0029] The present invention will be further described below with reference to the accompanying drawings and embodiments.
[0030] Please see Figure 1 The diagram shows the overall structure of a polarization-independent electro-optic modulator based on a thin-film lithium niobate multimode waveguide. The platform used in this modulator is an X-cut thin-film lithium niobate. The input straight waveguide 1, output straight waveguide 17, input mode converter 2, output mode converter 16, multimode beam splitter 5, first S-bend waveguide 6 and second S-bend waveguide 12, input tapered cone 7 of the multimode waveguide phase shift arm, output tapered cone 11 of the multimode waveguide phase shift arm, multimode waveguide phase shift arm 8, and multimode combiner 14 can be etched on the thin-film lithium niobate platform by electron beam or photolithography exposure and inductively coupled plasma etching technology. The metal electrodes, including the metal traveling wave ground electrode 9 and the metal traveling wave signal electrode 10, can be obtained by electron beam evaporation. When the TM0 mode is input from a certain port, it is converted to the TE1 mode by the input mode converter 2, and then the TE1 mode is split by 3dB by the multimode beam splitter 5. When the TE0 mode is input from the same port, its mode remains unchanged after passing through the input mode converter 2, and the 3dB beam split is completed by the same multimode beam splitter 5. In the modulation region, the metal traveling wave ground electrode 9 is located outside the two arms of the multimode waveguide phase shift arm 8, and the metal traveling wave signal electrode 10 is located between the two arms of the multimode waveguide phase shift arm 8. The electric field directions of the TE0 mode and TE1 mode are the same as the optical axis of lithium niobate. According to the electro-optic effect of lithium niobate, the refractive index of the lithium niobate crystal can be changed by changing the voltage on the electrode, thereby effectively modulating the phase of the transmitted TE0 mode and TE1 mode optical wave signals simultaneously. The phase-modulated TE0 mode and TE1 mode optical wave signals are output by interference at the multimode combiner 14, where the TE1 mode is converted to the TM0 mode by the output mode converter 16, while the TE0 mode remains unchanged.
[0031] See Figure 9As shown, all waveguide structures obtained by etching in this invention use a high-refractive-index thin film of lithium niobate as the core layer. The lithium niobate thin film layer 20 has a thickness of 400 nm and an etching depth of 200 nm. It uses air as a low-refractive-index upper cladding and a 3 µm thick buried oxide layer 21. The substrate is a silicon substrate layer 22. The selected thin film of lithium niobate is X-cut lithium niobate, and the waveguide direction is the Y-axis direction of lithium niobate.
[0032] The working principle of this invention is as follows: TE0 or TM0 mode light is input through an input straight waveguide (1) with a width of w1 = 0.8µm and transmitted to the input mode converter 2. TE0 mode light remains unchanged and is transmitted with low loss in the input mode converter 2, while TM0 mode light is converted to TE1 mode by the input mode converter 2 and then transmitted to the multimode beam splitter 5. According to... Figure 2The changes in equivalent refractive index of different modes with varying waveguide width reveal that hybridization occurs near a width w0 = 1.25µm between the TM0 and TE1 modes. At this location, the input width of the mode converter is selected as w1 = 0.8µm, and the output width as w2 = 2.5µm. A length of L1 = 300µm is chosen to ensure efficient conversion between the TM0 and TE1 modes. The TE0 or TE1 mode is then split by a 3dB beam splitter 5. The input tapered cone 3 on the multimode beam splitter 5 has a width of w3 = 2.8µm and a length of L2 = 50µm on the other side. The multimode interference structure has a width of w4 = 8µm and a length of L3 = 169.5µm to achieve low insertion loss in the multimode beam splitter 5. The multimode interference structure on the multimode beam splitter has the same output width as w3. Its output tapered cone 4 has an output width w5 = 1.5µm, which needs to be greater than the mode hybridization width w0. The length is L4 = 200µm to achieve adiabatic transmission. The TE0 or TE1 mode light is then transmitted through the first S-bend waveguide 6 with a width of 1.5µm, and then enters the multimode waveguide phase shifter arm 8 through the input tapered cone 7 of the multimode waveguide phase shifter arm, which is 250µm long and has a width that gradually changes from w5 = 1.5µm to w6 = 2.35µm. The metal traveling wave ground electrode 9 is 223µm wide, the metal traveling wave signal electrode 10 is 73µm wide, the length of the metal traveling wave ground and signal electrodes is 0.8cm, and the height of the metal traveling wave electrodes is 1.1µm. The height of the T-structure metal electrode 18 is 200nm, and a silicon substrate with a depth of w9=28µm is etched away through the small hole 19 between the T-structure metal electrodes 18 to achieve refractive index matching. The multimode waveguide 8 located between the metal traveling wave ground and signal electrodes has a width of w6=2.35µm, which is determined by the requirement that the TE0 or TE1 modes have the same half-wave voltage. The spacing between the T-structure metal electrodes 18 on both sides of the multimode waveguide is w8=4.6µm, and the distance from the waveguide to the T-structure metal electrode 18 is w7=1.125µm. The length of the portion of the T-structure metal electrode 18 extending to the metal traveling wave main electrode is h = 19µm, and the width is t = 5µm. The length of the T-structure metal electrode 18 parallel to the straight waveguide is r = 47µm, and the width is s = 3µm. The distance between the two T-structure metal electrodes 18 on the same side is c = 3µm. The length of the small hole 19 between the T-structure metal electrodes 18 is L = 36µm, and the width is w = 13µm. A microwave signal is applied to one end of the metal traveling wave electrode and propagates in the same direction as the light propagation. A 50-ohm load is connected to the other end of the metal traveling wave electrode. After modulation, the TE0 or TE1 mode light is interfered and output through the multimode combiner 14 connected to the second S-bend waveguide 12. The TE0 or TE1 mode is then output in TE0 or TM0 mode through the output mode converter 16 and the output straight waveguide 17.
[0033] Numerical simulation of light waves was performed using the Finite Difference Time Domain (FDTD) method. Figure 3 This shows the electric field amplitude distribution of light transmission in the input mode converter 2 when the light input is in TE0 mode and has a wavelength of 1.55μm. The transmission efficiency is 99.97%. Figure 4 This shows the electric field amplitude distribution of light transmission in the input mode converter 2 when the design is in TM0 mode and has a wavelength of 1.55μm as light input. The mode conversion efficiency is 99.91%. Figure 5 This shows the electric field amplitude distribution during optical transmission in the multimode beam splitter 5 when the light input is in TE0 mode and has a wavelength of 1.55 μm. Figure 6 The display shows the spectral response of the two ports when the TE0 mode light is input to the multimode beam splitter 5. The peak insertion loss is -3.03dB and the output deviation between the two ports is 0.003dB. Figure 7 This shows the electric field amplitude distribution during optical transmission in the multimode beam splitter 5 after the light is converted to TE1 mode by the input mode converter 2 when the light is input in TM0 mode and wavelength 1.55μm. Figure 8 The display shows the spectral response of the two ports when the light from the TE1 mode of this design is input into the multimode beam splitter 5. The peak insertion loss is -3.07dB and the output deviation between the two ports is 0.089dB.
[0034] This invention uses the "Electromagnetic Waves, Frequency Domain (ewfd)" interface of COMSOL software and HFSS electromagnetic simulation software to simulate the photoelectric performance of the modulation region. The optical field distribution under static voltage and the RF mode field distribution at a frequency of 100 GHz calculated by COMSOL and HFSS simulations are as follows: Figure 10 As shown. Based on the electro-optic material properties and structural dimensions of the designed modulator, the electric field intensity inside the electro-optic material is calculated, and the optical loss, group refractive index, and required half-wave voltage of the TE0 and TE1 modes under waveguides and electrodes of different sizes are calculated. Using HFSS software for device structure modeling, the structural parameters of the traveling wave electrode with T-structure and the phase-shifting arm of the multimode waveguide are optimized. Ultimately, low microwave and optical losses and low half-wave voltages are achieved simultaneously for both the TE0 and TE1 modes. By partially etching the silicon substrate, the microwave group refractive index is designed to be between the group refractive indices of the TE0 and TE1 modes in the multimode waveguide, reducing the refractive index mismatch between microwaves and optical waves, allowing both modes to achieve large electro-optic bandwidth modulation simultaneously. Simulation results show that when the modulator length is 0.8 cm, the half-wave voltage of both modes is 2.64 V. The effective refractive index of the TE0 mode is 1.818, and the group refractive index is 2.23; the effective refractive index of the TE1 mode is 1.759, and the group refractive index is 2.28. Figure 12As shown, the refractive index of microwaves lies between the refractive indices of the two optical wave modes. The simulated characteristic impedance matching value is approximately 51 ohms, as... Figure 11 As shown. The 3dB electro-optic bandwidth of both modes is greater than 110GHz, and the electro-optic bandwidth of the TE1 mode is slightly greater than that of the TE0 mode, as shown. Figure 13 As shown, this is because at high frequencies (such as above 80GHz), the refractive index of the TE1 mode is more compatible with that of microwaves.
[0035] Although the above embodiments have been described, those skilled in the art, once they understand the basic inventive concept, can make other changes and modifications to these embodiments. Therefore, the above descriptions are merely embodiments of the present invention and do not limit the scope of patent protection of the present invention. Any equivalent structural or procedural transformations made using the content of the present invention's specification and drawings, or direct or indirect applications in other related technical fields, are similarly included within the scope of patent protection of the present invention.
Claims
1. A polarization-independent electro-optic modulator based on a thin-film lithium niobate multimode waveguide, characterized in that, The structure includes a silicon substrate layer (22), a buried oxide layer (21), and a lithium niobate layer (20) stacked sequentially from bottom to top. A T-structure metal electrode (18), a metal traveling wave signal electrode (10), and a metal traveling wave ground electrode (9) are arranged above the lithium niobate layer (20). A thin-film lithium niobate optical waveguide is formed on the X-cut lithium niobate layer (20) using etching technology. The thin-film lithium niobate optical waveguide includes, in sequence, an input straight waveguide (1), an input mode converter (2), a multimode beam splitter (5), a first S-bend waveguide (6), an input tapered cone (7) of the multimode waveguide phase shift arm, a multimode waveguide phase shift arm (8), an output tapered cone (11) of the multimode waveguide phase shift arm, a second S-bend waveguide (12), a multimode combiner (14), an output mode converter (16), and an output straight waveguide (17). The multimode waveguide phase shift... Metal traveling wave electrodes are arranged around the arm (8). The metal traveling wave electrodes include: metal traveling wave signal electrodes (10) connected to a set of periodically arranged T-structure metal electrodes (18) on each side, and metal traveling wave ground electrodes (9) connected to a set of periodically arranged T-structure metal electrodes (18) on one side. A cavity (23) is formed by etching a portion of the silicon substrate layer (22) through a small hole (19) at the lithium niobate layer (20). Specifically, the metal traveling wave signal electrodes (10) connected to a set of periodically arranged T-structure metal electrodes (18) on each side are located between the two arms (8) of the multimode waveguide phase shift arm, while the metal traveling wave ground electrodes (9) connected to a set of periodically arranged T-structure metal electrodes (18) on one side are located outside the two arms of the multimode waveguide phase shift arm (8). The entire Mach-Zehnder modulation structure is placed symmetrically up and down and left and right. The input mode converter (2) and the output mode converter (16) have the same structure and are used to realize the mutual conversion between the narrow-end TM0 mode and the wide-end TE1 mode, while the TE0 mode remains unchanged. The narrow-end waveguide width W1 of the input mode converter (2) is smaller than the mode hybrid width of the TM0 mode and the TE1 mode, and supports TE0 and TM0 modes. The wide-end waveguide width W2 of the input mode converter (2) is larger than the mode hybrid width of the TM0 mode and the TE1 mode. The length L1 of the input mode converter (2) is determined by the mode evolution theory to achieve complete coupling between the narrow-end TM0 mode and the wide-end TE1 mode. The multimode beam splitter (5) and the multimode beam combiner (14) have the same structure. The multimode beam splitter consists of a 2×2 multimode interference structure and two sets of input and output tapered cones. The width W4 and length L3 of the interference structure of the multimode beam splitter are determined by the multimode interference self-imaging theory. When the TE0 or TE1 mode is input from one of the ports, the TE0 or TE1 mode is split by 3dB at the same time. The width W2 at the end of the input tapered cone (3) and the width W5 at the end of the output tapered cone (4) of the multimode beam splitter are greater than the hybrid width of the TM0 mode and the TE1 mode, so as to avoid the light from undergoing mode conversion after passing through the tapered cone. The output tapered cone (15) of the multimode beam combiner and the input tapered cone (3) of the multimode beam splitter have the same structure and are placed symmetrically. The input tapered cone (13) of the multimode beam combiner and the output tapered cone (4) of the multimode beam splitter have the same structure and are placed symmetrically.
2. The polarization-independent electro-optic modulator based on a thin-film lithium niobate multimode waveguide according to claim 1, characterized in that, The modulator material is selected as X-cut thin film lithium niobate on an insulator, comprising a silicon substrate layer (22), a low refractive index buried oxide layer (21), a high refractive index ridge-type lithium niobate layer (20), and an air cladding layer with a low refractive index.
3. The polarization-independent electro-optic modulator based on a thin-film lithium niobate multimode waveguide according to claim 1, characterized in that, The multimode waveguide phase shifter (8) on both sides of the metal traveling wave signal electrode (10) is a ridge-type multimode waveguide. The two ends of the multimode waveguide phase shifter (8) are connected to the first S-bend waveguide (6) and the second S-bend waveguide (12) respectively through the input tapered cone (7) and the output tapered cone (11) of the multimode waveguide phase shifter, so that the TE0 mode and the TE1 mode are transmitted in adiabatic mode respectively. The input tapered cone (7) and the output tapered cone (11) of the multimode waveguide phase shifter have the same structure and are placed symmetrically. The lithium niobate under the metal electrodes on both sides of the multimode waveguide phase shifter (8) is not etched to reduce metal absorption loss.
4. The polarization-independent electro-optic modulator based on a thin-film lithium niobate multimode waveguide according to any one of claims 1-3, characterized in that, In the modulation region, the width W6 of the multimode waveguide phase shift arm (8) is required to ensure that the half-wave voltages of the two modes are equal when modulating the light of TE0 mode and TE1 mode, and the width is as small as possible in order to achieve simultaneous high-efficiency modulation of light with different polarizations.
5. The polarization-independent electro-optic modulator based on a thin-film lithium niobate multimode waveguide according to claim 1, characterized in that, In the modulation region, the structural parameters of the metal traveling wave electrode with T-structure metal electrode (18) and the multimode waveguide phase shift arm (8) are optimized by using full-wave three-dimensional electromagnetic simulation software based on the electromagnetic field finite element method to analyze microwave engineering problems. This optimizes the TE0 mode and TE1 mode to achieve low microwave and optical wave losses and low half-wave voltage. The silicon substrate is partially etched, and the refractive index of the microwave group is designed to be between the refractive indices of the TE0 mode and the TE1 mode group of the multimode waveguide. This reduces the refractive index mismatch between microwave and optical waves, enabling both modes to achieve large electro-optic bandwidth modulation at the same time.
6. The polarization-independent electro-optic modulator based on a thin-film lithium niobate multimode waveguide according to claim 1, characterized in that, The T-structure metal electrode (18) has a thickness of 200 nm and is made of gold; the metal traveling wave ground electrode (9) and the metal traveling wave signal electrode (10) have a thickness of 1.1 μm and are made of gold.