A polarization independent electro-optic modulator based on thin film lithium niobate dense dual waveguide

Abstract

This invention discloses a polarization-independent electro-optic modulator based on a dense dual-waveguide lithium niobate film, comprising a silicon substrate, a buried oxide layer, a lithium niobate layer, and metal resistors and electrodes located on the lithium niobate layer. The lithium niobate layer includes input and output straight waveguides, input and output mode converters, a 1*3 beam splitter, a 1*2 beam splitter, a dual-waveguide phase-shifting arm, a 2*1 beam combiner, and a 3*1 beam combiner. When the TE0 mode is input from the input straight waveguide, it remains unchanged after passing through the input mode converter, and is split into two equal beams by the 1*3 and 1*2 beam splitters, placed in two channels of the inner waveguide. When the TM0 mode is input, it is converted to a first-order transverse electric mode (TE1 mode) by the input mode converter, split into two TE0 modes by the 1*3 beam splitter, placed in two channels of the outer waveguide, and finally converted to the TM0 mode output, while the TE0 mode remains unchanged at output. This invention provides a new scheme for realizing polarization-independent electro-optic modulators, effectively expanding the application range of electro-optic modulators.

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 dense dual waveguide of thin-film lithium niobate. 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] The Mach-Zehnder modulator is the most mature and widely used electro-optic modulator. It is an electro-optic modulator based on the Mach-Zehnder interference principle. It consists of two electro-optic phase modulators, two Y-branch waveguides, and corresponding driving electrodes. The two phase modulators achieve phase modulation of the light using the electro-optic effect of the crystal, the two Y-branch waveguides perform the function of splitting and combining the light, and the driving electrodes provide the driving voltage required to achieve the electro-optic effect.

[0004] 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.

[0005] 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

[0006] To overcome the shortcomings of existing technologies, the purpose of this invention is to provide a polarization-independent electro-optic modulator based on a dense dual-waveguide structure of thin-film lithium niobate. Through analysis from the perspective of optical electromagnetics theory, the modulator structure is designed and optimized, enabling the effective utilization and modulation of light waves in either TM or TE polarization states. This solves the polarization control problem and expands the application range of the electro-optic modulator.

[0007] To achieve the above objectives, the present invention adopts the following structural design:

[0008] A polarization-independent electro-optic modulator based on a densely packed dual-waveguide thin-film lithium niobate comprises, from bottom to top, a silicon substrate, a buried oxide layer, a lithium niobate layer, and metal resistors and electrodes on the lithium niobate layer. A thin-film lithium niobate optical waveguide is formed by etching the lithium niobate layer in an X-cut manner. The thin-film lithium niobate optical waveguide includes, in sequence, an input straight waveguide, an input mode converter, a 1*3 beamsplitter, a 1*2 beamsplitter, an S-bend waveguide, a dual-waveguide phase shift arm, an S-bend waveguide, a 2*1 beam combiner, a 3*1 beam combiner, an output mode converter, and an output straight waveguide. Heating resistors and traveling-wave metal electrodes are disposed on both sides of the dual-waveguide phase shift arm. The traveling-wave metal electrodes include: a traveling-wave metal signal electrode connected to a set of periodically arranged T-structure metal electrodes on each side, and a traveling-wave metal ground electrode connected to a set of periodically arranged T-structure metal electrodes on one side. A cavity is formed by etching a portion of the silicon substrate through a small hole in the lithium niobate layer. 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-shifting arms of the dual 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-shifting arms of the dual waveguide. The input straight waveguide, input mode converter, 1*3 beam splitter, and 1*2 beam splitter are structurally symmetrically placed with the output straight waveguide, output mode converter, 3*1 beam combiner, and 2*1 beam combiner.

[0009] In a further specific 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.

[0010] In a further specific embodiment, the 1*3 beamsplitter, 1*2 beamsplitter, 3*1 beam combiner, and 2*1 beam combiner have the same structure, but are placed in different positions. The 1*3 beamsplitter includes an input waveguide, a multimode interference structure, and three symmetrical output waveguides. When the TE0 mode is input, it can achieve self-imaging and output with low loss from the middle waveguide. When the TE1 mode is input, it is split into two equal beams and output from the TE0 mode of the two side waveguides. The middle waveguide of the 1*3 beamsplitter is connected to the input end of the 1*2 beamsplitter. The TE0 mode output from the middle waveguide of the 1*3 beamsplitter is split by 3dB by the 1*2 beamsplitter and finally output through two symmetrical tapered cones.

[0011] In a further specific embodiment, the modulator material is selected as an X-cut thin film lithium niobate on an insulator, comprising a high-refractive-index ridge core layer, a low-refractive-index buried oxide layer, and an upper cladding layer. The upper cladding layer can be selected from materials with a low refractive index, such as air.

[0012] In a further specific embodiment, the metal traveling wave ground-signal-ground electrode material containing the T-structure metal electrode is gold. The heating resistor is made of titanium and gold, with titanium having a thickness of 175 nm and gold a thickness of 5 nm. The heating resistor is 500 µm long and 4 µm wide.

[0013] In a further specific embodiment, the dual waveguide phase shift arms on both sides of the metal signal electrode are ridge waveguides. The two ends of the dual waveguide phase shift arms are connected by S-bend waveguides. The width of the S-bend waveguides gradually changes to accommodate the connection between the output waveguide of the beam splitter or the input waveguide of the beam combiner and the dual waveguide phase shift arms. Furthermore, the spacing between two adjacent S-waveguides on the inner and outer sides gradually decreases towards the connection point of the dual waveguide phase shift arms, enabling adiabatic transmission of the S-waveguide TEO mode to the corresponding waveguide. The lithium niobate beneath the electrodes on both sides of the dual waveguide phase shift arms is not etched, reducing metal absorption loss.

[0014] In a further specific embodiment, in the modulation region, two densely arranged straight waveguides, both transmitting the TEO mode, exist on both sides of the metal signal electrode, with the width W of the outer straight waveguide being... 10 Width W of the inner straight waveguide 11 They are different because their equivalent refractive indices are different, and the distance W between the two waveguides is different. 14 A smaller setting can be used to prevent coupling. At the halfway point of the dual-waveguide phase shift arm length, two tapered sections are used to interchange the waveguide width, ensuring that the average group velocity of the TEO mode light is equal in both the inner and outer waveguide channels. Additionally, the distance W between the outer straight waveguide and the T-structure metal electrode... 12 and the distance W between the inner straight waveguide and the TT structure metal electrode 13 Calculation optimization is required to achieve equal half-wave voltage modulation in the inner and outer waveguide channels.

[0015] In a further specific embodiment, in the modulation region, the parameters of the traveling wave electrode containing the T-structure metal electrode and the dual waveguide phase shifter structure 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 the dense dual waveguide phase shifter to achieve lower microwave and optical losses and lower half-wave voltage. By partially etching the silicon substrate, refractive index matching between microwaves and optical waves is achieved, improving the electro-optic bandwidth of the modulator. The traveling wave electrode is a gold electrode with two thicknesses: a 200nm thick T-structure metal electrode and a 1.1μm thick metal traveling wave ground and main signal electrode.

[0016] Unlike existing technologies, this invention has the following advantages: Utilizing the electro-optic effect of lithium niobate crystals and the principle that light from two densely packed waveguides can be modulated simultaneously, this invention proposes a polarization-independent electro-optic modulator based on thin-film lithium niobate dense dual waveguides. When the transverse electric fundamental mode (TE0 mode) is input from the input straight waveguide, it remains unchanged after passing through the input-end mode converter. It is then split into two equal beams by a 1*3 beam splitter and a 1*2 beam splitter, placing them in two channels of the inner waveguide. When the transverse magnetic fundamental mode (TM0 mode) is input, it is converted to a first-order transverse electric mode (TE1 mode) by the input-end mode converter. This is then split into two TE0 modes by a 1*3 beam splitter, placing them in two channels of the outer waveguide. This simultaneous modulation of the inner and outer waveguide channels is achieved. The output signal is then interfered through a 3*1 beam combiner and a 2*1 beam combiner. Finally, the TE1 mode is converted back to the TM0 mode by the output-end mode converter, while the TE0 mode remains unchanged during output.

[0017] This invention provides 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 light with different polarization modes. Attached Figure Description

[0018] Figure 1 This is a schematic diagram of the overall structure of a polarization-independent electro-optic modulator based on a dense dual waveguide of thin-film lithium niobate according to the present invention.

[0019] Figure 2 This is a graph showing the equivalent refractive index variation of different modes as the waveguide width changes.

[0020] Figure 3 This is a diagram of the optical field transmission when the narrow-end wavelength of the mode converter in this invention is 1.55μm as the input of the TE0 mode.

[0021] Figure 4 This is a diagram of the optical field transmission when the narrow-end wavelength of the mode converter in this invention is 1.55μm as the input TM0 mode.

[0022] Figure 5 The optical field diagram of the TE0 mode with a wavelength of 1.55 μm transmitted in the 1*3 beam splitter and 1*2 beam splitter of this invention.

[0023] Figure 6 The spectrum of the TE0 mode with a wavelength of 1.55 μm after passing through a 1*3 beam splitter and a 1*2 beam splitter in this invention is shown.

[0024] Figure 7 The optical field diagram of the 1.55μm wavelength TM0 mode after being converted to TE1 mode by the mode converter and transmitted through the 1*3 beam splitter in this invention.

[0025] Figure 8The spectrum of the TE1 mode with a wavelength of 1.55 μm is output from the 1*3 beam splitter in this invention.

[0026] Figure 9 This is a cross-sectional view of the modulation region in this invention.

[0027] Figure 10 The image shows the optical field distribution under static voltage calculated using COMSOL simulation, with V values ​​labeled for each mode. π *L value and group refractive index value.

[0028] Figure 11 The diagram shows the simulation results of the characteristic impedance of the traveling wave electrode in this invention.

[0029] Figure 12 The diagram shows the simulation results of the modulator microwave group refractive index in this invention. The dashed lines represent the group refractive indices of the two TEO mode optical waves in the double waveguide.

[0030] Figure 13 This is a diagram showing the simulation results of the modulator electro-optic bandwidth in this invention.

[0031] In the figure: 1. Input straight waveguide, 2. Input mode converter, 3. 1*3 beam splitter, 4. 1*2 beam splitter, 5. First S-bend waveguide, 6. Second S-bend waveguide, 7. Heating resistor, 8. Third S-bend waveguide, 9. Fourth S-bend waveguide, 10. Inner straight waveguide, 11. Outer straight waveguide, 12. Dual waveguide phase shift arm, 13. Metal traveling wave signal electrode, 14. Metal traveling wave ground electrode, 15. T-structure metal electrode, 16. Orifice, 17. Fifth S-bend waveguide, 18. Sixth S-bend waveguide, 19. 2*1 beam combiner, 20. 3*1 beam combiner, 21. Output mode converter, 22. Output straight waveguide, 23. Lithium niobate layer, 24. Buried oxide layer, 25. Silicon substrate layer, 26. Cavity. Detailed Implementation

[0032] The present invention will be further described below with reference to the accompanying drawings and embodiments. Example

[0033] Please see Figure 1The diagram shows the overall structure of a polarization-independent electro-optic modulator based on a dense dual-waveguide system of thin-film lithium niobate. The platform used in this modulator is thin-film lithium niobate. The input straight waveguide 1, input mode converter 2, 1*3 beam splitter (3), 1*2 beam splitter 4, S-bend waveguides (third S-bend waveguide 8, fourth S-bend waveguide 9, fifth S-bend waveguide 17, sixth S-bend waveguide 18), dual-waveguide phase shift arm 12, 3*1 beam combiner 20, 2*1 beam combiner 19, output mode converter 21, and output straight waveguide 22 can be directly etched onto the thin-film lithium niobate platform using inductively coupled plasma etching technology. The heating resistor 7 and the metal electrodes can be obtained by thermal evaporation. The metal electrodes are located on both sides of the waveguide arms. When a voltage is applied, according to the electro-optic effect of lithium niobate, the refractive index of the lithium niobate crystal can be changed by altering the voltage on the electrodes, thereby effectively modulating the transmitted optical signal.

[0034] 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 layer 23 has a thickness of 400 nm and an etching depth of 200 nm. It employs a low-refractive-index upper cladding of air and a 3 µm thick buried oxide layer 24. The substrate is a silicon substrate layer 25. The cross-sectional structure is as follows: Figure 9 As shown. The selected thin-film lithium niobate is an X-cut lithium niobate, with the waveguide direction aligned with the Y-axis of the lithium niobate.

[0035] Light in TE0 or TM0 mode enters through a straight input waveguide 1 with a width of w1 = 0.8µm and is 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 before being transmitted to the 1*3 beam splitter 3. According to... Figure 2The changes in equivalent refractive index of different modes with varying waveguide width indicate that hybridization occurs around a width w0 = 1.25µm. At this position, the input width of the mode converter 2 is selected as w1 = 0.8µm, and the output width is w2 = 2.5µm. The length L1 = 300µm is selected to satisfy the high-efficiency conversion between the TM0 and TE1 modes. The TE0 and TE1 modes are input to a 1*3 beam splitter 3 through a waveguide with a width of w3=2.5µm and a length of L2=50µm. At this time, the TE1 mode is split into two TE0 modes after being transmitted through the 1*3 beam splitter 3, and is output from the two outer ports respectively. The TE0 mode is output from the middle port of the 1*3 beam splitter 3 with low loss. The width of the multimode interference structure of the 1*3 beam splitter 3 is w4=8µm and the length is L3=88.5µm. The width of the two outer output ports is w5=1µm and the width of the middle output port is w6=1.8µm. The low-loss TE0 mode output is from the middle port of the 1*3 beam splitter 3. The TE0 mode output from the middle port of the 1*3 beam splitter 3 is transmitted through a waveguide with a width of w6=1.8µm to the 1*2 beam splitter 4 and then split by 3dB. The width of the multimode interference structure of the 1*2 beam splitter 4 is w7=5µm and the length is L4=17.8µm. The width of the two output ports of the 1*2 beam splitter 4 is w8=1.8µm, followed by a tapered cone with a length of 50µm and an output width of w9=1.2µm. The TE0 mode output from the two outer ports of the 1*3 beam splitter 3 and the two TE0 modes output from the 1*2 beam splitter 4 are transmitted through the first S-bend waveguide 5 and the second S-bend waveguide 6 to the space between the two heating resistors 7. Current is applied to the metal resistors, causing the temperature of the surrounding waveguides to rise, thereby adjusting the static operating points of the two channels respectively. Then, the light wave is transmitted through the third S-bend waveguide 8 and the fourth S-bend waveguide 9 to the inner straight waveguide 10 and the outer straight waveguide 11 between the two metal electrodes in the modulation region. There are two densely arranged waveguides between the metal traveling wave ground electrode 14 and the metal traveling wave signal electrode 13. The outer straight waveguide 11, closer to the metal traveling wave ground electrode, has a width of w. 10 =0.6µm, and the distance between it and the periodically arranged T-structure metal electrodes 15 connected to the metal traveling wave grounding electrode is w. 12 =1.5µm, while the other inner straight waveguide has a width of 10w. 11 =1.2µm, and the distance between it and the periodically arranged T-structure metal electrodes 15 connected to the metal traveling wave grounding electrode is w. 13 =2.8µm, the distance between the two waveguides is w 14=0.7µm. To ensure that the half-wave voltages of the TE0 mode in the two waveguides are equal during modulation, the widths of the two waveguides are interchanged at the midpoint of the two straight waveguides using two tapered sections of length L5=6µm. The width of the metal traveling wave ground electrode 14 is 223µm, the width of the metal traveling wave signal electrode 13 is 73µm, the length of the metal traveling wave electrode is 1cm, and the height of the metal traveling wave main electrode is 1.1µm. The height of the T-structure metal electrode 15 is 200nm, and the etching depth w is removed from the small hole 16 between the T-structure metal electrodes 15. 15 A silicon substrate with a refractive index of 29µm is used to achieve refractive index matching. The T-structure metal electrodes connected to the traveling wave ground and signal electrodes are spaced 6.8µm apart. The length of the T-structure metal electrode 15 to the main electrode is h=19µm and the width is t=5µm. The length of the T-structure metal electrode 15 parallel to the straight waveguide is r=47µm and the width is s=3µm. The distance between two T-structure metal electrodes 15 on the same side is c=3µm. The length of the small hole 16 between the T-structure metal electrodes is L=36µm and the width is w=13µm. The modulated TE0 mode light in the outer straight waveguide 11 is transmitted through the fifth S-bend waveguide 17, whose width gradually decreases from 1.2µm to 1µm, to the two outer ports of the 3*1 combiner 20 and combined into TE1 mode. Meanwhile, the TE0 mode light in the inner straight waveguide 10 is transmitted through the sixth S-bend waveguide 18, whose width gradually decreases from 0.6µm to 1.2µm, to the two ports of the 2*1 combiner 19 and combined into TE0 mode, before being output through the 3*1 combiner 20. The structures of the 3*1 combiner 20 and 2*1 combiner 19 are identical to those of the 1*3 beamsplitter 3 and 1*2 beamsplitter 4. The TE0 mode output from the 3*1 combiner 20 is output with low loss via the output mode converter 21 and the output straight waveguide 22, while the TE1 mode output from the 3*1 combiner 20 is converted to TM0 mode via the output mode converter 21 and then output via the output straight waveguide 22.

[0036] Numerical simulation of light waves was performed using the Finite Difference Time Domain (FDTD) method. Figure 3 The electric field amplitude distribution of the optical transmission in the input mode converter (2) is shown when the optical input is in TE0 mode and has a wavelength of 1.55 μm. The transmission efficiency is 99.97%. Figure 4 The electric field amplitude distribution of the optical transmission in the input mode converter (2) is shown when the optical input is in TM0 mode and has a wavelength of 1.55 μm. The transmission efficiency is 99.91%. Figure 5This shows the electric field amplitude distribution of the design transmitted through the 1*3 beam splitter (3) and the 1*2 beam splitter (4) when the optical input is in TE0 mode and has a wavelength of 1.55 μm. Figure 6 The spectral response of the TE0 mode with a wavelength of 1.55 μm is shown after it is transmitted from the 1*3 beam splitter (3) to the 1*2 beam splitter (4) and output from both ports, with a peak insertion loss of -3.221 dB. Figure 7 This shows the electric field amplitude distribution of the light transmission after it is converted from the input mode converter (2) to the TE1 mode and then transmitted to the 1*3 beam splitter (3) when the light input is in TM0 mode and has a wavelength of 1.55μm. Figure 8 The spectral response of the TE1 mode light input to the 1*3 beam splitter (3) is shown from the two outer ports, with a peak insertion loss of -3.179dB.

[0037] This invention utilizes the "Electromagnetic Waves, Frequency Domain (ewfd)" interface of COMSOL software for simulation. Materials can be added from the COMSOL material library or created from relevant literature to meet specific needs. Based on the electro-optic material properties and dimensions of the designed modulator, the required electric field strength within the electro-optic material is calculated, thereby determining the relevant positions of the electrodes and waveguides. The half-wave voltage required for modulation at different positions for waveguides of different sizes is then calculated. Device structure modeling is performed using HFSS software to optimize the electrode structure parameters and the overall device structure model, improving the matching degree between characteristic impedance and group refractive index, and expanding the electro-optic bandwidth of the modulator. Figure 10 As shown, simulation results indicate that when the modulator length is 1 cm, the average half-wave voltage values ​​for both polarization states are 3.72 V. The mode spot is located at a width of w. 10 The effective refractive index of the TE0 mode in a narrow waveguide with a width of 0.6µm is 1.704, the group refractive index is 2.23, and the mode spot is located at a width of w. 11 The effective refractive index of the TE0 mode in a wide waveguide with a diameter of 1.2µm is 1.78, and the group refractive index is 2.25. Figure 11 The simulation results indicate that the matched value of the characteristic impedance is approximately 53 ohms. Figure 12 The simulation results demonstrate the matching between the modulator's microwave refractive index and the group refractive index of the light wave. Figure 13 This indicates that the 3dB electro-optic bandwidth of the two polarization states of the modulator is equal, both greater than 110GHz.

[0038] 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 dense dual-waveguide structure of thin-film lithium niobate, characterized in that, The structure includes a silicon substrate layer (25), a buried oxide layer (24), and a lithium niobate layer (23) stacked sequentially from bottom to top. A heating resistor (7), a T-structure metal electrode (15), a metal traveling wave signal electrode (13), and a metal traveling wave ground electrode (14) are arranged above the lithium niobate layer (23). 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 an input straight waveguide (1), an input mode converter (2), and a 1*3 splitter connected sequentially. The waveguide includes a beam splitter (3), a 1*2 beam splitter (4), a first S-bend waveguide (5), a second S-bend waveguide (6), a third S-bend waveguide (8), a fourth S-bend waveguide (9), a dual-waveguide phase shifter (12), a fifth S-bend waveguide (17), a sixth S-bend waveguide (18), a 2*1 beam combiner (19), a 3*1 beam combiner (20), an output mode converter (21), and an output straight waveguide (22). A metal traveling-wave electrode is provided at the dual-waveguide phase shifter (12). The traveling wave electrode includes: a metal traveling wave signal electrode (13) with a set of periodically arranged T-structure metal electrodes (15) connected to each side; a metal traveling wave ground electrode (14) with a set of periodically arranged T-structure metal electrodes (15) connected to one side; and a cavity (26) is formed by etching a portion of the silicon substrate layer (25) through a small hole (16) at the lithium niobate layer (23). The metal traveling wave signal electrode (13) with a set of periodically arranged T-structure metal electrodes (15) connected to each side is located in the double... Between the two arms of the waveguide phase shift arm (12), two metal traveling wave grounding electrodes (14) connected on one side to a set of periodically arranged T-structure metal electrodes (15) are located outside the two arms of the double waveguide phase shift arm (12); the input straight waveguide (1), input mode converter (2), 1*3 beam splitter (3) and 1*2 beam splitter (4) are structurally symmetrically placed with the output straight waveguide (22), output mode converter (21), 3*1 beam combiner (20) and 2*1 beam combiner (19).

2. The polarization-independent electro-optic modulator based on a dense double waveguide of thin-film lithium niobate according to claim 1, characterized in that, The input mode converter (2) and the output mode converter (21) have the same structure and are used to realize the 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 less than the mode hybrid width of the TM0 mode and the TE1 mode, and supports TE0 and TM0 modes; the wider waveguide width W2 of the input mode converter (2) is greater 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 narrower TM0 mode and the wider TE1 mode.

3. The polarization-independent electro-optic modulator based on a dense double waveguide of thin-film lithium niobate according to claim 1, characterized in that, The 1*3 beam splitter (3) and 1*2 beam splitter (4) have the same structure as the 3*1 beam combiner (20) and 2*1 beam combiner (19), but are placed in different positions. The 1*3 beam splitter (3) includes an input waveguide, a multimode interference structure and three symmetrical output waveguides. When the TE0 mode is input, it can achieve self-imaging and output from the middle waveguide with low loss. When the TE1 mode is input, it is divided into two equal beams of TE0 mode light and output from the two side waveguides. The middle waveguide output by the 1*3 beam splitter (3) is connected to the input end of the 1*2 beam splitter (4). The TE0 mode output from the middle waveguide of the 1*3 beam splitter (3) is split by 3dB by the 1*2 beam splitter (4) and finally output through two symmetrical tapered cones.

4. The polarization-independent electro-optic modulator based on a dense double waveguide of thin-film lithium niobate according to claim 1, characterized in that, The modulator material is selected as X-cut thin film lithium niobate on an insulator, including a high refractive index ridge lithium niobate layer (23), a low refractive index buried oxide layer (24), and an air cladding with a low refractive index.

5. The polarization-independent electro-optic modulator based on a dense double waveguide of thin-film lithium niobate according to claim 1, characterized in that, The dual waveguide phase shift arms (12) on both sides of the metal traveling wave signal electrode (13) are ridge waveguides. The two ends of the dual waveguide phase shift arms (12) are connected by S-bend waveguides. The widths of the third S-bend waveguide (8), the fourth S-bend waveguide (9), the fifth S-bend waveguide (17), and the sixth S-bend waveguide (18) gradually change to accommodate the connection between the output waveguide of the beam splitter or the input waveguide of the beam combiner and the dual waveguide phase shift arms (12). The spacing between two adjacent S-bend waveguides on the inner and outer sides gradually decreases towards the connection of the dual waveguide phase shift arms (12), so that the S-bend waveguide TEO mode can be thermally transmitted to the corresponding waveguide. The lithium niobate under the electrodes on both sides of the dual waveguide phase shift arms (12) is not etched, which reduces metal absorption loss.

6. The polarization-independent electro-optic modulator based on a dense double waveguide of thin-film lithium niobate according to any one of claims 1-5, characterized in that, In the modulation region, there are two densely placed straight waveguides on both sides of the metal traveling wave signal electrode (13), both transmitting the TE0 mode. The width W of the outer straight waveguide (11) is... 10 Width W of the inner straight waveguide (10) 11 They are different because their equivalent refractive indices are different, and the distance W between the two waveguides is different. 14 The setup is small enough to prevent coupling. At half the length of the dual waveguide phase shift arm (12), the waveguide width is interchanged through two tapered sections to ensure that the average group velocity of the TEO mode light in the inner and outer waveguide channels is equal. In addition, the distance W between the outer straight waveguide (11) and the T-structure metal electrode (15) is... 12 and the distance W between the inner straight waveguide (10) and the T-structure metal electrode (15). 13 After optimization, equal half-wave voltage modulation is achieved in both the inner and outer waveguide channels.

7. The polarization-independent electro-optic modulator based on a dense dual-waveguide lithium niobate film according to claim 1, characterized in that, In the modulation region, the structural parameters of the metal traveling wave electrode containing the T-structure metal electrode (15) and the double waveguide phase shift arm (12) 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 enables the dense double waveguide phase shift arm (12) to achieve lower microwave and optical wave losses and lower half-wave voltage. By partially etching the silicon substrate, the refractive index matching of microwave and optical waves is achieved, thereby improving the electro-optic bandwidth of the modulator. The metal traveling wave electrode material is gold and includes two thicknesses. The first is the T-structure metal electrode (15) with a thickness of 200 nm, and the second is the metal traveling wave ground electrode (14) and the metal traveling wave signal electrode (13) with a thickness of 1.1 μm.

8. The polarization-independent electro-optic modulator based on a dense double waveguide of thin-film lithium niobate according to claim 1, characterized in that, Heating resistors (7) are set near the inner and outer channel waveguides respectively to adjust the static operating point of different channels so that the static operating points of TE and TM mode input are consistent.

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