CN114415400A

CN114415400A - Polarization-independent electro-optic modulator based on thin-film lithium niobate and preparation method thereof

Info

Publication number: CN114415400A
Application number: CN202210109306.5A
Authority: CN
Inventors: 蔡鑫伦; 潘颖
Original assignee: Sun Yat Sen University
Current assignee: Sun Yat Sen University
Priority date: 2022-01-28
Filing date: 2022-01-28
Publication date: 2022-04-29
Anticipated expiration: 2042-01-28
Also published as: CN114415400B

Abstract

The invention relates to the technical field of electro-optical modulators, and provides a polarization-independent electro-optical modulator based on thin-film lithium niobate and a preparation method thereof, wherein the polarization-independent electro-optical modulator comprises a substrate layer, a thin-film lithium niobate flat plate layer is arranged on the top of the substrate layer, and a first end face coupler, a polarization rotation beam splitter, two Mach-Zehnder interferometers, a polarization rotation beam combiner, a second end face coupler and an electrode, wherein the first end face coupler is used as an input waveguide, the second end face coupler is used as an output waveguide, and the electrode is used for inputting radio-frequency signals and bias voltage; the two Mach-Zehnder interferometers are symmetrically arranged on the thin film lithium niobate flat plate layer. The electrode and the Mach-Zehnder interferometer are combined to form the Mach-Zehnder modulator for realizing electro-optic modulation, and polarization-independent modulation can be carried out on any polarization state input on the lithium niobate film.

Description

Polarization-independent electro-optic modulator based on thin-film lithium niobate and preparation method thereof

Technical Field

The invention relates to the technical field of electro-optical modulators, in particular to a polarization-independent electro-optical modulator based on thin-film lithium niobate and a preparation method thereof.

Background

Electro-optic modulators are one of the major applications for thin film lithium niobate materials, and serve as very critical devices in optical communications, data centers, and wireless communication systems and various opto-electronic links. In particular, the lithium niobate thin film modulator has the characteristics of low loss, low driving voltage, ultrahigh electro-optical bandwidth, smaller structural size than the traditional lithium niobate modulator, and the like. Thin film lithium niobate modulators have been rapidly developed. However, for most of the present modulators to be polarization dependent, a large number of antenna elements and associated digital Application Specific Integrated Circuits (ASICs) need to be placed in close proximity to the electro-optic modulator in order to ensure electro-optic modulator performance and reliability, which results in a high probability of co-packaged or integrated laser sources being placed in high temperature environments (>100 ℃) (lasers typically need to operate in the temperature range of 0 ℃ to 70 ℃), which will cause a significant degradation of laser performance and solution reliability. One of the most cost effective approaches to this problem is to replace the existing polarization sensitive modulators with polarization insensitive modulators and to implement polarization independent modulation for arbitrary polarization state input. At the moment, the laser can be placed in a far-end temperature control room, and modulation can be realized only by connecting a far-end laser light source and the polarization-insensitive electro-optic modulator through standard single-mode optical fibers.

The prior polarization-insensitive electro-optical modulator is like a silicon-based micro-ring single-sideband polarization-insensitive modulator, but when a silicon material is adopted as the modulator, the electro-optical modulation is mainly realized through a free carrier effect, and the improvement of key performance parameters such as modulation signal quality, bandwidth, half-wave voltage, insertion loss and the like of the traditional silicon-based electro-optical modulator gradually meets the bottleneck. In addition, the silicon material is matched with the graphene or indium oxide material to assist in realizing polarization insensitive phase modulation, or the polymer material is used for realizing polarization independent modulation, the schemes need to be improved in process complexity, integration difficulty and modulation efficiency, and the problems of limitations of modulation efficiency, electro-optic bandwidth, driving voltage and the like exist.

Disclosure of Invention

The invention provides a polarization-independent electro-optic modulator based on thin-film lithium niobate and a preparation method thereof, aiming at overcoming the defects that the existing polarization-insensitive electro-optic modulator is low in modulation efficiency and has certain limitations.

In order to solve the technical problems, the technical scheme of the invention is as follows:

a polarization-independent electro-optic modulator based on thin-film lithium niobate comprises a substrate layer, wherein a thin-film lithium niobate flat plate layer is arranged on the top of the substrate layer, and a first end face coupler, a polarization rotation beam splitter, two Mach-Zehnder interferometers, a polarization rotation beam combiner, a second end face coupler and an electrode, wherein the first end face coupler, the polarization rotation beam splitter, the two Mach-Zehnder interferometers, the polarization rotation beam combiner and the second end face coupler are used as input waveguides; the two Mach-Zehnder interferometers are symmetrically arranged on the thin film lithium niobate flat plate layer.

In the using process, input light is input into a polarization rotation beam splitter through a first end face coupler, and the polarization rotation beam splitter converts the input light into two light components with the same polarization and transmits the two light components to two Mach-Zehnder interferometers through two waveguides respectively; radio frequency signals are input into the electrodes, single polarization state modulation is carried out on light components in the Mach-Zehnder interferometer, then the light components are combined through the polarization rotation beam combiner, and the light components are transmitted to the second end face coupler through the two waveguides for coupling and outputting, so that stable modulation, namely polarization-independent modulation, can be realized for any polarization state input.

In the technical scheme, the electrode and the Mach-Zehnder interferometer are combined to form the Mach-Zehnder modulator (MZM) for realizing electro-optic modulation, and polarization-independent modulation can be carried out on any polarization state input on the lithium niobate film.

Preferably, the mach-zehnder interferometer comprises a 1 × 2 multimode interferometer as an optical beam splitter and a 2 × 2 multimode interferometer as an optical beam combiner, and the 1 × 2 multimode interferometer and the 2 × 2 multimode interferometer are connected through two optical waveguides; and a first output end of the 2 x 2 multimode interferometer is connected with an input end of the polarization rotation beam combiner, and a second output end of the 2 x 2 multimode interferometer is connected with a grating coupler for monitoring the state of an output optical signal.

As a preferred scheme, the electrodes include a traveling wave electrode for inputting a radio frequency signal to realize modulation, and an electrically tunable electrode for adjusting a bias point of a modulator; the traveling wave electrode adopts one of GSG structure or GSGSGSG and GSSG differential electrode structure, and one optical waveguide in the Mach-Zehnder interferometer is arranged between the G traveling wave electrode and the S traveling wave electrode; the electric tuning electrode comprises a positive electrode and a negative electrode, and the other optical waveguide in the Mach-Zehnder interferometer is arranged between the positive electric tuning electrode and the negative electric tuning electrode.

Preferably, the traveling wave electrode adopts a T-shaped capacitance load type periodic electrode structure.

Preferably, the base layer comprises a substrate and an oxygen-buried layer which are sequentially arranged from bottom to top, the thin-film lithium niobate flat plate layer is arranged above the oxygen-buried layer, a silicon dioxide layer covers the thin-film lithium niobate flat plate layer, and the electrode is arranged above the silicon dioxide layer.

And as a preferred scheme, etching and removing the substrate corresponding to the traveling wave electrode position by adopting an isotropic etching technology.

Preferably, the first end face coupler and the second end face coupler respectively include an upper inverted cone waveguide and a lower inverted cone waveguide.

Preferably, the input end of the first end-face coupler and the output end of the second end-face coupler are coupled to be provided with tapered optical fibers.

Preferably, the polarization rotation beam splitter comprises an inverted cone mode converter, a directional coupler and a 1 × 1 multimode interferometer which are connected in sequence; the polarization rotation beam combiner comprises a 1 × 1 multimode interferometer, a directional coupler and an inverted cone mode converter which are sequentially connected.

Furthermore, the invention also provides a preparation method for preparing the polarization-independent electro-optical modulator based on the thin-film lithium niobate, which is provided by any technical scheme, and the preparation method comprises the following steps:

s1, preparing an upper waveguide of a device waveguide structure by utilizing photoetching and etching technologies, wherein the device waveguide structure comprises a first end face coupler serving as an input waveguide, a polarization rotation beam splitter, two Mach-Zehnder interferometers, a polarization rotation beam combiner, a second end face coupler serving as an output waveguide and a grating coupler;

s2, photoetching and manufacturing lower-layer waveguides of the first end-face coupler and the second end-face coupler on the sample obtained in the S1 step;

s3, spin-coating photoresist on the sample obtained in the step S2, and making a mask for protecting the waveguide structure of the device by utilizing ultraviolet lithography;

s4, etching and preparing other lower-layer waveguides of the waveguide structure of the device on the sample obtained in the step S3;

s5, cleaning the sample obtained in the step S4, and removing residual photoresist and masks;

s6, growing a covering layer on the sample obtained in the S5 step at a high temperature;

s7, spin-coating photoresist on the sample obtained in the step S6, and photoetching the photoresist to prepare a mask of the electrode;

s8, evaporating a layer of adhesion layer and a metal electrode on the sample obtained in the step S7 by using an electron beam, and removing redundant metal by using a metal stripping process;

s9, spin-coating photoresist on the sample obtained in the step S8, manufacturing a mask for protecting the metal electrode and the waveguide layer of the device by utilizing ultraviolet lithography, and forming an etching hole between the metal electrodes;

s10, etching the sample obtained in the step S9 to prepare a metal electrode suspension structure, and removing the substrate below the metal electrode;

and S11, manufacturing a polished end face of the sample obtained in the step S10 through a grinding and polishing process, and forming the complete polarization-independent electro-optic modulator.

Compared with the prior art, the technical scheme of the invention has the beneficial effects that: the invention adopts the thin-film lithium niobate material, converts two orthogonally polarized lights in input light into light components with the same polarization to be output by combining the polarization rotation beam splitter in an end face coupling mode, and electro-optically modulates the two light components by the Mach-Zehnder modulator consisting of the Mach-Zehnder interferometer and the electrodes respectively, so that polarization-independent modulation can be carried out on any polarization state input on the lithium niobate thin film, and the invention has the advantages of high modulation efficiency, large bandwidth, low driving voltage, high integration level, easy packaging, low cost and the like.

Drawings

Fig. 1 is a schematic diagram of the structure of a polarization-independent electro-optic modulator of embodiment 1.

Fig. 2 is a schematic structural diagram of the polarization rotation beam combiner in embodiment 1.

Fig. 3 is a polarization rotation schematic diagram of the polarization rotation beam combiner of embodiment 1.

Fig. 4 is a schematic diagram of the structure of the polarization-independent electro-optic modulator of embodiment 2.

FIG. 5 is a cross-sectional view of a polarization-independent electro-optic modulator of embodiment 2.

Fig. 6 is a schematic diagram of the single polarization state modulation of the polarization-independent electro-optic modulator of embodiment 2.

FIG. 7 is a flowchart of a method of making the polarization independent electro-optic modulator of example 3.

The optical waveguide device comprises a substrate layer 1, a substrate 101, an oxygen-buried layer 102, a thin-film lithium niobate flat plate layer 103, a silicon dioxide layer 104, a first end face coupler 2, a polarization rotating beam splitter 3, an inverted cone mode converter 301, a directional coupler 302, a multimode interferometer 303-1 multiplied by 1, a Mach Zehnder interferometer 4, a multimode interferometer 401-1 multiplied by 2, a multimode interferometer 402-2 multiplied by 2, an optical waveguide 403, a polarization rotating beam combiner 5, a second end face coupler 6, a grating coupler 7, a traveling wave electrode 8 and an electric tuning electrode 9.

Detailed Description

The drawings are for illustrative purposes only and are not to be construed as limiting the patent;

for the purpose of better illustrating the embodiments, certain features of the drawings may be omitted, enlarged or reduced, and do not represent the size of an actual product;

it will be understood by those skilled in the art that certain well-known structures in the drawings and descriptions thereof may be omitted.

The technical solution of the present invention is further described below with reference to the accompanying drawings and examples.

Example 1

This embodiment proposes a polarization-independent electro-optical modulator based on thin-film lithium niobate, and is a schematic structural diagram of the polarization-independent electro-optical modulator based on thin-film lithium niobate of this embodiment, as shown in fig. 1.

The polarization-independent electro-optical modulator based on thin-film lithium niobate provided by the embodiment comprises a substrate layer 1, wherein a thin-film lithium niobate flat plate layer 103 is arranged on the top of the substrate layer 1, and a first end-face coupler 2 serving as an input waveguide, a polarization rotation beam splitter 3, two mach-zehnder interferometers 4, a polarization rotation beam combiner 5, a second end-face coupler 6 serving as an output waveguide, and electrodes for inputting radio-frequency signals and bias voltage are arranged on the thin-film lithium niobate flat plate layer 103; the two mach-zehnder interferometers 4 are symmetrically arranged on the thin film lithium niobate flat plate layer 103.

The first end face coupler 2 and the second end face coupler 6 in this embodiment are used as an input waveguide and an output waveguide to realize the mode spot conversion and the end face coupling, and the end face coupling is provided with a tapered fiber for performing mode spot matching input or output with the tapered fiber.

Further, the first end face coupler 2 and the second end face coupler 6 in this embodiment use lithium niobate ridge waveguides with an upper and lower inverted-cone-shaped structure, and the end face couplers with the double inverted-cone-shaped waveguide structure may have coupling efficiencies of greater than or equal to 90% for both TE polarization and TM polarization.

Further, the polarization rotating beam splitter 3 and the polarization rotating beam combiner 5 in this embodiment have the same structure and opposite directions, and respectively include an inverse tapered mode converter 301, a directional coupler 302, and a 1 × 1 multimode interferometer 303. Fig. 2 and 3 are schematic structural diagrams and schematic diagrams of the polarization rotating beam splitter 3 according to the present embodiment.

The polarization rotation beam splitter 3 in this embodiment is used to input TE0 and TM0 polarized light from one end of the device at the same time, the TE0 component remains unchanged and is output through the output end of the 1 × 1 multimode interferometer 303, while the TM0 component is converted into the TE1 mode by the inverse tapered mode converter 301, and is output to the mach-zehnder interferometer 4 through the output end of the directional coupler 302 after being converted into the TE0 mode by the directional coupler 302.

Optionally, after the TM0 component in this embodiment is converted by the inverse tapered mode converter 301 and the directional coupler 302, the unfiltered TE1 component of the directional coupler 302 due to process errors or sidewall roughness may be removed by the 1 × 1 multimode interferometer 303, so as to improve the output purity of the eigenmode.

In the embodiment, the separation efficiency and the polarization rotation efficiency of the TE mode and the TM mode exceed 99%, and experimental test results show that the polarization crosstalk is lower than-23 dB, the working bandwidth is greater than or equal to 70nm, and the on-chip insertion loss is less than or equal to 0.12 dB.

The two mach-zehnder interferometers 4 in this embodiment are symmetrically arranged up and down on the upper surface of the thin film lithium niobate flat plate layer 103, and the two mach-zehnder interferometers 4 cooperate with electrodes for inputting radio frequency signals and bias voltages to form a mach-zehnder modulator (MZM) for realizing electro-optical modulation, and have the same modulation efficiency.

In a specific implementation process, input light is input into the polarization rotation beam splitter 3 through the first end-face coupler 2, and the polarization rotation beam splitter 3 converts two orthogonal polarization components into the same polarization and transmits the same polarization to the two mach-zehnder interferometers 4 through the two waveguides. Wherein, two orthogonal polarized light components are polarized as a basic transverse electric mode (TE0) and a basic transverse magnetic mode (TM0), the TE0 polarization state is kept unchanged after passing through the polarization rotation beam splitter 3, and the TM0 is converted into TE0 through polarization rotation. Then, radio frequency signals are input through the electrodes, the two paths of orthogonal polarized light components are subjected to single polarization state modulation in the Mach-Zehnder interferometer 4 respectively, then the light components are combined through the polarization rotation beam combiner 5, and the light components are transmitted to the second end face coupler 6 through the two waveguides to output optical signals carrying electric signals.

In this embodiment, a thin-film lithium niobate material is adopted, and the polarization rotation beam splitter 3 is combined in an end-face coupling manner to output two orthogonally polarized lights TE0 and TM0 in input light as TE0 and TE0, respectively, and after the two lights are simultaneously modulated by the mach-zehnder interferometer 4, the modulated orthogonally polarized lights are combined by the polarization rotation beam combiner 5, so that stable modulation, that is, polarization-independent modulation, can be realized for any polarization state input.

Example 2

The embodiment provides a polarization-independent electro-optical modulator based on thin-film lithium niobate, which comprises a substrate layer 1, wherein a thin-film lithium niobate flat plate layer 103 is arranged on the top of the substrate layer 1, and a first end face coupler 2 serving as an input waveguide, a polarization rotation beam splitter 3, two Mach-Zehnder interferometers 4, a polarization rotation beam combiner 5, a second end face coupler 6 serving as an output waveguide, an electrode for inputting radio-frequency signals and bias voltage, and a grating coupler 7 for monitoring the state of output optical signals are arranged on the thin-film lithium niobate flat plate layer 103; the two mach-zehnder interferometers 4 are symmetrically arranged on the thin film lithium niobate flat plate layer 103.

Further, the mach-zehnder interferometer 4 in the present embodiment includes a 1 × 2 multimode interferometer 401 as an optical beam splitter and a 2 × 2 multimode interferometer 402 as an optical beam combiner, and the 1 × 2 multimode interferometer 401 and the 2 × 2 multimode interferometer 402 are connected by two optical waveguides 403; a first output end of the 2 × 2 multimode interferometer 402 is connected to an input end of the polarization rotation beam combiner 5, and a second output end of the 2 × 2 multimode interferometer 402 is connected to a grating coupler 7.

Two optical waveguides 403 connecting the optical splitter and the optical combiner are respectively used for phase modulation and bias point control on polarization; and a grating coupler 7 connected to a second output of the 2 x 2 multimode interferometer 402 is used to monitor the output optical signal condition.

Further, the electrodes in this embodiment include a traveling wave electrode 8 for inputting a radio frequency signal to realize modulation, and an electrically tunable electrode 9 for adjusting a bias point of the modulator; the traveling wave electrode 8 adopts one of a G-S-G (ground-signal-ground) structure or a G-S-G-S-G, G-S-S-G differential electrode structure, and one optical waveguide 403 in the Mach-Zehnder interferometer 4 is arranged between the ground electrode of the G traveling wave electrode 8 and the signal electrode of the S traveling wave electrode 8; the electrically tunable electrode 9 includes a positive electrode and a negative electrode, and another optical waveguide 403 in the mach-zehnder interferometer 4 is disposed between the positive and negative electrically

tunable electrodes

9 and 9.

The traveling wave electrode 8 may have a G-S-G (ground-signal-ground) structure, or may have a differential driving electrode as a metallic microwave signal waveguide, and the metallic microwave signal electrode may have a GSGSG (ground-signal-ground) structure, or may have a differential electrode structure such as SS (signal-signal) or GSSG (ground-signal-ground), or may have a modified structure, for example, a differential electrode structure with a track formed by adding a track portion to an SS differential electrode structure, or may have various structures.

Further, the base layer 1 in this embodiment includes a substrate 101 and a buried oxide layer 102, which are sequentially disposed from bottom to top, the thin-film lithium niobate flat plate layer 103 is disposed above the buried oxide layer 102, the silicon dioxide layer 104 covers the thin-film lithium niobate flat plate layer 103, and the electrode is disposed above the silicon dioxide layer 104.

Further, the traveling wave electrode 8 in this embodiment adopts a T-type capacitance load periodic electrode structure, the substrate 101 corresponding to the traveling wave electrode 8 of the T-type capacitance load periodic electrode structure is etched and removed by an isotropic etching technique, and the silicon dioxide layer 104 between the traveling wave electrodes 8 is removed by the isotropic etching technique to form a suspended structure, which is helpful for reducing microwave loss.

The electric tuning electrode 9 and the traveling wave electrode 8 in the embodiment are made of gold materials.

In a specific implementation process, the present embodiment is further described with a G-S-G differential electrode structure, as shown in fig. 4 to 6, which are a schematic structural diagram and a single polarization state modulation schematic diagram of the thin-film lithium niobate-based polarization-independent electro-optical modulator of the present embodiment.

In the embodiment, a differential driving mode is adopted for modulation, an optical signal is converted into two beams of light components with the same polarization through a polarization rotation beam splitter 3 and then input into a mach-zehnder interferometer 4, a radio frequency signal and an adjusting bias voltage are respectively input to a traveling wave electrode 8 and an electric tuning electrode 9 which are arranged on two sides of an optical waveguide 403 through an external differential driving module, the modulator is ensured to be in an optimal modulation state, and the modulated signal is transmitted to a polarization rotation beam combiner 5 through the optical waveguide 403 to be converged and output. The differential driving module is connected with the traveling wave electrode 8 by using a gold wire, and an S electrode on the differential driving module is connected with an S electrode of the traveling wave electrode 8, and a G electrode on the differential driving module is connected with a G electrode of the traveling wave electrode 8; the electrically-tunable electrode 9 is connected with an external PCB by a gold wire bonding mode to realize voltage control.

In the embodiment, a traveling wave electrode 8, an electric tuning electrode 9 and a mach-zehnder interferometer 4 are combined to form a mach-zehnder modulator (MZM) for realizing electro-optic modulation, polarization-independent modulation can be carried out on any polarization state input on a lithium niobate film, and the invention has the advantages of high modulation efficiency, large bandwidth, low driving voltage, high integration level, easiness in packaging, low cost and the like. The optical signal in the Mach-Zehnder interferometer 4 is subjected to bias point adjustment through the electrically tunable electrode 9, and the tunable Mach-Zehnder interferometer has the characteristics of large tunable range, stable tuning, high tuning reliability and the like.

Example 3

This example proposes a method for manufacturing the thin film lithium niobate-based polarization-independent electro-optic modulator proposed in example 1 or example 2, and is a flowchart of the manufacturing method of this example, as shown in fig. 7.

The method for preparing the polarization-independent electro-optic modulator based on the thin-film lithium niobate provided by the embodiment comprises the following steps of:

s1, preparing an upper waveguide of a device waveguide structure by utilizing photoetching and etching technologies, wherein the device waveguide structure comprises a first end face coupler 2 serving as an input waveguide, a polarization rotation beam splitter 3, two Mach-Zehnder interferometers 4, a polarization rotation beam combiner 5, a second end face coupler 6 serving as an output waveguide and a grating coupler 7;

s2, photoetching and manufacturing lower-layer waveguides of the first end-face coupler 2 and the second end-face coupler 6 on the sample obtained in the S1 step;

s6, growing a layer of silicon dioxide as a covering layer on the sample obtained in the S5 step at a high temperature;

s10, etching the sample obtained in the step S9 to prepare a metal electrode suspension structure, and removing the substrate 101 below the metal electrode;

The polarization-independent electro-optic modulator prepared by the preparation method provided by the embodiment does not need extra off-chip optoelectronic devices, has the characteristic of simple preparation process, only needs the traditional photoetching and etching processes, and does not need complex processes such as bonding, transfer printing and the like. Compared with the polarization-independent modulator with more traditional discrete components, the thin-film lithium niobate polarization-independent electro-optic modulator has the characteristics of high integration level, simple manufacturing process, low cost, superior device performance and contribution to coupling packaging and mass production.

The same or similar reference numerals correspond to the same or similar parts;

the terms describing positional relationships in the drawings are for illustrative purposes only and are not to be construed as limiting the patent;

it should be understood that the above-described embodiments of the present invention are merely examples for clearly illustrating the present invention, and are not intended to limit the embodiments of the present invention. Other variations and modifications will be apparent to persons skilled in the art in light of the above description. And are neither required nor exhaustive of all embodiments. Any modification, equivalent replacement, and improvement made within the spirit and principle of the present invention should be included in the protection scope of the claims of the present invention.

Claims

1. The polarization-independent electro-optic modulator based on the thin-film lithium niobate is characterized by comprising a substrate layer (1), wherein a thin-film lithium niobate flat plate layer (103) is arranged on the top of the substrate layer (1), and a first end face coupler (2) serving as an input waveguide, a polarization rotation beam splitter (3), two Mach-Zehnder interferometers (4), a polarization rotation beam combiner (5), a second end face coupler (6) serving as an output waveguide and an electrode used for inputting radio-frequency signals and bias voltage are arranged on the thin-film lithium niobate flat plate layer (103); the two Mach-Zehnder interferometers (4) are symmetrically arranged on the thin film lithium niobate flat plate layer (103);

the optical fiber coupling device comprises a first end face coupler (2), a polarization rotation beam splitter (3), a second end face coupler (2), a third end face coupler and a fourth end face coupler, wherein input light is input into the polarization rotation beam splitter (3), the polarization rotation beam splitter (3) converts the input light into two light components with the same polarization and transmits the two light components to two Mach-Zehnder interferometers (4) through two waveguides; and radio frequency signals are input into the electrodes, the light components in the Mach-Zehnder interferometer (4) are subjected to single polarization state modulation, then the light components are combined through the polarization rotation beam combiner (5), and the light components are transmitted to the second end face coupler (6) through the two waveguides for coupling and outputting.

2. The thin film lithium niobate-based polarization-independent electro-optic modulator of claim 1, wherein the mach-zehnder interferometer (4) comprises a 1 x 2 multimode interferometer (401) as an optical beam splitter and a 2 x 2 multimode interferometer (402) as an optical beam combiner, and the 1 x 2 multimode interferometer (401) and the 2 x 2 multimode interferometer (402) are connected by two optical waveguides (403); the first output end of the 2 x 2 multimode interferometer (402) is connected with the input end of the polarization rotation beam combiner (5), and the second output end of the 2 x 2 multimode interferometer (402) is connected with a grating coupler (7).

3. The thin film lithium niobate-based polarization-independent electro-optic modulator of claim 2, wherein the electrodes comprise a traveling wave electrode (8) for input of a radio frequency signal to effect modulation, and an electrically tunable electrode (9) for adjusting a bias point of the modulator; the traveling wave electrode (8) adopts one of GSG structure or GSGSGSG and GSSG differential electrode structure, and one optical waveguide (403) in the Mach-Zehnder interferometer (4) is arranged between the G traveling wave electrode (8) and the S traveling wave electrode (8); the electrically tunable electrode (9) comprises a positive electrode and a negative electrode, and the other optical waveguide (403) in the Mach-Zehnder interferometer (4) is arranged between the positive electrically tunable electrode (9) and the negative electrically tunable electrode (9).

4. The thin film lithium niobate-based polarization-independent electro-optic modulator of claim 3, wherein the traveling wave electrode (8) employs a T-type capacitance loaded periodic electrode structure.

5. The thin-film lithium niobate-based polarization-independent electro-optic modulator according to claim 4, wherein the base layer (1) comprises a substrate (101) and an oxygen-buried layer (102) sequentially arranged from bottom to top, the thin-film lithium niobate flat plate layer (103) is arranged above the oxygen-buried layer (102), the thin-film lithium niobate flat plate layer (103) is covered with a silicon dioxide layer (104), and the electrode is arranged above the silicon dioxide layer (104).

6. The polarization-independent electro-optic modulator based on thin-film lithium niobate according to claim 5, wherein the substrate (101) corresponding to the traveling wave electrode (8) is removed by etching by an isotropic etching technique.

7. The thin film lithium niobate-based polarization-independent electro-optic modulator of any one of claims 1 to 6, wherein the first end-face coupler (2) and the second end-face coupler (6) comprise an upper and a lower layer of inverted tapered waveguides, respectively.

8. The thin film lithium niobate-based polarization-independent electro-optic modulator of any one of claims 1 to 6, wherein a tapered fiber is coupled to an input end of the first end-face coupler (2) and an output end of the second end-face coupler (6).

9. The thin film lithium niobate-based polarization-independent electro-optic modulator of any one of claims 1 to 6, wherein the polarization rotating beam splitter (3) comprises an inverted cone mode converter (301), a directional coupler (302) and a 1 x 1 multimode interferometer (303) connected in sequence; the polarization rotation beam combiner (5) comprises a 1 × 1 multimode interferometer, a directional coupler and an inverted cone mode converter which are sequentially connected.

10. A method of making a thin film lithium niobate based polarization independent electro-optic modulator of any of claims 1 to 9, comprising the steps of:

s1, preparing an upper waveguide of a device waveguide structure by utilizing photoetching and etching technologies, wherein the device waveguide structure comprises a first end face coupler (2) serving as an input waveguide, a polarization rotation beam splitter (3), two Mach-Zehnder interferometers (4), a polarization rotation beam combiner (5), a second end face coupler (6) serving as an output waveguide and a grating coupler (7);

s2, photoetching and manufacturing lower-layer waveguides of the first end-face coupler (2) and the second end-face coupler (6) on the sample obtained in the S1 step;

s10, etching the sample obtained in the step S9 to prepare a metal electrode suspension structure, and removing the substrate (101) below the metal electrode;