Lithium niobate film type FP (Fabry-Perot) cavity as well as preparation method and application thereof

Abstract

The invention discloses a lithium niobate film type FP cavity and a preparation method and application thereof.The lithium niobate film type FP cavity comprises a substrate layer, a lower wrapping layer and a waveguide layer which are sequentially stacked from bottom to top, and the waveguide layer comprises a lithium niobate film flat plate layer and a waveguide structure arranged on the surface of the film flat plate layer in a protruding mode; the waveguide structure comprises a first waveguide bragg grating and a second waveguide bragg grating which are arranged along the X direction, and an intermediate waveguide region formed between the first waveguide bragg grating structure and the second waveguide bragg grating, and one side of the first waveguide bragg grating is a light energy input port; one side of the second waveguide Bragg grating is a light energy output port, and electrodes are symmetrically arranged on the two sides of the middle waveguide area in the Z direction. According to the invention, the lithium niobate film material is introduced into the FP cavity, and the waveguide Bragg grating structure is manufactured for reflection and transmission, which is very beneficial to integration of richer functions on the same chip.

Description

Technical Field

[0001] The invention belongs to the field of photonic device design, and specifically relates to a lithium niobate thin film type FP cavity and a preparation method and application thereof. Background Art

[0002] With the development of optical chip integration technology, the hybrid integration of multiple materials is considered to be the future research direction of photonic chips. It can give full play to the advantages of different materials in the field of photonics, make up for the shortcomings of a single material, and realize richer functions on the same chip.

[0003] Lithium niobate material has excellent physical properties, including high electro-optic coefficient, piezoelectric coefficient, thermo-optic coefficient, acousto-optic coefficient and photoelastic coefficient. Therefore, it is widely used in various integrated photonic devices and is regarded as one of the most promising microwave photonic materials. However, the current research field of lithium niobate electro-optical Fabry-Perot (FP) cavity focuses on the study of bulk materials. However, this FP cavity has the disadvantages of low refractive index difference (0.001-0.04 for titanium diffusion process and ~0.02 for proton exchange process), large mode field (mode field diameter ~10um), half-wave voltage-length product, large volume, etc., which makes it difficult to integrate and limits the realization of richer functions on the same chip. The refractive index difference of lithium niobate thin film waveguide can be greater than 1 (lithium niobate refractive index @1550nm, no=2.211, ne=2.138, air refractive index n=1), small mode field (mode field diameter is less than 1um), high integration, low tuning voltage and low loss, etc., and has become a new research hotspot in the field of microwave photonics. Summary of the invention

[0004] In order to solve the above-mentioned defects in the prior art, the present invention proposes a lithium niobate thin film type FP cavity and its preparation method and application, introduces lithium niobate thin film material into the FP cavity, and prepares a waveguide Bragg grating structure for reflection and transmission, which is very beneficial for integrating more abundant functions on the same chip.

[0005] To achieve the above-mentioned object, the present invention provides a lithium niobate thin film type FP cavity in a first aspect, comprising a substrate layer, a lower cladding layer and a waveguide layer stacked in sequence from bottom to top, wherein the waveguide layer comprises a lithium niobate thin film slab layer and a waveguide structure protruding from the surface of the thin film slab layer, the waveguide structure comprises a first waveguide Bragg grating and a second waveguide Bragg grating arranged along the X direction and an intermediate waveguide region formed between the first waveguide Bragg grating structure and the second waveguide Bragg grating, one side of the first waveguide Bragg grating is a light energy input port, one side of the second waveguide Bragg grating is a light energy output port, and electrodes are symmetrically arranged on both sides of the intermediate waveguide region along the Z direction; the X direction and the Z direction are perpendicular to each other;

[0006] The wavelength satisfying the Bragg equation is reflected multiple times by the first waveguide Bragg grating and the second waveguide Bragg grating in the intermediate waveguide region to oscillate and coherently, and finally the coherent light energy is output through the light energy output port to form the transmission spectrum of the FP cavity; an electrical signal is applied to the electrode and the electro-optical effect of the intermediate waveguide region is used to realize the frequency shift of the transmission spectrum.

[0007] Furthermore, the FP cavity further includes an upper cladding layer arranged on the surface of the waveguide layer.

[0008] Furthermore, the waveguide structure is a ridge-type structure made of lithium niobate material, or a loaded strip-type structure made of thin film materials such as silicon nitride and silicon dioxide.

[0009] Furthermore, the first waveguide Bragg grating and the second waveguide Bragg grating include the determination of the grating period, the duty cycle and the number of grating periods, the duty cycle is fixedly selected as a 1:1 structure; the number of grating periods is determined by the reflectivity at both ends of the mature FP cavity, and the reflectivity of the FP cavity is usually not less than 80%; the grating period is determined by the grating equation, and the grating equation is as follows:

[0010]

[0011] Wherein, the central wavelength λ is 1550 nm, Λ is the grating period, n is the effective refractive index of the mode without grating structure (straight waveguide part), and m is the order of the Bragg grating.

[0012] Furthermore, the material of the lower cladding layer is silicon dioxide, with a thickness of 1.5-3.5 μm.

[0013] Furthermore, the material of the upper cladding layer is silicon dioxide, and the thickness is 0.5-4 μm.

[0014] Another aspect of the present invention provides a method for preparing a lithium niobate thin film FP cavity, comprising the following steps:

[0015] 1) forming a protruding overall outer contour of a waveguide structure on the surface of a lithium niobate thin film layer of a lithium niobate chip, wherein the chip comprises a substrate, a lower cladding layer, and a lithium niobate thin film layer stacked from bottom to top;

[0016] 2) forming the waveguide structure body into a structure including a first waveguide Bragg grating and a second waveguide Bragg grating at both ends, and an intermediate waveguide region between the first waveguide Bragg grating structure and the second waveguide Bragg grating;

[0017] 3) Electrodes are fabricated on both sides of the intermediate waveguide.

[0018] Furthermore, in step 1), the forming is selected from deposition, photolithography, development and etching processes.

[0019] Furthermore, in step 2), the forming is selected from processes such as photolithography, development and etching.

[0020] Furthermore, in step 3), the manufacturing is selected from processes such as photolithography, development, evaporation, electroplating or magnetron sputtering.

[0021] Furthermore, the waveguide layer is made of lithium niobate material, the width of the middle waveguide region is selected as a single-mode structure, the width is 800-1500nm, and the thickness is 100-600nm; the thickness of the first waveguide Bragg grating and the second waveguide Bragg grating are both 100-600nm.

[0022] Furthermore, the method further comprises step 4), forming an upper cladding layer on the upper surface of the waveguide layer for protecting the waveguide structure.

[0023] Another aspect of the present invention provides the use of the lithium niobate thin film FP cavity or the lithium niobate thin film FP cavity prepared by the above method, which is used in a photonic chip.

[0024] Compared with the prior art, the present invention has the following advantages:

[0025] The lithium niobate thin film type FP cavity of the present invention comprises a base layer, a lower cladding layer and a waveguide layer stacked in sequence from bottom to top, wherein the waveguide layer comprises a lithium niobate thin film flat layer and a waveguide structure protruding from the surface of the thin film flat layer, wherein the waveguide structure comprises an intermediate waveguide region formed along the first waveguide Bragg grating, the second waveguide Bragg grating, the first waveguide Bragg grating structure and the second waveguide Bragg grating, and electrodes symmetrically arranged on both sides of the intermediate waveguide region, wherein the first waveguide Bragg grating is a light energy input port, and the second waveguide Bragg grating is a light energy output port; wherein the wavelength satisfying the Bragg equation is reflected multiple times by the first waveguide Bragg grating and the second waveguide Bragg grating in the intermediate waveguide region to oscillate and coherent, and finally the coherent light energy is output through the light energy output port to form the transmission spectrum of the FP cavity; an electrical signal is applied to the electrode and the frequency shift of the transmission spectrum is realized by utilizing the electro-optical effect of the intermediate waveguide region. The lithium niobate thin film type FP cavity of the present invention has the advantages of low half-wave voltage and small volume.

[0026] In addition, the lithium niobate film-type FP cavity of the present invention can reflect a specific wavelength range, and the reflectivity can be adjusted. At the same time, the FP cavity is based on the lithium niobate film and can be integrated with other chip structures, and can achieve large-scale, multifunctional integration such as hybrid integration of the lithium niobate film platform or with the silicon photonic platform.

[0027] The lithium niobate thin film FP cavity structure of the present invention has wide applicability and can meet the design requirements of different hybrid integrated optical materials by targeted design of some parameters.

[0028] Other features and advantages of the present invention will be described in detail through the following detailed description. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings required for use in the description of the embodiments will be briefly introduced below. Obviously, the accompanying drawings described below are some embodiments of the present invention. For ordinary technicians in this field, other accompanying drawings can be obtained based on these accompanying drawings without paying any creative work.

[0030] Figure 1 It is a schematic diagram of the three-dimensional structure of a lithium niobate thin film type FP cavity applied to a photonic chip of the present invention in one embodiment;

[0031] Figure 2 yes Figure 1 Top view along the Y direction;

[0032] Figure 3 yes Figure 1 Side view along the Z direction;

[0033] Figure 4 It is a schematic diagram of the three-dimensional structure of the lithium niobate thin film type FP cavity applied to a photonic chip in another embodiment of the present invention;

[0034] Figure 5 yes Figure 4 Top view along the Y direction;

[0035] Figure 6 yes Figure 4 Side view along the Z direction;

[0036] Figure 7 It is a schematic diagram of the three-dimensional structure of the lithium niobate thin film type FP cavity applied to a photonic chip in another embodiment of the present invention;

[0037] Figure 8 yes Figure 4 Top view along the Y direction;

[0038] Fig. 9 yes Figure 4 Side view along the Z direction;

[0039] Fig.10 It is a preparation flow chart of the lithium niobate thin film FP cavity of the present invention in one embodiment.

[0040] Fig.11 It is a preparation flow chart of the lithium niobate thin film FP cavity in another embodiment of the present invention.

[0041] Fig.12It is a schematic diagram of the simulation results of the reflectivity of the Bragg gratings of the reflection structure at both ends of the FP cavity prepared in Examples 1-3 of the present invention.

[0042] Explanation of the reference numerals: substrate layer 1 , lower cladding layer 2 , waveguide layer 3 , first waveguide Bragg grating 4 , second waveguide Bragg grating 5 , intermediate waveguide region 6 , electrode 7 , thin film slab layer 8 . DETAILED DESCRIPTION

[0043] The specific embodiments of the present invention are described in detail below. It should be understood that the specific embodiments described herein are only used to illustrate and explain the present invention, and are not used to limit the present invention.

[0044] In the description of the present invention, it should be noted that the terms "center", "up", "down", "left", "right", "vertical", "horizontal", "inside", "outside" and the like indicate positions or positional relationships based on the positions or positional relationships shown in the accompanying drawings, and are only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore cannot be understood as limiting the present invention. In addition, the terms "first" and "second" are used for descriptive purposes only and cannot be understood as indicating or implying relative importance. Among them, the terms "first position" and "second position" are two different positions.

[0045] Unless otherwise clearly specified and limited, the terms "installed", "connected", "connected", and "fixed" should be understood in a broad sense, for example, it can be a fixed connection or a detachable connection; it can be a mechanical connection or an electrical connection; it can be a direct connection or an indirect connection through an intermediate medium, it can be the internal connection of two elements or the interaction relationship between two elements. For ordinary technicians in this field, the specific meanings of the above terms in the present invention can be understood according to specific circumstances.

[0046] A first aspect of the present invention provides a lithium niobate thin film type FP cavity, comprising: a substrate layer, a lower cladding layer and a waveguide layer stacked in sequence from bottom to top, wherein the waveguide layer comprises a lithium niobate thin film slab layer and a waveguide structure protruding from the surface of the thin film slab layer, the waveguide structure comprises a first waveguide Bragg grating and a second waveguide Bragg grating arranged along the X direction and an intermediate waveguide region formed between the first waveguide Bragg grating structure and the second waveguide Bragg grating, one side of the first waveguide Bragg grating is a light energy input port, one side of the second waveguide Bragg grating is a light energy output port, and electrodes are symmetrically arranged on both sides of the intermediate waveguide region along the Z direction;

[0047] The wavelength satisfying the Bragg equation is reflected multiple times by the first waveguide Bragg grating and the second waveguide Bragg grating in the intermediate waveguide region to oscillate and coherently, and finally the coherent light energy is output through the light energy output port to form the transmission spectrum of the FP cavity; an electrical signal is applied to the electrode and the electro-optical effect of the intermediate waveguide region is used to realize the frequency shift of the transmission spectrum.

[0048] In the present invention, the waveguide layer is used to realize the transmission and reflection of light. When the port on one side of the first waveguide Bragg grating is used as the input of light energy, after passing through the first waveguide Bragg grating, part of the light energy is transmitted to the second waveguide Bragg grating through the intermediate waveguide area for the first time, and is partially reflected, and the reflected light is transmitted to the first waveguide Bragg grating through the intermediate waveguide area, and part of the light energy is reflected for the second time and transmitted to the second waveguide Bragg grating through the intermediate waveguide area, and the light reflected by the second waveguide Bragg grating for the first and second time meets the coherence condition and is coherent. By analogy, the light that is reflected multiple times and meets the coherence condition oscillates and is coherent, and the final transmission spectrum is output from the port on one side of the second waveguide Bragg grating.

[0049] In some embodiments, the FP cavity further includes an upper cladding layer disposed on the surface of the waveguide layer, for protecting the waveguide structure and reducing the side wall scattering loss increased by etching the lithium niobate waveguide.

[0050] In some embodiments, the waveguide structure is a ridge waveguide made of a lithium niobate thin film material, or a loaded strip waveguide made of a thin film material such as silicon nitride or silicon dioxide.

[0051] In some embodiments, the first waveguide Bragg grating structure and the second waveguide Bragg grating structure include the determination of the grating period, the duty cycle and the number of grating periods. Preferably, the duty cycle is fixed to select a 1:1 structure; the number of grating periods is determined by the reflectivity of the two ends of the mature FP cavity, which is usually around 80%; the grating period is determined by the grating equation, which is as follows:

[0052]

[0053] Wherein, the central wavelength λ is 1550 nm, Λ is the grating period, n is the effective refractive index of the mode without grating structure (straight waveguide part), and m is the order of the Bragg grating.

[0054] In some embodiments, the first waveguide Bragg grating and the second waveguide Bragg grating have the same structure.

[0055] In some embodiments, the lower cladding layer is made of silicon dioxide with a thickness of 1.5-3.5 μm.

[0056] In some embodiments, the upper cladding layer is made of silicon dioxide and has a thickness of 0.5-4 μm.

[0057] In some embodiments, the substrate layer is a silicon substrate.

[0058] Another aspect of the present invention provides a method for preparing a lithium niobate thin film type FP cavity, wherein the lithium niobate thin film type FP cavity is applied to a hybrid integrated photonic chip and connected with other structures in the chip for use, specifically comprising the following steps:

[0059] 1) forming a protruding overall outer contour of a waveguide structure on the surface of a lithium niobate thin film layer of a lithium niobate chip, wherein the chip comprises a substrate, a lower cladding layer, and a lithium niobate thin film layer stacked from bottom to top;

[0060] 2) forming the waveguide structure body into a structure including a first waveguide Bragg grating and a second waveguide Bragg grating at both ends, and an intermediate waveguide region between the first waveguide Bragg grating structure and the second waveguide Bragg grating;

[0061] 3) Electrodes are symmetrically fabricated on both sides of the intermediate waveguide region along the Z direction.

[0062] In the present invention, the waveguide structure is manufactured by etching a lithium niobate thin film layer, or by loading a strip waveguide on the surface of a lithium niobate thin film layer.

[0063] In some embodiments, in step 1), the overall outer contour of the waveguide structure is formed by processes selected from deposition, photolithography, development and etching, and the specific implementation parameters refer to conventional processes in the art.

[0064] In some embodiments, in step 2), the waveguide structure body is formed by processes selected from photolithography, development and etching, and the specific implementation parameters refer to conventional processes in the art.

[0065] In some embodiments, in step 3), the electrode is prepared by processes selected from photolithography, development and evaporation or magnetron sputtering, and the specific implementation parameters refer to conventional processes in the art.

[0066] In some embodiments, the waveguide layer is made of lithium niobate material, the width of the intermediate waveguide region is selected as a single-mode structure, the width is 800-1500nm, and the thickness is 100-600nm; the thickness of the first waveguide Bragg grating structure and the second waveguide Bragg grating are both 100-600nm. The length of the intermediate waveguide region is usually determined according to the actual application requirements, and the length can be in the order of micrometers to millimeters. Exemplarily, the length of the intermediate waveguide region can be determined according to the working band (such as working at 1550, 1310nm, etc.), and the bandwidth, for example, the bandwidth is 20nm, 30nm, 40nm.

[0067] Herein, referring to the drawings, the length is the X direction of the coordinate axis, the width is the Z direction of the coordinate axis, and the thickness is the Y direction of the coordinate axis.

[0068] In some embodiments, the method further comprises step 4), manufacturing an upper cladding layer above the waveguide layer, which is manufactured by referring to conventional processes in the art, for example, by using a deposition process.

[0069] The present invention is further illustrated by the following examples, but the present invention is not limited thereto.

[0070] Example 1

[0071] A lithium niobate thin film FP cavity based on the variation of Bragg grating thickness, referred to as S1. Figure 1-3 As shown, it includes a silicon substrate layer (base layer 1), a silicon dioxide lower cladding layer 2, and a waveguide layer 3 stacked in sequence from bottom to top. The selected waveguide layer material is a lithium niobate film. The waveguide layer 3 includes a thin film slab layer 8, a first waveguide Bragg grating 4 and a second waveguide Bragg grating 5, as well as an intermediate waveguide region 6 and two side electrodes 7. Among them,

[0072] The thickness of the silicon dioxide lower cladding 2 is 2μm; the thickness of the waveguide layer 3 is 600nm, the thickness of the thin film slab layer 8 is 200nm, the thickness of the intermediate waveguide region 6 is 400nm, and the width is 1μm; the first waveguide Bragg grating 4 and the second waveguide Bragg grating 5 have the same structure, the grating width is 1μm, the thickest thickness (convex area) is 400nm, the thinnest thickness (concave area) is 200nm, the grating thickness changes according to the thickest thickness and the thinnest thickness period, the width remains unchanged, the grating period is 426nm, and the number of grating periods is 40. The reflectivity of the grating under this structure is 80%, the 3dB bandwidth is 54.55nm, and the results are as follows Fig.12 shown.

[0073] The fabrication process of the lithium niobate thin film FP cavity is as follows: Fig.10 As shown, the method comprises the following steps: selecting a thin-film lithium niobate chip original piece on an insulator, the chip original piece comprises a base layer 1, a lower cladding layer 2 (silicon dioxide), and a lithium niobate thin film layer stacked from bottom to top, and manufacturing a thin film flat layer 8 of a certain thickness, an intermediate waveguide region 7, and a first waveguide Bragg grating 5 and a second waveguide Bragg grating 6 with periodic thickness on the lithium niobate thin film layer of the chip original piece by photolithography, development, and etching processes. Finally, electrodes 8 are manufactured on both sides of the intermediate waveguide region by photolithography, development, evaporation, and electroplating processes.

[0074] Example 2

[0075] A lithium niobate thin film FP cavity based on the variation of Bragg grating width, referred to as S2; Figure 4-6As shown, it includes a silicon substrate layer 1, a silicon dioxide lower cladding layer 2, and a waveguide layer 3 stacked in sequence from bottom to top. The selected waveguide layer material is a lithium niobate film. The waveguide layer 3 includes a thin film slab layer 8, a first waveguide Bragg grating 4 and a second waveguide Bragg grating 5, as well as an intermediate waveguide region 6 and two side electrodes 7. Among them,

[0076] The thickness of the silicon dioxide lower cladding layer 2 is 2 μm; the thickness of the waveguide layer 3 is 600 nm, and the thickness of the thin film slab layer 8 is 300 nm; the thickness of the intermediate waveguide region 6 is 300 nm and the width is 1 μm; the first waveguide Bragg grating 4 and the second waveguide Bragg grating 5 have the same structure, the grating thickness is 300 nm, the minimum width is 0.5 μm, the maximum width is 1.5 μm, the grating width varies according to the maximum width and the minimum width period, the thickness remains unchanged, the grating period is 414 nm, and the number of grating periods is 35. The reflectivity of the grating under this structure is 80%, the 3dB bandwidth is 55.56 nm, and the results are as follows Fig.12 shown.

[0077] The fabrication process of the lithium niobate thin film FP cavity is as follows: Fig.10 As shown, the method comprises the following steps: selecting a thin-film lithium niobate chip original piece on an insulator, the chip original piece comprises a base layer 1, a lower cladding layer 2 (silicon dioxide), and a lithium niobate thin film layer stacked from bottom to top, and manufacturing a thin film flat layer 8 of a certain thickness, an intermediate waveguide region 7, and a first waveguide Bragg grating 5 and a second waveguide Bragg grating 6 with periodically varying widths on the lithium niobate thin film layer of the chip original piece by means of photolithography, development, and etching processes. Finally, electrodes 8 are manufactured on both sides of the intermediate waveguide region by means of photolithography, development, evaporation, and electroplating processes.

[0078] Example 3

[0079] A loaded strip waveguide lithium niobate film type FP cavity based on the variation of Bragg grating width, referred to as S3; Figure 7-9 As shown, it includes a silicon substrate layer 1, a silicon dioxide lower cladding layer 2, and a waveguide layer 3 stacked in sequence from bottom to top. The waveguide layer 3 includes a thin film slab layer 8 and a loaded strip waveguide. The loaded strip waveguide layer includes a first waveguide Bragg grating 4, a second waveguide Bragg grating 5, and an intermediate waveguide region 6. The waveguide layer material selected for the thin film slab layer 8 is a lithium niobate film, and the material selected for the loaded strip waveguide layer is a silicon nitride film. Among them,

[0080] The thickness of the silicon dioxide lower cladding 2 is 2 μm; the thickness of the thin film slab layer 8 is 300 nm, and the thickness of the loaded strip waveguide layer is 300 nm; the thickness of the intermediate waveguide region 6 is 300 nm and the width is 1 μm; the first waveguide Bragg grating 4 and the second waveguide Bragg grating structure 5 are the same, with a thickness of 300 nm, a minimum width of 0.5 μm, a maximum width of 1.5 μm, and a grating width that changes according to the maximum width and minimum width period, while the thickness remains unchanged, the grating period is 426 nm, and the number of grating periods is 30. The reflectivity of the grating under this structure is 80%, and the 3dB bandwidth is 55.56 nm. The results are as follows Fig.12 shown.

[0081] The fabrication process of the lithium niobate thin film FP cavity is as follows: Fig.11 As shown, the following steps are included: selecting an original thin-film lithium niobate chip on an insulator, and manufacturing a silicon nitride waveguide on the lithium niobate layer of the original thin-film lithium niobate chip on an insulator by deposition, photolithography, and etching processes, a first waveguide Bragg grating 4 and a second waveguide Bragg grating 5, an intermediate waveguide region 6 between the first waveguide Bragg grating 4 and the second waveguide Bragg grating 5, and finally manufacturing electrodes 8 on both sides of the intermediate waveguide region by photolithography, development, evaporation, and electroplating processes.

[0082] Obviously, the above embodiments of the present invention are merely examples for clearly explaining the present invention, and are not intended to limit the implementation methods of the present invention. For those skilled in the art, other different forms of changes or modifications can be made based on the above description. It is impossible to list all implementation methods here. All obvious changes or modifications derived from the technical solution of the present invention are within the spirit scope of the present invention.

Claims

1. A lithium niobate thin film FP cavity, characterized in that: The invention comprises a base layer, a lower cladding layer and a waveguide layer which are stacked in sequence from bottom to top, wherein the waveguide layer comprises a lithium niobate thin film slab layer and a waveguide structure protruding from the surface of the thin film slab layer, the waveguide structure comprises a first waveguide Bragg grating and a second waveguide Bragg grating arranged along the X direction and an intermediate waveguide region formed between the first waveguide Bragg grating structure and the second waveguide Bragg grating, one side of the first waveguide Bragg grating is a light energy input port, one side of the second waveguide Bragg grating is a light energy output port, and electrodes are symmetrically arranged on both sides of the intermediate waveguide region along the Z direction; the X direction and the Z direction are perpendicular to each other; The wavelength satisfying the Bragg equation is reflected multiple times by the first waveguide Bragg grating and the second waveguide Bragg grating in the intermediate waveguide region to oscillate and coherently, and finally the coherent light energy is output through the light energy output port to form the transmission spectrum of the FP cavity; an electrical signal is applied to the electrode and the electro-optical effect of the intermediate waveguide region is used to realize the frequency shift of the transmission spectrum.

2. The lithium niobate thin film FP cavity according to claim 1, characterized in that: The FP cavity further includes an upper cladding layer disposed on the surface of the waveguide layer.

3. The lithium niobate thin film FP cavity according to claim 1 or 2, characterized in that: The waveguide structure is a ridge waveguide made of lithium niobate material, or a loaded strip waveguide made of silicon nitride film material or silicon dioxide film material.

4. The lithium niobate thin film FP cavity according to any one of claims 1 to 3, characterized in that: The first waveguide Bragg grating and the second waveguide Bragg grating include the determination of the grating period, the duty cycle and the number of grating periods, the duty cycle is fixed to select a 1:1 structure; the number of grating periods is determined by the reflectivity at both ends of the mature FP cavity, which is usually greater than 80%; the grating period is determined by the grating equation, and the grating equation is as follows: Wherein, the central wavelength λ is 1550nm, Λ is the grating period, n is the effective refractive index of the mode without grating structure, and m is the order of the Bragg grating.

5. The lithium niobate thin film FP cavity according to any one of claims 1 to 4, characterized in that: The material of the lower cladding layer is silicon dioxide, and the thickness is 1.5-3.5 μm; and / or, The material of the upper cladding layer is silicon dioxide, and the thickness is 1-4 μm.

6. A method for preparing a lithium niobate thin film FP cavity according to any one of claims 1 to 5, characterized in that: The steps include: 1) forming a protruding overall outer contour of a waveguide structure on the surface of a lithium niobate thin film layer of a lithium niobate chip, wherein the chip comprises a substrate, a lower cladding layer, and a lithium niobate thin film layer stacked from bottom to top; 2) forming the waveguide structure body into a structure including a first waveguide Bragg grating and a second waveguide Bragg grating at both ends, and an intermediate waveguide region between the first waveguide Bragg grating structure and the second waveguide Bragg grating; 3) Electrodes are symmetrically fabricated on both sides of the intermediate waveguide region along the Z direction.

7. The method for preparing a lithium niobate thin film FP cavity according to claim 6, characterized in that: In step 1), the forming is selected from deposition, photolithography, development and etching processes; In step 2), the forming is selected from photolithography, development and etching processes; In step 3), the manufacturing is selected from photolithography, development and evaporation or magnetron sputtering process.

8. The method for preparing a lithium niobate thin film FP cavity according to claim 6 or 7, characterized in that: The waveguide layer is made of lithium niobate material, the width of the middle waveguide area is selected as a single-mode structure, the width is 800-1500nm, and the thickness is 100-600nm; the thickness of the first waveguide Bragg grating structure and the second waveguide Bragg grating are both 100-600nm.

9. The method for preparing a lithium niobate thin film FP cavity according to any one of claims 6 to 8, characterized in that: The method further comprises step 4), manufacturing an upper cladding layer on the upper surface of the waveguide layer for protecting the waveguide structure.

10. Use of the lithium niobate thin film FP cavity according to any one of claims 1 to 5 or the lithium niobate thin film FP cavity prepared by the method according to any one of claims 6 to 9, characterized in that: The lithium niobate thin film FP cavity is used in a photonic chip.

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