A phase-modulated laser Doppler velocimeter based on thin-film lithium niobate and graphene and a method of using the same

Abstract

The present application relates to a laser Doppler velocimeter based on thin-film lithium niobate and graphene phase modulation and a method for using the same. The velocimeter mainly comprises a semiconductor laser, an optical fiber coupler, an integrated thin-film lithium niobate, a graphene phase modulator, a spatial light coupling lens group, a photoelectric detector, and a signal solution and control module, which are sequentially connected according to the propagation direction of the optical path. The semiconductor laser comprises an infrared semiconductor laser and a visible light indicating semiconductor laser. The infrared semiconductor laser and the visible light indicating semiconductor laser are respectively connected to the first input end and the second input end of the optical fiber coupler through a polarization-maintaining optical fiber. The first output end of the optical fiber coupler is connected to the integrated thin-film lithium niobate and graphene phase modulator, and the second output end is idle. The light outputted from the first output end and the second output end of the integrated thin-film lithium niobate and graphene phase modulator is projected onto the object to be measured through the spatial light coupling lens group, and the return light enters the photoelectric detector through the spatial light coupling lens group.

Description

Technical Field

[0001] The present application relates to the field of optical technology, and in particular to a phase-modulated laser Doppler velocimeter based on thin-film lithium niobate and graphene and a method for using the same. Background Art

[0002] Laser Doppler frequency shift means that when the object to be measured has a moving speed, the scattered light excited by the laser incident on the surface of the object will produce a frequency shift proportional to the speed of the object. The instrument that calculates the speed of the object based on detecting the size of the frequency shift is a laser Doppler velocimeter. Since the frequency of the laser itself is relatively high and cannot be measured directly, the optical heterodyne method is usually used to combine two beams with a slight frequency difference and detect the periodic change frequency of the intensity of the interference light to obtain the frequency difference of the two beams. This double-beam double-scattering optical path structure is simple, and the detected light intensity is relatively large. In order to obtain a higher detection sensitivity and realize the simultaneous detection of positive and negative signals, it is often necessary to shift the frequency of one or both of the lights. When the object moves, the output will produce a positive or negative difference above the natural frequency, thereby solving the object's moving speed.

[0003] Traditional laser Doppler velocimeters mostly use acousto-optic frequency shifters, but due to the limitation of crystal material size, the frequency shift generated is usually more than tens of MHz. In order to obtain a frequency shift of several MHz or even lower, two acousto-optic frequency shifters are usually required to make a difference. At the same time, in order to improve the diffraction efficiency, a series of complex technical means such as increasing the bandwidth of the piezoelectric transducer are required, resulting in a large device size and complex structure, which makes it difficult to package, integrate and mass produce. It is difficult to further reduce the production cost, which limits the application market and development prospects of laser Doppler velocimeters.

[0004] The phase modulation method can also be used to achieve frequency shifting of dual laser beams, and the phase modulation method that provides appropriate zero-position bias for the velocimeter has the outstanding advantage of simple structure and function compared with the traditional acousto-optic frequency shifter. Among the existing optical modulators, the lithium niobate electro-optic modulator based on the linear electro-optic effect is widely used. It has high modulation efficiency, low production cost, and small size, which is very suitable for application in laser Doppler velocimeter frequency shifter. In recent years, with the advancement of processing technology, a series of processes such as deep ultraviolet lithography, electron beam lithography, argon ion dry etching, and inductively coupled plasma dry etching have been used to process mature thin-film lithium niobate ridge waveguides, and their transmission loss and light field area are greatly reduced. Compared with traditional lithium niobate body materials, electro-optic phase modulators made of thin-film lithium niobate can have higher modulation efficiency, more compact device size, and lower mass production cost. Therefore, the electro-optic modulator based on thin-film lithium niobate can fully play the role of frequency shifting in the velocimeter and realize the miniaturization and mass production of the entire instrument. However, the application of thin-film lithium niobate modulator in laser Doppler velocimeter is relatively rare at present. Summary of the invention

[0005] The present application aims to provide a thin film lithium niobate, graphene phase modulation laser Doppler velocimeter and its use method. The technical problem to be solved by the present application is achieved through the following technical solutions.

[0006] A thin-film lithium niobate and graphene phase-modulated laser Doppler velocimeter mainly comprises a semiconductor laser, an optical fiber coupler, an integrated thin-film lithium niobate, a graphene phase modulator, a spatial light coupling lens group, a photoelectric detector, and a signal solution and control module, which are sequentially connected according to the propagation direction of the optical path. The semiconductor laser comprises an infrared semiconductor laser and a visible light indicating semiconductor laser. The infrared semiconductor laser is a signal light source, and the visible light indicating semiconductor laser is an indicating light source. The infrared semiconductor laser and the visible light indicating semiconductor laser are respectively connected to the first input end and the second input end of the optical fiber coupler through a polarization-maintaining optical fiber. The first output end of the optical fiber coupler is connected to the integrated thin-film lithium niobate and graphene phase modulator, and the second output end is idle. The light outputted from the first output end and the second output end of the integrated thin-film lithium niobate and graphene phase modulator is projected onto the object to be measured through the spatial light coupling lens group, and the return light enters the photoelectric detector through the spatial light coupling lens group.

[0007] Preferably, end-face coupling or spatial optical coupling is adopted between the light source and the polarization-maintaining optical fiber.

[0008] Preferably, the pigtails connected to the input and output ends of all optical devices of the speed meter are polarization-maintaining optical fibers.

[0009] Preferably, the spatial optical coupling lens group includes a first collimating lens, a second collimating lens, and a receiving lens, wherein the receiving lens is located between the two collimating lenses, and then the photoelectric detector converts the optical signal into an electrical signal.

[0010] Preferably, the first collimating lens, the second collimating lens and the receiving lens are all one of a biconvex lens, a plano-convex lens, a concave-convex lens, an achromatic lens and an aplanatic lens.

[0011] Preferably, the integrated thin-film lithium niobate and graphene phase modulator integrates an input tapered waveguide, a 1×2MM I beam splitter, a large radius curved waveguide, two arms of the phase modulator, a metal electrode, and an output tapered waveguide according to the propagation direction of the optical path, wherein the tapered waveguide matches the input and output end pigtails.

[0012] Preferably, the distance between the two waveguides at the output end of the 1×2 MMI beam splitter is half the width of the multimode interference region, and both the input and output waveguides adopt a tapered waveguide structure.

[0013] Preferably, the tapered waveguides at the input and output ends of the integrated thin-film lithium niobate and graphene phase modulator adopt a three-time tapered structure of the ridge and the substrate.

[0014] Preferably, the integrated thin film lithium niobate and graphene phase modulator is composed of a silicon dioxide upper cladding layer, a thin film lithium niobate ridge waveguide layer, a silicon dioxide lower cladding layer, and a silicon substrate layer from top to bottom, wherein a bottom layer of graphene, a top layer of graphene, an aluminum oxide insulating layer, and a metal electrode are arranged above the silicon dioxide upper cladding layer.

[0015] Preferably, the metal electrode includes a signal electrode and a ground electrode, the aluminum oxide insulating layer separates the top graphene layer from the bottom graphene layer, the signal electrode is connected to the top graphene layer, and the ground electrode is connected to the bottom graphene layer.

[0016] Preferably, the metal electrode spacing is greater than 5μm, the modulation zone length is not less than 5mm, the electrode thickness is not less than 6μm, the electrode width is not less than 20μm, and the modulation signal of the integrated thin film lithium niobate and graphene phase modulator is a sawtooth wave, and the modulation depth is 0.5π.

[0017] Preferably, the input and output pigtails of the integrated thin-film lithium niobate and graphene phase modulator are 3μm optical fibers, the polarization-maintaining optical fibers used to connect to the optical fiber coupler and the spatial optical coupling lens group are 6μm optical fibers, and a 3μm-6μm mode field adapter is provided between the pigtail and the polarization-maintaining optical fiber.

[0018] A method for using a thin-film lithium niobate and graphene phase-modulated laser Doppler velocimeter, wherein the light emitted by an infrared semiconductor laser and a visible light indicating semiconductor laser is input into the first input end and the second input end of an optical fiber coupler through end-face coupling or spatial optical coupling via a polarization-maintaining optical fiber, and then input into an integrated thin-film lithium niobate and graphene phase modulator through the first output end of the optical fiber coupler, and is divided into two arms through a 1×2MM I beam splitter, and a 0.5π modulation depth sawtooth wave modulation signal is loaded and output into a spatial optical coupling lens group, and the signal light is projected onto the object to be measured through two collimating lenses that are respectively at an angle of θ with the object to be measured, and the scattered light signal is input into a photoelectric detector through a receiving lens and converted into an electrical signal, and the object moving speed is calculated by a signal solution and control module, and at the same time, the electrical signal for controlling the working current of the light source and the working voltage of the detector is fed back to the light source and the detector.

[0019] The present invention adopts the above-mentioned thin film lithium niobate, graphene phase modulation laser Doppler velocimeter and its working method, which has the following beneficial effects:

[0020] 1. The phase modulator is used to achieve frequency shift, replacing the traditional acousto-optic frequency shifter to provide zero bias for the system, reducing process complexity and production costs.

[0021] 2. The phase modulator uses an integrated thin-film lithium niobate electro-optic modulator and combines it with the graphene phase modulation method, which further realizes the miniaturization of the velocimeter and facilitates mass production.

[0022] 3. The present invention realizes the application of on-chip integrated thin-film lithium niobate and graphene modulators in laser Doppler velocimeters, which is compatible with traditional CMOS processes and provides a solution for the further integration and miniaturization of velocimeters in the future. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 This is a schematic diagram of the structure of a phase-modulated laser Doppler velocimeter based on thin-film lithium niobate and graphene.

[0024] Figure 2 Schematic diagram of the internal structure and components of the integrated thin-film lithium niobate and graphene phase modulator of the present invention

[0025] Figure 3 Schematic diagram of the transverse cross-section of the two-arm waveguide structure of the modulation area of ​​the phase modulator of the present invention

[0026] Figure 4 Schematic diagram of the special tapered waveguide structure used by the phase modulator of the present invention to reduce the coupling loss with the pigtail DETAILED DESCRIPTION

[0027] The technical solution of the present invention is further described in detail below through the accompanying drawings and embodiments.

[0028] Example

[0029] In order to make the purpose, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, not all of the embodiments. Generally, the components of the embodiments of the present invention described and shown in the drawings here can be arranged and designed in various different configurations.

[0030] Therefore, the following detailed description of the embodiments of the present invention provided in the accompanying drawings is not intended to limit the scope of the invention claimed for protection, but merely represents selected embodiments of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0031] It should be noted that similar reference numerals and letters denote similar items in the following drawings, and therefore, once an item is defined in one drawing, further definition and explanation thereof is not required in subsequent drawings.

[0032] In the description of the present invention, it should be noted that the terms "upper", "lower", "inside", "outside", etc. indicate directions or positional relationships based on the directions or positional relationships shown in the accompanying drawings, or are directions or positional relationships in which the product of the invention is usually placed when in use. They are only for the convenience of describing the present invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific direction, be constructed and operated in a specific direction, and therefore should not be understood as a limitation on the present invention.

[0033] In the description of the present invention, it is also necessary to explain that, unless otherwise clearly specified and limited, the terms "setting", "installation" and "connection" should be understood in a broad sense, for example, it can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection or an electrical connection; it can be a direct connection, or it can be an indirect connection through an intermediate medium, or it can be the internal communication of two components. For ordinary technicians in this field, the specific meanings of the above terms in the present invention can be understood according to specific circumstances.

[0034] The embodiments of the present invention are described in detail below in conjunction with the accompanying drawings.

[0035] Example

[0036] like Figure 1As shown, a thin film lithium niobate and graphene phase modulated laser Doppler velocimeter mainly includes a semiconductor laser, an optical fiber coupler, an integrated thin film lithium niobate, a graphene phase modulator, a spatial light coupling lens group, a photoelectric detector, and a signal solution and control module, which are sequentially connected according to the propagation direction of the optical path, wherein the semiconductor laser includes an infrared semiconductor laser and a visible light indication semiconductor laser, the infrared semiconductor laser is a signal light source, the visible light indication semiconductor laser is an indication light source, the infrared semiconductor laser and the visible light indication semiconductor laser are respectively connected to the first input end and the second input end of the optical fiber coupler through a polarization-maintaining optical fiber, the first output end of the optical fiber coupler is connected to the integrated thin film lithium niobate and graphene phase modulator, and the second output end is idle, and the light outputted from the first output end and the second output end of the integrated thin film lithium niobate and graphene phase modulator is projected onto the object to be measured through the spatial light coupling lens group, and the return light enters the photoelectric detector through the spatial light coupling lens group.

[0037] Furthermore, end-face coupling or spatial optical coupling is adopted between the light source and the polarization-maintaining optical fiber.

[0038] Furthermore, the pigtails connected to the input and output ends of all optical devices of the speed meter are polarization-maintaining optical fibers.

[0039] Furthermore, the spatial optical coupling lens group includes a first collimating lens, a second collimating lens, and a receiving lens, wherein the receiving lens is located between the two collimating lenses, and then the photoelectric detector converts the optical signal into an electrical signal.

[0040] Furthermore, the first collimating lens, the second collimating lens and the receiving lens are all one of a biconvex lens, a plano-convex lens, a concave-convex lens, an achromatic lens and an aplanatic lens.

[0041] like Figure 2 As shown, the integrated thin-film lithium niobate and graphene phase modulator integrates an input tapered waveguide, a 1×2MM I beam splitter, a large radius curved waveguide, two arms of a phase modulator, a metal electrode, and an output tapered waveguide according to the propagation direction of the optical path, wherein the tapered waveguide matches the input and output end pigtails.

[0042] Furthermore, the distance between the two waveguides at the output end of the 1×2 MMI beam splitter is half the width of the multimode interference region, and both the input and output waveguides adopt a tapered waveguide structure.

[0043] Furthermore, the integrated thin-film lithium niobate and graphene phase modulator has a tapered waveguide at the input and output ends that adopts a three-time tapered structure of the ridge and the substrate.

[0044] like Figure 3As shown, the integrated thin-film lithium niobate and graphene phase modulator is composed of a silicon dioxide upper cladding layer, a thin-film lithium niobate ridge waveguide layer, a silicon dioxide lower cladding layer, and a silicon substrate layer from top to bottom, wherein a bottom graphene layer, a top graphene layer, an aluminum oxide insulating layer, and a metal electrode are arranged above the silicon dioxide upper cladding layer. In this embodiment, the refractive index of the silicon material is about 3.5, the refractive index of the silicon dioxide lower cladding material is about 1.4, the refractive index of the thin-film lithium niobate material is about 2.2, and the refractive index of the silicon dioxide upper cladding material is about 1.4. There is enough refractive index difference between the core layer and the cladding layer to ensure that the light field has a sufficiently strong binding effect and minimize the transmission loss.

[0045] Furthermore, the metal electrode includes a signal electrode and a ground electrode, the aluminum oxide insulating layer separates the top graphene layer from the bottom graphene layer, the signal electrode is connected to the top graphene layer, and the ground electrode is connected to the bottom graphene layer.

[0046] Furthermore, the metal electrode spacing is greater than 5μm, the modulation zone length is not less than 5mm, the electrode thickness is not less than 6μm, the electrode width is not less than 20μm, and the modulation signal of the integrated thin film lithium niobate and graphene phase modulator is a sawtooth wave, and the modulation depth is 0.5π.

[0047] In this embodiment, in order to ensure that light has high modulation efficiency and low transmission loss when transmitted in the integrated thin-film lithium niobate and graphene phase modulator, the length of the tapered waveguide at the input end and the two output ends is at least greater than 700μm, the waveguide width at the input and output ends of the 1×2MMI beam splitter is at least greater than 2μm, the width of the multimode interference zone is at least greater than 8μm, and the spacing between the two output waveguides is strictly half of the width of the multimode interference zone. In order to ensure that no optical crosstalk occurs when the output light of the two arms of the modulator passes through the spatial optical coupling lens group, the bending radius of the two curved waveguides is at least 700μm. In order to ensure sufficient modulation depth, the spacing between the metal electrodes in the modulation zone is at least 5μm, the electrode length is at least greater than 5mm, and the electrode width is not less than 30μm, and the electrode thickness is not less than 5μm.

[0048] Furthermore, the input and output pigtails of the integrated thin-film lithium niobate and graphene phase modulator are 3μm optical fibers, the polarization-maintaining optical fibers used to connect to the optical fiber coupler and the spatial optical coupling lens group are 6μm optical fibers, and a 3μm-6μm mode field adapter is provided between the pigtail and the polarization-maintaining optical fiber.

[0049] like Figure 4 As shown, in order to maximize the efficient coupling between the integrated chip and the polarization-maintaining fiber pigtail and reduce the transmission loss in the coupling process, the thin-film lithium niobate ridge waveguides at the input and output ends of the phase modulator port adopt a special tapered structure. First, the waveguide ridge layer is tapered for the first time, and then the substrate layer is tapered twice. The size of each tapered waveguide is strictly designed.

[0050] A method for using a thin-film lithium niobate and graphene phase-modulated laser Doppler velocimeter, wherein the light emitted by an infrared semiconductor laser and a visible light indicating semiconductor laser is input into the first input end and the second input end of an optical fiber coupler through end-face coupling or spatial optical coupling via a polarization-maintaining optical fiber, and then input into an integrated thin-film lithium niobate and graphene phase modulator through the first output end of the optical fiber coupler, and is divided into two arms through a 1×2MM I beam splitter, and a 0.5π modulation depth sawtooth wave modulation signal is loaded and output into a spatial optical coupling lens group, and the signal light is projected onto the object to be measured through two collimating lenses that are respectively at an angle of θ with the object to be measured, and the scattered light signal is input into a photoelectric detector through a receiving lens and converted into an electrical signal, and the object moving speed is calculated by a signal solution and control module, and at the same time, the electrical signal for controlling the working current of the light source and the working voltage of the detector is fed back to the light source and the detector.

[0051] The three lenses in the spatial optical coupling lens group use biconvex lenses, plano-convex lenses, concave-convex lenses, achromatic lenses, aplanatic lenses, etc., and the two collimating lenses have a fixed angle of 2θ. The expression of the relationship between the resulting Doppler frequency shift and the object's motion speed is: The signal calculation and control module can not only demodulate the speed of the object, but also adjust the working current of the infrared semiconductor laser and the working voltage of the photodetector in real time according to the size of the detected DC component and AC component to adapt to the objects to be tested with different surface reflectivities, thereby ensuring the relative constancy of the detected signal intensity.

[0052] It should be noted that the above detailed descriptions are exemplary and are intended to provide further explanation of the present application. Unless otherwise specified, all technical and scientific terms used herein have the same meaning as commonly understood by those of ordinary skill in the art to which the present application belongs.

[0053] It should be noted that the terms used herein are only for describing specific embodiments and are not intended to limit the exemplary embodiments described in the present application. As used herein, unless the context clearly indicates otherwise, the singular form is also intended to include the plural form. In addition, it should also be understood that when the terms "comprise" and / or "include" are used in this specification, it indicates the presence of features, steps, operations, devices, components and / or combinations thereof.

[0054] It should be noted that the terms "first", "second", etc. in the specification and claims of the present application and the above-mentioned drawings are used to distinguish similar objects, and are not necessarily used to describe a specific order or sequence. It should be understood that the terms used in this way can be interchangeable where appropriate, so that the embodiments of the present application described herein can be implemented in an order other than those illustrated or described herein.

[0055] In addition, the terms "include" and "have" and any variations thereof are intended to cover non-exclusive inclusions. For example, a process, method, system, product, or apparatus that includes a series of steps or units is not necessarily limited to those steps or units explicitly listed, but may include other steps or units that are not explicitly listed or inherent to these processes, methods, products, or apparatuses.

[0056] For ease of description, spatially relative terms, such as "above", "above", "on the upper surface of", "above", etc., may be used herein to describe the spatial positional relationship between a device or feature and other devices or features as shown in the figure. It should be understood that spatially relative terms are intended to include different orientations of the device in use or operation in addition to the orientation described in the figure. For example, if the device in the accompanying drawings is inverted, the device described as "above other devices or structures" or "above other devices or structures" will be positioned as "below other devices or structures" or "below other devices or structures". Thus, the exemplary term "above" may include both "above" and "below". The device may also be positioned in other different ways, such as rotated 90 degrees or in other orientations, and the spatially relative descriptions used herein are interpreted accordingly.

[0057] In the above detailed description, reference is made to the accompanying drawings, which form a part of this document. In the accompanying drawings, similar symbols typically identify similar components unless the context indicates otherwise. The illustrated embodiments described in the detailed description, drawings, and claims are not meant to be limiting. Other embodiments may be used, and other changes may be made, without departing from the spirit or scope of the subject matter presented herein.

[0058] The above description is only the preferred embodiment of the present application and is not intended to limit the present application. For those skilled in the art, the present application may have various modifications and variations. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present application shall be included in the protection scope of the present application.

Claims

1. A phase-modulated laser Doppler velocimeter based on thin-film lithium niobate and graphene, characterized in that: It mainly includes semiconductor lasers, optical fiber couplers, integrated thin-film lithium niobate, graphene phase modulators, spatial light coupling lens groups, photoelectric detectors, and signal solution and control modules, which are sequentially connected according to the propagation direction of the optical path. The semiconductor lasers include infrared semiconductor lasers and visible light indicating semiconductor lasers. The infrared semiconductor lasers are signal light sources, and the visible light indicating semiconductor lasers are indicating light sources. The infrared semiconductor lasers and the visible light indicating semiconductor lasers are respectively connected to the first input end and the second input end of the optical fiber coupler through polarization-maintaining optical fibers. The first output end of the optical fiber coupler is connected to the integrated thin-film lithium niobate and graphene phase modulators, and the second output end is idle. The light output from the first output end and the second output end of the integrated thin-film lithium niobate and graphene phase modulators is projected onto the object to be measured through the spatial light coupling lens group, and the return light enters the photoelectric detector through the spatial light coupling lens group. The integrated thin-film lithium niobate and graphene phase modulator is composed of a silicon dioxide upper cladding layer, a thin-film lithium niobate ridge waveguide layer, a silicon dioxide lower cladding layer, and a silicon substrate layer from top to bottom, wherein a bottom layer of graphene, a top layer of graphene, an aluminum oxide insulating layer, and a metal electrode are arranged above the silicon dioxide upper cladding layer; The metal electrode comprises a signal electrode and a ground electrode. The aluminum oxide insulating layer separates the top graphene layer from the bottom graphene layer. The signal electrode is connected to the top graphene layer, and the ground electrode is connected to the bottom graphene layer.

2. The phase-modulated laser Doppler velocimeter based on thin-film lithium niobate and graphene according to claim 1, characterized in that: The light source and the polarization-maintaining optical fiber are coupled by end face coupling or spatial light coupling.

3. The phase-modulated laser Doppler velocimeter based on thin-film lithium niobate and graphene according to claim 1, characterized in that: The pigtails connected to the input and output ends of all optical devices of the speed meter are polarization-maintaining optical fibers.

4. The phase-modulated laser Doppler velocimeter based on thin-film lithium niobate and graphene according to claim 1, characterized in that: The spatial optical coupling lens group includes a first collimating lens, a second collimating lens, and a receiving lens, wherein the receiving lens is located between the two collimating lenses, and then the photoelectric detector converts the optical signal into an electrical signal.

5. The phase-modulated laser Doppler velocimeter based on thin-film lithium niobate and graphene according to claim 4, characterized in that: The first collimating lens, the second collimating lens and the receiving lens are all one of a biconvex lens, a plano-convex lens, a concave-convex lens, an achromatic lens and an aplanatic lens.

6. The phase-modulated laser Doppler velocimeter based on thin-film lithium niobate and graphene according to claim 1, characterized in that: The integrated thin-film lithium niobate and graphene phase modulator integrates an input tapered waveguide, a 1×2 MMI beam splitter, a large-radius curved waveguide, two arms of a phase modulator, a metal electrode, and an output tapered waveguide according to the propagation direction of the optical path, wherein the tapered waveguide matches the input and output end pigtails.

7. The phase-modulated laser Doppler velocimeter based on thin-film lithium niobate and graphene according to claim 6, characterized in that: The distance between the two waveguides at the output end of the 1×2 MMI beam splitter is half the width of the multi-mode interference region, and both the input and output waveguides adopt a tapered waveguide structure.

8. The phase-modulated laser Doppler velocimeter based on thin-film lithium niobate and graphene according to claim 6, characterized in that: The integrated thin-film lithium niobate and graphene phase modulator has a tapered waveguide at the input and output ends that adopts a three-time tapered structure of the ridge and the substrate.

9. The phase-modulated laser Doppler velocimeter based on thin-film lithium niobate and graphene according to claim 1, characterized in that: The metal electrode spacing is greater than 5μm, the modulation area length is not less than 5mm, the electrode thickness is not less than 6μm, the electrode width is not less than 20μm, the integrated thin film lithium niobate and graphene phase modulator modulation signal is a sawtooth wave, and the modulation depth is 0.5π.

10. The phase-modulated laser Doppler velocimeter based on thin-film lithium niobate and graphene according to claim 1, characterized in that: The input and output pigtails of the integrated thin-film lithium niobate and graphene phase modulator are 3μm optical fibers, the polarization-maintaining optical fibers used to connect to the optical fiber coupler and the spatial optical coupling lens group are 6μm optical fibers, and a 3μm-6μm mode field adapter is provided between the pigtails and the polarization-maintaining optical fibers.

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