Thin film lithium niobate chip for hybrid integrated laser Doppler velocimeter

Abstract

The invention relates to a thin-film lithium niobate chip for a hybrid integrated laser Doppler velocimeter, a first group of mode field adapters, a 2 * 1 coupler, a polarizer, a phase modulator and a second group of mode field adapters are sequentially cascaded in the chip along an incident light path, and the first group of mode field adapters comprises a first mode field adapter and a second mode field adapter. The first mode field adapter and the second mode field adapter are respectively connected with the input end of the 2 * 1 coupler so as to combine the signal light and the indication light, the output end of the 2 * 1 coupler is connected with the input end of the polarizer, the polarizer is used for filtering the TE0 mode in the signal light, the stray light absorber is used for absorbing the TE0 mode in the signal light, and the stray light absorber is used for absorbing the TE0 mode in the signal light. The output end of the polarizer is connected with the input end of a phase modulator, multiple groups of micro-rings are cascaded in the phase modulator, the output end of the phase modulator is connected with a second group of mode field adapters, and the second group of mode field adapters comprises a third mode field adapter and a fourth mode field adapter which are respectively connected with two output ends of the phase modulator. The chip further comprises a scattered light loop, the scattered light loop comprises a fifth mode field adapter and a vertical coupler which are cascaded, and a III-V group detector is attached to the surface of the vertical coupler.

Description

TECHNICAL FIELD

[0001] The application relates to the technical field of a velocity meter, in particular to a thin film lithium niobate chip for a hybrid integrated laser Doppler velocimeter. BACKGROUND

[0002] A laser Doppler velocimeter is an optical sensing system for measuring the velocity of an object based on the principle of Doppler shift. When the object moves, the scattered light excited by the incident signal light on the surface of the object will produce a frequency shift, and the size of the frequency shift can be detected to calculate the velocity of the object. The traditional laser Doppler velocimeter mainly uses discrete devices, including a signal light source, an indicator light source, a fiber coupler, a coupling lens group, a body material lithium niobate multifunctional modulator, a detector, a solving and control circuit chip and the like. The connection between all optical devices needs to pass through a polarization maintaining optical fiber, which has a high production cost, a large volume, involves multiple functional devices, has a complex structure and is difficult to realize mass production.

[0003] With the progress of micro-nano processing technology, integrated optics is gradually becoming a hot topic. An integrated photon chip based on the principle of guided wave optics can realize a variety of complex functions and has no obvious performance degradation compared with original discrete devices. Further, the integrated photon chip, the light source die, the photoelectric detector and the microlens are co-packaged in the same metal shell through a hybrid integration method, only through a tail fiber, a lens or a module pin to exchange data with the outside world, so that the integration of many traditional optical systems can be realized, and the structural complexity is reduced. The existing integrated optical technology can meet the working requirements of the laser Doppler velocimeter, so it is necessary to explore the integration method of part or all functional devices of the traditional velocimeter, so as to reduce the production cost of the velocimeter, reduce the size of the instrument, realize mass production and improve the market competitiveness.

[0004] Waveguide is the basic structure of integrated photonic chip, and is also the bridge for realizing the optical interconnection of various functional devices in the chip. Preparing a waveguide with low loss is the basis for realizing the integration of traditional optical systems. After years of process exploration, it has been found that many core materials can be used to prepare low-loss waveguides. Lithium niobate material has a high electro-optic coefficient and excellent linear electro-optic effect, and is widely used in the production of optical modulators. The lithium niobate modulator prepared by traditional lithium niobate bulk material using proton diffusion process and other processes has a large size and is difficult to integrate with other devices. With the advancement of processing technology, thin film lithium niobate ridge waveguide with greater refractive index contrast with the cladding material can be prepared based on electron beam lithography, inductively coupled reactive ion etching and other processes. This waveguide structure has stronger binding ability to the optical field, smaller transmission loss and absorption loss during modulation, smaller mode field area and smaller device size, thus promoting the development of other lithium niobate functional devices other than lithium niobate modulators. Many passive structures have been widely used, which makes it possible to realize the hybrid integrated laser Doppler velocimeter based on thin film lithium niobate photonic chip, but few people apply thin film lithium niobate chip to hybrid integrated laser Doppler velocimeter. SUMMARY

[0005] The present application aims to provide a thin film lithium niobate chip for a hybrid integrated laser Doppler velocimeter to solve the problems existing in the prior art. The technical problems to be solved by the present application are solved by the following technical solutions.

[0006] A thin film lithium niobate chip for a hybrid integrated laser Doppler velocimeter, the chip inside cascades a first group of mode field adapters, a 2x1 coupler, a polarizer, a phase modulator and a second group of mode field adapters along the incident light path, wherein the first group of mode field adapters includes a mode field adapter one and a mode field adapter two, the mode field adapter one and the mode field adapter two are connected to the input end of the 2x1 coupler to combine signal light and indicator light, the output end of the 2x1 coupler is connected to the input end of the polarizer, the polarizer is used to filter out the TE0 mode in the signal light, and the TE0 mode in the signal light is absorbed by a stray light absorber connected to the polarizer, the output end of the polarizer is connected to the input end of the phase modulator, a plurality of groups of micro rings are cascaded in the phase modulator, the output end of the phase modulator is connected to the second group of mode field adapters, the second group of mode field adapters includes a mode field adapter three and a mode field adapter four, the mode field adapter three and the mode field adapter four are connected to the two output ends of the phase modulator respectively, and the chip inside further includes a scattered light circuit, the scattered light circuit includes a mode field adapter five and a vertical coupler cascaded, and a group III-V detector is attached to the upper surface of the vertical coupler.

[0007] Preferably, the mode field adapters are coupled with microlenses.

[0008] Preferably, the mode field adapter, 2x1 coupler, polarizer, phase modulator and detector are all based on thin-film lithium niobate ridge waveguide and are optically interconnected through the waveguide.

[0009] Preferably, the ridge waveguide structure used by the thin-film lithium niobate chip is made of a z-cut thin-film lithium niobate wafer, the crystal axis corresponding to the maximum electro-optic coefficient r33 is perpendicular to the wafer plane, and the waveguide structure from bottom to top is silicon substrate layer, silicon dioxide lower cladding layer, thin-film lithium niobate ridge waveguide layer and silicon dioxide upper cladding layer. The ground electrode of the modulation region is located inside the silicon dioxide lower cladding layer, covering the entire modulation region. The positive and negative signal electrodes are located inside the silicon dioxide upper cladding layer, covering only the same side micro-ring above. Each electrode is connected to the pad on the chip surface through a via hole for applying a spatial electric field perpendicular to the wafer surface to the modulation region.

[0010] Preferably, the mode field adapter is composed of a thin-film lithium niobate ridge waveguide and a substrate layer trapezoidal waveguide, and the trapezoidal waveguide and the ridge waveguide satisfy the transverse mode matching condition. The width of the substrate layer of the ridge waveguide is slightly smaller than the width of the trapezoidal waveguide. The width of the trapezoidal waveguide gradually narrows in the direction away from the coupling region. The mode field adapter one and the mode field adapter two are used to receive input light of the signal light source and the indicator light source, respectively. The mode field adapter three and the mode field adapter four collimate the light output from the chip through the micro-lens. The mode field adapter five receives the scattered light carrying the object motion speed. The ridge waveguide structure of each mode field adapter is the same, but the width and structure of the trapezoidal waveguide are different.

[0011] Preferably, the 2x1 coupler is an MMI coupler or a directional coupler, and the length of the coupling region is an even multiple of the three times the beat length of the TE0 and TM0 input modes of the signal light source corresponding wavelength, and at the same time is an odd multiple of the three times the beat length of the TE0 and TM0 input modes of the indicator light source corresponding wavelength.

[0012] Preferably, the polarizer is composed of a left 2x2 MMI, an intermediate phase control region and a right 2x1 MMI. The input light first passes through two straight waveguiders of different lengths in the phase control region. The length difference between the two waveguiders is one quarter of the effective wavelength of the signal light TE0 mode. The sub-wavelength grating reflects all the TE0 modes of the signal light. After phase control, the TE0 mode of the two-arm signal light has a pi phase difference with the input, which is output from the lower port of the 2x2 MMI into the stray light absorber. The TM0 mode of the input signal light and the TE0 and TM0 modes of the indicator light are not reflected by the sub-wavelength grating and are output from the two MMI output ports with low loss. The stray light absorber is composed of a thin-film lithium niobate ridge waveguide and a pair of metal electrodes grown above the waveguide substrate layer and close to the ridge layer.

[0013] Preferably, the phase modulator is provided with a plurality of micro-ring combinations for folding the optical path in each of the two arms, each micro-ring combination is composed of three micro-rings, and each micro-ring satisfies the resonance condition of the signal light but does not satisfy the resonance condition of the indicator light, the signal light is sequentially established resonance relationship in the three micro-rings after input and is transmitted multiple times, the spatial electric field applied by the two arms only covers the position where the micro-ring waveguide exists, and the direction is opposite and determined by the electrode polarity inside the upper cladding layer, the modulation signal of the modulator is a sawtooth wave, and the modulation depth is 0.5pi.

[0014] Preferably, the vertical coupler is a chirped grating structure, the signal light of the five-input vertical coupler through the mode field adapter is gradually expanded in the mode field through a plurality of tapered waveguides, then enters the subwavelength chirped grating scattering area, and is coupled into the III-V group detector attached to the upper surface to be converted into an electrical signal, and the period of the subwavelength chirped grating is alternately changed to broaden the spectral width of the scattered light.

[0015] The thin film lithium niobate chip for the mixed integrated laser Doppler velocimeter has the following beneficial effects:

[0016] 1. The overall size of the laser Doppler velocimeter is further reduced, and the cost is reduced, the chip is further integrated with a coupler, a polarizer, a detector and various functional devices on the basis of the original bulk material lithium niobate modulator, and various optical interconnection devices for mode field adaptation and vertical coupling are integrated, combined with the existing micro-lens coupling packaging process, and the mixed integration with a light source is easy to realize, the process flow of the traditional velocimeter is simplified, the production batch is realized, and the overall precision and response bandwidth of the key parameters of the original discrete device velocimeter are equivalent.

[0017] 2. All functional devices of the thin film lithium niobate chip except the detector are based on the thin film lithium niobate ridge waveguide and utilize the waveguide to realize optical interconnection, the excellent high-speed modulation characteristics of the thin film lithium niobate are fully utilized, and the overall performance of the velocimeter is not excessively reduced compared with the discrete device.

[0018] 3. The chip scheme and process provided by the application are compatible with the traditional CMOS process, and can be further heterogeneously integrated with a light source, and the future optoelectromechanical integration is preliminarily explored. BRIEF DESCRIPTION OF DRAWINGS

[0019] Figure 1 It is a schematic diagram of the internal structure and optical interconnection method of the thin film lithium niobate chip of the velocimeter of the application;

[0020] Figure 2 It is a schematic diagram of the internal structure of one of the mode field adapters of the thin film lithium niobate chip of the application;

[0021] Figure 3 It is a schematic diagram of the internal structure of the 2*1 coupler of the thin film lithium niobate chip of the application;

[0022] Figure 4 Fig. 1 is a schematic diagram of the internal structure of a thin-film lithium niobate chip phase modulator according to the present application;

[0023] Figure 5 Fig. 2 is a schematic diagram of the internal structure of a thin-film lithium niobate chip micro-ring cascade phase modulator according to the present application;

[0024] Figure 6 Fig. 3 is a schematic diagram of the internal structure of a thin-film lithium niobate chip sub-wavelength chirped grating vertical coupler according to the present application. DETAILED DESCRIPTION

[0025] The technical solutions of the present application will be further described in detail below with reference to the accompanying drawings and examples.

[0026] EXAMPLE

[0027] In order to make the objects, technical solutions and advantages of the embodiments of the present application clearer, the technical solutions of the embodiments of the present application will be described clearly and completely below with reference to the accompanying drawings of the embodiments of the present application. Obviously, the described embodiments are only some of the embodiments of the present application, but not all of the embodiments of the present application. The components of the embodiments of the present application described and shown in the accompanying drawings can be arranged and designed in various different configurations.

[0028] Therefore, the detailed description of the embodiments of the present application provided below in the accompanying drawings is not intended to limit the scope of the claimed present application, but only represents selected embodiments of the present application. All other embodiments obtained by those of ordinary skill in the art based on the embodiments in the present application without making creative efforts fall within the scope of protection of the present application.

[0029] It should be noted that: similar reference numerals and letters represent similar items in the following drawings, and therefore, once an item is defined in one drawing, it does not need to be further defined and explained in subsequent drawings.

[0030] In the description of the present application, it should be noted that the terms "upper", "lower", "inner", "outer" and the like indicate the orientation or positional relationship based on the orientation or positional relationship shown in the drawings, or the orientation or positional relationship in which the product of the present application is usually placed, and are only for the convenience of describing the present application and simplifying the description, and therefore cannot be understood as indicating or implying that the indicated device or element must have a particular orientation, be constructed and operated in a particular orientation, and therefore cannot be understood as limiting the present application.

[0031] In the description of the present invention, it should also be noted that, unless otherwise expressly specified or limited, the terms "disposed," "installed," and "connected" should be understood broadly. For example, they may refer to fixed, detachable, or integral connections; mechanical or electrical connections; direct or indirect connections through an intermediate medium; and internal communication between two components. Those skilled in the art will understand the specific meanings of the above terms in the present invention based on the specific circumstances.

[0032] The embodiments of the present invention will be described in detail below with reference to the accompanying drawings.

[0033] like Figure 1 As shown, the thin film lithium niobate chip of the present application includes an incident light path and a scattered light circuit.

[0034] The incident light path includes a first group of mode field adapters, a 2×1 coupler, a polarizer, a phase modulator, and a second group of mode field adapters cascaded in sequence along the light path direction, wherein the first group of mode field adapters includes mode field adapter 1 and mode field adapter 2, and the mode field adapter 1 and mode field adapter 2 are respectively connected to the input end of the 2×1 coupler to combine the signal light and the indicator light, and the output end of the 2×1 coupler is connected to the input end of the polarizer, the polarizer is used to filter out the TE0 mode in the signal light, and a stray light absorber connected to the polarizer is used to absorb the TE0 mode in the signal light, the output end of the polarizer is connected to the input end of the phase modulator, multiple groups of microrings are cascaded in the phase modulator, and the output end of the phase modulator is connected to the second group of mode field adapters, and the second group of mode field adapters includes mode field adapter 3 and mode field adapter 4, and the mode field adapter 3 and mode field adapter 4 are respectively connected to the two output ends of the phase modulator.

[0035] The scattered light loop comprises a cascaded mode field adapter 5 and a vertical coupler, and a III-V group detector is mounted on the vertical coupler.

[0036] Mode field adapters 1 and 2 receive the signal light and indicator light, respectively, collimated by microlenses. They combine these light beams through a 2×1 coupler, then pass through a polarizer to filter out the TE0 mode in the signal light to accommodate the polarization selectivity of the lithium niobate material's modulation efficiency. After the input light enters the modulation region, the signal light, meeting the microring resonance conditions, propagates back and forth multiple times within each microring combination, carrying the modulated signal. However, the indicator light, however, does not meet the resonance conditions and passes directly through the straight waveguide. The signal and indicator light exit the chip through mode field adapters 3 and 4, where they are collimated by a microlens assembly and illuminate the surface of the object under test. Scattered light returns to the chip through mode field adapter 5 and, via a vertical coupler, enters the detector, where it is converted into an electrical signal.

[0037] The thin film lithium niobate chip of the present application is based on thin film lithium niobate ridge waveguide to realize and interconnect the mode field adapter, 2x1 coupler, polarizer, phase modulator and detector.

[0038] The ridge waveguide structure used by the thin film lithium niobate chip is made of a z-cut thin film lithium niobate wafer, the crystal axis corresponding to the maximum electro-optic coefficient r33 is perpendicular to the wafer plane, and the waveguide structure from bottom to top is silicon substrate layer, silicon dioxide lower cladding layer, thin film lithium niobate ridge waveguide layer and silicon dioxide upper cladding layer, the ground electrode of the modulation region is located inside the silicon dioxide lower cladding layer, the coverage range includes the entire modulation region, the positive and negative signal electrodes are located inside the silicon dioxide upper cladding layer, the coverage range only includes the same side micro-ring above, each electrode is led out to the chip surface by means of a via hole, and is used for applying a spatial electric field perpendicular to the wafer surface to the modulation region, wherein the thin film lithium niobate ridge waveguide layer is composed of a substrate layer with infinite width and a ridge layer with limited width.

[0039] As shown in Figure 2 The mode field adapters are coupled with the micro-lenses.

[0040] The mode field adapters are composed of thin film lithium niobate ridge waveguides and substrate layer trapezoidal waveguides, the trapezoidal waveguides and the ridge waveguides satisfy the transverse mode matching condition, the substrate layer width of the ridge waveguide is slightly smaller than the trapezoidal waveguide width, the trapezoidal waveguide width gradually narrows in the direction away from the coupling region, and the mode field can be further expanded, the mode field adapter one and the mode field adapter two are respectively used for receiving input light of the signal light source and the indication light source, the mode field adapter three and the mode field adapter four output the light from the chip through the micro-lenses, and the mode field adapter five receives the scattered light carrying the object motion speed, the ridge waveguide structures of each mode field adapter are the same, but the trapezoidal waveguide widths and structures are different.

[0041] As shown in Figure 3 The 2x1 coupler is an MMI coupler or a directional coupler, the length of the coupling region is an even multiple of the three times beat length of the TE0 and TM0 input modes corresponding to the wavelength of the signal light source, and is an odd multiple of the three times beat length of the TE0 and TM0 input modes corresponding to the wavelength of the indication light source at the same time, so as to ensure that the signal light still transmits in the same side waveguide after passing through the coupler, and the indication light is coupled into the opposite side waveguide.

[0042] As shown in Figure 4As shown, the polarizer of the thin film lithium niobate chip is composed of a left 2x2 MMI, a middle phase control region and a right 2x1 MMI, the input light first passes through two straight waveguides with different lengths in the phase control region, the length difference of the two waveguides is one quarter of the effective wavelength of the signal light TE0 mode, the subwavelength grating can reflect all the TE0 mode of the signal light, after phase control, the TE0 mode of the two-arm signal light has a pi phase difference compared with the input, and will be output from the left lower port of the left 2x2 MMI into the stray light absorber, while the TM0 mode of the input signal light and the TE0 and TM0 modes of the indicator light will not be reflected by the subwavelength grating and will be output from the right port with low loss. The stray light absorber is composed of a thin film lithium niobate ridge waveguide and a pair of metal electrodes grown on the upper layer of the waveguide substrate and very close to the ridge layer.

[0043] As shown in Figure 5 , the phase modulator of the thin film lithium niobate chip is provided with a plurality of micro-ring combinations inside the two arms for folding the optical path to reduce the overall size of the chip, each micro-ring combination is composed of a first to third micro-ring, and each micro-ring satisfies the resonance condition of the signal light, but does not satisfy the resonance condition of the indicator light, and the signal light input establishes resonance relationship in the three micro-rings in turn and transmits multiple times. The spatial electric field applied to the two arms only covers the position where the micro-ring waveguide exists, and the direction is opposite, and is determined by the polarity of the electrode inside the upper cladding layer, the modulation signal of the modulator is a sawtooth wave, and the modulation depth is 0.5pi.

[0044] As shown in Figure 6 , the vertical coupler is a chirped grating structure, the signal light input into the vertical coupler through the fifth mode field adapter gradually expands the mode field through multiple tapered waveguides, then enters the subwavelength chirped grating scattering area, and is coupled into the III-V group detector on the upper surface to convert into an electrical signal, the period of the subwavelength chirped grating alternately changes, which can broaden the scattering optical spectrum width, and ensure that most of the returned signal light after frequency shift can be absorbed by the detector.

[0045] The working process of the present application is as follows:

[0046] The light emitted by the signal light source and the indicator light source is collimated by the microlens and then input into the mode field adapter one and the mode field adapter two respectively and coupled into the chip, combined into a bundle through the specially designed 2x1 coupler, filtered through the phase control polarizer to remove the TE0 mode of the signal light, then loaded with a modulation signal through the cascaded micro-ring phase modulator, and then output from the chip through the mode field adapter three and the mode field adapter four, collimated by the microlens combination and then irradiated to the surface of the object to be measured, the scattered light carrying the object motion speed information returns to the chip through the mode field adapter five, and enters the III-V group detector through the subwavelength chirped grating vertical coupler to convert into an electrical signal for subsequent data processing to calculate the object motion speed, and the output light intensity of the two light sources is subjected to feedback control.

[0047] It should be noted that the foregoing detailed description is illustrative, and is not to be considered as limiting the application. Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs. The entirety of any patent or publication cited herein can be incorporated by reference in its entirety for the teachings and disclosures provided.

[0048] It should be noted that the foregoing detailed description is illustrative, and is not to be considered as limiting the application. Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs. The entirety of any patent or publication cited herein can be incorporated by reference in its entirety for the teachings and disclosures provided.

[0049] It should be noted that the foregoing detailed description is illustrative, and is not to be considered as limiting the application. Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs. The entirety of any patent or publication cited herein can be incorporated by reference in its entirety for the teachings and disclosures provided.

[0050] In addition, the terms "comprise", "comprising", "having", "including", and "contains", and variations thereof, are intended to cover a non-exclusive inclusion. For example, a process, method, system, product, or apparatus that comprises a list of steps or elements is not necessarily limited to those steps or elements but can include other not expressly listed steps or elements.

[0051] For purposes of the description hereinafter, the terms "upper", "lower", "right", "left", "vertical", "horizontal", "top", "bottom", "lateral", "medial", "apical", "basal" and derivatives thereof shall relate to the application as it is shown in the drawings and described herein. However, it is to be understood that the application can assume various alternative orientations and, accordingly, such terms are not to be taken as limitations of the present application or any embodiments thereof described herein or is shown in the drawings.

[0052] In the detailed description above, reference is made to the accompanying drawings, which form a part hereof. In the drawings, similar symbols typically identify similar components, unless context dictates otherwise. The illustrative embodiments described in the detailed description, drawings, and claims are not meant to be limiting. Other embodiments can be used, and other changes can be made, without departing from the spirit or scope of the subject matter presented herein.

[0053] The specific embodiments described herein have many advantages over conventional methods and techniques. These and other embodiments will become readily apparent to those skilled in the art from the following detailed description, wherein embodiments are shown and described by way of illustrative example. As will be realized, the subject matter disclosed herein is capable of modification in various obvious respects, all without departing from the spirit and scope of the present subject matter. Accordingly, the drawings and description are to be regarded as illustrative in nature, and not as restrictive.

Claims

1. A thin-film lithium niobate chip for a hybrid integrated laser Doppler velocimeter, characterized in that: A first group of mode field adapters, a 2×1 coupler, a polarizer, a phase modulator, and a second group of mode field adapters are sequentially cascaded inside the chip along the incident light path. The first group of mode field adapters includes mode field adapter 1 and mode field adapter 2, which are respectively connected to the input end of the 2×1 coupler to combine the signal light with the indicator light. The output end of the 2×1 coupler is connected to the input end of the polarizer. The polarizer is used to filter out the TE0 mode in the signal light, and a stray light absorber connected to the polarizer is used to absorb the TE0 mode in the signal light. The output end of the polarizer is connected to the input end of the phase modulator. Multiple groups of microrings are cascaded in the phase modulator. The output end of the phase modulator is connected to the second group of mode field adapters. The second group of mode field adapters includes mode field adapter 3 and mode field adapter 4, which are respectively connected to the two output ends of the phase modulator. The chip also includes a scattered light circuit, which includes a cascaded mode field adapter 5 and a vertical coupler. A III-V group detector is surface-mounted on the vertical coupler.

2. The thin-film lithium niobate chip for a hybrid integrated laser Doppler velocimeter according to claim 1, characterized in that: The mode field adapters are all coupled with the microlenses.

3. The thin-film lithium niobate chip for a hybrid integrated laser Doppler velocimeter according to claim 1, characterized in that: The mode field adapter, 2×1 coupler, polarizer, phase modulator and detector are all implemented based on a thin film lithium niobate ridge waveguide and are optically interconnected through the waveguide.

4. The thin-film lithium niobate chip for a hybrid integrated laser Doppler velocimeter according to claim 3, characterized in that: The ridge waveguide structure used in the thin-film lithium niobate chip is made of a Z-cut thin-film lithium niobate wafer. The crystal axis corresponding to the maximum electro-optic coefficient r33 is perpendicular to the wafer plane. The waveguide structure consists of a silicon base layer, a silicon dioxide lower cladding, a thin-film lithium niobate ridge waveguide layer, and a silicon dioxide upper cladding from bottom to top. The ground electrode of the modulation area is located inside the silicon dioxide lower cladding, and its coverage range includes the entire modulation area. The positive and negative signal electrodes are located inside the silicon dioxide upper cladding, and their coverage range only includes the top of the microring on the same side. Each electrode uses a through-hole to lead the pad to the chip surface, which is used to apply a spatial electric field perpendicular to the wafer surface to the modulation area.

5. The thin-film lithium niobate chip for a hybrid integrated laser Doppler velocimeter according to claim 3, characterized in that: The mode field adapter consists of a thin-film lithium niobate ridge waveguide and a substrate layer trapezoidal waveguide. The trapezoidal waveguide and the ridge waveguide meet the transverse mode matching condition. The substrate layer width of the ridge waveguide is slightly smaller than the width of the trapezoidal waveguide, and the width of the trapezoidal waveguide gradually narrows in the direction away from the coupling region. The first and second mode field adapters are respectively used to receive input light from the signal light source and the indicator light source. The third and fourth mode field adapters collimate the light output chip through a microlens. The fifth mode field adapter receives scattered light carrying the object's motion speed. The ridge waveguide structure of each mode field adapter is the same, but the width and structure of the trapezoidal waveguide are different.

6. The thin-film lithium niobate chip for a hybrid integrated laser Doppler velocimeter according to claim 1, characterized in that: The 2×1 coupler is an MMI coupler or a directional coupler, and the length of its coupling region is an even multiple of three times the beat length of the input mode of the signal light source corresponding to the wavelength TE0 and TM0, and at the same time an odd multiple of three times the beat length of the input mode of the indicator light source corresponding to the wavelength TE0 and TM0.

7. The thin-film lithium niobate chip for a hybrid integrated laser Doppler velocimeter according to claim 1, characterized in that: The polarizer consists of a 2×2 MMI on the left, a phase-controlled region in the middle, and a 2×1 MMI on the right. In the phase-controlled region, the input light first passes through two straight waveguides of unequal lengths, with the length difference between the two waveguides being one-quarter of the effective wavelength of the signal light's TE0 mode. The subwavelength grating reflects all of the signal light's TE0 mode. After phase control, the TE0 modes of the two arms of the signal light have a pi phase difference compared to the input, and are output from the port below the 2×2 MMI into a stray light absorber. The TM0 mode of the input signal light and the TE0 and TM0 modes of the indicator light are not reflected by the subwavelength grating and are output through two MMI output ports with low loss. The stray light absorber consists of a thin-film lithium niobate ridge waveguide and a pair of metal electrodes grown above the waveguide substrate layer and close to the ridge layer.

8. The thin-film lithium niobate chip for a hybrid integrated laser Doppler velocimeter according to claim 1, characterized in that: The phase modulator is provided with multiple micro-ring combinations for folding the optical path on the inner sides of both arms. Each micro-ring combination is composed of three micro-rings, and each micro-ring meets the resonance conditions of the signal light, but does not meet the resonance conditions of the indicator light. After the signal light is input, a resonance relationship is established in the three micro-rings in sequence and transmitted multiple times. The spatial electric field applied by the two arms only covers the location where the micro-ring waveguide exists, and is in opposite directions and is determined by the polarity of the electrodes inside the upper cladding. The modulation signal of the modulator is a sawtooth wave, and the modulation depth is 0.5π.

9. The thin-film lithium niobate chip for a hybrid integrated laser Doppler velocimeter according to claim 1, characterized in that: The vertical coupler is a chirped grating structure. The signal light passing through the five-input vertical coupler of the mode field adapter gradually expands the mode field through multiple tapered waveguides and enters the subwavelength chirped grating scattering region. It is then coupled into the III-V group detector mounted on the surface and converted into an electrical signal. The period of the subwavelength chirped grating is alternately changed to broaden the scattering optical spectrum.

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