A thin-film lithium niobate chip for hybrid integrated laser Doppler velocimeters

Abstract

This application relates to a thin-film lithium niobate chip for a hybrid integrated laser Doppler velocimeter. Internally, along the incident light path, a first set of mode field adapters, a 2×1 coupler, a polarizer, a phase modulator, and a second set of mode field adapters are cascaded sequentially. The first set of mode field adapters includes mode field adapter one and mode field adapter two, which are respectively connected to the input of the 2×1 coupler to combine the signal light and the indicator light. The output of the 2×1 coupler is connected to the input of the polarizer, which is used to filter out T-waves in the signal light. The E0 mode is used, and the TEO mode in the signal light is absorbed by a stray light absorber. The output of the polarizer is connected to the input of the phase modulator. Multiple micro-rings are cascaded in the phase modulator. The output of the phase modulator is connected to the second set of mode field adapters. The second set of mode field adapters includes mode field adapter three and mode field adapter four, which are respectively connected to the two outputs of the phase modulator. The chip also includes a scattering light circuit. The scattering light circuit includes a cascaded mode field adapter five and a vertical coupler. A III-V group detector is attached to the surface of the vertical coupler.

Description

Technical Field

[0001] This application relates to the field of velocimeter technology, and in particular to a thin-film lithium niobate chip for use in a hybrid integrated laser Doppler velocimeter. Background Technology

[0002] A laser Doppler velocimeter is an optical sensing system that measures the velocity of a moving object based on the Doppler frequency shift principle. When an object moves, the scattered light excited by the incident signal light on its surface will produce a frequency shift. By detecting the magnitude of this frequency shift, the velocity of the object can be calculated. Traditional laser Doppler velocimeters mostly use discrete components, specifically including a signal source, an indicator source, an optical fiber coupler, a coupling lens group, a multifunctional modulator made of lithium niobate, a detector, and a calculation and control circuit chip. All optical components must be connected to each other through polarization-maintaining optical fibers, resulting in high production costs, large size, and complex structure involving multiple functional components, making mass production difficult.

[0003] With advancements in micro- and nano-fabrication technologies, integrated optics is becoming a hot topic. Integrated photonic chips based on waveguide optics can achieve a variety of complex functions without significant performance degradation compared to existing discrete devices. Furthermore, by co-encapsulating integrated photonic chips, light source dies, photodetectors, and microlenses within a single metal housing using a hybrid integration approach, and exchanging data with the outside world only through fiber optic cables, lenses, or module pins, many traditional optical systems can be integrated, reducing their structural complexity. Existing integrated optical technologies can meet the operational requirements of laser Doppler velocimeters; therefore, it is necessary to explore methods for integrating some or all functional components of traditional velocimeters, thereby reducing production costs, minimizing instrument size, enabling mass production, and enhancing market competitiveness.

[0004] Waveguides are the fundamental structure of integrated photonic chips and serve as the bridge for optical interconnection between various functional devices within the chip. Fabricating low-loss waveguides is fundamental to the integration of traditional optical systems. After years of process exploration, many core materials capable of fabricating low-loss waveguides have been discovered. Among them, lithium niobate exhibits a high electro-optic coefficient and excellent linear electro-optic effect, making it widely used in optical modulators. However, traditional lithium niobate modulators fabricated using processes such as proton diffusion are relatively large and difficult to integrate with other devices. With advancements in processing technology, thin-film lithium niobate ridge waveguides with a greater refractive index contrast to the cladding material can be fabricated using processes such as electron beam lithography and inductively coupled reactive ion etching. This waveguide structure has a stronger ability to confine the light field, resulting in lower light transmission loss and absorption loss during modulation, smaller mode area, and smaller device size. This has promoted the development of other lithium niobate functional devices besides lithium niobate modulators, and many passive structures have already been widely used. This makes it possible to realize a hybrid integrated laser Doppler velocimeter based on thin-film lithium niobate photonic chips. However, currently, few researchers have applied thin-film lithium niobate chips to hybrid integrated laser Doppler velocimeters. Summary of the Invention

[0005] The present invention aims to provide a thin-film lithium niobate chip for hybrid integrated laser Doppler velocimeters to overcome the shortcomings of the prior art. The technical problem to be solved by the present invention is achieved through the following technical solution.

[0006] A thin-film lithium niobate chip for a hybrid integrated laser Doppler velocimeter comprises, internally, a first set of mode field adapters, a 2×1 coupler, a polarizer, a phase modulator, and a second set of mode field adapters cascaded sequentially along the incident light path. The first set of mode field adapters includes mode field adapter one and mode field adapter two, which are respectively connected to the input of the 2×1 coupler to combine the signal light and the indicator light. The output of the 2×1 coupler is connected to the input of the polarizer, which filters out the TEO mode from the signal light and uses a filter connected to the polarizer to filter out noise. The astigmatism absorber absorbs the TEO mode in the signal light. The output of the polarizer is connected to the input of the phase modulator. Multiple micro-rings are cascaded in the phase modulator. The output of the phase modulator is connected to a second set of mode field adapters. The second set of mode field adapters includes mode field adapter three and mode field adapter four. Mode field adapter three and mode field adapter four are respectively connected to the two outputs of the phase modulator. The chip also includes a scattering light circuit. The scattering light circuit includes a cascaded mode field adapter five and a vertical coupler. A III-V group detector is attached to the surface of the vertical coupler.

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

[0008] Preferably, the mode field adapter, 2×1 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 in the thin-film lithium niobate chip is fabricated from 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, from bottom to top, consists of a silicon substrate layer, a silicon dioxide lower cladding layer, a thin-film lithium niobate ridge waveguide layer, and a silicon dioxide upper cladding layer. The ground electrode of the modulation area is located inside the silicon dioxide lower cladding layer, covering the entire modulation area. The positive and negative signal electrodes are located inside the silicon dioxide upper cladding layer, covering only the area above the micro-ring on the same side. Each electrode has a pad led out to the chip surface via a via, which is used to apply a spatial electric field perpendicular to the wafer surface to the modulation area.

[0010] Preferably, the mode field adapter consists of a thin-film lithium niobate ridge waveguide and a substrate trapezoidal waveguide. The trapezoidal waveguide and the ridge waveguide satisfy the lateral mode matching condition. The width of the substrate of the ridge waveguide is slightly smaller than the width of the trapezoidal waveguide. The width of the trapezoidal waveguide gradually narrows along the direction away from the coupling region. Mode field adapter one and mode field adapter two are used to receive the input light from the signal light source and the indicator light source, respectively. Mode field adapter three and mode field adapter four collimate the light output chip through a microlens. Mode field adapter five receives the scattered light carrying the velocity of the moving object. 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 2×1 coupler is an MMI coupler or a directional coupler, and its coupling region length is an even multiple of three times the beat length of the input mode corresponding to the wavelengths TE0 and TM0 of the signal light source, and at the same time, it is an odd multiple of three times the beat length of the input mode corresponding to the wavelengths TE0 and TM0 of the indicator light source.

[0012] Preferably, the polarizer consists of a 2×2 MMI on the left, a phased array region in the middle, and a 2×1 MMI on the right. The input light first passes through two straight waveguides of unequal length in the phased array region. The length difference between the two waveguides is one-quarter of the effective wavelength of the TE0 mode of the signal light. The subwavelength grating reflects all the TE0 mode of the signal light. After phase control, the TE0 mode of the two arms of the signal light has a phase difference of pi compared to the input. It will be output from the lower port of the 2×2 MMI and enter the stray light absorber. 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 through the 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 on the waveguide substrate layer and close to the ridge layer.

[0013] Preferably, the inner sides of both arms of the phase modulator are provided with multiple micro-ring combinations for folding the optical path. Each micro-ring combination consists 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. After the signal light is input, it establishes a resonance relationship in the three micro-rings in sequence and is transmitted multiple times. The spatial electric field applied by the two arms only covers the location where the micro-ring waveguide exists, and the directions are opposite and determined by the polarity of the electrodes inside the upper cladding. The modulation signal of the modulator is a sawtooth wave with a modulation depth of 0.5π.

[0014] Preferably, the vertical coupler is a chirped grating structure. The signal light from the mode field adapter five input vertical coupler enters the subwavelength chirped grating scattering region after the mode field is gradually expanded through multiple tapered waveguides, and is coupled into the III-V group detector attached to its surface and converted into an electrical signal. The period of the subwavelength chirped grating changes alternately to broaden the scattering optical spectrum.

[0015] The thin-film lithium niobate chip for hybrid integrated laser Doppler velocimeters of the present invention has the following beneficial effects:

[0016] 1. Further reduce the overall size and cost of the laser Doppler velocimeter. Based on the original bulk material lithium niobate modulator, the chip further integrates multiple functional devices such as couplers, polarizers, and detectors, as well as various optical interconnect devices for mode field adaptation and vertical coupling. Combined with the existing microlens coupling packaging process, it is easy to integrate with the light source, which simplifies the process flow of the traditional velocimeter and enables mass production. It also achieves key parameters such as overall accuracy and response bandwidth comparable to the original discrete device velocimeter.

[0017] 2. Except for the detector, all functional devices of the thin-film lithium niobate chip are based on the thin-film lithium niobate ridge waveguide and use the waveguide to realize optical interconnection. This fully utilizes the excellent high-speed modulation characteristics of thin-film lithium niobate and ensures that the overall performance of the speed meter does not decrease too much compared with discrete devices.

[0018] 3. The chip solution and process provided by this invention are compatible with traditional CMOS processes and can be further integrated with light sources in a heterogeneous manner, thus making a preliminary exploration of future opto-mechatronics integration. Attached Figure Description

[0019] Figure 1 This is a schematic diagram of the internal structure and optical interconnection method of the thin-film lithium niobate chip in the velocimeter of the present invention;

[0020] Figure 2 This is a schematic diagram of the internal structure of one of the mode field adapters of the thin-film lithium niobate chip of the present invention;

[0021] Figure 3 This is a schematic diagram of the internal structure of the 2×1 coupler for the thin-film lithium niobate chip of the present invention;

[0022] Figure 4 This is a schematic diagram of the internal structure of the phase-controlled polarizer for the thin-film lithium niobate chip of the present invention;

[0023] Figure 5 This is a schematic diagram of the internal structure of the thin-film lithium niobate chip microring cascaded phase modulator of the present invention;

[0024] Figure 6 This is a schematic diagram of the internal structure of the subwavelength chirped grating vertical coupler for the thin-film lithium niobate chip of the present invention. Detailed Implementation

[0025] The technical solution of the present invention will be further described in detail below with reference to the accompanying drawings and embodiments.

[0026] Example

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

[0028] Therefore, the following detailed description of the embodiments of the invention provided in the accompanying drawings is not intended to limit the scope of the claimed invention, but merely to illustrate selected embodiments of the invention. All other embodiments obtained by those skilled in the art based on the embodiments of the invention without inventive effort are within the scope of protection of the invention.

[0029] It should be noted that similar labels and letters in the following figures indicate similar items. Therefore, once an item is defined in one figure, it does not need to be further defined and explained in subsequent figures.

[0030] In the description of this invention, it should be noted that the terms "upper," "lower," "inner," "outer," etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings, or the orientation or positional relationship in which the product of this invention is usually placed when in use. They are only for the convenience of describing this invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limiting this invention.

[0031] In the description of this invention, it should also be noted that, unless otherwise explicitly specified and limited, the terms "set," "install," and "connect" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral connection; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; and they can refer to the internal connection of two components. Those skilled in the art can understand the specific meaning of the above terms in this invention based on the specific circumstances.

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

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

[0034] The incident optical path includes a first set of mode field adapters, a 2×1 coupler, a polarizer, a phase modulator, and a second set of mode field adapters cascaded along the optical path direction. The first set of mode field adapters includes mode field adapter one and mode field adapter two. Mode field adapter one and mode field adapter two are respectively connected to the input end of the 2×1 coupler to combine the signal light and 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 TEO mode in the signal light and uses a stray light absorber connected to the polarizer to absorb the TEO mode in the signal light. The output end of the polarizer is connected to the input end of the phase modulator. The phase modulator has multiple sets of micro-rings cascaded in it. The output end of the phase modulator is connected to the second set of mode field adapters. The second set of mode field adapters includes mode field adapter three and mode field adapter four. Mode field adapter three and mode field adapter four are respectively connected to the two output ends of the phase modulator.

[0035] The scattered light circuit includes a cascaded mode field adapter and a vertical coupler, with a III-V group detector attached to the upper surface of the vertical coupler.

[0036] Mode field adapters one and two receive the signal light and indicator light, respectively, after being collimated by microlenses. These are then combined into a single beam via a 2×1 coupler and enter a polarizer to filter out the TEO mode in the signal light, adapting to the polarization selectivity of the lithium niobate material's modulation efficiency. After entering the modulation region, the signal light, satisfying the microring resonance condition, will repeatedly transmit the modulated signal within each microring assembly, while the indicator light, not satisfying the resonance condition, passes directly through a straight waveguide. The signal light and indicator light, after being output from the chip via mode field adapters three and four, are collimated by a microlens array and projected onto the surface of the object under test. The scattered light returns to the chip via mode field adapter five and enters the detector via a vertical coupler, where it is converted into an electrical signal.

[0037] In this application, the thin-film lithium niobate chip, except for the detector, includes a mode field adapter, a 2×1 coupler, a polarizer, a phase modulator, and a detector, all of which are based on a thin-film lithium niobate ridge waveguide and optically interconnected through the waveguide.

[0038] The ridge waveguide structure used in the thin-film lithium niobate chip is fabricated from 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, from bottom to top, consists of a silicon substrate layer, a silicon dioxide lower cladding layer, a thin-film lithium niobate ridge waveguide layer, and a silicon dioxide upper cladding layer. The ground electrode of the modulation area is located inside the silicon dioxide lower cladding layer, covering the entire modulation area. The positive and negative signal electrodes are located inside the silicon dioxide upper cladding layer, covering only the area above the micro-ring on the same side. Each electrode has a pad brought out to the chip surface through a via, which is used to apply a spatial electric field perpendicular to the wafer surface to the modulation area. The thin-film lithium niobate ridge waveguide layer is composed of a substrate layer with infinite width and a ridge layer with finite width.

[0039] like Figure 2 As shown, all the mode field adapters are coupled to the microlenses.

[0040] The mode field adapter consists of a thin-film lithium niobate ridge waveguide and a substrate trapezoidal waveguide. The trapezoidal waveguide and the ridge waveguide satisfy the lateral mode matching condition. The width of the substrate of the ridge waveguide is slightly smaller than the width of the trapezoidal waveguide. The width of the trapezoidal waveguide gradually narrows along the direction away from the coupling region, which can further expand the mode field. Mode field adapter one and mode field adapter two are used to receive the input light from the signal light source and the indicator light source, respectively. Mode field adapter three and mode field adapter four collimate the light output chip through a microlens. Mode field adapter five receives the scattered light carrying the velocity of the moving object. The ridge waveguide structure of each mode field adapter is the same, but the width and structure of the trapezoidal waveguide are different.

[0041] like Figure 3 As shown, the 2×1 coupler is an MMI coupler or a directional coupler. Its coupling region length is an even multiple of three times the beat length of the input mode corresponding to the wavelengths TE0 and TM0 of the signal light source, and at the same time, it is an odd multiple of three times the beat length of the input mode corresponding to the wavelengths TE0 and TM0 of the indicator light source. This ensures that after passing through the coupler, the signal light still propagates inside the waveguide on the same side, while the indicator light couples into the waveguide on the opposite side.

[0042] like Figure 4As shown, the polarizer of the thin-film lithium niobate chip consists of a 2×2 MMI on the left, a phased array region in the middle, and a 2×1 MMI on the right. The input light first passes through two straight waveguides of unequal length in the phased array region. The length difference between the two waveguides is one-quarter of the effective wavelength of the TE0 mode of the signal light. The subwavelength grating can completely reflect the TE0 mode of the signal light. After phase control, the TE0 modes of the two arms of the signal light have a phase difference of pi compared to the input, and will be output from the lower left port of the left 2×2 MMI into the stray light absorber. The TM0 mode of the input signal light and both the TE0 and TM0 modes of the indicator light are not reflected by the subwavelength grating and are output from the right port through the two MMIs with low loss. The stray light absorber consists of a thin-film lithium niobate ridge waveguide and a pair of metal electrodes grown on the waveguide substrate layer and very close to the ridge layer.

[0043] like Figure 5 As shown, the phase modulator of the thin-film lithium niobate chip has multiple micro-rings arranged on the inner sides of both arms to fold the optical path and reduce the overall size of the chip. Each micro-ring assembly consists of three micro-rings, from the first to the third. Each micro-ring satisfies the resonance condition for the signal light, but not the resonance condition for the indicator light. After the signal light is input, it establishes a resonance relationship in the three micro-rings sequentially and is transmitted multiple times. The spatial electric field applied to the two arms only covers the location where the micro-ring waveguide exists, and the directions are opposite, determined by the polarity of the electrodes inside the upper cladding. The modulation signal of the modulator is a sawtooth wave with a modulation depth of 0.5π.

[0044] like Figure 6 As shown, the vertical coupler is a chirped grating structure. The signal light input to the vertical coupler through the fifth 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 attached above it and converted into an electrical signal. The period of the subwavelength chirped grating changes alternately, which can broaden the scattering optical spectrum and ensure that most of the returned signal light after the frequency shift can be absorbed by the detector.

[0045] The application process is as follows:

[0046] The light emitted from the signal light source and the indicator light source is collimated by microlenses and then input into mode field adapter 1 and mode field adapter 2 respectively, and coupled into the chip. It is then combined into a beam by a specially designed 2×1 coupler. After the TEO mode of the signal light is filtered out by a phased polarizer, it enters the cascaded micro-ring phase modulator to load the modulation signal. After being output from the chip by mode field adapter 3 and mode field adapter 4, it is collimated by a combination of microlenses and then illuminates the surface of the object under test. The scattered light carries the object's motion velocity information and returns to the chip through mode field adapter 5. It then enters the III-V group detector through a subwavelength chirped grating vertical coupler and is converted into an electrical signal for subsequent data processing to calculate the object's motion velocity and to apply feedback control to the output light intensity of the two light sources.

[0047] It should be noted that the above detailed descriptions are exemplary and intended to provide further explanation of this application. Unless otherwise specified, 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 pertains.

[0048] It should be noted that the terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the exemplary embodiments described in this application. As used herein, the singular form is intended to include the plural form as well, unless the context clearly indicates otherwise. Furthermore, it should be understood that when the terms "comprising" and / or "including" are used in this specification, they indicate the presence of features, steps, operations, devices, components, and / or combinations thereof.

[0049] It should be noted that the terms "first," "second," etc., used in the specification, claims, and accompanying drawings of this application are used to distinguish similar objects and are not necessarily used to describe a specific order or sequence. It should be understood that such terms can be used interchangeably where appropriate so that the embodiments of this application described herein can be implemented in sequences other than those illustrated or described herein.

[0050] Furthermore, the terms “comprising” and “having”, and any variations thereof, are intended to cover non-exclusive inclusion. 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 that are explicitly listed, but may include other steps or units that are not explicitly listed or that are inherent to such process, method, product, or apparatus.

[0051] For ease of description, spatial relative terms such as "above," "on top of," "on the upper surface of," "above," etc., are used herein to describe the spatial positional relationship of a device or feature as shown in the figures to other devices or features. It should be understood that spatial relative terms are intended to encompass different orientations in use or operation beyond the orientation of the device as described in the figures. For example, if the device in the figures were inverted, a device described as "above" or "on top of" other devices or structures would subsequently be positioned as "below" or "under" other devices or structures. Thus, the exemplary term "above" can 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 spatial relative descriptions used herein will be interpreted accordingly.

[0052] In the detailed description above, reference has been made to the accompanying drawings, which form part of this document. In the drawings, similar symbols typically identify similar parts unless the context otherwise indicates otherwise. The illustrated embodiments described in the detailed specification, drawings, and claims are not intended 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.

[0053] The above description is merely a preferred embodiment of this application and is not intended to limit this application. Various modifications and variations can be made to this application by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of this application should be included within the protection scope of this application.

Claims

1. A thin-film lithium niobate chip for use in a hybrid integrated laser Doppler velocimeter, characterized in that: The chip internally cascades a first set of mode field adapters, a 2×1 coupler, a polarizer, a phase modulator, and a second set of mode field adapters along the incident light path. The first set of mode field adapters includes mode field adapter one and mode field adapter two, which are respectively connected to the input of the 2×1 coupler to combine the signal light and the indicator light. The output of the 2×1 coupler is connected to the input of the polarizer, which is used to filter out the TEO mode in the signal light and uses a stray light absorber connected to the polarizer to absorb the TEO mode in the signal light. The output of the polarizer is connected to the input of the phase modulator, which has multiple sets of micro-rings cascaded in it. The output of the phase modulator is connected to the second set of mode field adapters, which includes mode field adapter three and mode field adapter four, which are respectively connected to the two outputs of the phase modulator. The chip also includes a scattering light circuit, which includes a cascaded mode field adapter five and a vertical coupler. A III-V group detector is attached to the surface of the vertical coupler.

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

3. A 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 based on thin-film lithium niobate ridge waveguide and are optically interconnected through the waveguide.

4. A 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 fabricated from 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, from bottom to top, consists of a silicon substrate, a silicon dioxide lower cladding, a thin-film lithium niobate ridge waveguide layer, and a silicon dioxide upper cladding. The ground electrode of the modulation area is located inside the silicon dioxide lower cladding, covering the entire modulation area. The positive and negative signal electrodes are located inside the silicon dioxide upper cladding, covering only the area above the micro-ring on the same side. Each electrode has a pad brought out to the chip surface via a via, which is used to apply a spatial electric field perpendicular to the wafer surface to the modulation area.

5. A 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 trapezoidal waveguide. The trapezoidal waveguide and the ridge waveguide satisfy the lateral mode matching condition. The width of the substrate of the ridge waveguide is slightly smaller than the width of the trapezoidal waveguide. The width of the trapezoidal waveguide gradually narrows away from the coupling region. Mode field adapter one and mode field adapter two are used to receive the input light from the signal light source and the indicator light source, respectively. Mode field adapter three and mode field adapter four collimate the light output chip through a microlens. Mode field adapter five receives the scattered light carrying the velocity of the moving object. The ridge waveguide structure of each mode field adapter is the same, but the width and structure of the trapezoidal waveguide are different.

6. A 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 its coupling region length is an even multiple of three times the beat length of the input mode corresponding to the wavelengths TE0 and TM0 of the signal light source, and at the same time, it is an odd multiple of three times the beat length of the input mode corresponding to the wavelengths TE0 and TM0 of the indicator light source.

7. A 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 phased array region in the middle, and a 2×1 MMI on the right. The input light first passes through two straight waveguides of unequal length in the phased array region. The length difference between the two waveguides is one-quarter of the effective wavelength of the TE0 mode of the signal light. The subwavelength grating reflects all the TE0 mode of the signal light. After phase control, the TE0 mode of the two arms of the signal light has a phase difference of pi compared to the input. It will be output from the lower port of the 2×2 MMI and enter the stray light absorber. 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 through the output ports of the two MMIs with low loss. The stray light absorber consists of a thin-film lithium niobate ridge waveguide and a pair of metal electrodes grown on the waveguide substrate layer and close to the ridge layer.

8. A thin-film lithium niobate chip for a hybrid integrated laser Doppler velocimeter according to claim 1, characterized in that: The phase modulator has multiple micro-ring combinations for folding optical paths on the inner sides of both arms. Each micro-ring combination consists 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. After the signal light is input, it establishes a resonance relationship in the three micro-rings in sequence and is transmitted multiple times. The spatial electric field applied by the two arms only covers the location where the micro-ring waveguide exists, and the directions are opposite and determined by the polarity of the electrodes inside the upper cladding. The modulation signal of the modulator is a sawtooth wave with a modulation depth of 0.5π.

9. A 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 from the mode field adapter five input vertical coupler enters the subwavelength chirped grating scattering region after the mode field is gradually expanded through multiple tapered waveguides. It is then coupled into the III-V group detector attached to the grating and converted into an electrical signal. The period of the subwavelength chirped grating changes alternately to broaden the scattering optical spectrum.

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