Frequency modulation continuous wave laser radar

Abstract

The embodiment of the invention discloses a frequency-modulated continuous wave laser radar which comprises a light source module, a transceiving module and a scanning system. The light source module is used for emitting detection light, the transceiving module comprises a transceiving lens, a transceiving channel and a beam expanding structure, the transceiving lens is used for collimating the detection light, the transceiving channel is used for receiving the collimated detection light and transmitting the collimated detection light to the scanning system, and the scanning system is used for reflecting the collimated detection light to the beam expanding structure. The beam expanding structure is used for expanding the diameter of the probe light emitted by the scanning system. The scanning system comprises a one-dimensional galvanometer and a rotating mirror, and the rotating mirror is a regular polygon frustum. When the lower bottom surface of the rotating mirror is parallel to the horizontal plane and is upward, the one-dimensional galvanometer is positioned at the lower left part or the lower right part of the rotating mirror; and when the lower bottom surface of the rotating mirror is parallel to the horizontal plane downwards, the one-dimensional galvanometer is positioned at the upper left part or the upper right part of the rotating mirror. According to the laser radar provided by the embodiment of the invention, the Doppler broadening of the detection light can be reduced, the signal-to-noise ratio of the echo signal is improved, and the range finding capability of the laser radar system is improved.

Description

Technical Field

[0001] The present application relates to the field of laser detection technology, and in particular to a frequency modulated continuous wave laser radar system. Background Art

[0002] LiDAR detects the position, speed and other characteristic quantities of the target by emitting laser beams. Frequency Modulated Continuous Wave (FMCW) LiDAR can measure distance and speed at the same time. It is widely used in the fields of smart transportation and unmanned driving, and can provide more reliable distance and speed information for autonomous driving or assisted driving. Its working principle is to use the continuous wave obtained by frequency modulation as the local oscillator signal and detection signal, emit the detection signal to the target object, and then compare the received signal (echo signal) reflected from the target object with the local oscillator signal, and obtain the distance, direction, height, speed, attitude, and even shape of the target object after appropriate processing. Compared with the time of flight (ToF) speed and distance measurement solution, the FMCW laser detection and ranging system (Light Detection and Ranging, LiDAR) can identify the target object faster and transmit it to the vehicle control system to make risk avoidance operations in advance.

[0003] For FMCW LiDAR, which uses a one-dimensional galvanometer combined with a polygonal rotating mirror as a scanning system, the energy of the detection light and the echo light has a certain spatial distribution, and the radial components of the outgoing light or incident light at different points on the light spot are different relative to the outgoing or incident direction, so the Doppler frequency deviation values ​​hitting the polygonal rotating mirror are different, and finally reflected as Doppler broadening on the echo signal. This system combined with a transceiver module using a single lens will face the following problems: First, for silicon photonic chips that integrate transmission and reception, when the rotating mirror speed and focal length are too large, the light walk-off effect of the system is obvious, causing the receiving efficiency of the silicon photonic waveguide to drop sharply, which in turn seriously affects the ranging capability of the FMCW LiDAR. Second, the spot size received by the transceiver waveguide is large. When the rotating mirror is responsible for fast axis scanning, the spot will introduce strong Doppler broadening, which will broaden the originally sharp echo peak signal into a very wide wave packet signal, greatly reducing the signal-to-noise ratio of the echo signal. Summary of the invention

[0004] The embodiment of the present application aims to provide an improved solution for a frequency modulated continuous wave laser radar, which solves the problems of large size and severe Doppler broadening of existing frequency modulated continuous wave laser radars and improves the ranging capability of FMCW LiDAR.

[0005] The embodiment of the present application provides a frequency modulated continuous wave laser radar, including a light source module, a transceiver module and a scanning system, wherein the light source module is used to emit detection light. The transceiver module includes a transceiver lens, a transceiver channel and a beam expansion structure, wherein the transceiver lens is used to collimate the detection light, the transceiver channel is used to receive the collimated detection light and transmit it to the scanning system, the scanning system is used to reflect the collimated detection light to the beam expansion structure, and the beam expansion structure is used to expand the diameter of the detection light emitted by the scanning system. The scanning system includes a one-dimensional galvanometer and a rotating mirror, wherein the rotating mirror is a regular polygonal pyramid.

[0006] In some embodiments, the beam expansion structure includes a first lens and a second lens, the focus of the first lens coincides with the focus of the second lens, and the collimated detection light passes through the first lens and the second lens in sequence. In some embodiments, the first lens is a negative lens or a positive lens, and the second lens is a positive lens. The beam expansion system can expand the size of the output light spot by any multiple, and at the same time can increase the system echo light energy, which is conducive to reducing the system transmission power and improving the reception of light energy.

[0007] In some embodiments, when the bottom surface of the rotating mirror is parallel to the horizontal plane and facing upward, the one-dimensional galvanometer is located at the lower left or lower right of the rotating mirror. This structure can transform the horizontal detection light from horizontal to vertical through the one-dimensional galvanometer, and then transform the vertical light to horizontal through the rotating mirror before emitting.

[0008] In some embodiments, when the bottom surface of the rotating mirror is parallel to the horizontal plane and facing downward, the one-dimensional galvanometer is located at the upper left or upper right of the rotating mirror. The vertical arrangement of the rotating mirror and the one-dimensional galvanometer can improve the internal space utilization of the laser radar, reduce the height of the entire system, and help to compress the volume of the laser radar.

[0009] In some embodiments, the vertex angle of the regular polygonal pyramid is in the range of 15° to 65°. When the vertex angle of the regular polygonal pyramid is in this range, the radial velocity component of the scanning system on the light transmission and reception path is reduced, which can reduce the Doppler broadening and frequency shift introduced by the rotating mirror.

[0010] In some embodiments, the number of the transceiver channels is n, where n≥1, and the n transceiver channels do not overlap in the vertical direction. The embodiment of the present application supports an n-channel FMCW LiDAR, where more than one channel is staggered in the vertical direction of the scan, to reduce crosstalk between channels and expand the scanning angle.

[0011] In some embodiments, the light source module includes a laser and a beam splitter, the laser is used to generate an original light beam, and the beam splitter is used to obtain the detection light through the original light beam.

[0012] In some embodiments, the transceiver module also includes a circulator, a mixer and a photodetector. The circulator is used to guide the echo light reflected by the target object to the mixer. The mixer is used to receive the original light beam and the echo light except the detection light, so that the local oscillator light and the echo light beat each other to obtain a beat signal. The photodetector is used to convert the beat signal into an electrical signal. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] In order to more clearly illustrate the embodiments of the present application or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present application. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0014] Figure 1 It is a schematic diagram of the system architecture of the frequency modulated continuous wave laser radar provided in an embodiment of the present application;

[0015] Figure 2 It is a structural schematic diagram of a transceiver module and a scanning system of a frequency modulated continuous wave laser radar provided in an embodiment of the present application;

[0016] Figure 3 It is a schematic diagram of the effect of Doppler broadening on the signal-to-noise ratio.

[0017] Explanation of the reference numerals: 1. Scanning system; 2. Beam expansion structure; 10. Rotating mirror; 11. One-dimensional galvanometer; 21. Transceiver lens; 22. First lens; 23. Second lens; 3. Signal without Doppler broadening; 4. Signal with Doppler broadening. DETAILED DESCRIPTION

[0018] In order to make the objectives, technical solutions and advantages of the present application more clear, the embodiments of the present application will be further described in detail below with reference to the accompanying drawings.

[0019] When the following description refers to the drawings, unless otherwise indicated, the same numbers in different drawings represent the same or similar elements. The embodiments described in the following exemplary embodiments do not represent all embodiments consistent with the present application. Instead, they are only examples of devices and methods consistent with some aspects of the present application as detailed in the attached claims.

[0020] Figure 1The system architecture of the frequency modulated continuous wave laser radar provided in this embodiment is shown. The frequency modulated continuous wave laser radar includes a light source module, a transceiver module and a scanning system 1, wherein the light source module is used to emit detection light. In some embodiments, the light source module includes a laser and a spectrometer, the laser is used to generate an original light beam, and the spectrometer is used to receive the original light beam generated by the laser and divide the original light beam into local oscillator light and detection light. The transmission of optical signals between the spectrometer and the laser can be achieved by means of a waveguide, an optical fiber or a spatial optical path. Among them, the laser can be a gas laser, a semiconductor laser, a wavelength tunable solid laser, and an optical fiber frequency modulated laser, etc. The frequency modulated continuous wave generated by the laser can be internally modulated or externally modulated, and can have a nonlinear calibration path or not.

[0021] The transceiver module includes a transceiver lens 21, a transceiver channel and a beam expansion structure 2. The transceiver lens 21 is used to collimate the emitted detection light and converge the echo light onto the receiving waveguide or photodetector. When the FMCW LiDAR uses only one lens for transmission and reception, if the ultra-long-distance target object is to be measured, the equivalent transceiver aperture of the transceiver lens 21 needs to be very large, and the focal length is correspondingly larger. The larger the diameter of the light spot received by the transceiver waveguide, the more serious the Doppler broadening. Therefore, reducing the diameter of the light spot on the rotating mirror 10 can reduce the Doppler broadening. To solve this problem, a small-diameter transceiver lens 21 is used in the embodiments of the present application, and the diameter of the transceiver lens 21 is d. When the numerical aperture of the optical fiber is equal or the same waveguide is used, the diameter of the transceiver lens 21 is reduced to 1 / a of the original, the size of the emitted light spot is reduced to 1 / a before, and the Doppler broadening and frequency shift are also reduced to the original 1 / a. After the exit spot becomes smaller, the size of the rotating mirror 10 and other optical devices can be reduced accordingly, which can not only further reduce the Doppler broadening of the system but also reduce the volume of the laser radar. The use of the same optical path for transmission and reception reduces the hardware configuration of the laser radar and reduces the cost of the laser radar. The detection light is converted into parallel light after being collimated by the transceiver lens 21, and the parallel light is output to the scanning system 1 through the transceiver channel. In addition, the transceiver channel is also used to receive the echo light transmitted by the scanning system 1. In some embodiments, the number of transceiver channels is n, where n≥1, and the n transceiver channels do not overlap in the vertical direction. The transceiver channel can be a planar optical waveguide, an optical fiber, a grating, or a silicon waveguide on a chip, which is not limited in the embodiments of the present application. Due to the use of a small-aperture transceiver lens 21, if scanning detection is performed directly, the echo light energy of the system is insufficient, the echo signal-to-noise ratio is low, and the detection distance will be limited. For this reason, a beam expansion structure 2 is added downstream of the scanning system 1 along the transmission direction of the detection light to amplify the detection light emitted by the scanning system 1. The beam expansion structure 2 can reduce the divergence angle of the laser beam and make the focused spot smaller to improve its detection capability. The beam expansion structure 2 is input with a collimated beam and output with a collimated beam, and the magnification MP is expressed as the ratio of the output beam waist diameter to the input beam waist diameter. The detection beam waist diameter input to the beam expansion structure 2 is small but the divergence angle is large, and the detection beam waist diameter after beam expansion is large but the divergence angle is small. In some embodiments, the beam expansion structure 2 includes a first lens 22 and a second lens 23, and the focus of the first lens 22 coincides with the focus of the second lens 23, and the collimated detection light passes through the first lens 22 and the second lens 23 in sequence. Among them, the focal length of the first lens 22 is f1, the focal length of the second lens 23 is f2, and the magnification MP of the beam expansion structure 2 = f2 / f1. In one embodiment, the first lens 22 is a negative lens and the second lens 23 is a positive lens. The light is input through the negative lens and output from the positive lens, and the distance between the first lens 22 and the second lens 23 is equal to the difference between the focal lengths of the two lenses. This setup not only maintains the beam direction, but also reduces the spherical aberration of the output beam through the positive and negative lens combination.In one embodiment, the first lens 22 is a positive lens, and the second lens 23 is also a positive lens. The light beam is focused between the two lenses, and the distance between the first lens 22 and the second lens 23 is equal to the sum of the focal lengths of the two lenses. For the transmitting optical path, the divergence angle of the detection light becomes smaller, so the detection light can propagate a longer distance without exceeding the maximum beam diameter limit, thereby improving the detection capability of the detection light, thereby improving the detection capability of the frequency modulated continuous wave radar. For the receiving optical path, the beam expansion structure 2 can improve the reception of light energy, making the light spot falling on the transceiver lens 21 smaller, and improving the detection accuracy of the radar. The transceiver module uses a small-aperture transceiver lens 21 combined with a beam expansion structure 2, which can increase the equivalent transceiver aperture of the outgoing light spot and the incident light spot without changing the system scanning range, and does not introduce additional Doppler broadening or frequency shift, and does not introduce light walk-off effect. Although the diameter of the transceiver lens 21 is d, the equivalent transceiver capability of the transceiver module is equivalent to that of a transceiver module with an aperture of MP*d, and the volume of the laser radar system is greatly reduced, which is convenient for integration. According to the laser radar equation, if the transceiver aperture is doubled, the system echo light energy will increase fourfold. The increase in the equivalent transceiver aperture can reduce the system transmission power to a certain extent, which is beneficial to ensure the FMCW laser radar detection effect and reduce the total power consumption of the system.

[0022] The scanning system 1 is used to reflect the collimated detection light to the beam expansion structure 2, so that the detection light forms a detection field of view outside the laser radar after beam expansion, and transmits the echo light reflected by the target object. The scanning system 1 includes a one-dimensional galvanometer 11 and a rotating mirror 10. The one-dimensional galvanometer 11 is responsible for the slow axis scanning in the vertical direction, and the rotating mirror 10 is responsible for the fast axis scanning in the horizontal direction. Among them, the shape of the rotating mirror 10 is a regular polygonal pyramid, and the number of edges of the regular polygonal pyramid is M, where M is an integer greater than or equal to three. In some embodiments, the vertex angle of the regular polygonal pyramid is in the angle range of 15° to 65°, including 15° and 65°. Figure 2As shown, in one embodiment, the multifaceted rotating mirror 10 is a right quadrangular prism with a top angle of 45°. The lower bottom surface of the rotating mirror 10 is parallel to the horizontal plane and faces upward, and the one-dimensional galvanometer 11 is located at the lower left of the rotating mirror 10. During the emission of the detection light, the collimated detection light is horizontally incident from the right side of the scanning system 1, and is reflected by the one-dimensional galvanometer 11 to the 45-degree inclined reflection surface of the rotating mirror 10. Since the one-dimensional galvanometer 11 performs slow axis scanning, its radial velocity component in the light transmission and receiving path is very small, so the generated Doppler broadening can be ignored. The rotating mirror 10 has a radial velocity component in the incident direction of the detection light. When the light spot hits the rotating mirror 10, the radial velocity components at different positions are different, and the Doppler frequency deviation values ​​are different, so Doppler broadening is generated. The light spot collimated by the transceiver lens 21 is reflected once on the rotating mirror 10, which will introduce two Doppler broadening and frequency shift. During the incident process of the echo light, the light beam is horizontally incident on the rotating mirror 10, and is vertically emitted downward after being reflected by the 45° inclined surface. During the whole process, the scanning system 1 does not introduce a radial velocity component, so Doppler broadening will not be generated. The scanning system 1 composed of the one-dimensional galvanometer 11 and the 45° regular quadrangular prism rotating mirror 10 can convert the horizontal detection light from horizontal to vertical, and then convert the vertical light to horizontal through the 45° regular quadrangular prism rotating mirror 10. Compared with the existing scanning system of the one-dimensional galvanometer 11 combined with the vertical polygonal prism, the scanning scheme in the embodiment of the present application can reduce the Doppler broadening introduced in the transmission and reception process by half. In the process of scanning using the vertical polygonal prism as a rotating mirror, similar to the above-mentioned detection light emission process, when the vertical polygonal prism receives the echo light, two Doppler broadenings and frequency shifts will be introduced. In the detection light emission and reception paths, the vertical polygonal rotating mirror generates a total of 4 Doppler broadenings and frequency shifts, which seriously reduces the signal-to-noise ratio of the echo signal. Doppler broadening is the change in frequency or wavelength caused by the movement of a light source or object. This change is mainly caused by the Doppler effect. When there is a certain angle between the direction of motion of the receiver and the direction of incidence of the incident light, the Doppler effect will cause components of different frequencies or wavelengths to be broadened to different degrees (see Figure 3). Therefore, Doppler broadening can be reduced by reducing the radial velocity component of the scanning system 1 on the light receiving and emitting path. In one embodiment, the rotating mirror 10 is a regular hexagonal prism with a top angle of 60°. The lower bottom surface of the rotating mirror 10 is parallel to the horizontal plane and faces upward, and the one-dimensional galvanometer 11 is located at the lower right of the rotating mirror 10. In one embodiment, the rotating mirror 10 is a regular octagonal prism with a top angle of 30°. The lower bottom surface of the rotating mirror 10 is parallel to the horizontal plane and faces downward, and the one-dimensional galvanometer 11 is located at the upper left of the rotating mirror 10. In one embodiment, the rotating mirror 10 is a regular dodecagonal prism with a top angle of 40°. The lower bottom surface of the rotating mirror 10 is parallel to the horizontal plane and faces downward, and the one-dimensional galvanometer 11 is located at the upper right of the rotating mirror 10. The relative position relationship between the rotating mirror 10 and the one-dimensional galvanometer 11 has no effect on the scanning accuracy. When the lower bottom surface of the rotating mirror 10 is parallel to the horizontal plane and upward, the detection field of view formed by the scanning is wide at the top and narrow at the bottom; when the lower bottom surface of the rotating mirror 10 is parallel to the horizontal plane and downward, the detection field of view formed by the scanning is wide at the bottom and narrow at the top. The design and installation can be set according to the needs of the actual use scenario. The flexibility of the placement of the rotating mirror 10 and the one-dimensional galvanometer 11 can make more efficient use of space, reduce the height of the entire system, and is conducive to the miniaturization of the laser radar.

[0023] In some embodiments, the transceiver module also includes a circulator, a mixer and a photodetector. The circulator guides the echo light reflected by the target object to the mixer, and the mixer receives the original light beam and the echo light except the detection light, so that the original light beam except the detection light and the echo light beat each other therein to obtain a beat signal, and the photodetector performs photoelectric conversion on the beat signal to obtain an electrical signal related to the beat signal. Among them, the photodetector can be a photomultiplier tube, a semiconductor photodiode, an avalanche photodiode, an infrared and visible light multi-element detection device, etc., and the embodiments of the present application do not limit this. The transceiver module realizes the separation of the detection light path and the echo light path. The echo light will not return to the light source module along the original light path, but will flow to the mixer from another port, thereby ensuring the accuracy and reliability of the transmission of the detection light and the echo light.

[0024] In some embodiments, the FMCW laser radar also includes a conditioning circuit and a signal acquisition and data processing module. The conditioning circuit converts the current signal into a voltage signal, performs secondary amplification, and transmits it to the signal acquisition and data processing module. The signal acquisition and data processing module collects the signal output by the conditioning circuit, and calculates information such as the distance and speed of the target object based on the collected data to complete the detection of the FMCW laser radar. The signal acquisition and data processing module may include a processor, an analog-to-digital converter, a time-to-digital converter, a memory, and the like. In addition, the signal acquisition and data processing module may be connected to the active devices in the above-mentioned modules to control the active devices to work in an orderly manner.

[0025] The FMCW laser radar provided in the embodiment of the present application significantly reduces the Doppler broadening introduced by the scanning system 1 by reducing the radial velocity component of the scanning system 1 on the light receiving and light path and the diameter of the light receiving and light spot on the polygonal rotating mirror 10, thereby improving the signal-to-noise ratio of the echo signal, thereby improving the ranging accuracy of the frequency modulated continuous wave laser radar, and can achieve fast, efficient and accurate ranging and speed measurement of target objects. It can be applied to any field that requires ranging and speed measurement, such as smart transportation, aerospace, resource exploration, urban planning, agricultural development, water conservancy projects, land use, environmental monitoring, metallurgical manufacturing, textile manufacturing, etc.

[0026] In the description of the present application, it should be understood that the terms "first", "second", etc. are used for descriptive purposes only and should not be understood as indicating or implying relative importance. For those of ordinary skill in the art, the specific meanings of the above terms in the present application can be understood according to the specific circumstances. In addition, in the description of the present application, unless otherwise specified, "multiple" refers to two or more. "And / or" describes the association relationship of associated objects, indicating that three relationships may exist. For example, A and / or B can represent: A exists alone, A and B exist at the same time, and B exists alone. The character " / " generally indicates that the objects associated before and after are in an "or" relationship.

[0027] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as those commonly understood by those skilled in the art to which this application belongs. The terms used in this specification are only for the purpose of describing specific embodiments and are not intended to limit this application. The term "and / or" used herein includes any and all combinations of one or more of the related listed items.

[0028] In the description of the present application, it should be understood that the terms "center", "longitudinal", "lateral", "length", "width", "thickness", "up", "down", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "inside", "outside", "clockwise", "counterclockwise", "axial", "radial", "circumferential" and the like indicating orientations or positional relationships are based on the orientations or positional relationships shown in the drawings, which are only for the convenience of describing the present application and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation. Therefore, the terms describing the positional relationship in the drawings are only for illustrative purposes and should not be understood as limitations on the present application.

[0029] In the present application, unless otherwise expressly specified and limited, a first feature being "above" or "below" a second feature may include that the first and second features are in direct contact, or may include that the first and second features are not in direct contact but are in contact through another feature between them. Moreover, a first feature being "above", "above" and "above" a second feature includes that the first feature is directly above and obliquely above the second feature, or simply indicates that the first feature is higher in level than the second feature. A first feature being "below", "below" and "below" a second feature includes that the first feature is directly below and obliquely below the second feature, or simply indicates that the first feature is lower in level than the second feature. For those of ordinary skill in the art, the specific meanings of the above terms can be understood according to specific circumstances.

[0030] The above are only preferred embodiments of the present application and are not intended to limit the present application. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present application should be included in the protection scope of the present application.

Claims

1. A frequency modulated continuous wave laser radar, characterized in that: The frequency modulated continuous wave laser radar includes a light source module, a transceiver module and a scanning system; the light source module is used to emit detection light, the transceiver module includes a transceiver lens, a transceiver channel and a beam expansion structure, the transceiver lens is used to collimate the detection light, the transceiver channel is used to receive the collimated detection light and transmit it to the scanning system, the scanning system is used to reflect the collimated detection light to the beam expansion structure, and the beam expansion structure is used to expand the diameter of the detection light emitted by the scanning system; the scanning system includes a one-dimensional galvanometer and a rotating mirror, wherein the rotating mirror is a regular polygonal frustum.

2. The frequency modulated continuous wave laser radar according to claim 1, characterized in that: The beam expansion structure includes a first lens and a second lens, the focus of the first lens coincides with the focus of the second lens, and the collimated detection light passes through the first lens and the second lens in sequence.

3. The frequency modulated continuous wave laser radar as claimed in claim 2, characterized in that: The first lens is a negative lens or a positive lens, and the second lens is a positive lens.

4. The frequency modulated continuous wave laser radar according to claim 1, characterized in that: When the lower bottom surface of the rotating mirror is parallel to the horizontal plane and faces upward, the one-dimensional galvanometer mirror is located at the lower left or lower right of the rotating mirror.

5. The frequency modulated continuous wave laser radar according to claim 1, characterized in that: When the lower bottom surface of the rotating mirror is parallel to the horizontal plane and faces downward, the one-dimensional galvanometer mirror is located at the upper left or upper right of the rotating mirror.

6. The frequency modulated continuous wave laser radar according to claim 1, characterized in that: The vertex angle of the regular polygonal pyramid is in the range of 15° to 65°.

7. The frequency modulated continuous wave laser radar according to claim 1, characterized in that: The number of the transceiver channels is n, where n≥1, and the n transceiver channels do not overlap in the vertical direction.

8. The frequency modulated continuous wave laser radar according to any one of claims 1 to 7, characterized in that: The light source module includes a laser and a spectrometer. The laser is used to generate an original light beam. The spectrometer is used to obtain the detection light through the original light beam. The laser is also used to emit the detection light.

9. The frequency modulated continuous wave laser radar as claimed in claim 8, characterized in that: The transceiver module also includes a circulator, a mixer and a photodetector. The circulator is used to guide the echo light reflected by the target object to the mixer. The mixer is used to receive the original light beam and the echo light except the detection light. The original light beam and the echo light except the detection light beat in the mixer to obtain a beat signal. The photodetector is used to convert the beat signal into an electrical signal.

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