WO2022134136A1

WO2022134136A1 - Frequency-modulated continuous-wave lidar system and lidar scanning method

Info

Publication number: WO2022134136A1
Application number: PCT/CN2020/140331
Authority: WO
Inventors: 疏达; 李鲲; 杨野; 李�远
Original assignee: 北醒（北京）光子科技有限公司
Priority date: 2020-12-25
Filing date: 2020-12-28
Publication date: 2022-06-30
Also published as: CN112764007A

Abstract

A frequency-modulated continuous-wave lidar system and a lidar scanning method, which relate to the technical field of measurements. The frequency-modulated continuous-wave lidar system comprises: a frequency modulation device (30) performs frequency modulation on laser light emitted by a laser (20); an interference device (40) divides the frequency-modulated laser light into local oscillator light and signal light, wherein the signal light is emitted from an optical waveguide, and then scanned and emitted successively by at least two wedge mirrors in a wedge mirror scanning device (50), and the interference device (40) performs interference on the signal light reflected by a target and the local oscillator light to generate a beat frequency signal; and a signal receiving and processing device (60), which converts the beat frequency signal into an electrical signal, processes the converted electrical signal to obtain frequency components in the beat frequency signal, and calculates measurement results according to the frequency components. The lidar system is suitable for medium and long-distance measurement, and is able to ameliorate the problem of reduced coupling between echo signals and receiving devices caused by time-of-flight.

Description

A frequency modulated continuous wave lidar system and lidar scanning method

This application claims the priority of the Chinese patent application filed on December 25, 2020 with the application number 202011566650.4 and the invention title is "A Frequency Modulated Continuous Wave Lidar System and Lidar Scanning Method", the entire contents of which are approved by Reference is incorporated in this application.

technical field

The present application relates to the field of measurement technology, and in particular, to a frequency-modulated continuous wave laser radar system and a laser radar scanning method.

Background technique

Frequency Modulation Continuous Wave Lidar (FMCW Lidar) is essentially heterodyne interferometry. The principle is to linearly modulate the laser frequency and divide the laser into two parts, one for the local oscillator light, one for the signal light, and the other for the signal light. It is collimated and reflected by the target into the receiving system. It interferes with the local oscillator light to generate a beat frequency signal. The signal carries the frequency tuning amount produced by the time of flight and the Doppler frequency shift produced by the relative motion. The beat frequency is measured to calculate Distance and speed information.

At present, the scanning mechanism in the FMCW lidar system mainly scans based on reflection methods such as galvanometers and prisms. When this scanning method is used for medium and long-distance measurement, it is easy to cause the problem that the coupling between the echo signal and the receiving device caused by the time-of-flight is reduced, thereby affecting the imaging quality.

SUMMARY OF THE INVENTION

In view of this, embodiments of the present application provide a frequency-modulated continuous wave laser radar system and a laser radar scanning method, which are convenient to improve the problem of reduced coupling between echo signals and receiving devices caused by time-of-flight.

In order to achieve the above purpose, in the first aspect, an embodiment of the present application provides a frequency modulation continuous wave lidar system, including: a laser, a frequency modulation device, an interference device, a wedge mirror scanning device, and a signal receiving and processing device; the frequency modulation device, using For frequency modulation of the laser light emitted by the laser; the interference device is used to divide the frequency-modulated laser light into local oscillator light and signal light, wherein the signal light is emitted by the optical waveguide, and then passes through the wedge mirror scanning device. At least two wedge-shaped mirrors are scanned and emitted successively, wherein the interference device is also used for interfering the signal light reflected by the target and the local oscillator light to generate a beat frequency signal; the signal receiving and processing device, using Converting the beat signal into an electrical signal, processing the converted electrical signal to obtain a frequency component in the beat signal, and calculating a measurement result according to the frequency component.

According to a specific implementation of the embodiment of the present application, the laser is a narrow linewidth laser; the frequency modulation device includes: a direct digital frequency synthesizer and a two-way Mach-Zehnder interference modulator; the direct digital frequency synthesizer is used for Two orthogonal radio frequency signals are generated to drive a two-way Mach-Zehnder interferometric modulator; the two-way Mach-Zehnder interferometric modulator is used to perform linear frequency modulation on the laser light emitted by the laser according to the driving of the radio frequency signal .

According to a specific implementation of the embodiment of the present application, the interference device includes: a splitter, a polarization controller, a circulator, and an optical waveguide; the splitter divides the frequency-modulated laser into a first path of laser light and an optical waveguide. The second path of laser light; the first path of laser light becomes local oscillator light after passing through the polarization controller; the second path of laser light is signal light, and the signal light is emitted through the optical waveguide after passing through the circulator; The signal light reflected by the target is coupled to the optical waveguide, and mixed with the local oscillator light after passing through the circulator to obtain a beat frequency signal.

According to a specific implementation of the embodiment of the present application, the lidar system further includes an optical splitter, and the optical splitter is used to divide the frequency-modulated laser light into N laser beams; the optical waveguide is composed of N bundles of optical fibers side by side an optical fiber array; the splitter divides the N paths of laser light formed by the splitter into the first path of laser light and the second path of laser light; the second path of laser light has N paths, which pass through The fiber array exits; wherein, the N is a natural number ≥ 2.

According to a specific implementation of the embodiment of the present application, the wedge mirror scanning device includes: a collimating lens, a first wedge mirror and a second wedge mirror; the collimating lens collimates the signal light emitted by the interference device The collimated signal light is scanned and emitted through the first wedge mirror and the second wedge mirror; the signal light reflected by the target passes through the second wedge mirror and the first wedge mirror, and passes through the collimation A lens is coupled to the optical waveguide.

According to a specific implementation of the embodiment of the present application, the signal receiving and processing device includes: a photodetector, an analog-to-digital converter, and a processing module; the photodetector converts the beat frequency signal into an electrical signal, and the The analog-to-digital converter samples the converted electrical signal, and the processing module obtains the frequency component in the beat signal according to the sampled signal, and calculates the measurement distance and/or the measurement speed according to the frequency component.

According to a specific implementation manner of the embodiment of the present application, the frequency-modulated continuous wave laser radar system further includes a fiber amplifier for amplifying the frequency-modulated laser light.

In a second aspect, an embodiment of the present application further provides a measurement method based on a frequency-modulated continuous wave lidar, including: frequency-modulating the laser light emitted by the laser; dividing the frequency-modulated laser light into a local oscillator light and a signal light, wherein the After the signal light is emitted from the optical waveguide, it is scanned and emitted successively through at least two wedge mirrors; the signal light reflected by the target interferes with the local oscillator light to generate a beat frequency signal; the beat frequency signal is converted into an electrical signal, and the The converted electrical signal is processed to obtain frequency components in the beat signal, and a measurement result is calculated according to the frequency components.

According to a specific implementation of the first embodiment of the present application, the dividing the frequency-modulated laser light into local oscillator light and signal light includes: dividing the frequency-modulated laser light into N lasers; dividing the N lasers into all The first path laser and the second path laser light; the first path laser light becomes local oscillator light after passing through the polarization controller, the second path laser light is signal light, and the second path laser light has N paths; After the signal light is emitted from the optical waveguide, it is scanned and emitted successively through at least two wedge mirrors. The wedge mirror scans out.

According to a specific implementation of the embodiment of the present application, the signal light reflected by the target interferes with the local oscillator light to generate a beat frequency signal, including: the signal light reflected by the target passes through the second wedge mirror and After the first wedge-shaped mirror, it is coupled to the light-emitting surface of the optical fiber array through the collimating lens.

In the FM continuous wave laser radar system and the laser radar scanning method according to the embodiments of the present application, after the signal light is emitted from the optical waveguide, it is scanned by at least two wedge-shaped mirrors in the wedge-shaped mirror scanning device, and a scanning trajectory symmetrical around the center of the optical axis can be obtained. The scanning speed is the smallest at the edge of the field of view, and the maximum scanning speed is only in the central field of view, so the average speed of the entire scanning process is relatively small, and the image shift is relatively small during most of the scanning time, thereby improving the overall time of flight. The resulting problem is that the coupling between the echo signal and the optical waveguide is reduced.

Description of drawings

In order to more clearly illustrate the embodiments of the present application or the technical solutions in the prior art, the following briefly introduces the accompanying drawings required for the description of the embodiments or the prior art. Obviously, the drawings in the following description are only These are some embodiments of the present application. For those of ordinary skill in the art, other drawings can also be obtained based on these drawings without any creative effort.

FIG. 1 is a schematic diagram of a frequency-modulated continuous wave laser ranging and speed measurement in an embodiment of the present application.

2a and 2b are schematic diagrams of receiving echo signals in a non-scanning state and a scanning state, respectively, based on reflection methods such as galvanometers and prisms.

Figure 3a is a schematic diagram of the light path transmission of a single wedge mirror.

Figure 3b is a schematic diagram of the scanning trajectory of a single wedge mirror.

Figure 3c Schematic diagram of the scanning principle of the double wedge mirror.

Figure 3d Schematic diagram of the scanning trajectory of the double wedge mirror.

FIG. 4 is a schematic structural diagram of a frequency-modulated continuous wave laser radar system according to an embodiment of the present application.

FIG. 5 is a schematic structural diagram of a frequency-modulated continuous wave laser radar system according to another embodiment of the present application.

FIG. 6 is a schematic diagram of linear modulation of the frequency of a radio frequency driving signal over time in an embodiment of the present application.

FIG. 7 is a schematic diagram of a DPMZID two-way modulation in an embodiment of the present application.

FIG. 8 is a schematic structural diagram of a frequency-modulated continuous wave laser radar system according to another embodiment of the present application.

FIG. 9 is a schematic diagram of the arrangement of an optical fiber array in an embodiment of the present application.

FIG. 10 is a schematic diagram of an optical path after exiting through an optical fiber array according to an embodiment of the present application.

FIG. 11 is a flowchart of a measurement method based on a frequency-modulated continuous wave lidar according to an embodiment of the present application.

Detailed ways

The embodiments of the present application will be described in detail below with reference to the accompanying drawings. It should be clear that the embodiments described herein are only a part of the embodiments of the present application, but not all of the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those of ordinary skill in the art without creative work fall within the protection scope of the present application.

The FMCW lidar system, different from the pulse radar to measure the time of flight of the pulse, is essentially a heterodyne interferometric measurement device. The light source uses a narrow linewidth laser, and linearly changes the wavelength of the laser by means of internal modulation or external modulation, so as to achieve linear modulation of the laser frequency. The modulated laser is divided into two paths, one is the local oscillator light and the other is the signal light. An optical antenna (optical lens) is used to collimate the signal light to increase the spatial direction angle gain. The collimated signal light illuminates the target for reflection and interferes with the local oscillator light. A photodetector is used to convert the interference light signal (also known as a beat frequency signal) into an electrical signal, and the beat frequency signal is amplified by the amplifier circuit and then sampled by an A/D (analog-to-digital converter), and the digital signal is processed in the processing module , and perform frequency estimation through algorithms such as fast Fourier transform to obtain the frequency components of the beat frequency. Among them, the frequency components include time-of-flight heterodyne frequency shift and Doppler frequency shift, and through simple operations, the target distance and axial velocity measurement can be achieved.

Figure 1 is a schematic diagram of the frequency-modulated continuous wave laser ranging and speed measurement. According to Figure 1, it can be found that the laser is subjected to triangular wave linear frequency modulation, A is the local oscillator light, and B is the echo (also called signal light). The echo and the local oscillator light interfere to generate a beat frequency signal. In order to ensure the frequency estimation accuracy of the beat frequency signal, a measurement time of at least one frequency modulation period is required. In a triangular wave frequency modulation cycle, there are two frequency components of the triangular wave rising and falling. If the measured object and the laser transceiver components are stationary, the two frequencies are equal, that is, the frequency shift generated during the echo flight time, and the frequency shift is the frequency modulation slope. Multiply by the flight time of the echo. If the measured object and the transceiver components move axially, a Doppler frequency shift will occur. Compared with microwave radar, the Doppler frequency shift of lidar is obvious. The beat frequency increases.

Therefore, the frequency of distance and velocity can be obtained through the beat frequency information generated by interference. The common mode frequency of the beat frequency of the rising edge and the falling edge is the heterodyne frequency generated by the distance, and the differential mode frequency is the Doppler frequency of the axial movement. Le frequency. The heterodyne frequency is positively related to the slope of the modulation frequency and the time of flight to obtain the time of flight, that is, the distance information; while the Doppler frequency shift is proportional to the axial velocity and inversely proportional to the wavelength to obtain the velocity information.

The scanning mechanism in the FMCW lidar system can scan based on reflection methods such as galvanometers and prisms. However, scanning based on reflection methods such as galvanometers and prisms may not be suitable for medium and long distance measurements. Because the scanning range is usually large in medium and long distance measurement, in order to ensure the refresh rate of the point cloud, the fast axis needs to scan at an extremely fast scanning speed, and the light used for measurement has a long flight time, which is easy to The echo signal deviates from the receiving surface of the optical waveguide, causing the offset of the imaging point, so that the receiving surface of the optical waveguide cannot receive the echo signal, or causes the echo signal to decay rapidly, which affects the measurement accuracy.

2a and 2b are schematic diagrams of receiving echo signals in a non-scanning state and a scanning state, respectively, based on reflection methods such as galvanometers and prisms. In Fig. 2a, when the scanning is not performed, the emitted light and the received light are coaxial (ie, the receiving and transmitting light is coaxial), and the signal light returns in the same way. At this time, the imaging point is on the receiving optical axis. In Fig. 2b, during scanning, the receiving optical axis (shown by the solid line) rotates during the flight time, and the measured object moves relative to the receiving optical axis at this time, which is equivalent to the image of the imaging point on the receiving surface. move the imaging point away from the optical axis. When the off-axis height of the imaging point away from the optical axis is greater than the size boundary of the receiving surface, the receiving surface cannot receive echo information.

In order to effectively reduce the problem of reduced coupling between the echo signal and the receiving device (optical waveguide) caused by the flight time, the embodiments of the present application provide a frequency-modulated continuous wave laser radar system and a laser radar scanning method.

Wedge mirror scanning principle:

When the optics pass through a wedge mirror, the light is deflected. The deflection angle is related to the index of refraction and the included angle of the wedge. As shown in Figure 3a, the refraction angle sinβ=n sinα, and the deflection angle of the outgoing light relative to the vertical incident light is φ=β-α; when the wedge mirror rotates around the vertical axis at an angle of γ, the The incident light rotates the included angle γ along the vertical axis, and always maintains the included angle φ with the vertical axis, which forms a circular scanning trajectory. FIG. 3b is a schematic diagram of a circular scanning trajectory formed when a wedge-shaped mirror rotates.

When the two wedge mirrors overlap and rotate according to their respective periods, their deflection angle φ and rotation angle γ are also superimposed respectively, forming a complex scanning trajectory. Under the respective specific scanning periods of the double wedge mirrors, full coverage aperiodic scanning of the circular area can be achieved, see Fig. 3c and Fig. 3d.

4 is a schematic structural diagram of a frequency-modulated continuous wave laser radar system according to an embodiment of the present application. Referring to FIG. 4 , the frequency-modulated continuous wave laser radar system 10 of this embodiment may include: a laser 20 , a frequency modulation device 30 , an interference device 40 , and a wedge mirror scanning device 50 and signal receiving and processing means 60 .

The frequency modulation device 30 is used to frequency modulate the laser light emitted by the laser 20; the interference device 40 is used to divide the frequency modulated laser light into local oscillator light and signal light. At least two wedge-shaped mirrors in the wedge-shaped mirror scanning device 50 are successively scanned and emitted, wherein the interference device is also used for interfering the signal light reflected by the target and the local oscillator light to generate a beat frequency signal; the signal receiving and processing device 60, It is used to convert the beat frequency signal into an electrical signal, process the converted electrical signal, obtain the frequency component in the beat frequency signal, and calculate the measurement result according to the frequency component.

In this embodiment, after the signal light is emitted from the optical waveguide, it is scanned by at least two wedge-shaped mirrors in the wedge-shaped mirror scanning device, so that a scanning trajectory that is symmetrical around the center of the optical axis can be obtained. The scanning speed is the smallest at the edge of the field of view, and the maximum scanning speed is only in the central field of view, so the average speed of the entire scanning process is relatively small, and the image shift is relatively small during most of the scanning time, thereby improving the overall time of flight. The resulting problem is that the coupling between the echo signal and the optical waveguide is reduced.

In addition, the use of wedge mirror scanning eliminates the need to allocate the number of scanning lines in the horizontal and vertical directions in the scanning space, and the scanning trajectory is rotated without periodic dislocation, and under multiple frames (that is, the scanned images obtained with the passage of scanning time) High point density and complete coverage of the entire scanning area can be achieved.

Furthermore, when the wedge mirror is used for scanning, since it rotates around the optical axis during scanning, a larger clear aperture can be obtained.

The laser 20 serves as a light source. In some embodiments, the laser 20 can be a narrow linewidth laser, which restricts the number of longitudinal moduli oscillating in the gain spectrum through wavelength selectors such as tunable filters, F-B filters, Bragg gratings, etc., so that only certain conditions are met. There are only a few longitudinal modes, and even only one longitudinal mode lasing occurs. The output light of narrow linewidth lasers has extremely high temporal coherence and extremely low phase noise. For the design needs of ultra-large dynamic range and long-distance imaging lidar, narrow linewidth lasers with linewidths below 10kHz can be used, the coherence length is 19km, and the coherence time is 65us.

The frequency modulation device 30 chirps the laser light emitted by the laser 20 . The FM linearity directly affects the ranging accuracy, and the long-distance high-precision ranging imaging lidar has higher requirements on the FM linearity. The time-of-flight of long-distance ranging results in a larger ranging scale, and it is necessary to ensure better FM linearity in the entire large dynamic range.

5, in some embodiments, the frequency modulation device 30 may include: a direct digital frequency synthesizer 301 (Direct Digital Synthesizer, DDS) and a two-way Mach-Zehnder Interferometer Modulator 302 (Double Mach-Zehnder Interferometer Modulator, DMZIM), The DDS is used to generate two orthogonal radio frequency signals to drive the DMZIM; the DMZIM is used to perform linear frequency modulation on the laser light emitted by the laser 20 according to the driving of the radio frequency signals.

Using DDS as a frequency source, two orthogonal RF signals are generated, and the frequency of the RF signals is linearly modulated by a triangular wave over time, as shown in Figure 6. After being amplified by the power amplifier, the radio frequency signal can drive the two MZI arms of the DPMZID respectively. At the same time, the voltages are configured for the two circuits (b, b' and p, p') of each MZI arm respectively to make it work at the lowest transmittance point. The voltage configuration makes it work at the half-wave voltage to complete the single-sideband carrier suppression modulation, as shown in Figure 7.

Continuing to refer to FIG. 5 , in some embodiments, the interference device 40 may include: a first splitter 401 , a polarization controller 402 , a circulator 403 and an optical waveguide 404 .

Among them, a splitter, which may also be called an optical splitter or an optical splitter, is a device used to realize the splitting of light wave energy according to a predetermined splitting ratio. The splitting ratio is the ratio of the output power of each output port of the splitter. In this embodiment, the first splitter 401 divides the frequency-modulated laser into two paths, that is, into a first path of laser light and a second path of laser light.

The first path of laser light becomes the local oscillator after passing through the polarization controller 402; the second path of laser light is the signal light, and the signal light passes through the circulator 403 and then exits through the optical waveguide 404; the signal light reflected by the target is coupled to the optical waveguide 404, After passing through the circulator 403, it is mixed with the local oscillator light (which can be mixed by the coupler 405) to obtain a beat frequency signal. The circulator 403 therein may also be referred to as a fiber optic circulator 403 .

The optical waveguide 404 is a medium device that guides light waves to propagate in it, and is also called a medium optical waveguide. The number of channels (also referred to as optical channels) of the optical waveguide may be one.

To increase the point cloud density of the obtained measurement image, the optical waveguide 404 may have multiple channels.

The lidar system may also include a second splitter 70 when the optical waveguide 404 has multiple channels. The frequency-modulated laser is first divided into N lasers through the second splitter 70, where N is a natural number ≥ 2, for example, N is 8, 12, or 16.

Each of the N lasers is divided by the second splitter, and then divided into two lasers by a first splitter 401 , namely, the aforementioned first laser and second laser. Each of the first splitters 401 may form an array of first splitters 401 .

The light-emitting end faces of the multiple channels of the optical waveguide 404 may be arranged in an array arrangement.

The optical waveguide 404 can take a variety of structural forms. In some embodiments, the optical waveguide 404 is in the form of an optical fiber bundle (also referred to as an optical fiber array), that is, in a structural form in which a plurality of optical fibers are combined side by side, wherein each optical fiber may be a single-mode optical fiber. In the fiber bundle, the light-emitting end faces of the fibers are arranged in an array.

The signal receiving and processing device 60 needs to collect the interference signal of the intrinsic light and the signal light. The interference signal needs to accurately and stably feed back the frequency change information of the measured signal. Stable and efficient interference requires similar deflection characteristics, similar spatial distribution and similar wavefront information. If the echo light is collected by ordinary optical systems, stable and efficient interference with the intrinsic light cannot occur. The part that cannot interfere will become signal noise. The measured object information carried by the echo signal will be submerged in noise and interference. In this embodiment, a single optical fiber can realize a coaxial optical path system, and the end face of the optical fiber is both the light emitting surface and the echo (signal light reflected by the target) receiving surface. Single-mode fiber enables control and screening of the wavelength, spatial distribution, and wavefront of the beam propagating within it, resulting in stable and efficient interference.

In some embodiments, before dividing the frequency-modulated laser light into N-channel laser light through the second splitter 70, a fiber amplifier may also be used to amplify the frequency-modulated laser light.

Continuing to refer to FIG. 5 , in some embodiments, the wedge-shaped mirror scanning device 50 may include: a collimating lens 501 , a first wedge-shaped mirror 502 and a second wedge-shaped mirror 503 .

The collimating lens 501 collimates the signal light emitted by the interference device, and the collimated signal light is scanned and emitted through the first wedge mirror 502 and the second wedge mirror 503; the signal light reflected by the target passes through the second wedge mirror 503. After the wedge mirror 503 and the first wedge mirror 502 , they are coupled to the optical waveguide 404 through the collimating lens 501 .

The first wedge-shaped mirror 502 and the second wedge-shaped mirror 503 are arranged at intervals along the optical axis of the collimating lens 501, and can rotate around the optical axis of the collimating lens 501 respectively, and the two can be rotated at different rotational speeds. The direction of rotation can be the same or different.

When the optical waveguide 404 is an optical fiber array, the light emitted by the light exit end face of one of the optical fibers, the signal light reflected by the target, is coupled to the light exit end face of the same optical fiber through the collimating lens 501, Optical fiber receiving can realize coaxial light emission and reception, which is beneficial to improve the accuracy of measurement.

Continuing to refer to FIG. 5, in some embodiments, the signal receiving and processing device 60 may include: a photodetector 601, an analog-to-digital converter 602 (Analog-to-digital converter, ADC) and a processing module 603; the photodetector 601, The beat frequency signal is converted into an electrical signal, the analog-to-digital converter 602 samples the converted electrical signal, and the processing module 603 obtains the frequency component in the beat frequency signal according to the sampled signal, and calculates the measurement distance and/or the measured distance according to the frequency component. Measure speed.

The photodetector 601 may be a balanced detector. The processing module 603 can be a field programmable gate array (Field Programmable Gate Array, FPGA) or a digital signal processor (digital signal processor, DSP).

In the following, a specific example is used to illustrate the frequency-modulated continuous wave laser radar system based on the embodiment of the present application with reference to FIG. 8 .

Referring to Figure 8, in this embodiment, the laser light emitted by a narrow linewidth laser with a linewidth below 10kHz (coherence length is 19km, coherence time is 65us) is used to generate a chirp signal by using DDS to drive a two-channel Mach-Zehnder interferometric modulator. (referred to as dual MZ) to linear frequency modulation of the laser.

The linear frequency-modulated laser is amplified to 10W by an Erbium-doped Optical Fiber Amplifer (EDFA), and divided into 12 channels by a 1*12 splitter. Considering the coupling loss, the output of each channel is guaranteed to be 1W. Each path passes through a 1*2 splitter, with a split ratio of 1:99. The weak light becomes a local oscillator after passing through the polarization controller, and is output to the balanced detector port. The energy of the local oscillator is 1mW; 99% of the energy is used as the main optical path , the output power is close to 600mW.

The main optical path laser passes through the circulator and then exits through the fiber array. The 12 channels share an optical lens and are collimated by the optical lens. After scanning and illuminating the target with a double wedge (the first wedge mirror and the second wedge mirror), the reflection is re-coupled to the fiber. , after passing through the circulator, it is mixed with the local oscillator light, and the mixed signal is received by a balanced detector.

The balanced detector converts the optical signal into an electrical signal and amplifies it. The main amplifier uses a variable gain amplifier VGA for time-sensitive linear gain control, and then uses 12 ADCs for synchronous sampling, and uses FPGA to sample through fast Fourier transform and other algorithms Perform frequency estimation, convert the frequency information into distance and upload it to the host computer.

The selection of the optical lens is mainly based on the mode coupling and the influence of the coma aberration at the edge of the array on the coupling efficiency. Taking the imaging requirement of 5km as an example, the outer diameter of the optical lens is 60mm, and the effective diameter is 56mm. Considering the effect of coma, a telephoto lens is required. Considering the overall coupling efficiency, the Gaussian-like distribution of the light intensity on the fiber end face, and the total length of the system, the focal length can be 220mm, and the coupling matching with the single-mode fiber is almost close to the diffraction limit.

The core diameter of the single-mode fiber is 8um, the mode field diameter is 10.4um, the focal length of the receiving lens is 220mm, and the output of the fiber is approximately Gaussian light, the exit aperture of the collimated light is about 42mm, and the divergence angle after collimation is about 0.05mrad. That is 0.003°. At the same time, considering the distribution of 12 channels, reducing the coma aberration and coupling efficiency caused by the deflection of the optical axis, the outgoing fiber array is arranged in a circular distribution, and the fiber array is arranged as shown in Figure 9. The spacing between each fiber is 250um, and the angle of view between the two fibers is 0.07°, as shown in Figure 10.

The insertion loss of each channel is 12dB through the splitter, and the single-channel light output is about 700mW. After the splitter of 0.5:99.5, the local oscillator light is about 3.5mW, and the power of the main optical path is about 28.3dBm. The optical fiber circulator 403 is incident from port 1 and exits from port 2, with an insertion loss of 0.8dB and an exit of about 27.5dBm, and exits through a collimating lens. The echo is also received by the collimating lens, re-coupled to the fiber circulator 403, and exits from the 3-port, and the local oscillator light passes through the two-port 50:50 coupler, irradiated to the balanced detector, and converted into an electrical signal for amplification and analysis .

Since the local oscillator light energy is 3.5mW, that is, 5.4dBm, which is an obvious DC component, it will be in a saturated state when a single detector is used for detection. Therefore, it is necessary to constrain the local oscillator light intensity noise to -55dBm for effective detection. The intensity noise of 20 is about -120dB/sqrt(Hz) and needs to be detected with a balanced detector.

Through lidar equation calculation and data analysis, the energy of the main lobe of the signal is from -45dBm to -31dBm, and the amplification factor of the balanced detector is about 53dB, so a balanced detector with a bandwidth of 70MHz can be used. The current equivalent noise is 7pW/sqrt(Hz), and within the main lobe signal bandwidth, the noise is about 1.2nW, which is close to the detection limit. After transimpedance amplification of the balanced detector, the signal power increases to 8dBm to 22dBm, that is, from 6mV to 150mV, while the noise output of the balanced detector is 15mV, which requires signal processing to analyze the signal in the frequency domain.

The main amplifier can be used to build a second-order low-pass filter, and fit the curve of the beat frequency signal energy and flight time to ensure the time-dependent gain, amplify the signal to 1.5V, and increase the gain from 10 times to 250 times to ensure that the ADC input maintains At the signal input of 1.5V, but considering that the equivalent noise output of TIA is 15mV, and the equivalent noise input of time gain is 5nV/sqrt(Hz), which is equivalent to the noise input of 0.7mV, it is necessary to ensure that the dynamic range of the waveform is within Within the dynamic range of the ADC, the gain needs to be reduced by a factor of 90 to meet the linear input below 3.3V.

The multi-channel ADC is used for data sampling, and the sampling rate is greater than twice the signal bandwidth, which satisfies the Nyquist sampling law. The digital signal is submitted to the FPGA for parallel calculation to complete the frequency estimation. Perform spectrum estimation on the rising edge and falling edge of the frequency modulation of the triangular wave at 4097 points, respectively, to obtain f _1F1 and f _1F2 , and through calculation, the common mode frequency is the measurement distance, and the differential mode frequency is the axis between the target and the radar direction velocity, as shown in the following formula:

2f _Dis =f _1F1 +f _1F2 ;

2f _Dop = f _1F2 -f _1F1 ;

where t is the laser flight time, k is the sweep slope, c is the speed of light, λ is the laser wavelength, v is the axial velocity, and R is the target distance.

And the position information of the double wedge is obtained through the zero position of the double wedge (the first wedge mirror and the second wedge mirror) and the photoelectric encoder, so as to use the FPGA to analyze the angle information of the laser, with the distance information and speed of the above analysis. information, so as to obtain long-distance 4D high-precision imaging information.

FIG. 11 is a flowchart of a measurement method based on a frequency-modulated continuous wave laser radar according to an embodiment of the present application. Referring to FIG. 11 , the measurement method of this embodiment includes steps:

S100, frequency modulate the laser light emitted by the laser;

S102. Divide the frequency-modulated laser light into local oscillator light and signal light, and after the signal light is emitted by the optical waveguide, it is scanned and emitted successively through at least two wedge mirrors;

S104, the signal light reflected by the target interferes with the local oscillator light to generate a beat frequency signal;

S106: Convert the beat signal into an electrical signal, process the converted electrical signal, obtain a frequency component in the beat signal, and calculate a measurement result according to the frequency component.

The measurement method in this embodiment can be applied to the foregoing FM continuous wave laser radar system embodiment, and its implementation manner and beneficial effects are basically the same, which will not be repeated here.

In some embodiments, dividing the frequency-modulated laser light into local oscillator light and signal light may include: dividing the frequency-modulated laser light into N lasers; dividing the N lasers into a first laser and a second laser Laser; the first laser becomes the local oscillator after passing through the polarization controller, the second laser is the signal light, and the second laser has N paths; after the signal light is emitted by the optical waveguide, it is scanned successively by at least two wedge mirrors The emitting includes: the second laser beam is emitted through the light-emitting surface of the optical fiber array, and after being collimated by the collimating lens 501, it is scanned and emitted through the first wedge mirror and the second wedge mirror.

In some embodiments, the signal light reflected by the target interferes with the local oscillator light to generate a beat frequency signal, including: the signal light reflected by the target passes through the second wedge mirror and the first wedge mirror, and passes through a collimating lens Coupled to the light exit surface of the fiber array.

In the measurement method of this embodiment, the wavelength of the laser light is linearly changed by means of internal modulation or external modulation for the laser light emitted by the laser, so as to realize the linear modulation of the laser frequency. The modulated laser is divided into two paths, one is the local oscillator light and the other is the signal light. The signal light is collimated by an optical lens, the collimated signal is scanned and emitted by the first wedge mirror and the second wedge mirror, and after being reflected by the target, it interferes with the local oscillator light. The interference optical signal is converted into an electrical signal, and after sampling the beat signal, the frequency is estimated by algorithms such as fast Fourier transform, and the frequency component of the beat frequency is obtained. Among them, the frequency components include time-of-flight heterodyne frequency shift and Doppler frequency shift, and through simple operations, the target distance and axial velocity measurement can be achieved.

It should be noted that, in this document, the terms “upper”, “lower” and the like indicated in terms of orientation or positional relationship are only for the convenience of describing the present application and simplifying the description, rather than indicating or implying that the referred device or element must be It has a specific orientation, is constructed and operates in a specific orientation, and therefore should not be construed as a limitation of the present application. Unless otherwise expressly specified and limited, the terms "installed", "connected" and "connected" should be understood in a broad sense, for example, it may be a fixed connection, a detachable connection, or an integral connection; it may be directly connected, It can also be indirectly connected through an intermediary. Relational terms such as, first and second, etc. are only used to distinguish one entity or operation from another and do not necessarily require or imply any such actual relationship between these entities or operations or order. Moreover, the terms "comprising", "comprising" or any other variation thereof are intended to encompass a non-exclusive inclusion such that a process, method, article or device that includes a list of elements includes not only those elements, but also includes not explicitly listed or other elements inherent to such a process, method, article or apparatus. Without further limitation, an element qualified by the phrase "comprising a..." does not preclude the presence of additional identical elements in the process, method, article, or device that includes the element. For those of ordinary skill in the art, it can be understood by specific situations.

The above are only specific embodiments of the present application, but the protection scope of the present application is not limited thereto. Any changes or substitutions that can be easily thought of by those skilled in the art within the technical scope disclosed in the present application should be Covered within the scope of protection of this application. Therefore, the protection scope of the present application shall be subject to the protection scope of the claims.

Claims

A frequency-modulated continuous wave laser radar system, characterized in that it includes: a laser, a frequency-modulation device, an interference device, a wedge mirror scanning device, and a signal receiving and processing device;

The frequency modulation device is used for frequency modulation of the laser light emitted by the laser;

The interference device is used to divide the frequency-modulated laser light into local oscillator light and signal light. After the signal light is emitted by the optical waveguide, it is successively scanned and emitted by at least two wedge-shaped mirrors in the wedge-shaped mirror scanning device. , wherein the interference device is also used for interfering the signal light reflected by the target with the local oscillator light to generate a beat frequency signal;

The signal receiving and processing device is configured to convert the beat signal into an electrical signal, process the converted electrical signal, obtain a frequency component in the beat signal, and calculate a measurement result according to the frequency component.
The frequency-modulated continuous wave lidar system according to claim 1, wherein the laser is a narrow linewidth laser;

The frequency modulation device includes: a direct digital frequency synthesizer and a two-way Mach-Zehnder interferometric modulator;

The direct digital frequency synthesizer is used to generate two quadrature radio frequency signals to drive a two-way Mach-Zehnder interferometric modulator;

The two-channel Mach-Zehnder interferometric modulator is used to perform linear frequency modulation on the laser light emitted by the laser according to the driving of the radio frequency signal.
The FM continuous wave lidar system according to claim 1, wherein the interference device comprises: a splitter, a polarization controller, a circulator and an optical waveguide;

The splitter divides the frequency-modulated laser into a first-channel laser and a second-channel laser; the first-channel laser becomes a local oscillator after passing through the polarization controller; the second-channel laser It is signal light, which passes through the circulator and is emitted through the optical waveguide; the signal light reflected by the target is coupled to the optical waveguide, and is mixed with the local oscillator light after passing through the circulator The beat frequency signal is then obtained.
The frequency-modulated continuous wave lidar system according to claim 3, wherein:

The lidar system further includes an optical splitter, and the optical splitter is used to divide the frequency-modulated laser light into N lasers;

The optical waveguide is an optical fiber array composed of N bundles of optical fibers side by side;

The splitter divides the N paths of laser light formed by the splitter into the first path of laser light and the second path of laser light; the second path of laser light has N paths, which pass through the optical fiber Array exit; wherein, the N is a natural number ≥ 2.
The frequency modulated continuous wave lidar system according to claim 1 or 4, wherein the wedge mirror scanning device comprises: a collimating lens, a first wedge mirror and a second wedge mirror;

The collimating lens collimates the signal light emitted by the interference device, and the collimated signal light is scanned and emitted by the first wedge mirror and the second wedge mirror; the signal light reflected by the target passes through the The second wedge mirror and the first wedge mirror are coupled to the optical waveguide through the collimating lens.
The FM continuous wave lidar system according to claim 1, wherein the signal receiving and processing device comprises: a photodetector, an analog-to-digital converter and a processing module;

The photodetector converts the beat frequency signal into an electrical signal, the analog-to-digital converter samples the converted electrical signal, and the processing module obtains the beat frequency signal according to the sampled signal The frequency components in , and the measurement distance and/or the measurement speed are calculated according to the frequency components.
The frequency-modulated continuous wave laser radar system according to claim 1, further comprising a fiber amplifier for amplifying the frequency-modulated laser light.
A measurement method based on frequency-modulated continuous wave lidar, characterized in that it includes:

Frequency modulation of the laser light emitted by the laser;

The frequency-modulated laser is divided into local oscillator light and signal light, wherein the signal light is emitted by the optical waveguide, and then scanned and emitted successively by at least two wedge mirrors;

The signal light reflected by the target interferes with the local oscillator light to generate a beat frequency signal;

The beat frequency signal is converted into an electrical signal, the converted electrical signal is processed, the frequency component in the beat frequency signal is obtained, and the measurement result is calculated according to the frequency component.
The measurement method according to claim 8, wherein the dividing the frequency-modulated laser light into a local oscillator light and a signal light, comprising:

Divide the frequency modulated laser into N lasers;

The N-channel lasers are divided into the first-channel laser and the second-channel laser; the first-channel laser becomes local oscillator light after passing through a polarization controller, the second-channel laser is signal light, and the first-channel laser is signal light. Two-channel laser has N channels;

Wherein, after the signal light is emitted by the optical waveguide, it is scanned and emitted successively by at least two wedge mirrors, including: the second laser light is emitted through the light exit surface of the optical fiber array, and after being collimated by a collimating lens, it is passed through the first wedge mirror. And the second wedge mirror scans out.
The measurement method according to claim 9, wherein the signal light reflected by the target interferes with the local oscillator light to generate a beat frequency signal, comprising:

The signal light reflected by the target, after passing through the second wedge mirror and the first wedge mirror, is coupled to the light emitting surface of the optical fiber array through the collimating lens.