CN112764007A - Frequency modulation continuous wave laser radar system and laser radar scanning method - Google Patents
Frequency modulation continuous wave laser radar system and laser radar scanning method Download PDFInfo
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- G—PHYSICS
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- G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
- G01S7/00—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00
- G01S7/48—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00 of systems according to group G01S17/00
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- G01S7/4817—Constructional features, e.g. arrangements of optical elements relating to scanning
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
- G01S17/00—Systems using the reflection or reradiation of electromagnetic waves other than radio waves, e.g. lidar systems
- G01S17/02—Systems using the reflection of electromagnetic waves other than radio waves
- G01S17/06—Systems determining position data of a target
- G01S17/08—Systems determining position data of a target for measuring distance only
- G01S17/32—Systems determining position data of a target for measuring distance only using transmission of continuous waves, whether amplitude-, frequency-, or phase-modulated, or unmodulated
- G01S17/34—Systems determining position data of a target for measuring distance only using transmission of continuous waves, whether amplitude-, frequency-, or phase-modulated, or unmodulated using transmission of continuous, frequency-modulated waves while heterodyning the received signal, or a signal derived therefrom, with a locally-generated signal related to the contemporaneously transmitted signal
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
- G01S17/00—Systems using the reflection or reradiation of electromagnetic waves other than radio waves, e.g. lidar systems
- G01S17/02—Systems using the reflection of electromagnetic waves other than radio waves
- G01S17/50—Systems of measurement based on relative movement of target
- G01S17/58—Velocity or trajectory determination systems; Sense-of-movement determination systems
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Abstract
The embodiment of the application discloses a frequency modulation continuous wave laser radar system and a laser radar scanning method, relates to the technical field of measurement, and aims to solve the problem of reduced coupling between an echo signal and a receiving device caused by flight time. A frequency modulated continuous wave lidar system comprising: the frequency modulation device is used for carrying out frequency modulation on laser emitted by the laser; the interference device divides the laser light subjected to frequency modulation into local oscillation light and signal light, wherein the signal light is emitted by the optical waveguide and then sequentially scanned and emitted by at least two wedge mirrors in the wedge mirror scanning device, and the interference device interferes the signal light reflected by a target with the local oscillation light to generate a beat frequency signal; and the signal receiving and processing device is used for converting the beat frequency signal into an electric signal, processing the converted electric signal to obtain frequency components in the beat frequency signal, and calculating a measurement result according to the frequency components. The embodiment of the application can be suitable for measuring medium and long distances.
Description
Technical Field
The application relates to the technical field of measurement, in particular to a frequency modulation continuous wave laser radar system and a laser radar scanning method.
Background
The principle of the Frequency modulated Continuous Wave Lidar is that heterodyne interference measurement is performed, a laser is linearly modulated, laser is divided into two parts, one part is local oscillator light, the other part is signal light, the signal light is collimated and emitted and enters a receiving system through target reflection, interference is performed on the signal light and the local oscillator light to generate beat Frequency signals, the signals carry Frequency tuning amount generated by flight time and Doppler Frequency shift generated by relative motion, and the beat Frequency is measured, so that distance and speed information is calculated.
At present, a scanning mechanism in an FMCW laser radar system mainly scans based on reflection modes such as a galvanometer and a prism. When the scanning mode is used for medium and long distance measurement, the problem of reduced coupling between an echo signal and a receiving device caused by flight time is easily caused, and thus the imaging quality is influenced.
Disclosure of Invention
In view of this, embodiments of the present application provide a frequency modulated continuous wave lidar system and a lidar scanning method, which are convenient for improving the problem of reduced coupling between an echo signal and a receiving device caused by flight time.
In order to achieve the above object, in a first aspect, an embodiment of the present application provides a frequency modulated continuous wave lidar system, including: the device comprises a laser, a frequency modulation device, an interference device, a wedge-shaped mirror scanning device and a signal receiving and processing device; the frequency modulation device is used for carrying out frequency modulation on laser emitted by the laser; the interference device is used for dividing the laser subjected to frequency modulation into local oscillator light and signal light, wherein the signal light is emitted by the optical waveguide and then sequentially scanned and emitted by at least two wedge mirrors in the wedge mirror scanning device, and the interference device is also used for interfering the signal light reflected by a target with the local oscillator light to generate a beat frequency signal; and the signal receiving and processing device is used for converting the beat frequency signal into an electric signal, processing the converted electric signal to obtain frequency components in the beat frequency signal, and calculating a measurement result according to the frequency components.
According to a specific implementation manner of the embodiment of the application, the laser is a narrow linewidth laser; the frequency modulation device comprises: a direct digital frequency synthesizer and a two-way mach-zehnder interferometric modulator; the direct digital frequency synthesizer is used for generating two orthogonal radio frequency signals and driving the double-path Mach-Zehnder interferometric modulator; and the double-path Mach-Zehnder interferometric modulator is used for carrying out linear frequency modulation on laser emitted by the laser according to the driving of the radio-frequency signal.
According to a specific implementation manner of the embodiment of the present application, the interference device includes: the polarization controller is connected with the optical waveguide; the splitter divides the laser subjected to frequency modulation into a first path of laser and a second path of laser; the first path of laser becomes local oscillation light after passing through the polarization controller; the second path of laser is signal light, and the signal light passes through the circulator and then is emitted through the optical waveguide; and the signal light reflected by the target is coupled to the optical waveguide, passes through the circulator and then is mixed with the local oscillator light to obtain a beat frequency signal.
According to a specific implementation manner of the embodiment of the present application, the laser radar system further includes an optical splitter, where the optical splitter is configured to split the frequency-modulated laser into N paths of laser light; the optical waveguide is an optical fiber array formed by N bundles of optical fibers in parallel; the splitter splits 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 has N paths and is emitted out through the optical fiber array; wherein N is a natural number more than or equal to 2.
According to a specific implementation manner of the embodiment of the present application, the wedge mirror scanning apparatus includes: the device comprises a collimating lens, a first wedge-shaped mirror and a second wedge-shaped mirror; the collimating lens collimates the signal light emitted by the interference device, and the collimated signal light is emitted by scanning through the first wedge-shaped mirror and the second wedge-shaped mirror; and the signal light reflected by the target passes through the second wedge-shaped mirror and the first wedge-shaped mirror and then is coupled to the optical waveguide through the collimating lens.
According to a specific implementation manner of the embodiment of the present application, the signal receiving and processing apparatus includes: the device comprises a photoelectric detector, an analog-digital converter and a processing module; the photoelectric detector converts the beat frequency signal into an electric signal, the analog-digital converter samples the converted electric signal, the processing module obtains frequency components in the beat frequency signal according to the sampled signal, and the measuring distance and/or the measuring speed are/is calculated according to the frequency components.
According to a specific implementation manner of the embodiment of the present application, the frequency modulated continuous wave lidar system further includes an optical fiber amplifier, configured to amplify the frequency modulated laser.
In a second aspect, an embodiment of the present application further provides a measurement method based on a frequency modulated continuous wave lidar, including: carrying out frequency modulation on laser emitted by a laser; dividing the laser subjected to frequency modulation into local oscillation light and signal light, wherein the signal light is emitted by the optical waveguide and then is sequentially scanned and emitted by at least two wedge-shaped mirrors; interfering the signal light reflected by the target with the local oscillator light to generate a beat signal; and converting the beat frequency signal into an electric signal, processing the converted electric signal to obtain frequency components in the beat frequency signal, and calculating a measurement result according to the frequency components.
According to a specific implementation manner of the embodiment of the present application, dividing the laser light subjected to frequency modulation into the local oscillator light and the signal light includes: dividing the laser subjected to frequency modulation into N paths of laser; dividing the N paths of laser into a first path of laser and a second path of laser; the first path of laser becomes local oscillator light after passing through a polarization controller, the second path of laser is signal light, and the second path of laser has N paths; after the signal light is emitted from the optical waveguide, the signal light is successively scanned and emitted through at least two wedge-shaped mirrors, and the scanning and the emitting process comprises the following steps: and the second path of laser is emitted through the light emitting surface of the optical fiber array, is collimated by the collimating lens and then is emitted by scanning through the first wedge-shaped mirror and the second wedge-shaped mirror.
According to a specific implementation manner of the embodiment of the present application, the generating a beat signal by interfering the signal light reflected by the target with the local oscillator light includes: and the signal light reflected by the target passes through the second wedge-shaped mirror and the first wedge-shaped mirror and is coupled to the light-emitting surface of the optical fiber array through the collimating lens.
According to the frequency modulation continuous wave laser radar system and the laser radar scanning method, after the signal light is emitted from the optical waveguide, the signal light is scanned through at least two wedge-shaped mirrors in the wedge-shaped mirror scanning device, and the scanning track symmetrical around the center of the optical axis can be obtained. The scanning speed is minimum in the edge field, and the scanning speed is maximum only in the central field, so that the average speed in the whole scanning process is relatively lower, the image shift in most scanning time is relatively smaller, and the problem of reduced coupling between an echo signal and an optical waveguide caused by flight time is integrally solved.
Drawings
In order to more clearly illustrate the embodiments of the present application or the technical solutions in the prior art, the drawings used in the description of the embodiments or the prior art will be briefly described below, it is obvious that the drawings in the following description are only some embodiments of the present application, and other drawings can be obtained by those skilled in the art without creative efforts.
Fig. 1 is a schematic diagram of frequency modulated continuous wave laser ranging and speed measurement in the embodiment of the present application.
Fig. 2a and 2b are schematic diagrams of echo signal reception in a non-scanning state and a scanning state, respectively, based on reflection methods such as a galvanometer and a prism.
Fig. 3a is a schematic diagram of optical path propagation of a single wedge mirror.
FIG. 3b is a schematic diagram of a single wedge mirror scan trajectory.
FIG. 3c is a schematic diagram of the scanning principle of the double wedge mirror.
FIG. 3d is a schematic diagram of a scanning trajectory of a double wedge mirror.
Fig. 4 is a schematic structural diagram of a frequency modulated continuous wave lidar system according to an embodiment of the present application.
Fig. 5 is a schematic structural diagram of a frequency modulated continuous wave lidar system according to another embodiment of the present application.
FIG. 6 is a diagram illustrating linear modulation of the frequency of the RF driving signal with time according to 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 lidar system according to yet another embodiment of the present application.
Fig. 9 is a schematic diagram of an arrangement of optical fiber arrays according to an embodiment of the present application.
Fig. 10 is a schematic diagram of an optical path after exiting through the optical fiber array according to an embodiment of the present application.
Fig. 11 is a flowchart of a measurement method based on frequency modulated continuous wave lidar according to an embodiment of the present application.
Detailed Description
The embodiments of the present application will be described in detail below with reference to the accompanying drawings.
It should be understood that the embodiments described herein are only a few embodiments of the present application, and not all embodiments. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present application.
The FMCW laser radar system is different from a pulse radar in measuring the flight time of pulses and is essentially a heterodyne interference measuring device. The light source uses a narrow linewidth laser, and the wavelength of the laser is linearly changed in an internal modulation or external modulation mode, so that the linear modulation of the laser frequency is realized. The modulated laser is divided into two paths, one path is local oscillator light, and the other path is signal light. The signal light is collimated by using an optical antenna (optical lens), the spatial directional angle gain is increased, and the collimated signal light irradiates a target to be reflected and interferes with the local oscillation light. The method comprises the steps of converting interference optical signals (also called beat frequency signals) into electric signals by using a photoelectric detector, amplifying the beat frequency signals by using an amplifying circuit, then sampling by using an A/D (analog-to-digital converter), processing the digital signals in a processing module, and performing frequency estimation by algorithms such as fast Fourier transform and the like to obtain frequency components of beat frequency. The frequency components comprise time-of-flight heterodyne frequency shift and Doppler frequency shift, and target distance and axial speed measurement is achieved through simple operation.
Fig. 1 is a schematic diagram of a frequency modulated continuous wave laser ranging and speed measurement, and it can be found from fig. 1 that triangular wave linear frequency modulation is performed on a laser, where a is local oscillator light and B is echo (which may also be referred to as signal light). The echo and the local oscillator light interfere to generate a beat frequency signal, and the measurement time of one frequency modulation period is at least needed to ensure the frequency estimation precision of the beat frequency signal. Within a triangular wave frequency modulation period, there are two frequency components of triangular wave rising and falling, if the object to be measured and the laser transceiver are stationary, the two frequencies are equal, i.e. the frequency shift amount generated within the flight time of the echo, and the frequency shift is the frequency modulation slope multiplied by the flight time of the echo. If the measured object and the receiving and transmitting component move axially, Doppler frequency shift is generated, compared with a microwave radar, the Doppler frequency shift of a laser radar is obvious, the beat frequency is reduced when the measured object and the receiving and transmitting component move oppositely, and the beat frequency is increased when the measured object and the receiving and transmitting component move oppositely.
Therefore, the frequency quantity of distance and speed can be obtained through beat frequency information generated by interference, the common mode frequency of the beat frequency of the rising edge and the falling edge is heterodyne frequency generated by the distance, and the differential mode frequency is Doppler frequency of axial movement. Heterodyne frequency is positively correlated with modulation frequency slope and flight time, thereby obtaining flight time, i.e. distance information; and the Doppler (Doppler) shift is proportional to the axial velocity and inversely proportional to the wavelength, thereby obtaining velocity information.
The scanning mechanism in the FMCW lidar system may perform scanning based on a reflection method such as a galvanometer or a prism. However, such scanning based on reflection methods such as galvanometer and prism may not be suitable for long and medium distance measurement. Because the scanning range is usually large under the medium-long distance measurement, in order to ensure the refreshing 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, so that the echo signal is easy to deviate from the receiving surface of the optical waveguide, which causes the deviation of an imaging point, and the receiving surface of the optical waveguide cannot receive the echo signal, or causes the echo signal to be attenuated quickly, which affects the measurement accuracy.
Fig. 2a and 2b are schematic diagrams of echo signal reception in a non-scanning state and a scanning state, respectively, based on reflection methods such as a galvanometer and a prism. In fig. 2a, when the scanning is not performed, the transmitting light and the receiving light are coaxial (i.e., coaxial transmission and reception), and the signal light returns in the original path, where the imaging point is on the receiving optical axis. In fig. 2b, during scanning, the receiving optical axis (shown by a solid line) rotates in the flight time, and the object to be measured moves relative to the receiving optical axis, which is equivalent to the image shift of the imaging point on the receiving surface, and the imaging point is far away from the optical axis. When the off-axis height of the imaging point far away from the optical axis is larger 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 an echo signal and a receiving device (optical waveguide) caused by flight time, the embodiment of the application provides a frequency modulation continuous wave laser radar system and a laser radar scanning method.
Wedge mirror scanning principle:
the light rays are deflected when the light rays pass through the wedge-shaped lens. The deflection angle is related to the refractive index and the wedge angle. As shown in fig. 3a, the refraction angle sin β = n sin α, and the deflection angle of the outgoing ray with respect to the vertical incident light is Φ = β - α; when the wedge-shaped mirror rotates around the vertical shaft by an angle gamma, the emergent light can rotate by an included angle gamma along the vertical shaft and always keep an included angle phi with the vertical shaft, and a circular scanning track is formed. FIG. 3b is a schematic diagram of a circular scan trajectory formed by the rotation of a wedge mirror.
When the two wedge-shaped mirrors are overlapped and rotate according to respective periods, the deflection angle phi and the rotation included angle gamma of the two wedge-shaped mirrors are respectively overlapped, and a complex scanning track is formed. Full coverage non-periodic scanning of the circular area can be achieved at each specific scanning period of the double wedge mirror, see fig. 3c and 3 d.
Fig. 4 is a schematic structural diagram of an fm cw lidar system according to an embodiment of the present disclosure, and referring to fig. 4, the fm cw lidar system 10 according to this embodiment may include: a laser 20, a frequency modulation device 30, an interference device 40, a wedge mirror scanning device 50 and a signal receiving and processing device 60.
The frequency modulation device 30 is configured to perform frequency modulation on laser light emitted by the laser 20; the interference device 40 is configured to divide the laser light subjected to frequency modulation into local oscillator light and signal light, where the signal light is emitted from the optical waveguide and then sequentially scanned and emitted by at least two wedge mirrors in the wedge mirror scanning device 50, and the interference device is further configured to interfere the signal light reflected by the target with the local oscillator light to generate a beat signal; and a signal receiving and processing device 60, configured to convert the beat frequency signal into an electrical signal, process the converted electrical signal to obtain frequency components in the beat frequency signal, and calculate a measurement result according to the frequency components.
In this embodiment, after the signal light is emitted from the optical waveguide, the signal light is scanned by at least two wedge mirrors in the wedge mirror scanning device, so that a scanning track symmetrical around the center of the optical axis can be obtained. The scanning speed is minimum in the edge field, and the scanning speed is maximum only in the central field, so that the average speed in the whole scanning process is relatively lower, the image shift in most scanning time is relatively smaller, and the problem of reduced coupling between an echo signal and an optical waveguide caused by flight time is integrally solved.
In addition, by adopting the wedge-shaped mirror scanning, the number of scanning lines in the transverse direction and the longitudinal direction in the scanning space is not required to be distributed, the scanning track is rotated without period dislocation, and the high-point density complete coverage of the whole scanning area can be realized under the condition of multiple frames (namely scanning images obtained along with the scanning time).
Moreover, when the wedge-shaped mirror is adopted for scanning, the wedge-shaped mirror rotates around the optical axis during scanning, so that a larger light-passing aperture can be obtained.
A laser 20 as a light source. In some embodiments, the laser 20 may be a narrow linewidth laser that is configured to be lasing in only a few longitudinal modes, even only one longitudinal mode, that satisfy certain conditions by limiting the number of longitudinal modes that oscillate in the gain spectrum by a wavelength selector such as a tunable filter, F-B filter, Bragg grating, etc. The output light of a narrow linewidth laser has extremely high temporal coherence and extremely low phase noise. Aiming at the design requirements of the ultra-large dynamic range and long-distance imaging laser radar, a narrow linewidth laser with linewidth below 10kHz can be adopted, the coherence length is 19km, and the coherence time is 65 us.
The frequency modulation device 30 chirps the laser light emitted by the laser 20. The frequency modulation linearity directly influences the distance measurement accuracy, and the requirement of the long-distance high-precision distance measurement imaging laser radar on the frequency modulation linearity is higher. The range measurement scale brought by the flight time of the long-distance range measurement is larger, and better frequency modulation linearity needs to be ensured in the whole large dynamic range.
Referring to fig. 5, in some embodiments, the frequency modulation device 30 may include: a Direct Digital Synthesizer (DDS) and a Double-path Mach-Zehnder interferometric Modulator (DMZIM) 302, wherein the DDS is configured to generate two orthogonal rf signals to drive the DMZIM; DMZIM is used to chirp the laser light emitted by the laser 20 according to the driving of the radio frequency signal.
Using DDS as a frequency source, two orthogonal radio frequency signals are generated, the frequency of which is linearly triangular wave modulated over time, as shown in fig. 6. The rf signal may be amplified by a power amplifier and then separately drive the two MZI arms of the DPMZID. Meanwhile, the two paths (b, b 'and p, p') of each MZI arm are respectively configured with voltage to work at the lowest transmittance point, and the two arms (c, d) of the mother MZI are configured with voltage to work at a half-wave voltage to complete single-sideband carrier suppression modulation, as shown in FIG. 7.
With continued reference to FIG. 5, in some embodiments, the interference device 40 may comprise: a first splitter 401, a polarization controller 402, a circulator 403, and an optical waveguide 404.
Here, the splitter, which may also be referred to as an optical splitter or an optical splitter, is a device for splitting optical 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 splits the frequency-modulated laser into two paths, that is, a first path of laser and a second path of laser.
The first path of laser becomes local oscillation light after passing through the polarization controller 402; the second path of laser is signal light, and the signal light is emitted through the optical waveguide 404 after passing through the circulator 403; the signal light reflected by the target is coupled to the optical waveguide 404, and is mixed with the local oscillator light (which may be mixed by the coupler 405) by the circulator 403 to obtain a beat signal. Wherein the circulator 403 may also be referred to as a fiber circulator 403.
Optical waveguide 404 (optical waveguide) is a dielectric device, also called a dielectric optical waveguide, that guides light waves to propagate therein. 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 measurement image obtained, the optical waveguide 404 may have multiple channels.
Where optical waveguide 404 has multiple channels, the lidar system may also include a second splitter 70. The frequency modulated laser is first split into N laser paths by the second splitter 70, where N is a natural number greater than or equal to 2, for example, N is 8, 12, or 16.
Each of the N laser beams is divided by the second splitter, and is divided by the first splitter 401 into two laser beams, that is, the first laser beam and the second laser beam. Each first splitter 401 may make up a first splitter 401 array.
The light exit 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 a fiber bundle (also referred to as a fiber array), that is, in the form of a plurality of optical fibers combined together in parallel, wherein each optical fiber may be a single-mode optical fiber. In the optical fiber bundle, the light-emitting end surfaces of the optical fibers are arranged in an array.
The signal receiving and processing device 60 needs to collect interference signals of the intrinsic light and the signal light. The interference signal needs to accurately and stably feed back the frequency change information of the detected signal. Stable and efficient interference requires similar deflection characteristics, similar spatial distribution and similar wavefront information. If a common optical system is used to collect the echo light, stable and efficient interference with the intrinsic light cannot be generated. The portion that cannot interfere becomes signal noise. The information of the measured object carried by the echo signal is submerged in noise and interference. In this embodiment, a coaxial optical path system can be realized by a single optical fiber, and the end face of the optical fiber is not only a light emitting surface, but also an echo (signal light reflected by a target) receiving surface. The single-mode optical fiber can realize control and screening of the wavelength, spatial distribution and wavefront of a transmitted light beam in the single-mode optical fiber, thereby realizing stable and efficient interference.
In some embodiments, the frequency modulated laser may be amplified by a fiber amplifier before being split into N laser beams by the second splitter 70.
With continued reference to FIG. 5, in some embodiments, a wedge mirror scanning device 50 may comprise: a collimating lens 501, a first wedge mirror 502 and a second wedge mirror 503.
The collimating lens 501 collimates the signal light emitted by the interference device, and the collimated signal light is emitted by scanning through the first wedge mirror 502 and the second wedge mirror 503; the signal light reflected by the target passes through the second wedge 503 and the first wedge 502, and is 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 disposed at intervals in the front-back direction of 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 rotate at different rotation speeds, and the rotation directions of the two can be the same or different.
When the optical waveguide 404 is an optical fiber array, the light emitted from the light emitting end surface of one of the optical fibers is coupled to the light emitting end surface of the same optical fiber through the collimating lens 501, and is received by the same optical fiber, so that the light emitting and receiving are coaxial, and the measurement accuracy is improved.
With continued reference to fig. 5, in some embodiments, the signal receiving and processing device 60 may comprise: a photodetector 601, an Analog-to-digital converter 602 (ADC) and a processing module 603; the photodetector 601 converts the beat frequency signal into an electrical signal, the analog-to-digital converter 602 samples the converted electrical signal, the processing module 603 obtains a frequency component in the beat frequency signal according to the sampled signal, and calculates a measurement distance and/or a measurement speed according to the frequency component.
Wherein the photodetector 601 may be a balanced detector. The processing module 603 may be a Field Programmable Gate Array (FPGA) or a Digital Signal Processor (DSP).
The following describes an embodiment of the present application based on a frequency modulated continuous wave lidar system with reference to fig. 8 as a specific example.
Referring to fig. 8, in this embodiment, a laser (with a coherence length of 19km and a coherence time of 65 us) emitted from a narrow linewidth laser with a linewidth of 10kHz or less is used to generate a chirp signal using a DDS, and a dual-path mach-zehnder interferometric modulator (abbreviated as dual-path MZ) is driven to chirp the laser.
Amplifying the laser light which is subjected to linear frequency modulation to 10W by an Erbium-doped Fiber amplifier (EDFA), dividing the laser light into 12 channels by a 1 x 12 splitter, ensuring the output of 1W of each channel by considering the coupling loss, enabling each channel to pass through a 1 x 2 splitter with the splitting ratio of 1:99, enabling the weak light to pass through a polarization controller to become local oscillation light, outputting the local oscillation light to a port of a balanced detector, and enabling the local oscillation light to be 1 mW; 99% of energy is used as a main light path, and the output power is close to 600 mW.
The laser of a main light path is emitted through the optical fiber array after passing through the circulator, the 12 channels share the optical lens, the laser is collimated through the optical lens, is reflected and recoupled to the optical fiber after being scanned and irradiated on a target through the double optical wedges (the first wedge-shaped mirror and the second wedge-shaped mirror), is mixed with the local oscillator light after passing through the circulator, and a mixing signal is received by using the balance detector.
The balance detector converts optical signals into electric signals and amplifies the electric signals, the main amplifier uses the variable gain amplifier VGA to carry out time-sensitive linear gain control, then 12 ADCs are used for carrying out synchronous sampling, FPGA is used for carrying out frequency estimation on the sampling through algorithms such as fast Fourier transform and the like, and frequency information is converted into distance and uploaded to an upper computer.
The choice of optical lens depends mainly on the mode coupling and the effect of coma 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 aperture is 56 mm. In consideration of the influence of coma, a telephoto lens needs to be used. The coupling efficiency, the light intensity Gaussian distribution of the end face of the optical fiber and the total length size of the system are integrated, the focal length can be 220mm, and the coupling matching with the single-mode optical fiber is almost close to the diffraction limit at the moment.
The diameter of the fiber core of the single-mode optical fiber is 8um, the diameter of the mode field is 10.4um, the focal length of the receiving lens is 220mm, the emergent light of the optical fiber is approximately Gaussian light, the emergent aperture of collimated light is about 42mm, and the divergence angle after collimation is about 0.05mrad, namely 0.003 degrees. Meanwhile, in consideration of 12-channel distribution, coma aberration and reduction of coupling efficiency caused by optical axis deflection are reduced, and outgoing optical fiber arrays are distributed in a circular shape, and are arranged as shown in fig. 9. Each fiber spacing is 250um, and the fiber field angles are spaced 0.07 ° from each other, as shown in fig. 10.
Each channel is subjected to 12dB insertion loss through a splitter, the single channel emits light with the power of about 700mW, and the power of each channel passes through 0.5: the splitter of 99.5 splits, the local oscillator light is about 3.5mW, the power of the main optical path is about 28.3dBm, the local oscillator light enters from the port 1 through the optical fiber circulator 403 with low insertion loss, the local oscillator light exits from the port 2, the insertion loss is 0.8dB, the exit is about 27.5dBm, and the exit is emitted through the collimating lens. The echo is received by the collimating lens, is coupled into the optical fiber circulator 403 again, is emitted from the 3-port, and is irradiated to the balance detector together with the local oscillation light through the coupler with the double ports being 50:50, and is converted into an electric signal for amplification analysis.
Since the local oscillator light energy is 3.5mW, i.e. 5.4dBm, and is an obvious dc component, the local oscillator light energy is detected by using a single detector, and the local oscillator light intensity noise is limited to-55 dBm to be detected effectively, and the intensity noise of the laser 20 is about-120 dB/sqrt (hz), and a balanced detector is required to be used for detection.
Through calculation and data analysis of a laser radar equation, the energy of a main lobe of a signal ranges from-45 dBm to-31 dBm, the amplification factor of a balanced detector is about 53dB, and therefore the balanced detector with the bandwidth of 70MHz can be used. The current equivalent noise is 7pW/sqrt (Hz), the noise is about 1.2nW in the bandwidth of the main lobe signal, and the detection limit is already approached. After transimpedance amplification of the balanced detector, the signal power increases to 8dBm to 22dBm, i.e., 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.
And a main amplifier can be used for building second-order low-pass filtering, a curve of beat frequency signal energy and flight time is fitted, time-dependent gain is guaranteed, signals are amplified to 1.5V, the gain is increased from 10 times to 250 times, the ADC input is guaranteed to be kept at 1.5V, but considering that the equivalent noise output of TIA is 15mV, the equivalent noise input of time gain is 5nV/sqrt (Hz), the equivalent noise input is 0.7mV, and the gain needs to be reduced by 90 times to guarantee that the waveform dynamic range is within the dynamic range of the ADC, so that the linear input below 3.3V is met.
And the multichannel ADC is used for sampling data, the sampling rate is more than twice of the signal bandwidth, the Nyquist sampling law is satisfied, and the digital signals are handed to the FPGA for parallel calculation to complete frequency estimation. Respectively carrying out frequency spectrum estimation on the rising edge and the falling edge of the frequency modulation of the triangular wave at 4097 points to obtainf 1F1 Andf 1F2 and through calculation, the common mode frequency is the measurement distance, and the differential mode frequency is the axial speed between the target and the radar, as shown in the following formula:
where t is the laser time of flight, 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 optical wedges is obtained through the zero positions of the double optical wedges (the first wedge-shaped mirror and the second wedge-shaped mirror) and the photoelectric encoder, so that the angle information of the laser is analyzed by using the FPGA, and the long-distance 4D high-precision imaging information is obtained by matching the analyzed distance information and speed information.
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, and referring to fig. 11, the measurement method according to the embodiment includes the steps of:
s100, performing frequency modulation on laser emitted by a laser;
s102, dividing the laser subjected to frequency modulation into local oscillation light and signal light, wherein the signal light is emitted by the optical waveguide and then is sequentially scanned and emitted through at least two wedge-shaped mirrors;
s104, interfering the signal light reflected by the target with local oscillator light to generate beat frequency signals;
and S106, converting the beat frequency signal into an electric signal, processing the converted electric signal to obtain frequency components in the beat frequency signal, and calculating a measurement result according to the frequency components.
The measurement method of this embodiment can be applied to the foregoing embodiments of the frequency modulated continuous wave lidar system, and the implementation manner and the beneficial effects thereof are substantially the same, and are not described herein again.
In some embodiments, dividing the frequency-modulated laser light into the local oscillator light and the signal light may include: dividing the laser subjected to frequency modulation into N paths of laser; dividing the N paths of laser into a first path of laser and a second path of laser; the first path of laser becomes local oscillator light after passing through a polarization controller, the second path of laser is signal light, and the second path of laser has N paths; wherein, after the signal light is emergent by the optical waveguide, successively scan through two at least wedge mirrors and emit including: the second path of laser light is emitted through the light emitting surface of the optical fiber array, collimated by the collimating lens 501, and then emitted by scanning through the first wedge mirror and the second wedge mirror.
In some embodiments, the generating the beat signal by interfering the signal light reflected by the target with the local oscillator light includes: the signal light reflected by the target passes through the second wedge-shaped mirror and the first wedge-shaped mirror and is coupled to the light-emitting surface of the optical fiber array through the collimating lens.
In the measuring method of this embodiment, the wavelength of the laser light emitted by the laser is linearly changed by internal modulation or external modulation, so as to realize linear modulation of the laser frequency. The modulated laser is divided into two paths, one path is local oscillator light, and the other path is signal light. And collimating the signal light by using an optical lens, scanning and emitting the collimated signal by using a first wedge-shaped mirror and a second wedge-shaped mirror, and interfering with the local oscillation light after the collimated signal is reflected by a target. The interference optical signal is converted into an electric signal, the beat frequency signal is sampled, and frequency estimation is carried out through algorithms such as fast Fourier transform and the like, so that the frequency component of the beat frequency is obtained. The frequency components comprise time-of-flight heterodyne frequency shift and Doppler frequency shift, and target distance and axial speed measurement is achieved through simple operation.
It should be noted that the terms "upper", "lower", and the like, herein indicate orientations or positional relationships and are used for convenience in describing the present application and for simplicity in description, but do not indicate or imply that the referenced devices or elements must have a particular orientation, be constructed and operated in a particular orientation, and therefore should not be construed as limiting the present application. Unless expressly stated or limited otherwise, the terms "mounted," "connected," and "connected" are intended to be inclusive and mean, for example, either fixedly, detachably, or integrally connected, directly or indirectly connected through intervening elements. Relational terms such as first and second, and the like may be used solely to distinguish one entity or action from another entity or action without necessarily requiring or implying any actual such relationship or order between such entities or actions. Also, the terms "comprises," "comprising," or any other variation thereof, are intended to cover a non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements does not include only those elements but may include other elements not expressly listed or inherent to such process, method, article, or apparatus. Without further limitation, an element defined by the phrase "comprising an … …" does not exclude the presence of other identical elements in a process, method, article, or apparatus that comprises the element. As will be appreciated by one of ordinary skill in the art, the situation may be specified.
The above description is only for the specific embodiments of the present application, but the scope of the present application is not limited thereto, and any changes or substitutions that can be easily conceived by those skilled in the art within the technical scope of the present application should be covered within the scope of the present application. Therefore, the protection scope of the present application shall be subject to the protection scope of the claims.
Claims (10)
1. A frequency modulated continuous wave lidar system, comprising: the device comprises a laser, a frequency modulation device, an interference device, a wedge-shaped mirror scanning device and a signal receiving and processing device;
the frequency modulation device is used for carrying out frequency modulation on laser emitted by the laser;
the interference device is used for dividing the laser subjected to frequency modulation into local oscillator light and signal light, wherein the signal light is emitted by the optical waveguide and then sequentially scanned and emitted by at least two wedge mirrors in the wedge mirror scanning device, and the interference device is also used for interfering the signal light reflected by a target with the local oscillator light to generate a beat frequency signal;
and the signal receiving and processing device is used for converting the beat frequency signal into an electric signal, processing the converted electric signal to obtain frequency components in the beat frequency signal, and calculating a measurement result according to the frequency components.
2. A frequency modulated continuous wave lidar system according to claim 1, wherein the laser is a narrow linewidth laser;
the frequency modulation device comprises: a direct digital frequency synthesizer and a two-way mach-zehnder interferometric modulator;
the direct digital frequency synthesizer is used for generating two orthogonal radio frequency signals and driving the double-path Mach-Zehnder interferometric modulator;
and the double-path Mach-Zehnder interferometric modulator is used for carrying out linear frequency modulation on laser emitted by the laser according to the driving of the radio-frequency signal.
3. A frequency modulated continuous wave lidar system according to claim 1, wherein the interference device comprises: the polarization controller is connected with the optical waveguide;
the splitter divides the laser subjected to frequency modulation into a first path of laser and a second path of laser; the first path of laser becomes local oscillation light after passing through the polarization controller; the second path of laser is signal light, and the signal light passes through the circulator and then is emitted through the optical waveguide; and the signal light reflected by the target is coupled to the optical waveguide, passes through the circulator and then is mixed with the local oscillator light to obtain a beat frequency signal.
4. A frequency modulated continuous wave lidar system according to claim 3,
the laser radar system also comprises an optical splitter, wherein the optical splitter is used for splitting the laser subjected to frequency modulation into N paths of laser;
the optical waveguide is an optical fiber array formed by N bundles of optical fibers in parallel;
the splitter splits 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 has N paths and is emitted out through the optical fiber array; wherein N is a natural number more than or equal to 2.
5. A frequency modulated continuous wave lidar system according to claim 1 or 4, wherein the wedge mirror scanning apparatus comprises: the device comprises a collimating lens, a first wedge-shaped mirror and a second wedge-shaped mirror;
the collimating lens collimates the signal light emitted by the interference device, and the collimated signal light is emitted by scanning through the first wedge-shaped mirror and the second wedge-shaped mirror; and the signal light reflected by the target passes through the second wedge-shaped mirror and the first wedge-shaped mirror and then is coupled to the optical waveguide through the collimating lens.
6. A frequency modulated continuous wave lidar system according to claim 1, wherein the signal receiving and processing means comprises: the device comprises a photoelectric detector, an analog-digital converter and a processing module;
the photoelectric detector converts the beat frequency signal into an electric signal, the analog-digital converter samples the converted electric signal, the processing module obtains frequency components in the beat frequency signal according to the sampled signal, and the measuring distance and/or the measuring speed are/is calculated according to the frequency components.
7. A frequency modulated continuous wave lidar system according to claim 1, further comprising a fiber amplifier for amplifying the frequency modulated laser light.
8. A measuring method based on frequency modulation continuous wave laser radar is characterized by comprising the following steps:
carrying out frequency modulation on laser emitted by a laser;
dividing the laser subjected to frequency modulation into local oscillation light and signal light, wherein the signal light is emitted by the optical waveguide and then is sequentially scanned and emitted by at least two wedge-shaped mirrors;
interfering the signal light reflected by the target with the local oscillator light to generate a beat signal;
and converting the beat frequency signal into an electric signal, processing the converted electric signal to obtain frequency components in the beat frequency signal, and calculating a measurement result according to the frequency components.
9. The method according to claim 8, wherein the dividing the frequency-modulated laser light into local oscillation light and signal light includes:
dividing the laser subjected to frequency modulation into N paths of laser;
dividing the N paths of laser into a first path of laser and a second path of laser; the first path of laser becomes local oscillator light after passing through a polarization controller, the second path of laser is signal light, and the second path of laser has N paths;
after the signal light is emitted from the optical waveguide, the signal light is successively scanned and emitted through at least two wedge-shaped mirrors, and the scanning and the emitting process comprises the following steps: and the second path of laser is emitted through the light emitting surface of the optical fiber array, is collimated by the collimating lens and then is emitted by scanning through the first wedge-shaped mirror and the second wedge-shaped mirror.
10. The method according to claim 9, wherein the interference between the signal light reflected by the target and the local oscillator light generates a beat signal, comprising:
and the signal light reflected by the target passes through the second wedge-shaped mirror and the first wedge-shaped mirror and is coupled to the light-emitting surface of the optical fiber array through the collimating lens.
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| CN115356742A (en) * | 2022-08-08 | 2022-11-18 | 深圳市圳阳精密技术有限公司 | High-precision externally-adjusted FMCW laser ranging system and method based on phase splicing |
| CN115356742B (en) * | 2022-08-08 | 2023-09-29 | 深圳市圳阳精密技术有限公司 | High-precision external-adjustment FMCW laser ranging system and method based on phase splicing |
| CN115480263A (en) * | 2022-09-30 | 2022-12-16 | 深圳市速腾聚创科技有限公司 | Detection method of laser detection device, laser detection device and storage medium |
| CN116520293A (en) * | 2023-06-30 | 2023-08-01 | 深圳市速腾聚创科技有限公司 | Laser radar detection method and device and laser radar |
| CN116520293B (en) * | 2023-06-30 | 2023-10-31 | 深圳市速腾聚创科技有限公司 | Laser radar detection method and device and laser radar |
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