CN114152951A - Frequency-adjustable continuous wave laser radar detection method and system - Google Patents
Frequency-adjustable continuous wave laser radar detection method and system Download PDFInfo
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Abstract
The application discloses a frequency-adjustable continuous wave laser radar detection method, which comprises the following steps: carrying out linear frequency modulation on the continuous light wave output by the laser by using the symmetrical triangular wave signal to generate frequency-modulated continuous wave probe light; periodically changing the direction of the frequency modulation continuous wave probe light through an optical structure to scan a spatial view field angle region; performing cooperative control on the frequency scanning period and the angle scanning period, performing angle scanning and frequency scanning at the same time, and completing one or more times of frequency scanning in each time period for completing the angle scanning; interfering the sampling of the frequency-modulated continuous wave detection light and the sampling of the reflected light from each detection target point in the angular region of the spatial field of view to generate a beat frequency signal; and resolving the beat frequency signal to generate a three-dimensional scanning image. The application also includes a device implementing the method. The frequency-adjustable continuous wave laser radar solves the problems of poor linear stability and calculation delay.
Description
Technical Field
The application relates to the technical field of laser radars, in particular to a frequency-adjustable continuous wave laser radar and a detection method thereof.
Background
The laser coherent detection has the advantages of high sensitivity, abundant carried information, capability of effectively filtering stray background light and the like, and is widely applied and developed in the fields of military affairs, surveying and mapping, communication and the like. In particular, the FMCW laser radar has strong anti-interference capability, low power consumption and high safety factor. However, to acquire a complete image of a frame, the FMCW lidar needs to scan the field angle FOV point by point, which requires a high scan rate and a smaller step length of the scanning mechanism. In order to increase the distance resolution, the high-frequency modulation period can be adjusted, but the turning times of the triangular wave can be increased by increasing the modulation period, so that the non-constant value area is increased, more useless signals are introduced, and more noise is introduced. In addition, a complete triangular wave modulation cycle is required to be finished for realizing decoupling calculation of distance and speed, and an application scene with high speed requirement cannot be met.
Disclosure of Invention
In order to solve the problems of poor linear stability and calculation delay of the frequency-adjustable continuous wave laser radar, the invention provides a frequency-adjustable continuous wave laser radar detection method and system, which can realize the detection and acquisition of high frame rate, low delay and high signal-to-noise ratio target distance and speed information.
The embodiment of the application provides a frequency-adjustable continuous wave laser radar detection method, which comprises the following steps:
carrying out linear frequency modulation on continuous light waves output by a laser by using symmetrical triangular wave signals to generate frequency-modulated continuous wave probe light and realize frequency scanning;
periodically changing the direction of the frequency modulation continuous wave probe light through an optical structure, and scanning a spatial view field angle region to realize angle scanning;
performing cooperative control on the frequency scanning period and the angle scanning period of the frequency modulation continuous wave probe light, performing angle scanning and frequency scanning simultaneously, and completing one or more times of frequency scanning in each time period for completing the angle scanning;
interfering the sampling of the frequency-modulated continuous wave detection light and the sampling of the reflected light from each detection target point in the angular region of the spatial field of view to generate a beat frequency signal;
and resolving the beat frequency signal to generate a three-dimensional scanning image.
Preferably, the triangular wave signal is corrected by adopting a predistortion technology, so that the optical frequency linear output under the open loop control is achieved.
Preferably, the scanning of the set angle of view by the signal light is realized using a regular prism of a line scanning prism.
Further preferably, 2 frequency-modulated continuous waves with different central wavelengths are used for detecting optical signals, and a spatial view field angle area is scanned at the same time to realize angle scanning; the frequency conversion directions of the 2 frequency modulation continuous wave detection optical signals are opposite.
The embodiment of the application also provides a frequency-adjustable continuous wave laser radar detection system, which is used for realizing the method in any one embodiment of the application and comprises a control and signal processing system, a modulation signal generation system, an optical interference system and a linear scanning servo driving system.
And the frequency modulation signal generation system is used for modulating the optical frequency by using the symmetrical triangular wave to generate frequency modulation continuous wave detection light. The optical interference system uses a Fabry-Perot interference structure, the detection light sampling and the reflected light sampling interfere, and the detector receives a beat frequency signal. And the linear scanning servo driving system is used for controlling the prism to rotate through the servo motor so as to realize scanning of a set field angle. The control and signal processing system generates laser modulation signal control parameters, servo motor control parameters and calculates beat frequency signals.
Further, the modulation signal generation system comprises a laser and a signal generator. The signal generator adopts direct digital frequency synthesis technology to generate symmetrical triangular wave signals, modulates the light frequency of the laser to generate frequency modulation continuous wave detection light,
further, the linear scanning servo driving system comprises a linear scanning prism and a servo motor. The line scanning prism adopts a regular prism structure and is controlled by a servo motor to rotate at a constant speed, so that the scanning of a set field angle is realized.
Further, the optical interference system comprises a fiber circulator, a fiber collimator, a half-transmitting and half-reflecting mirror, a photoelectric detector and a filter amplifier. The optical fiber circulator is respectively connected with the laser, the optical fiber collimator and the photoelectric detector; the output light of the optical fiber collimator is coupled to the semi-transparent semi-reflecting mirror through space; the two-dimensional linear scanning prism is arranged behind the semi-transparent semi-reflecting mirror and is connected with the servo motor; the output signal of the photoelectric detector enters a control and signal processing system through a filter amplifier.
The embodiment of the application also provides a frequency-adjustable continuous wave laser radar detection system, on the basis of the embodiment, a wavelength division multiplexing coupler is further used for coupling two lasers and a detector, wherein the two lasers have different central wavelengths; one laser emits the scanning frequency of the uplink section, and the other laser emits the scanning frequency of the downlink section; the scan rates of the two lasers are the same or different.
Preferably, in the method or system of the present application, the line-scan prism satisfies a set scan field angle; and under the set sensitivity condition of the photoelectric detector, adjusting the radius of the circumscribed circle until the system error of the distance and/or the speed of the detected target point is lower than a set threshold value.
The embodiment of the application adopts at least one technical scheme which can achieve the following beneficial effects:
the laser radar can realize the detection and acquisition of the target distance and speed information with high signal-to-noise ratio by adopting a coherent detection technology; by utilizing a laser internal modulation method, linear output of optical frequency can be realized by laser by adopting a predistortion technology, the complexity of a system is effectively reduced, and the detection precision is improved; the scanning speed and the calculating speed of the system can be effectively improved by adopting a two-dimensional line scanning prism structure and a wavelength division multiplexing coupling technology. The invention solves the problems of poor anti-interference effect, limited precision and low calculation efficiency of the existing distance measurement method, and can realize a three-dimensional distance and speed point cloud scanning imaging method with high target precision, low time delay and high signal-to-noise ratio.
Drawings
The accompanying drawings, which are included to provide a further understanding of the application and are incorporated in and constitute a part of this application, illustrate embodiment(s) of the application and together with the description serve to explain the application and not to limit the application. In the drawings:
FIG. 1 is a schematic diagram of detection of a symmetrical triangular-wave FMCW lidar;
FIG. 2 is a flow chart of an embodiment of the method of the present application;
FIG. 3 is a schematic view of the reflected light of a rotating prism using a regular prism as an example;
FIG. 4 is a schematic diagram of a reflective frequency and angle scanning architecture;
FIG. 5 is a schematic diagram of a tunable continuous wave lidar system;
FIG. 6 is a diagram of a ranging and speed measuring optical path of a single-channel frequency-modulated laser in an embodiment;
fig. 7 is a structural diagram of an embodiment of a dual-channel frequency-modulated laser coupled distance-measuring and speed-measuring optical circuit.
Detailed Description
In order to make the objects, technical solutions and advantages of the present application more apparent, the technical solutions of the present application will be described in detail and completely with reference to the following specific embodiments of the present application and the accompanying drawings. It should be apparent that the described embodiments are only some of the embodiments of the present application, and not all of the 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 technical solutions provided by the embodiments of the present application are described in detail below with reference to the accompanying drawings.
Fig. 1 is a diagram of the detection principle of a symmetrical triangular wave FMCW laser radar in the prior art.
The detection principle of a Frequency Modulated Continuous Wave (FMCW) laser radar developed on the basis of coherent detection is that signal light and reference light are subjected to interference beat frequency through linear modulation of laser frequency, and the beat frequency signal is resolved, so that the distance and speed information of a target is obtained.
In order to eliminate the coupling of the distance and the distance, a modulation and demodulation mode of symmetrical triangular waves is adopted. Using linear frequency modulation for distance measurement and Doppler effect for speed measurement, the frequency corresponding to the target distance
And velocity Doppler shift fd2v/λ, wherein R is the distance from the transmitting antenna to the target; b isFThe frequency modulation bandwidth is adopted; t ismIs the modulation period (i.e. the period of the triangular wave, the period of the frequency sweep); c is the speed of light in the transmission medium; v is the velocity of the target; λ is the wavelength of the probe light.
For a moving target, the Doppler frequency shift is superposed on the total beat frequency signal, and the frequency shift of the corresponding falling modulation section of the triangular wave
And rising modulation segment frequency shift
Using the above formula, the beat frequency of the range information can be expressed as
And the Doppler shift of the velocity information may be expressed as
Thus, the target distance and velocity may be expressed as:
by detecting the frequency shift of the descending and ascending frequency modulation sections of the symmetrical triangular wave, the distance and speed information of the target can be calculated, therebyAnd the target distance and speed acquisition under certain interference environment is realized. Wherein the distance resolution is Δ R ═ c/(2B)F) I.e. by the bandwidth BFAnd determining that the distance resolution can reach centimeter level under the modulation bandwidth of 10 GHz.
In the present application, the rising frequency modulation segment is referred to as "up-sweep"; the falling frequency modulation segment is called "down sweep". The period of 1 frequency sweep includes 1 up sweep and 1 down sweep.
Fig. 2 is a flowchart of an embodiment of the method of the present application.
An embodiment of a method of the present application comprises the steps of:
preferably, a predistortion technology is adopted to correct the triangular wave signal, so that the linearity of the output of the laser is improved, and the linear output of the optical frequency under the open-loop control is achieved.
preferably, a regular prism line scanning prism can be used, and the scanning of the set angle of view by the signal light is realized under the driving of a high-precision servo motor.
103, cooperatively controlling the frequency scanning period and the angle scanning period of the frequency modulation continuous wave probe light, and carrying out angle scanning and frequency scanning simultaneously so as to complete one or more times of frequency scanning in each time period for completing the angle scanning;
the angle scanning period TaThis is the time it takes to complete a scan of the angular region of the spatial field of view (which is done by rotating the prism one edge). The primary frequency scanning means finishing primary up-frequency scanning or down-frequency scanning; the multiple frequency scans are performed by at least 1 frequency sweep and at least 1 frequency sweep. That is, the number of frequency sweeps is an integer multiple of the number of times the angular sweep is completed in a unit time. The frequency sweepPeriod of drawing TfIt means the time taken to complete 1 up sweep and 1 down sweep. That is, the unit time T0The number of internally completed frequency scans is 2T0/Tf。
Step 104, interfering the detection light sample of the frequency modulation continuous wave detection light and the reflected light sample from each detection target point in the space view field angle area to generate a beat frequency signal;
and 105, resolving the beat frequency signal to generate a three-dimensional scanning image. The three-dimensional scan image at least includes information of the velocity and position (distance) of the detection target point.
Based on the method and the device, radar imaging based on frequency modulation continuous wave detection light is achieved.
Further, in order to improve the frequency scanning efficiency, 2 frequency-modulated continuous waves with different central wavelengths are used for detecting optical signals, and meanwhile, a space view field angle area is scanned to realize angle scanning; the frequency conversion directions of the 2 frequency modulation continuous wave detection optical signals are opposite.
It should be noted that, when scanning the spatial view angle region by using the frequency modulated continuous wave probe optical signal with a single wavelength, signal resolution should occur after the end of a frequency sweep period; when 2 frequency-modulated continuous wave detection optical signals with different center wavelengths are used, because the frequency conversion directions of the 2 detection optical signals are opposite, the receiver can simultaneously receive the reflected light of the upper frequency-sweep detection signal and the reflected light of the lower frequency-sweep detection signal at any time, and therefore, the signal settlement can be carried out in real time.
In the following embodiments of the present application, to implement the method of steps 101 to 105, the control and signal processing system sends a control parameter to the signal generator, so that the signal generator generates a frequency modulation signal of a symmetric triangular wave, and sends a driving signal to control the high-precision servo motor to work so that the line scanning prism scans;
the laser generates a linear frequency modulation laser signal under the drive of the signal generator, laser enters through an I port of the optical circulator and is output to the optical fiber collimator through an II port, and meanwhile, a reflected light beat frequency signal enters through the II port and is coupled to the photoelectric detector from an III port;
the collimated and shaped laser signal is divided into two beams at the half-transmitting and half-reflecting mirror, one beam is used as detection light to be transmitted to the line scanning prism, the other beam is used as reference light to return according to the original optical path (namely sampling of the detection light), and the other beam interferes with the reflected signal light (namely sampling of reflected light from each detection target point in the space view field angle area), so that a beat frequency signal is generated.
The line scanning prism is driven by a high-precision servo motor driven by the control and signal processing system to realize the scanning of the detection light on the set field angle, the detection light is reflected by a target and returns along the original light path, and the detection light interferes with the reference light in the light path;
and the photoelectric detector receives the beat frequency signal, realizes photoelectric conversion, outputs the beat frequency signal to the filter amplifier, temporarily stores the beat frequency signal to the control and signal processing system for resolving, and outputs the distance and speed information of the target.
Fig. 3 is a schematic diagram of light reflected by a rotating prism using a regular prism as an example.
The angle range of the laser reflected by each surface in the rotation process of the prism is obtained by the reflection law of light, namely the field of view (FOV) thetah＝2αn＝720°/Nm. Wherein alpha isnFor maximum angle of rotation of each face, NmThe number of prism facets. Regular octahedral prism alpha as shown in FIG. 3an＝45°，θh＝90°。
As shown in fig. 3b, the non-linear effect and vignetting effect of the polygon mirror on the reflection point are related to the rotation angle θ and the radius r of the circumscribed circle, and the signal-to-noise ratio can be improved by adjusting the parameters.
Specifically, in the process of rotating the reflecting mirror surface AB, positions A 'B', AB and A 'B' under three time nodes are selected, when transient rotation angles are theta, 0 and-theta, intersection points of incident light and the mirror surface are P, M and K respectively, and distances from an incident light source to the three point positions are unequal, so that the optical path of the detection light is subjected to nonlinear change due to the rotation of a prism, different optical path differences can be introduced under different angles, and further, speed and distance differences caused in the calculation need to be compensated, and the calculation complexity is increased to influence the calculation efficiency; the nonlinear error can be reduced by reducing the radius of the circumscribed circle of the prism, and the calculation efficiency is prevented from being influenced by compensation calculation; in addition, the rotation period can be changed by factors such as vibration, the larger the radius is, the larger the outermost linear velocity is, and therefore, the system error caused by vibration can be reduced by reducing the radius of the circumscribed circle.
On the other hand, at the set angle of view θhAnd under the rotating speed, the radius of the circumscribed circle is positively correlated with the scanning efficiency, namely the smaller the radius is, the lower the scanning efficiency is. This is because the vignetting phenomenon in the scanning process causes energy loss, and when the radius of the circumscribed circle is reduced, the effective signal quality is reduced, the signal-to-noise ratio is reduced, and the sensitivity of the photodetector needs to be improved. In order to ensure that the incident beam does not generate vignetting, namely, the light beam is ensured to be completely irradiated on the mirror surface to the maximum extent, the energy utilization rate is improved, the radius of the circumscribed circle has the theoretical minimum value, namely, the measurement precision is reduced by continuously reducing the radius of the circumscribed circle.
In summary, the present invention designs the maximum rotation angle αnSatisfy the scanning field angle thetahThe prism of (2) optimizes the radius r of the circumscribed circle. For example, under the set sensitivity condition of the photoelectric detector, the radius of the circumscribed circle is adjusted until the system error of the distance and the speed is lower than the set threshold value. Therefore, the calculation complexity is reduced, and the signal resolving speed is ensured; the signal to noise ratio of the signal is improved, and the requirement on a sensor is reduced; the system error is reduced, and the measurement precision is improved;
the present system gives the following examples: under the conditions that the rotation speed of a regular octahedral rotating prism is 3000r/min, namely the rotation period of the prism is 20ms, the scanning range of a field angle is 90 degrees, an incident light source is 1cm away from an circumscribed circle, and the diameter of a light spot is 2mm, the radius of the circumscribed circle is reduced to 1cm from 10cm, only a one-way light path is calculated, so that the introduced maximum distance error can be reduced to 0.1 cm from 0.7 cm, and is reduced by 85%; the maximum speed error is reduced from 34.5m/s to 3.14m/s, the reduction is 91%, the energy loss is negligible, the system performance can be further improved by considering the round trip of the optical path, and the system volume can be reduced by reducing the radius of the prism.
As shown in fig. 4, the two-dimensional line-scan prism scanning structure needs to be explained, in a unit time, the frequency scanning is completed by integral multiple of the angular scanning. For example, the first scanning mode in the figure: when the light path sweeps a certain side of the prism, the frequency scanning is completed in a half period, namely when the light path sweeps two adjacent sides of the prism, the frequency scanning is respectively completed, namely, the frequency scanning is completed up and down, so that the complete frequency scanning is realized, and the signal noise caused by the frequency scanning turning is filtered by introducing the physical turning; for another example, the frequency scanning method two in the figure: when the optical path sweeps a certain side of the prism, the plane completes single-period or multi-period frequency scanning, namely, one or more times of up-scanning and down-scanning are realized when the prism rotates a side surface.
By adopting the prism scanning mode, when the prism completes one omnidirectional rotation, the multi-time angle scanning can be realized, and the scanning is determined by the edge number of the prism; and the frequency scanning times which are integral multiple of the angle scanning times are determined by the scanning times of the frequency on the single side surface, thereby playing a frequency doubling role, reducing the requirement on a servo driving mechanism, effectively avoiding noise signals introduced by frequency turning and improving the signal resolving speed.
The invention provides a laser radar system based on frequency modulation continuous wave, the system composition schematic diagram of which is shown in fig. 5, and the system comprises a control and signal processing system 10, a modulation signal generating system 20, an optical interference system 30 and a linear scanning servo driving system 40.
In the frequency modulation signal generation system 20, the frequency modulation signal modulates the optical frequency by using the symmetrical triangular wave, and the frequency resolution is above 0.5 Hz. Preferably, the dual-channel frequency modulation signal can be output, and the dual-channel frequency modulation signal generates a mirror symmetry triangular wave frequency modulation signal under the control of the control and signal processing system.
The mirror symmetry triangular wave modulation signal further preferably takes the influence of temperature on laser into consideration, and the triangular wave signal is corrected by adopting a predistortion technology, so that the linearity of the laser output is improved, and the optical frequency linear output under open loop control is achieved.
The optical interference system 30 uses a fabry-perot interference structure. For example, in another embodiment of the present application, the optical fiber circulator and the half-mirror are used as the main devices of the interference structure, and the optical signal coherence is realized in the open space. In the interference structure, laser is output after optical shaping of a laser beam, for example, the laser passes through a half-transmitting half-reflecting mirror, a part of the laser is reflected as reference light, a part of the laser is projected and transmitted to a detection target, reflected detection signal light interferes with the reference light, a beat frequency signal is received by a detector, and signal acquisition is realized through high-linearity laser coherence.
The linear scanning servo driving system 40 uses a high-precision servo motor to control the prism to rotate so as to realize information acquisition of a certain field angle, and meanwhile, the frequency scanning times are integral multiples of the angle scanning times. Preferably, the prism used in the system of the present application is a regular prism structure, which can realize the scan of setting the angle of view, and simultaneously, the control and signal processing system corrects the final result by considering the nonlinear effect of the reflection point introduced by the rotation of the prism.
The control and signal processing system 10 uses a high-performance FPGA as a core to generate a laser modulation signal, a servo motor control parameter and a demodulation beat frequency signal, and cooperates with a linear scanning servo driving system to realize fast and low-delay signal demodulation and processing. The modulation signal control module and the servo control module respectively drive the modulation signal generation system and the line scanning servo drive system; the signal resolving and processing module resolves the beat frequency signal.
Fig. 6 is a structure diagram of a distance and speed measuring electric path of a single-channel frequency modulation laser. As an embodiment of the tunable continuous wave lidar detection system of the present application, the control and signal processing system 10, the signal generator 22, and the laser 23 are sequentially connected, and the optical fiber circulator is respectively connected to the laser, the optical fiber collimator 34, and the photodetector 35. The fiber collimator output light is spatially coupled to a half mirror 36. The two-dimensional linear scanning prism 47 is installed behind the half-mirror and is connected with a servo motor 48. The output signal of the photodetector enters the control and signal processing system via a filter amplifier 39.
In the embodiment shown in fig. 6, the servo motor and the signal generator are connected to the signal processing system, the sweep frequency signal of the signal generator and the process of angular scanning driven by the servo motor are synchronously controlled, so that the angular scanning of the optical signal output by the line-scanning prism is performed simultaneously with the frequency scanning, and the frequency scanning is completed by integral multiple of the number of angular scanning in unit time. Specifically, for example, the control and signal processing system sends control parameters to enable the arbitrary waveform signal generator to generate a modulation signal, wherein the modulation signal has been subjected to signal pre-distortion processing in a calibration test, the signal is loaded on a large-bandwidth DFB laser to generate a chirped laser signal, and the control and signal processing system controls the servo motor to enable the two-dimensional servo line scanning prism to rotate, so that the frequency scanning times are integral multiples of the angle scanning times in the same time period.
In the embodiment shown in fig. 6, the modulation signal generation system comprises a laser and a signal generator; the signal generator adopts a direct digital frequency synthesis technology, utilizes a high-performance double-channel DDS chip to generate a high-resolution frequency driving current signal, modulates the optical frequency of the laser to generate frequency-modulated continuous wave detection light serving as a light source of an optical interference system, and further improves the detection precision of the system.
Further: the signal generator can be any waveform signal generator, and the output port is connected with the large-bandwidth DFB laser. The random waveform signal generator can generate symmetrical triangular wave signals, the preferred signal output is double channels, and mirror symmetry triangular waves can be output. In order to satisfy the linear output of optical frequency, the modulation signal can be subjected to predistortion treatment. The large-bandwidth DFB laser adopts an internal modulation optical frequency modulation mode, the center wavelength is 1550nm, the modulation bandwidth is not lower than 10GHz, and high-precision signal acquisition can be realized.
Preferably, in any embodiment of the present application, the laser modulation adopts an internal modulation technique, and the frequency modulation is realized by changing the injection current, so that the large-bandwidth laser frequency modulation can be realized, and the bandwidth modulation bandwidth of the laser is required to be not less than 10 GHz. Further preferably, a TEC temperature control module is added to the laser driving circuit to control the temperature of the laser and improve the linearity of laser frequency output.
In the embodiment shown in fig. 6, the linear scanning servo drive system comprises a linear scanning prism and a servo motor. The direction of the signal light is adjusted by configuring the number of prism faces and the scanning speed, so that large-field-angle scanning is realized, and the interference system outputs beat frequency signals at different spatial positions. Further: the line scanning prism adopts a regular prism structure, and is controlled by a servo motor to rotate at a constant speed, so that the scanning of a set field angle is realized. Different from a galvanometer scanning mode, the prism rotary scanning does not introduce dynamic acceleration brought by switching scanning directions, so that the result is more accurate, and the calculation time is shorter.
In the embodiment shown in fig. 6, the optical interference system comprises a fiber circulator, a fiber collimator, a half-mirror, a photodetector and a filter amplifier; the optical interference system outputs high-linearity collimation shaping signal light and generates a beat frequency signal with target information, beat frequency signal acquisition of a simple light path is achieved, the signal is transmitted to a control and signal processing system, and the anti-interference capability of the system is improved through four-dimensional information including three-dimensional distance and speed calculated.
Further: as shown in fig. 6, the output port of the laser is connected to the I port of the optical fiber circulator, the input port of the optical fiber collimator is connected to the II port of the optical fiber circulator, the semi-transparent semi-reflective mirror is installed in the output direction of the optical fiber collimator, the two-dimensional servo line scanning prism is installed in the transmission direction of the semi-transparent semi-reflective mirror and connected to the servo motor, the photodetector is connected to the III port of the optical fiber circulator, the input port of the filter amplifier is connected to the output port of the photodetector, and the control and signal processing system is simultaneously connected to the arbitrary waveform signal generator, the filter amplifier and the servo motor.
Laser output by a laser is incident to an I port of the optical fiber circulator and is output to an optical fiber collimator from an II port to shape the light beam, the collimated signal light is split at a semi-transparent semi-reflective mirror, one part of the light is returned as reference light along an original light path, and the other part of the transmitted light is incident to a two-dimensional linear scanning prism as signal light to scan a space view field angle region.
The reflected signal light returns along the original light path and generates interference beat frequency with the reference light, the reflected signal light is incident from the port II of the optical fiber circulator and is output from the port III of the optical fiber circulator after passing through the optical fiber circulator, the reflected signal light is further coupled into the photoelectric detector to realize photoelectric conversion, the electric signal is conditioned through the filtering and amplifying circuit, and finally the signal is resolved by the signal processing control and signal processing system to realize three-dimensional point cloud imaging of target distance and speed information.
As a preferred embodiment of the present application, the laser scanning bandwidth is 500GHz, and the period is 5ms (time length of one up sweep + one down sweep); the angle scanning mechanism is a regular octahedral rotating prism, the rotating speed is 3000r/min, namely the scanning period of the prism is 20ms, the upper frequency scanning and the lower frequency scanning of the frequency scanning range can be respectively realized on two adjacent sides of the prism, the equivalent field angle scanning period is 2.5ms, the field angle scanning range is 90 degrees, and the equivalent data acquisition frequency is 200 Hz.
Fig. 7 is a structural diagram of an embodiment of a dual-channel frequency-modulated laser coupled distance-measuring and speed-measuring optical circuit.
In order to increase the measurement rate, real-time measurement is realized. As shown in fig. 7, a wavelength division multiplexing coupler is used to couple two lasers and a detector, so as to realize dual-unit cooperative detection, wherein the two lasers have different central wavelengths to facilitate signal differentiation; the two sweep signals are output by the angle scanning mechanism after being coupled by the wavelength division multiplexing coupler, and the reflected light is separated by the wavelength division multiplexing coupler and returns to the respective detection unit according to the difference of the central wavelength. The two distance measuring units adopt mirror symmetry modulation triangular wave frequency, namely, in the scanning process of the linear scanning prism, one laser emits an upper sweep frequency modulation signal, and the other laser emits a lower sweep frequency modulation signal. The scanning rates of the two lasers may be the same or different, preferably the scanning rates are the same. Because the modulated triangular wave frequencies of the two ranging units are in mirror symmetry, each scanning simultaneously comprises two groups of data of an upper scanning frequency band and a lower scanning frequency band, the data can be directly used for resolving distance and speed after being received by the control and signal processing system, the total measuring speed can be increased by at least two times, and distance and speed information can be directly output in real time in each measurement.
On the basis of the embodiment shown in fig. 6, as shown in fig. 7, a wavelength division multiplexing coupler may be used to couple two lasers and a detector, specifically, the wavelength division multiplexing coupler includes two groups of lasers (a laser 1 and a laser 2), a circulator, a photodetector (a detector 1 and a detector 2), and a filter amplifier (a filter amplifier 1 and a filter amplifier 2), a signal generator generates two groups of modulation signals, respectively modulates output light of the laser 1 and the laser 2, generates two channels of frequency modulated continuous wave detection light signals, respectively outputs the two channels of frequency modulated continuous wave detection light signals to the wavelength division multiplexing coupler through the circulator, and combines the two channels of frequency modulated continuous wave detection light signals to enter a collimator; the reflected waves are branched by the wavelength division multiplexing coupler and then respectively enter the circulator, are respectively output to the detector 1 and the detector 2, and then respectively enter the control and signal processing system through the filter amplifier 1 and the filter amplifier 2. Wherein, the two lasers have different central wavelengths to be beneficial to distinguishing signals; the two sweep signals are coupled by the wavelength division multiplexing coupler and then output by the angle scanning mechanism, and the reflected light is separated by the wavelength division multiplexing coupler and returns to the respective detection unit. The two groups of distance measuring units adopt reverse laser triangular wave frequency, namely, in the scanning process of the angle scanning mechanism, one laser emits the scanning frequency of an uplink section, and the other laser emits the scanning frequency of a downlink section. The scanning rates of the two lasers may be the same or different, preferably the scanning rates are the same. Because the triangular wave frequency scanning of the two groups of distance measuring units is reversed, each scanning simultaneously comprises two groups of data of upper frequency scanning and lower frequency scanning, the data can be directly used for calculating distance and speed, the distance and speed information can be directly output in real time, and further the calculation delay is reduced and the calculation efficiency is improved.
It should also be noted that 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 like elements in a process, method, article, or apparatus that comprises the element.
The above description is only an example of the present application and is not intended to limit the present application. Various modifications and changes may occur to those skilled in the art. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present application should be included in the scope of the claims of the present application.
Claims (10)
1. A frequency-adjustable continuous wave laser radar detection method is characterized by comprising the following steps:
carrying out linear frequency modulation on the continuous light wave output by the laser by using the symmetrical triangular wave signal to generate frequency-modulated continuous wave probe light;
periodically changing the direction of the frequency modulation continuous wave probe light through an optical structure to scan a spatial view field angle region;
performing cooperative control on the frequency scanning period and the angle scanning period of the frequency modulation continuous wave probe light, performing angle scanning and frequency scanning simultaneously, and completing one or more times of frequency scanning in each time period for completing the angle scanning;
interfering the sampling of the frequency-modulated continuous wave detection light and the sampling of the reflected light from each detection target point in the angular region of the spatial field of view to generate a beat frequency signal;
and resolving the beat frequency signal to generate a three-dimensional scanning image.
2. The tunable continuous wave lidar detection method of claim 1,
the triangular wave signal is corrected by adopting a predistortion technology, so that the optical frequency linear output under the open-loop control is achieved.
3. The tunable continuous wave lidar detection method of claim 1,
the scanning of the set angle of view by the signal light is realized by using a regular prism.
4. The tunable continuous wave lidar detection method of claim 1,
2 frequency-modulated continuous waves with different central wavelengths are used for detecting optical signals, and a spatial view field angle area is scanned at the same time to realize angle scanning; the frequency conversion directions of the 2 frequency modulation continuous wave detection optical signals are opposite.
5. The tunable continuous wave lidar detection method of claim 3, wherein the line-scan prism satisfies a set scan field angle; and under the set sensitivity condition of the photoelectric detector, adjusting the radius of the circumscribed circle until the system error of the distance and/or the speed of the detected target point is lower than a set threshold value.
6. A frequency-adjustable continuous wave laser radar detection system for realizing the method of any one of claims 1 to 5, which is characterized by comprising a control and signal processing system, a modulation signal generation system, an optical interference system and a linear scanning servo driving system;
the frequency modulation signal generation system is used for modulating the optical frequency by using a symmetrical triangular wave to generate frequency modulation continuous wave detection light;
the optical interference system adopts a Fabry-Perot interference structure, the detection light sampling and the reflected light sampling interfere with each other, and a detector receives a beat frequency signal;
the linear scanning servo driving system is used for controlling the prism to rotate through the servo motor so as to realize scanning of a set field angle;
the control and signal processing system generates laser modulation signal control parameters, servo motor control parameters and calculates beat frequency signals.
7. The tunable continuous wave lidar detection system of claim 6,
the modulation signal generating system comprises a laser and a signal generator; the signal generator adopts a direct digital frequency synthesis technology to generate a symmetrical triangular wave signal, modulates the light frequency of the laser and generates frequency modulation continuous wave detection light.
8. The tunable continuous wave lidar detection system of claim 6,
the linear scanning servo driving system comprises a linear scanning prism and a servo motor;
the line scanning prism adopts a regular prism structure and is controlled by a servo motor to rotate at a constant speed, so that the scanning of a set field angle is realized.
9. The tunable continuous wave lidar detection system of claim 6,
the optical interference system comprises an optical fiber circulator, an optical fiber collimator, a semi-transparent semi-reflecting mirror, a photoelectric detector and a filter amplifier;
the optical fiber circulator is respectively connected with the laser, the optical fiber collimator and the photoelectric detector; the output light of the optical fiber collimator is coupled to the semi-transparent semi-reflecting mirror through space; the two-dimensional linear scanning prism is arranged behind the semi-transparent semi-reflecting mirror and is connected with the servo motor; the output signal of the photoelectric detector enters a control and signal processing system through a filter amplifier.
10. The tunable continuous wave lidar detection system of claim 6,
coupling two lasers and a detector using a wavelength division multiplexing coupler, wherein the two lasers have different center wavelengths; one laser emits the scanning frequency of the uplink section, and the other laser emits the scanning frequency of the downlink section; the scan rates of the two lasers are the same or different.
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