High-precision laser ranging system and method based on pulsed laser calibration

Abstract

The present invention provides a high-precision laser ranging system and method based on pulse laser calibration. The system includes a ranging optical path and a calibration optical path. In the ranging optical path, an arbitrary waveform generator frequency-modulates the laser signal of a semiconductor laser. The laser signal is split into two beams by an optical fiber coupler and respectively incident on a ranging unit and the calibration optical path. The ranging unit transmits the laser signal to a target for ranging. In the calibration optical path, a pulse laser emits frequency-calibration laser, which is transmitted to an optical fiber coupler II. The frequency-calibration laser is coupled with the laser signal to obtain a mixed-frequency optical signal. The processor obtains a ranging result based on the ranging signal, determines the variation curve of the laser signal frequency with time based on the frequency-calibration laser, obtains the non-linearity of frequency modulation, and calibrates the ranging result based on the non-linearity of frequency modulation. The present invention uses a pulse laser to calibrate the output frequency of a semiconductor laser, corrects the ranging result, and improves the ranging accuracy.

Description

Technical Field

[0001] The present invention relates to the technical field of lidar, and more specifically, to a high-precision laser ranging system and method based on pulse laser calibration. Background Art

[0002] Lidar is an active detection technology that uses laser as the light source and adopts optoelectronic detection technology to measure the distance to the target. It has the advantages of high precision, high speed, small volume, etc., and is widely used in the fields of autonomous driving, unmanned aerial vehicles, intelligent robots, three-dimensional modeling, geographical mapping, etc.

[0003] In the existing lidar ranging methods, when using a semiconductor laser for FMCW ranging, since the frequency modulation of the semiconductor laser is mainly achieved by adjusting the injection current, when the injection current changes, the carrier density in the laser also changes, and the change in carrier density will affect the refractive index, thereby changing the frequency of the laser. However, the relationship between carrier density and refractive index is not linear, which will lead to non-linear errors during the frequency modulation process; when using triangular wave modulation, due to the non-linearity of the frequency response, non-linear errors will occur during the frequency modulation process, further leading to a decrease in ranging accuracy.

[0004] To improve the ranging accuracy, the prior art has proposed: 1. The external reference source method, which uses a stable external frequency source (such as a high-precision frequency standard or a reference laser) with a known and stable frequency. By comparing the frequency of the FMCW laser with the external reference source, the output frequency of the laser can be calibrated; 2. The frequency meter method, which uses a precision frequency meter to directly measure the output frequency of the laser and compares it with the set target frequency; 3. The interference method, which uses interference technology to interfere the output of the laser with a reference laser with a known frequency, and adjusts the laser frequency by analyzing the frequency difference in the interference pattern; 4. The spectral analysis method, which uses a spectral analyzer to directly measure the output spectrum of the laser and calibrates it according to the deviation between the spectral line position and the target frequency. However, the prior art also has the following defects: limited accuracy, traditional methods rely on external reference sources or measurement devices, and the accuracy is limited by the performance of these devices; single-point calibration, most methods can only calibrate a single frequency point and cannot cover the entire frequency range at the same time; poor real-time performance, it is difficult to monitor and calibrate the frequency drift in real time, and multiple manual adjustments are required; low degree of automation, the traditional calibration process usually requires manual operation and cannot achieve automation and continuous monitoring.

[0005] When the frequency of a semiconductor laser is modulated by an electric current, the change in the frequency of the semiconductor laser with respect to the drive current is not strictly linear, and there is frequency non-linearity. This non-linearity causes the frequency change of the frequency-modulated signal to deviate from the expected linear relationship. In the frequency-modulated continuous-wave (FMCW) ranging method, non-linearity causes errors in distance measurement. In heterodyne interference, non-linearity affects the stability and accuracy of the interference signal, thereby reducing the ranging accuracy.

[0006] As the requirements for ranging accuracy continue to increase, existing ranging systems and methods can no longer meet the needs. For the above reasons, it is necessary to invent a more convenient and high-precision laser ranging system and method. Summary of the Invention

[0007] The object of the present invention is to provide a high-precision laser ranging system and method based on the calibration of a pulsed laser. By using the pulsed laser to calibrate the output frequency of the semiconductor laser, the non-linear amount of frequency modulation can be obtained, and the ranging result can be corrected to obtain a higher-precision ranging result.

[0008] To achieve the above object, the present invention provides the following solution:

[0009] A high-precision laser ranging system based on the calibration of a pulsed laser, comprising a ranging optical path and a calibration optical path;

[0010] The ranging optical path includes an arbitrary waveform generator, a semiconductor laser, an optical fiber coupler I, a ranging unit, a photodetector I, a data collector, and a processor;

[0011] The calibration optical path includes a pulsed laser, an optical fiber coupler II, a photodetector II, and a filter;

[0012] The arbitrary waveform generator generates a frequency-modulated signal, and modulates the laser signal of the semiconductor laser by controlling the drive current of the semiconductor laser;

[0013] The frequency-modulated laser signal is split by the optical fiber coupler I. One beam of the laser signal is incident on the ranging unit, and the other beam of the laser signal is incident on the optical fiber coupler II;

[0014] The ranging unit transmits the laser signal to the target for ranging; the photodetector I acquires the ranging signal, converts it into an electrical signal, and transmits it to the data collector;

[0015] The pulsed laser emits frequency calibration laser and transmits it to the optical fiber coupler II. The optical fiber coupler II couples the frequency calibration laser with the laser signal to obtain a mixed optical signal, and the mixed optical signal is sequentially transmitted to the data collector through the photodetector II and the filter;

[0016] The data collector receives the electrical signals output by the first photodetector and the filter respectively, and transmits them to the processor;

[0017] The processor obtains a ranging result based on the ranging signal, determines the variation curve of the laser signal frequency with time based on the frequency-calibrated laser, obtains the non-linearity of frequency modulation, and calibrates the ranging result based on the non-linearity of frequency modulation.

[0018] Further, the ranging unit includes a loop unit and an optical fiber collimation unit.

[0019] The frequency-modulated laser signal is split by an optical fiber coupler. One beam of the laser signal is incident on the loop unit, and the other beam of the laser signal is incident on the second optical fiber coupler.

[0020] The loop unit is respectively connected to the optical fiber collimation unit and the first photodetector; the laser signal is transmitted to the target through the optical fiber collimation unit for ranging.

[0021] Further, the ranging signal is the object light and the reference light transmitted in a common path.

[0022] The loop unit is connected to the optical fiber collimation unit through an optical fiber and is used to transmit the laser signal to the optical fiber collimation unit.

[0023] The optical fiber collimation unit is used to transmit a part of the laser signal to the target, collect the object light reflected by the target, and reflect a part of the laser signal to form a reference light. The object light and the reference light received by the optical fiber collimation unit are transmitted in a common path.

[0024] The loop unit receives the object light and the reference light transmitted back by the optical fiber collimation unit and transmits them to the first photodetector.

[0025] The first photodetector is used to convert the intensity signal of the interference light of the object light and the reference light into an electrical signal and transmit it to the data collector.

[0026] Further, the loop unit adopts an optical fiber circulator, which is provided with three interfaces, namely port a, port b, and port c. Among them, port a is used to receive the laser signal from the first optical fiber coupler, port b is used to output the laser signal to the optical fiber collimation unit and receive the object light and the reference light, and port c is used to transmit the object light and the reference light to the first photodetector.

[0027] The optical fiber collimation unit adopts an optical fiber collimator with a PC-type interface, a lens with transmission and reflection functions, or a combined structure of an optical fiber and a collimating mirror.

[0028] Further, an electro-optic modulator is arranged between the pulsed laser and the second optical fiber coupler, and the electro-optic modulator is used to modulate the signal of the frequency-calibrated laser.

[0029] Furthermore, the arbitrary waveform generator is also used to generate a synchronous electrical signal and transmit it to the data collector, and the synchronous electrical signal has the same frequency as the frequency modulation signal.

[0030] Furthermore, the pulsed laser is a mode-locked laser.

[0031] The present invention also provides a high-precision laser ranging method based on pulsed laser calibration, which is applied to the above high-precision laser ranging system based on pulsed laser calibration, and includes the following steps:

[0032] Calibrate the laser based on frequency to obtain the frequency spectrum line of the pulsed laser;

[0033] Based on the frequency spectrum line, couple the frequency-calibrated laser with the laser signal to obtain a mixed-frequency optical signal, including a difference-frequency signal and a sum-frequency signal. The sum-frequency signal is ignored, and the expression of the difference-frequency signal is

[0034]

[0035] where is the repetition frequency, which is equal to the repetition rate of the laser pulses; is the carrier-envelope phase shift frequency, which represents the rate of change of the relative position between the carrier phase of a single pulse and the pulse envelope, that is, the phase shift rate between the carrier light wave and the pulse envelope, and this value is less than ; is the frequency of the laser signal of the semiconductor laser; n is a positive integer;

[0036] After the difference-frequency signal passes through the filter, there is an output when the frequency of the difference-frequency signal meets the filter bandwidth range;

[0037] During the frequency modulation process of the frequency signal of the semiconductor laser, when the frequency difference between the equally spaced discrete spectral lines of the frequency-calibrated laser meets the filter bandwidth range, a pair of output signals are generated. By using the equally spaced characteristics of the discrete spectral lines and the characteristic that the bandwidth of the filter is not affected by the frequency modulation of the semiconductor laser, the interval between each output of the difference-frequency signal and the width of a single output are obtained, and the frequency modulation rate of the semiconductor laser frequency modulation signal at different times is obtained, thereby obtaining the non-linearity of the frequency modulation;

[0038] The ideal frequency of the semiconductor laser is:

[0039] ; ; ;

[0040] where represents the initial frequency of the laser, is the frequency modulation rate, i.e., the slope of the rising edge of the triangular wave, is the frequency modulation bandwidth, is the frequency modulation period;

[0041] After calibration, the detected actual curve is:

[0042] , , ,

[0043] Among them, is the non-linear part in the actual laser frequency. After mixing, the beat frequency signal frequency is corrected to {f}_{b}\left ( {t} \right )=\alpha \tau +\left [ {{f}_{n}\left ( {t} \right )-{f}_{n}\left ( {t-\tau} \right )} \right ] , and the phase of the beat frequency signal is corrected to , is the time corresponding to the optical path difference between the object light and the reference light at the position where the target is located;

[0044] After obtaining the result of a single measurement, according to the relatively low-precision time obtained for the first time, substitute it to correct the frequency image of the beat frequency signal, obtain a more accurate time interval, and then iterate to obtain a more accurate ranging result.

[0045] Furthermore, when the pulsed laser is a mode-locked laser, based on the optical frequency comb technology, the mode-locked laser generates an optical frequency comb, and the frequencies of the corresponding frequency spectral lines are expressed as follows:

[0046]

[0047] Among them: is the frequency of the n th spectral line of the optical frequency comb, is the repetition frequency, equal to the repetition rate of the pulse train, which is determined by the cavity length of the mode-locked laser, is the carrier-envelope phase shift frequency, which represents the rate of change of the relative position between the carrier phase of a single pulse and the pulse envelope, that is, the phase offset rate between the carrier light wave and the pulse envelope. This value is less than .

[0048] Furthermore, based on the frequency spectral line characteristics of the pulsed laser, the Fourier transform of the periodic pulse converts the pulse signal in the time domain into a frequency domain representation. If the periodic pulse train is periodic in time, the Fourier transform will generate discrete frequency components, forming discrete frequency spectral lines.

[0049] According to the specific embodiments provided by the present invention, the following technical effects are disclosed: The high-precision laser ranging system and method based on pulse laser calibration provided by the present invention can improve the ranging accuracy by using a pulse laser to calibrate the output frequency of a semiconductor laser. A pulse laser (especially a mode-locked laser) can generate ultrashort, high-repetition-rate, and stable optical pulses. By using the pulse laser as a reference source, the frequency drift of the semiconductor laser under different working conditions is measured and recorded, thereby obtaining the nonlinear amount of frequency modulation. The ranging result of the semiconductor laser is corrected by using the nonlinear amount of frequency modulation to improve the ranging accuracy.

[0050] High precision: The pulse laser as a reference source has very high frequency stability and can provide a high-precision calibration benchmark.

[0051] Real-time correction: By real-time monitoring and calibration, the ranging result can be dynamically adjusted under different working conditions, improving the ranging accuracy of the overall system.

[0052] Wide application range: This method is not only applicable to the laboratory environment but also can be applied to industrial sites to improve the ranging accuracy in practical applications. In applications that require high-precision ranging, such as lidar and optical rangefinders; in the fields of optical experiments and spectroscopy, for precise measurement and calibration.

[0053] Through the present invention, the ranging error caused by the nonlinear frequency response and temperature sensitivity of the semiconductor laser can be effectively reduced, thereby improving the overall performance of the ranging system. Brief Description of the Drawings

[0054] In order to more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required in the embodiments. Obviously, the drawings in the following description are only some embodiments of the present invention. For those of ordinary skill in the art, other drawings can be obtained based on these drawings without creative efforts.

[0055] Figure 1 It is a schematic structural diagram of the high-precision laser ranging system based on pulse laser calibration in Embodiment 1 of the system of the present invention;

[0056] Figure 2 It is a schematic structural diagram of the high-precision laser ranging system based on pulse laser calibration in Embodiment 2 of the system of the present invention;

[0057] Figure 3 It is a spectrogram of the object light and the reference light in the embodiment of the present invention;

[0058] Figure 4 It is a frequency image of the beat signal in the embodiment of the present invention;

[0059] Figure 5 Images of the comb function and the rectangular periodic function according to the embodiments of the present invention;

[0060] Figure 6 Images of the comb function modulated by the rectangular periodic function according to the embodiments of the present invention;

[0061] Figure 7 Images of the mode-locked laser filtered by the rectangular periodic function according to the embodiments of the present invention;

[0062] Figure 8 According to the embodiments of the present invention Frequency domain schematic diagram;

[0063] Explanation of reference numerals:

[0064] 1. Arbitrary waveform generator; 2. Semiconductor laser; 3. First optical fiber coupler; 4. Loop unit (divided into three ports a, b, and c, input from port a and output from port b, input from port b and output from port c); 5. Optical fiber collimation unit; 6. First photodetector; 7. Data collector (three-channel input); 8. Processor; 9. Pulse laser; 10. Second optical fiber coupler; 11. Second photodetector; 12. Filter; 13. Electro-optic modulator: T. Target. Specific embodiments

[0065] Next, the technical solutions in the embodiments of the present invention will be clearly and completely described in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the protection scope of the present invention.

[0066] The purpose of the present invention is to provide a high-precision laser ranging system and method based on pulse laser calibration. By using a pulse laser to calibrate the output frequency of a semiconductor laser, the non-linear quantity of frequency modulation can be obtained, and the ranging result can be corrected to obtain a higher-precision ranging result.

[0067] To make the above objects, features, and advantages of the present invention more obvious and understandable, the present invention will be further described in detail below in conjunction with the accompanying drawings and specific embodiments.

[0068] System Embodiment 1

[0069] As Figure 1 shown, the high-precision laser ranging system based on pulse laser calibration provided by System Embodiment 1 of the present invention includes a ranging optical path and a calibration optical path;

[0070] The ranging optical path includes an arbitrary waveform generator 1, a semiconductor laser 2, a first optical fiber coupler 3, a loop unit 4, an optical fiber collimation unit 5, a first photodetector 6, a data collector 7, and a processor 8;

[0071] The calibration optical path includes a pulsed laser 9, a second optical fiber coupler 10, a second photodetector 11, and a filter 12;

[0072] The arbitrary waveform generator 1 generates a frequency modulation signal, and by controlling the drive current of the semiconductor laser 2, frequency modulates the laser signal of the semiconductor laser 2;

[0073] The frequency-modulated laser signal is split by the first optical fiber coupler 3. One beam of the laser signal is incident on the loop unit 4, and the other beam of the laser signal is incident on the second optical fiber coupler 10;

[0074] The loop unit 4 is respectively connected to the optical fiber collimation unit 5 and the first photodetector 6; the optical fiber collimation unit 5 transmits the laser signal to the target T for ranging; the first photodetector 6 acquires the ranging signal, converts it into an electrical signal, and transmits it to the data collector 7;

[0075] The pulsed laser 9 emits frequency calibration laser and transmits it to the second optical fiber coupler 10. The second optical fiber coupler 10 couples the frequency calibration laser with the laser signal to obtain a mixed optical signal. The mixed optical signal is sequentially transmitted to the data collector 7 through the second photodetector 11 and the filter 12;

[0076] The data collector 7 respectively receives the electrical signals output by the first photodetector 6 and the filter 12 and transmits them to the processor 8;

[0077] The processor 8 obtains the ranging result based on the ranging signal, determines the change curve of the laser signal frequency over time based on the frequency calibration laser, obtains the non-linearity of the frequency modulation, and calibrates the ranging result based on the non-linearity of the frequency modulation.

[0078] Specifically, the ranging signal is the object light and the reference light transmitted in a common path;

[0079] The loop unit 4 is connected to the optical fiber collimation unit 5 through an optical fiber and is used to transmit the laser signal to the optical fiber collimation unit 5;

[0080] The optical fiber collimation unit 5 is used to transmit a part of the laser signal to the target and collect the object light reflected by the target. At the same time, it reflects a part of the laser signal to form a reference light. The object light and the reference light received by the optical fiber collimation unit 5 are transmitted in a common path;

[0081] The loop unit 4 receives the object light and the reference light returned by the optical fiber collimation unit 5 and transmits them to the first photodetector 6;

[0082] The photodetector 6 is used to convert the intensity signal of the interference light of the object light and the reference light into an electrical signal and transmit it to the data collector 7.

[0083] The present invention combines the characteristics of a common-path optical path. The object light received by the fiber collimation unit and the reference light reflected from the end face of the fiber collimation unit are transmitted in a common path and form interference light (the spectrograms of the object light and the reference light are as Figure 3 shown). It is transmitted to the photodetector through the loop unit. When measuring the distance, based on the intensity signals of the interference light corresponding to the positions where the target is located before and after moving, the phase difference is calculated. Since the object light and the reference light are transmitted in a common path, the influence of external environments (such as temperature, pressure, vibration, etc.) on the object light and the reference light is the same. When calculating the phase difference, the external interference can be mutually cancelled by phase subtraction, maximizing the measurement accuracy.

[0084] The processor 8 receives the electrical signal of the photodetector 6, and can calculate the phase difference based on the intensity signals of the interference light corresponding to the positions where the target is located before and after moving, and obtain the relative distance measurement value, where the relative distance measurement value is the distance that the target moves.

[0085] When measuring the distance at the position where the target is located before moving, the time-domain expression of the intensity of the difference-frequency signal light obtained after the interference of the object light and the reference light is:

[0086]

[0087]

[0088] In the formula, , are respectively the light intensities of the rising edge and the falling edge of the difference-frequency signal obtained after the interference of the object light and the reference light when measuring the distance at the position where the target is located before moving; , are respectively the light intensities of the reference light and the object light; is the time difference corresponding to the optical path difference between the object light and the reference light when measuring the distance at the position where the target is located before moving; is the frequency modulation rate, that is, the slope of the rising edge of the triangular wave, is the frequency modulation bandwidth, is the frequency modulation period;

[0089] Then, when measuring the distance at the position where the target is located before moving, the phase difference between the light intensities of the rising edge and the falling edge is ;

[0090] Similarly, assuming that the time difference corresponding to the optical path difference between the object light and the reference light when measuring the distance at the position where the target is located after moving is , the time-domain expression of the intensity of the difference-frequency signal light obtained after the interference of the object light and the reference light is:

[0091]

[0092]

[0093] Wherein, and are respectively the light intensities of the rising edge and the falling edge of the difference frequency signal obtained after the interference of the object light and the reference light when measuring the distance at the position where the target is located after moving;

[0094] Then, when measuring the distance at the position where the target is located after moving, the phase difference between the light intensities of the rising edge and the falling edge is ;

[0095] Calculate the combined phase difference based on the intensity signals of the interference light at the positions where the target is located before and after moving:

[0096] ;

[0097] From , it is obtained that ;

[0098] Among them, because the phase difference between the light intensities of the rising edge and the falling edge is amplified by one time, the corresponding measured phase difference also needs to be amplified by one time, which is ;

[0099] Then, the calculation formula for the relative distance measurement value is as follows:

[0100]

[0101] Wherein, represents the relative distance measurement value at the positions where the target is located before and after moving;

[0102] is the time difference corresponding to the optical path difference between the object light and the reference light when measuring the distance at the position where the target is located before moving;

[0103] is the time difference corresponding to the optical path difference between the object light and the reference light when measuring the distance at the position where the target is located after moving;

[0104] is the phase difference actually measured based on the intensity signals of the interference light at the positions where the target is located before and after moving;

[0105] is the initial frequency of the laser signal; is the speed of light.

[0106] In addition, the beat frequency signal of the interference light can also be extracted based on the electrical signal. According to the principle of frequency modulated continuous wave ranging, the absolute distance can be obtained. After the target moves, the updated absolute distance can be obtained; the relative distance measurement value described above is used to correct the absolute distances measured twice to obtain the corrected absolute distances measured twice, specifically including:

[0107] According to the frequency modulation ranging principle, the absolute distances before and after the target moves are respectively and . Based on the absolute distances and , the theoretical value of the relative distance is obtained as follows:

[0108]

[0109] Based on the error analysis method, the absolute error or relative error between the measured value of the relative distance and the theoretical value of the relative distance is obtained. Based on the absolute error or relative error, the absolute distances and are respectively corrected. The specific correction scheme is as follows:

[0110] Compare the measured value of the relative distance with the theoretical value of the relative distance to analyze the error distribution law;

[0111] According to the error distribution law, through the data fitting method, an error model is established to obtain the absolute error correction parameter or relative error correction coefficient, and the systematic deviation in frequency modulation ranging (such as FMCW ranging) (including sensor calibration error, frequency response error, etc.) is found. For example, if the difference between the absolute distances and obtained by FMCW ranging and the measured value of the relative distance is a linear relationship within a certain distance range, then the relative error correction coefficient can be obtained through linear fitting. Or, if the difference between the absolute distances and obtained by FMCW ranging and the measured value of the relative distance is a fixed error value within a certain distance range, then the absolute error correction parameter calculated by the error model can be used;

[0112] Based on the absolute error correction parameter or relative error correction coefficient, the absolute distances and are respectively corrected. Specifically, multiplying the measured value of the relative distance by the relative error correction coefficient can obtain the absolute distance obtained by FMCW ranging. Or, subtracting or adding the absolute error correction parameter from the measured value of the relative distance can obtain the absolute distance obtained by FMCW ranging.

[0113] In practical applications, the error models of FMCW ranging and relative distance measurement may not be globally consistent, that is, within different ranging ranges, the error characteristics of the two may be different. Therefore, the error model can be divided into multiple different ranging intervals, and different correction factors are used for different intervals. The FMCW ranging results can be continuously corrected through a dynamic error compensation algorithm (such as Kalman filtering or other adaptive filtering methods).

[0114] Among them, the error analysis methods and data fitting methods involved can all adopt the existing technical principles in this field, and will not be elaborated here.

[0115] Exemplarily, the loop unit 4 adopts an optical fiber circulator, which is provided with three interfaces, namely port a, port b, and port c. Among them, port a is used to receive the laser signal from the optical fiber coupler 3, port b is used to output the laser signal to the optical fiber collimation unit 5, and receive the object light and the reference light, and port c is used to transmit the object light and the reference light to the first photodetector 6.

[0116] The optical fiber collimation unit 5 adopts an optical fiber collimator with a pc-type interface, a lens with transmission and reflection functions, or a combined structure of an optical fiber and a collimating mirror. The present invention does not limit the selection of the optical fiber collimation unit 5, as long as the optical fiber collimation unit 5 can realize the functions of transmitting and reflecting the laser signal. The laser signal is reflected at the end face of the optical fiber collimation unit 5 to form the reference light. After the object light passes through the optical fiber collimation unit 5, it is transmitted to the first photodetector 6 through the loop unit 4 in the same optical path as the reference light, making the measured optical path difference between the reference light and the object light closer to the distance to be measured and reducing the measurement error.

[0117] System Embodiment 2

[0118] As Figure 2 shown, in System Embodiment 2, an electro-optic modulator 13 is provided between the pulsed laser 9 and the optical fiber coupler 10, and the electro-optic modulator 13 is used to modulate the signal of the frequency-calibrated laser.

[0119] The pulsed laser 9 can adopt a mode-locked laser. The modulation means of existing low-repetition-rate mode-locked lasers are limited, and it is impossible to flexibly realize rapid adjustment and selective control of the frequency, which limits its application scope in complex application scenarios. By introducing the electro-optic modulator 13, the existing high-repetition-rate mode-locked lasers can be frequency-selected and adjusted to reduce the system's dependence on the physical cavity length, thereby realizing miniaturization, integration, and higher anti-interference ability.

[0120] The arbitrary waveform generator 1 is also used to generate a synchronous electrical signal and transmit it to the data collector 7, and the synchronous electrical signal has the same frequency as the frequency-modulated signal.

[0121] Method Embodiment 1:

[0122] The present invention also provides a high-precision laser ranging method based on pulse laser calibration, which is applied to the above-mentioned high-precision laser ranging system based on pulse laser calibration. The pulse laser can adopt a mode-locked laser. The method specifically includes the following steps:

[0123] When the pulse laser adopts a mode-locked laser, based on the optical frequency comb technology, the mode-locked laser generates an optical frequency comb, and the frequencies of the corresponding frequency spectral lines are expressed as follows:

[0124]

[0125] Where: is the frequency of the nth spectral line of the optical frequency comb, is the repetition frequency, equal to the repetition rate of the pulse train, which is determined by the cavity length of the mode-locked laser, is the carrier-envelope phase shift frequency, representing the rate of change of the relative position between the carrier phase of a single pulse and the pulse envelope, that is, the phase offset rate between the carrier light wave and the pulse envelope. This value is less than ; n is a positive integer.

[0126] The frequency of the laser signal of the semiconductor laser is , , , ;

[0127] Wherein, represents the initial frequency of the laser, is the frequency modulation rate, that is, the slope of the rising edge of the triangular wave, is the frequency modulation bandwidth. After mixing the mode-locked laser and the semiconductor laser, a difference frequency signal and a sum frequency signal will be obtained. The frequency of the sum frequency signal is too high and can be ignored. Then the expression of the difference frequency signal is:

[0128]

[0129] It can be seen that, as shown in Figure 8 , Figure 8The frequency-modulated laser in this application is the semiconductor laser in this application. Taking the aforementioned laser as an example, the difference frequency signal shows a periodic distribution and a discrete distribution. The mode-locked laser in the system selects a mode-locked laser with a repetition frequency of about 8 MHz, and the filter selects a band-pass filter with a center bandwidth of 2 MHz and a filtering bandwidth of 1 MHz. After the difference frequency signal passes through the filter, there will be an output only when the difference frequency signal frequency is between 1.5 - 2.5 MHz. Therefore, there will be an output only when the frequency modulation spectrum line is close to or far from the mode-locked laser spectrum line, and each spectrum line corresponds to a pair of output signals. By analyzing the data at the output end of the band-pass filter, the frequency modulation rate of the frequency modulation spectrum line at different times can be obtained from the interval between each output and the data width of a single output, so as to obtain the non-linearity of frequency modulation and the impact on ranging.

[0130] Assume that the frequency modulation bandwidth is 800 MHz, then there are 200 outputs in one frequency modulation period. Taking the distance between two outputs as a variable, a curve reflecting the linearity of frequency modulation can be obtained after fitting to correct the ranging result. For phase ranging, since the triangular wave is synchronously output at the arbitrary waveform generator, the frequency spectrum line corresponding to the measurement time point can be judged using the triangular wave waveform. When the frequency modulation spectrum line is in an ideal situation, the beat frequency signal obtained should be a sine wave except for the first and last segments, that is Figure 4 the frequency difference in

[0131] After the difference frequency signal passes through the filter, there is an output when the difference frequency signal frequency meets the filter bandwidth range;

[0132] During the frequency modulation process of the frequency signal of the semiconductor laser, when the frequency difference from the equally spaced discrete spectrum lines of the frequency calibration laser meets the bandwidth range of the filter, a pair of output signals are generated. By using the equally spaced characteristics of the discrete spectrum lines and the characteristic that the bandwidth of the filter is not affected by the frequency modulation of the semiconductor laser, the interval between each output of the difference frequency signal and the width of a single output are obtained, and the frequency modulation rate of the frequency modulation signal of the semiconductor laser at different times is obtained, so as to obtain the non-linear quantity of frequency modulation.

[0133] For short-distance measurement, the time for beat frequency transformation is very short. During processing, a certain truncation can be performed on the front and back of the beat frequency signal to obtain the complete sine wave part. By corresponding the measured frequency modulation spectrum line with the actually obtained sine wave, the measured sine wave signal can be corrected to make the obtained signal closer to the theoretical signal and the obtained phase more accurate.

[0134] The ideal frequency of the semiconductor laser is:

[0135] , , ,

[0136] where represents the initial frequency of the laser, the frequency modulation rate, i.e., the slope of the rising edge of the triangular wave, is the frequency modulation bandwidth;

[0137] and the actually detected curve after calibration is:

[0138] , , ,

[0139] wherein, is the non - linear part in the actual laser frequency. After mixing, the beat - frequency signal frequency is corrected to {f}_{b}\left ( {t} \right )=\alpha \tau+\left [ {{f}_{n}\left ( {t} \right )-{f}_{n}\left ( {t - \tau} \right )} \right ] , and the phase of the beat - frequency signal is corrected to , is the time corresponding to the optical path difference between the object light and the reference light at the position of the target;

[0140] After obtaining the result of a single measurement, according to the relatively low - precision time obtained for the first time, substitute it to correct the image, obtain a more accurate time interval, and then iterate to obtain a more accurate ranging result.

[0141] In an embodiment of a high - precision laser ranging method based on pulse laser calibration, the specific calibration scheme is as follows:

[0142] First, use a mode - locked laser and a single - frequency laser to obtain the frequency - modulation curve of the single - frequency laser;

[0143] Couple the signals emitted by the mode - locked laser and the semiconductor laser. The frequency of the mode - locked laser can be expressed as

[0144]

[0145] wherein, is the frequency of the n th spectral line of the optical frequency comb, is the repetition frequency, equal to the repetition rate of the pulse train, determined by the cavity length of the mode - locked laser, is the carrier - envelope phase - shift frequency, representing the rate of change of the relative position between the carrier phase of a single pulse and the pulse envelope, i.e., the phase - shift rate between the carrier light wave and the pulse envelope. This value is less than .

[0146] , Both of them can be obtained through measurement and are known quantities. The semiconductor laser can be frequency calibrated with this known quantity.

[0147] For the rising edge period of frequency modulation, the frequency expression of the semiconductor laser is known as

[0148] ,

[0149] Then the difference frequency between the mode-locked laser and the semiconductor laser is

[0150]

[0151] It can be seen that its beat frequency structure is also comb-shaped, but the carrier frequency of the comb-shaped structure changes with time. Considering the positive and negative axis properties of the difference frequency signal, that is, there is no negative value part in the difference frequency, the value range of the first spectral line of the difference frequency between the single-frequency signal and the mode-locked laser is , and the corresponding physical meaning is that if this part of the spectrum is observed, it reflects the frequency difference between the single spectral line and the nearest mode-locked laser spectral line. Therefore, observing this part of the spectrum can obtain the movement of the single spectral line and obtain the frequency modulation spectral line. Or use a band-pass filter (the passband is in ), and obtain the output of each single spectral line passing through a certain frequency band. The physical meaning is that there will be an output when the single spectral line arrives and leaves a certain frequency modulation spectral line. The obtained pulse pair can be used to fit the frequency modulation spectral line.

[0152] After obtaining the frequency modulation curve, correct the correct single-frequency frequency modulation curve to ;

[0153] For phase-based ranging, the image of the beat frequency needs to be used for phase calculation. Therefore, consider correcting the time axis of non-linear frequency modulation t to an ideal linear time axis , satisfying Map the beat frequency signal into a new beat frequency image. This beat frequency image corresponds to the correct linear time axis and eliminates the non-linearity.

[0154] The relationship between the frequency of the non-linear signal and time is . By establishing a time mapping function , convert the non-linear time axis to a linear time axis . Satisfy:

[0155]

[0156] It is necessary to inversely find the corresponding , that is, solve

[0157]

[0158] This equation can be solved using numerical methods:

[0159] I. Sample generation

[0160] Establish a curve based on the experimental data of the known mode-locked laser and single-frequency laser :

[0161] 1. Generate a set of discrete sample points on the time axis, and generate the corresponding frequency values according to the curve to generate a set of discrete sample points:

[0162]

[0163] 2. Calculate the ideal time corresponding to the discrete time points

[0164]

[0165] II. Interpolation function

[0166] Use the discrete points to construct an interpolation function . Common methods include:

[0167] Linear interpolation: Suitable for simple cases with fewer points.

[0168] Spline interpolation: Suitable for cases with more points and requirements for smoothness.

[0169] The form of the interpolation function is:

[0170]

[0171] Then, resample or interpolate the measured beat frequency signal according to the time mapping to obtain the corrected phase.

[0172] In this embodiment, a mode-locked laser is used for frequency calibration, which can avoid an additional complex reference optical path, is convenient for integration, and has a volume advantage. Using a mode-locked laser for frequency calibration has the advantages of real-time monitoring, and the obtained frequency information can not only be used to correct the frequency in frequency ranging, but also be used to correct the obtained beat frequency information, making the accuracy of the obtained phase information higher. And after obtaining the offset of the frequency modulation, it can be selected to be fed back to the laser for correction, or the result can be directly corrected through the frequency spectrum line, which can adapt to different application scenarios.

[0173] Method Embodiment 2:

[0174] The present invention also provides a high-precision laser ranging method based on pulse laser calibration, which is applied to the above-mentioned high-precision laser ranging system based on pulse laser calibration, and includes the following steps:

[0175] First, discuss the frequency spectrum characteristics of the pulse laser. The Fourier transform of a periodic pulse can convert the pulse signal in the time domain into a frequency domain representation. If the periodic pulse train is periodic in time, the Fourier transform will produce discrete frequency components. The specific steps are as follows:

[0176] Fourier transform of a single pulse: Assume that the duration of a single pulse is The Fourier transform of a single rectangular pulse is:

[0177]

[0178] Among them, ;

[0179] Periodic pulse train: If the pulse repeats with a period , the overall pulse train can be regarded as the convolution of a single pulse and a periodic impulse function . In the Fourier transform, the time-domain convolution is equal to the frequency-domain product, which will cause the pulse spectrum to become a discrete spectrum line, and the spectral line interval is the repetition frequency ;

[0180] Spectral characteristics: The frequency-domain signal after Fourier transform presents discrete frequency components with an interval of . The envelope shape of each frequency component is determined by the Fourier transform of a single pulse (such as the function of a rectangular pulse).

[0181] Based on the frequency spectrum characteristics of the pulse laser, the Fourier transform of the periodic pulse converts the pulse signal in the time domain into a frequency domain representation. If the periodic pulse train is periodic in time, the Fourier transform will produce discrete frequency components, forming discrete frequency spectrum lines. It can be seen that the frequency correction for ranging can also be theoretically performed on the pulse signal spectrum.

[0182] The subsequent principle is the same as that of the mode-locked laser calibration described above. However, the intensity of the pulse pair obtained after passing through the filter will change, but we do not measure the intensity, so it does not affect the application.

[0183] In some cases where it is not convenient to use a mode-locked laser, or when considering the cost and complexity of a mode-locked laser, a general pulse laser and its above principle can be adopted.

[0184] Based on the above method and principle, after receiving various signals in the processor, the following steps are executed:

[0185] Step 1: Pass the square wave obtained from an arbitrary waveform generator through a comparator to change the high level of the square wave to 1 and the low level to 0;

[0186] Step 2: Multiply the square wave obtained in Step 1 by the beat signal obtained from the first photodetector to obtain the rising edge of the triangular wave; then invert the square wave obtained in Step 1 and multiply it by the beat signal obtained from the first photodetector to obtain the falling edge of the triangular wave:

[0187] Step 3: According to the slope of the triangular wave of the arbitrary waveform generator, the rising edge and falling edge of the obtained triangular wave can be filtered to remove the linear component;

[0188] Step 4: Multiply the square wave obtained in Step 1 by the pulse signal obtained from the data collector to obtain the pulse signal corresponding to the rising edge of the triangular wave; then invert the square wave obtained in Step 1 and multiply it by the pulse signal obtained from the data collector to obtain the pulse signal corresponding to the falling edge of the triangular wave:

[0189] The following processing methods for the rising edge and falling edge are the same. Taking the rising edge signal as an example:

[0190] Step 5: Use the rising edge pulse signal obtained in Step 4 to draw a scatter plot. When processing, determine the time node corresponding to the pulse signal according to the full width at half maximum of the pulse signal;

[0191] Step 6: According to the ideal frequency of the semiconductor laser being , , , , and the actually detected curve after calibration is , , , , where is the non-linear part in the actual laser frequency. In the ideal case, the scatter plot obtained in Step 5 should be a straight line. Fit the curve of the scatter plot drawn in Step 5 to obtain the expression;

[0192] Step 7: Repeat Steps 5 and 6 for the falling edge signal;

[0193] Step 8: According to the beat signal obtained in Step 4, truncate the small non-sinusoidal parts at the beginning and end of each period, and then use the zero-crossing method to calculate its frequency, that is, obtain the frequency through the time interval between two zero points;

[0194] Step 9: Substitute the frequency of the beat into the formula for the beat signal frequency and time delay formula , to obtain the time delay , substituting the corrected beat signal frequency {f}_{b}(t)=\alpha\tau+[{f}_{n}(t)-{f}_{n}(t - \tau)] , obtaining the new frequency and the new time delay again and performing iteration, setting the number of iterations and the iteration conditions according to the target accuracy, and finally obtaining the updated frequency and the time delay ;

[0195] Step 10: Substituting the obtained time delay into , the corrected phase term can be obtained.

[0196] In addition, the repetition rate of the mode-locked laser can be reduced by performing a rectangular modulation with a lower repetition rate on the mode-locked laser through an electro-optic modulator.

[0197] After modulation, the mode-locked laser waveform is as Figure 6 shown.

[0198] From Figure 6 , it can be seen that when using the electro-optic modulator for frequency selection, the switching duration of the electro-optic modulator needs to be less than the repetition period of the mode-locked laser. The switching time of the electro-optic modulator is usually in the range of picoseconds to nanoseconds, depending on the material and application, while the repetition rate of the mode-locked laser is generally in GHz and the repetition period is in the nanosecond range, so it is feasible. Adjusting the switching time of the electro-optic modulator so that there is only one pulse during conduction, there is:

[0199] Step 1: Determine the switching time of the electro-optic modulator according to the repetition rate of the mode-locked laser to ensure that the switching time of the electro-optic modulator is less than the repetition period;

[0200] Step 2: Determine the switching frequency of the electro-optic modulator according to the frequency modulation bandwidth of the semiconductor laser. It is expected that there will be a better effect when the frequency of the electro-optic modulator is below two percent of the frequency modulation bandwidth, that is, the repetition rate of the mode-locked laser is reduced.

[0201] Using the method of reducing the repetition rate to achieve calibration can avoid the disadvantages of weak anti-interference ability and insufficient modulation flexibility of long-cavity mode-locked lasers, and the mode-locked laser has the advantages of low cost and small volume in structure. In the signal processing method, since the curve of frequency modulation nonlinearity has been obtained, the data can be post-processed after measurement, or the information can be negatively fed back to change the modulation current to remove the frequency modulation nonlinearity; and the processing process is relatively simple.

[0202] In this article, specific examples are used to illustrate the principles and implementation methods of the present invention. The description of the above embodiments is only for helping to understand the method of the present invention and its core idea; at the same time, for those of ordinary skill in the art, according to the idea of the present invention, there will be changes in the specific implementation methods and application scopes. To sum up, the content of this specification should not be construed as a limitation to the present invention.

Claims

1. A high-precision laser ranging system based on pulse laser calibration, characterized in that: Including ranging optical path and calibration optical path; The distance measuring optical path comprises an arbitrary waveform generator (1), a semiconductor laser (2), an optical fiber coupler (3), a distance measuring unit, a photoelectric detector (6), a data acquisition device (7) and a processor (8); The calibration optical path comprises a pulse laser (9), a second optical fiber coupler (10), a second photodetector (11), and a filter (12); The arbitrary waveform generator (1) generates a frequency modulation signal, and performs frequency modulation on the laser signal of the semiconductor laser (2) by controlling the driving current of the semiconductor laser (2); The frequency modulated laser signal is split by the optical fiber coupler 1 (3), one laser signal is incident on the distance measuring unit, and the other laser signal is incident on the optical fiber coupler 2 (10); The distance measuring unit transmits a laser signal to the target for distance measurement; the photoelectric detector (6) obtains the distance measuring signal, converts it into an electrical signal, and transmits it to the data acquisition device (7); The pulse laser (9) emits a frequency calibration laser and transmits it to the second optical fiber coupler (10). The second optical fiber coupler (10) couples the frequency calibration laser with the laser signal to obtain a mixed optical signal. The mixed optical signal is sequentially transmitted to the data acquisition device (7) via the second photoelectric detector (11) and the filter (12). The data collector (7) receives the electrical signals output by the photoelectric detector (6) and the filter (12) respectively, and transmits them to the processor (8); The processor (8) obtains a distance measurement result based on the distance measurement signal, determines a curve of a change in the frequency of the laser signal over time based on a frequency calibration laser, obtains a nonlinear amount of frequency modulation, and calibrates the distance measurement result based on the nonlinear amount of frequency modulation; The distance measuring unit comprises a loop unit (4) and an optical fiber alignment unit (5). The frequency modulated laser signal is split by the optical fiber coupler 1 (3), one beam of the laser signal is incident on the loop unit (4), and the other beam of the laser signal is incident on the optical fiber coupler 2 (10); The loop unit (4) is respectively connected to the optical fiber collimation unit (5) and the photoelectric detector (6); the laser signal is transmitted to the target via the optical fiber collimation unit (5) for distance measurement.

2. The high-precision laser ranging system based on pulse laser calibration according to claim 1, characterized in that: The distance measurement signal is the co-transmitted object light and reference light; The loop unit (4) is connected to the optical fiber collimation unit (5) via an optical fiber, and is used to transmit the laser signal to the optical fiber collimation unit (5); The optical fiber collimation unit (5) is used to transmit a part of the laser signal to the target, collect the object light reflected by the target, and reflect a part of the laser signal to form reference light. The object light received by the optical fiber collimation unit (5) and the reference light are transmitted in the same way. The loop unit (4) receives the object light and the reference light transmitted back by the optical fiber collimation unit (5), and transmits them to a photoelectric detector 1 (6); The photoelectric detector 1 (6) is used to convert the intensity signal of the interference light of the object light and the reference light into an electrical signal and transmit it to the data acquisition device (7).

3. The high-precision laser ranging system based on pulse laser calibration according to claim 2, characterized in that: The loop unit (4) adopts an optical fiber circulator and is provided with three interfaces, namely, port a, port b, and port c, wherein port a is used to receive a laser signal from an optical fiber coupler (3), port b is used to output the laser signal to an optical fiber collimation unit (5) and receive object light and reference light, and port c is used to transmit the object light and reference light to a photoelectric detector (6); The optical fiber collimation unit (5) adopts an optical fiber collimator with a PC-type interface, a lens with transmission and reflection functions, or a combined structure of an optical fiber and a collimator.

4. The high-precision laser ranging system based on pulse laser calibration according to claim 1, characterized in that: An electro-optic modulator (13) is arranged between the pulse laser (9) and the second optical fiber coupler (10), and the electro-optic modulator (13) is used to modulate the signal of the frequency calibration laser.

5. The high-precision laser ranging system based on pulse laser calibration according to claim 1, characterized in that: The arbitrary waveform generator (1) is also used to generate a synchronous electrical signal, which is transmitted to a data collector (7), wherein the synchronous electrical signal has the same frequency as the frequency modulation signal.

6. The high-precision laser ranging system based on pulse laser calibration according to claim 1, characterized in that: The pulse laser (9) is a mode-locked laser.

7. A high-precision laser ranging method based on pulse laser calibration, applied to the high-precision laser ranging system based on pulse laser calibration according to any one of claims 1 to 6, characterized in that: The steps include: Based on the frequency calibration laser, the frequency spectrum line of the pulsed laser is obtained; Based on the frequency spectrum, the frequency calibration laser is coupled with the laser signal to obtain a mixed optical signal, including a difference frequency signal and a sum frequency signal. The sum frequency signal is negligible, and the difference frequency signal expression is Δf = n·f r +f c -f e (t) Among them, f r is the repetition frequency, which is equal to the repetition rate of the laser pulse; f c is the carrier envelope phase shift frequency, which indicates the rate at which the relative position between the carrier phase and the pulse envelope of a single pulse changes, that is, the phase shift rate between the carrier light wave and the pulse envelope. This value is less than f r ;f e (t) is the frequency of the laser signal of the semiconductor laser; n is a positive integer; After the difference frequency signal passes through the filter, there is an output when the difference frequency signal frequency meets the filter bandwidth range; During the frequency modulation process of the frequency signal of the semiconductor laser, when the frequency difference between the equally spaced discrete spectral lines of the frequency calibration laser meets the bandwidth range of the filter, a pair of output signals is generated. By utilizing the equally spaced characteristics of the discrete spectral lines and the characteristics that the bandwidth of the filter is not affected by the frequency modulation of the semiconductor laser, the interval between each output of the difference frequency signal and the width of a single output are calculated, and the frequency modulation rate of the semiconductor laser frequency modulation signal at different times is obtained, thereby obtaining the nonlinear amount of the frequency modulation; The ideal frequency of a semiconductor laser is: Where f0 represents the initial frequency of the laser, α=2B / T is the frequency modulation rate, that is, the slope of the rising edge of the triangle wave, and B is the frequency modulation bandwidth; After calibration, the actual curve detected is: Among them, f n (t) is the nonlinear part of the actual laser frequency. After mixing, the beat signal frequency is corrected to f b (t) = ατ + [f n (t)-f n (t-τ)], the phase correction of the beat frequency signal is τ is the time corresponding to the optical path difference between the object light and the reference light at the target location; After obtaining the result of a single measurement, the beat signal frequency image is corrected based on the time τ with lower precision obtained for the first time to obtain a more accurate time interval, and then a more accurate distance measurement result is obtained by iteration.

8. The high-precision laser ranging method based on pulse laser calibration according to claim 7, characterized in that: When a pulsed laser uses a mode-locked laser, based on the optical frequency comb technology, the mode-locked laser generates an optical frequency comb, and the frequency of the corresponding frequency spectrum line is expressed as follows: f=n·f r +f c Where: f is the frequency of the nth spectral line of the optical frequency comb, f r is the repetition frequency, which is equal to the repetition rate of the pulse train and is determined by the cavity length of the mode-locked laser, f c is the carrier envelope phase shift frequency, which indicates the rate at which the relative position between the carrier phase and the pulse envelope of a single pulse changes, that is, the phase shift rate between the carrier light wave and the pulse envelope. This value is less than f r .

9. The high-precision laser ranging method based on pulse laser calibration according to claim 7, characterized in that: Based on the frequency spectrum characteristics of the pulsed laser, the Fourier transform of the periodic pulse converts the pulse signal in the time domain into a frequency domain representation. If the periodic pulse train is periodic in time, the Fourier transform will produce discrete frequency components and form discrete frequency spectrum lines.

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