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

Abstract

The invention provides a pulse laser calibration-based high-precision laser ranging system and method. The system comprises a ranging light path and a calibration light path, in the distance measurement light path, an arbitrary waveform generator performs frequency modulation on a laser signal of a semiconductor laser, the laser signal is divided into two beams through an optical fiber coupler, the two beams are respectively incident to a distance measurement unit and a calibration light path, and the distance measurement unit transmits the laser signal to a target for distance measurement; in the calibration light path, a pulse laser emits frequency calibration laser, the frequency calibration laser is transmitted to a second optical fiber coupler, and the frequency calibration laser is coupled with the laser signal to obtain a frequency mixing light signal; and the processor obtains a distance measurement result based on the distance measurement signal, determines a change curve of the laser signal frequency along with time based on the frequency calibration laser, obtains a non-linear quantity of frequency modulation, and calibrates the distance measurement result based on the non-linear quantity of frequency modulation. The pulse laser is used for calibrating the output frequency of the semiconductor laser, the distance measurement result is corrected, and the distance measurement precision is improved.

Description

Technical Field

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

[0002] LiDAR uses laser as light source and photoelectric detection technology to realize active detection of target distance. It has the advantages of high precision, high speed and small size. It is widely used in autonomous driving, drones, intelligent robots, 3D modeling, geographic surveying and mapping and other fields.

[0003] In the existing laser radar ranging method, when using semiconductor lasers for FMCW ranging, since the frequency modulation of semiconductor lasers 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 affects 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 nonlinear errors in the frequency modulation process; when using triangular wave modulation, due to the nonlinearity of the frequency response, it will lead to nonlinear errors in the frequency modulation process, further leading to a decrease in ranging accuracy.

[0004] In order to improve the ranging accuracy, the existing technology proposes: 1. External reference source method, using a stable external frequency source (such as a high-precision frequency standard or reference laser) whose frequency is known and stable. By comparing the frequency of the FMCW laser with the external reference source, the output frequency of the laser can be calibrated; 2. Frequency meter method, using a precision frequency meter to directly measure the output frequency of the laser and compare it with the set target frequency; 3. Interference method, using interference technology, the output of the laser is interfered with a reference laser of known frequency, and the laser frequency is adjusted by analyzing the frequency difference in the interference pattern; 4. Spectral analysis method, using a spectrum analyzer to directly measure the output spectrum of the laser, and calibrate according to the deviation of the spectral line position from the target frequency. However, the existing technology also has the following defects: limited accuracy, traditional methods rely on external reference sources or measurement equipment, 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, real-time monitoring and calibration of frequency drift are difficult, 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 semiconductor laser is frequency modulated by electric current, the frequency of the semiconductor laser is not strictly linear with the change of the driving current. There is frequency nonlinearity, which will cause the frequency change of the frequency modulated signal to deviate from the expected linear relationship. In the frequency modulated continuous wave (FMCW) ranging method, nonlinearity will cause errors in distance measurement. In heterodyne interferometry, nonlinearity will affect the stability and accuracy of the interference signal, thereby reducing the ranging accuracy.

[0006] As people's requirements for ranging accuracy continue to increase, existing ranging systems and methods can no longer meet the needs. Based on 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 purpose of the present invention is to provide a high-precision laser ranging system and method based on pulse laser calibration. The pulse laser is used to calibrate the output frequency of a semiconductor laser, the nonlinear amount of frequency modulation can be obtained, the ranging result can be corrected, and a higher-precision ranging result can be obtained.

[0008] To achieve the above object, the present invention provides the following solutions: A high-precision laser ranging system based on pulse laser calibration, comprising a ranging optical path and a calibration optical path; The distance measurement optical path includes an arbitrary waveform generator, a semiconductor laser, a fiber coupler, a distance measurement unit, a photoelectric detector, a data acquisition device and a processor; The calibration optical path includes a pulse laser, a second optical fiber coupler, a second photodetector, and a filter; The arbitrary waveform generator generates a frequency modulation signal, and performs frequency modulation on the laser signal of the semiconductor laser by controlling the driving current of the semiconductor laser; The frequency modulated laser signal is split by the optical fiber coupler 1, one laser signal is incident on the distance measuring unit, and the other laser signal is incident on the optical fiber coupler 2; The ranging unit transmits a laser signal to the target for ranging; the photoelectric detector obtains the ranging signal, converts it into an electrical signal, and transmits it to the data collector; The pulse laser emits a frequency calibration laser and transmits it to the second optical fiber coupler, which couples the frequency calibration laser with the laser signal to obtain a mixed frequency optical signal, which is then transmitted to the data collector via the second photoelectric detector and the filter. The data collector receives the electrical signals output by the photodetector 1 and the filter respectively, and transmits them to the processor; The processor obtains a distance measurement result based on the distance measurement signal, determines a curve of a change in the laser signal frequency 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.

[0009] Furthermore, the distance measuring unit includes a loop unit and an optical fiber alignment unit. The frequency modulated laser signal is split by the optical fiber coupler 1, one laser signal is incident on the loop unit, and the other laser signal is incident on the optical fiber coupler 2; The loop unit is connected to the optical fiber collimation unit and the photoelectric detector respectively; the laser signal is transmitted to the target through the optical fiber collimation unit for distance measurement.

[0010] Furthermore, the distance measurement signal is the co-transmitted object light and reference light; The loop unit is connected to the optical fiber collimation unit via an optical fiber, and is used to transmit the laser signal to the optical fiber collimation unit; 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 received by the optical fiber collimation unit and the reference light are transmitted in the same way. 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; The photoelectric detector 1 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.

[0011] Furthermore, the loop unit 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 1, port b is used to output the laser signal to an optical fiber collimation unit and receive object light and reference light, and port c is used to transmit the object light and reference light to a photodetector 1; 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 collimator.

[0012] Furthermore, an electro-optic modulator is arranged between the pulse laser and the second optical fiber coupler, and the electro-optic modulator is used to modulate the signal of the frequency calibration laser.

[0013] Furthermore, the arbitrary waveform generator is also used to generate a synchronous electrical signal, which is transmitted to a data collector, and the synchronous electrical signal has the same frequency as the frequency modulation signal.

[0014] Furthermore, the pulse laser is a mode-locked laser.

[0015] 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 comprises the following steps: 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: in, is the repetition frequency, which is equal to the repetition rate of the laser pulses; 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 ; 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: , , , in, represents the initial frequency of the laser, is the frequency modulation rate, that is, the slope of the rising edge of the triangle wave, is the FM bandwidth, is the frequency modulation period; After calibration, the actual curve detected is: , , , in, is the nonlinear part of the actual laser frequency. After mixing, the beat 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 ] , the phase correction of the beat frequency signal 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 time with lower accuracy obtained first is The frequency image of the post-corrected beat frequency signal is brought in to obtain a more precise time interval, and then a more accurate ranging result is obtained by iteration.

[0016] Furthermore, 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 frequency of the corresponding frequency spectrum line is expressed as follows: in: It is the optical frequency comb n The frequency of the spectral line, 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. 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 .

[0017] Furthermore, 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.

[0018] According to the specific embodiments provided by the present invention, the present invention discloses the following technical effects: the high-precision laser ranging system and method based on pulse laser calibration provided by the present invention uses a pulse laser to calibrate the output frequency of a semiconductor laser, which can improve the ranging accuracy. Pulse lasers (especially mode-locked lasers) can generate ultra-short, high-repetition-frequency and stable light 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 nonlinear amount of frequency modulation is used to correct the ranging result of the semiconductor laser to improve the ranging accuracy.

[0019] High Precision: Pulsed lasers as reference sources have very high frequency stability and can provide a high-precision calibration benchmark.

[0020] Real-time correction: Through real-time monitoring and calibration, the ranging results can be dynamically adjusted under different working conditions to improve the ranging accuracy of the overall system.

[0021] Wide range of applications: This method is not only suitable for laboratory environments, but can also be applied to industrial sites to improve the ranging accuracy in practical applications. In applications that require high-precision ranging, such as lidar, optical rangefinders, etc.; in optical experiments, spectroscopy and other fields, it is used for precise measurement and calibration.

[0022] The present invention can effectively reduce the distance measurement error caused by the frequency response nonlinearity and temperature sensitivity of the semiconductor laser, thereby improving the overall performance of the distance measurement system. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative labor.

[0024] Figure 1 This is a schematic diagram of the structure of a high-precision laser ranging system based on pulse laser calibration in system embodiment 1 of the present invention; Figure 2 This is a schematic diagram of the structure of a high-precision laser ranging system based on pulse laser calibration in system embodiment 2 of the present invention; Figure 3 Spectrum diagram of object light and reference light according to an embodiment of the present invention; Figure 4 It is a frequency image of a beat frequency signal according to an embodiment of the present invention; Figure 5 The images of the comb function and the rectangular periodic function according to the embodiment of the present invention; Figure 6 An image in which the comb function of an embodiment of the present invention is modulated by a rectangular periodic function; Figure 7 An image of a mode-locked laser screened by a rectangular periodic function according to an embodiment of the present invention; Figure 8 For the embodiment of the present invention Frequency domain schematic diagram; Description of reference numerals: 1. Arbitrary waveform generator; 2. Semiconductor laser; 3. Fiber coupler 1; 4. Loop unit (divided into three ports a, b, and c, port a input and port b output, port b input and port c output); 5. Fiber collimation unit; 6. Photodetector 1; 7. Data acquisition unit (three-way input); 8. Processor; 9. Pulse laser; 10. Fiber coupler 2; 11. Photodetector 2; 12. Filter; 13. Electro-optic modulator: T, target. DETAILED DESCRIPTION

[0025] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0026] The purpose of the present invention is to provide a high-precision laser ranging system and method based on pulse laser calibration. The pulse laser is used to calibrate the output frequency of a semiconductor laser, the nonlinear amount of frequency modulation can be obtained, the ranging result can be corrected, and a higher-precision ranging result can be obtained.

[0027] In order to make the above-mentioned objects, features and advantages of the present invention more obvious and easy to understand, the present invention is further described in detail below with reference to the accompanying drawings and specific embodiments.

[0028] System Example 1 like Figure 1 As 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; The distance measurement optical path includes an arbitrary waveform generator 1, a semiconductor laser 2, a fiber coupler 3, a loop unit 4, a fiber collimation unit 5, a photoelectric detector 6, a data acquisition device 7 and a processor 8; The calibration optical path includes a pulse laser 9, a fiber coupler 10, a 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 loop unit 4, and the other laser signal is incident on the optical fiber coupler 2 10; The loop unit 4 is connected to the optical fiber collimation unit 5 and the photoelectric detector 6 respectively; the optical fiber collimation unit 5 transmits the laser signal to the target T for ranging; the photoelectric detector 6 obtains the ranging signal, converts it into an electrical signal, and transmits it to the data collector 7; The pulse laser 9 emits a frequency calibration laser and transmits it to the optical fiber coupler 10. The optical fiber coupler 10 couples the frequency calibration laser with the laser signal to obtain a mixed frequency optical signal. The mixed frequency optical signal is sequentially transmitted to the data collector 7 via the photoelectric detector 11 and the filter 12. The data collector 7 receives the electrical signals output by the photodetector 1 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 laser signal frequency 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.

[0029] Specifically, the ranging 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 a reference light. The object light received by the optical fiber collimation unit 5 and the reference light are transmitted together. 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 the photoelectric detector 6; The photoelectric detector 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.

[0030] The present invention combines the characteristics of a common optical path. The object light received by the optical fiber collimation unit and the reference light reflected at the end face of the optical fiber collimation unit are transmitted in the same path and form interference light (the spectrum diagram of the object light and the reference light is shown in FIG. Figure 3 As shown in the figure), it is transmitted to the photodetector through the loop unit. When measuring the distance, the phase difference is calculated based on the intensity signal of the interference light corresponding to the position before and after the target moves. Since the object light and the reference light are transmitted in the same way, the interference of the external environment (such as temperature, pressure, vibration, etc.) has the same effect on the object light and the reference light. When calculating the phase difference, the external interference can be offset by phase subtraction, thereby maximizing the measurement accuracy.

[0031] The processor 8 receives the electrical signal from the photodetector 6 and can calculate the phase difference based on the intensity signal of the interference light corresponding to the position of the target before and after movement to obtain a relative distance measurement value, where the relative distance measurement value is the distance the target moves.

[0032] When measuring the distance at the position where the target moves before, the time domain expression of the difference frequency signal intensity obtained after the interference of the object light and the reference light is: In the formula, , They are respectively the light intensity at the rising and falling edges of the difference frequency signal obtained after the interference of the object light and the reference light when measuring the distance at the position before the target moves; , are the intensities of the reference light and the object light, respectively; The time difference corresponding to the optical path difference between the object light and the reference light when measuring the distance to the position of the target before it moves; is the frequency modulation rate, that is, the slope of the rising edge of the triangle wave, is the FM bandwidth, is the frequency modulation period; When measuring the distance at the position where the target moved before, the phase difference between the rising and falling edges is ; Similarly, when measuring the distance at the position where the target moves, the time difference corresponding to the optical path difference between the object light and the reference light is , the time domain expression of the difference frequency signal intensity obtained after the interference of the object light and the reference light is: In the formula, , They are the light intensity at the rising and falling edges of the difference frequency signal obtained after the interference of the object light and the reference light when measuring the distance to the position of the target after it moves; When measuring the distance at the position where the target moves, the phase difference between the rising and falling edges is ; The combined phase difference is calculated based on the intensity signal of the interference light at the position before and after the target moves: ; Depend on , we can conclude ; Among them, because the phase difference of the light intensity at the rising edge and the falling edge is doubled, the corresponding measured phase difference also needs to be doubled, which is ; Then, the calculation formula for the relative distance measurement value is as follows: In the formula, Relative distance measurement indicating the position of the target before and after the movement; The time difference corresponding to the optical path difference between the object light and the reference light when measuring the distance to the position of the target before it moves; The time difference corresponding to the optical path difference between the object light and the reference light when measuring the distance to the target after it moves; It is the phase difference actually measured based on the intensity signal of the interference light at the position of the target before and after movement; is the initial frequency of the laser signal; The speed of light.

[0033] In addition, the beat frequency signal of the interference light can be extracted based on the electrical signal, and the absolute distance can be obtained according to the frequency modulated continuous wave ranging principle. After the target moves, the updated absolute distance can be obtained; the absolute distances of the two measurements are corrected using the aforementioned relative distance measurement values ​​to obtain the corrected two absolute distances, which specifically includes: According to the frequency modulation ranging principle, the absolute distances before and after the target movement are obtained as follows: , , according to the absolute distance and Get the theoretical value of relative distance : Based on the error analysis method, the relative distance measurement value is obtained Theoretical value of relative distance The absolute error or relative error between , The specific amendment plans are as follows: Comparing relative distance measurements Relative distance theoretical value , analyze the error distribution law; According to the error distribution law, the error model is established through data fitting method to obtain the absolute error correction parameter or relative error correction coefficient, and the system deviation (including sensor calibration error, frequency response error, etc.) in frequency modulation ranging (such as FMCW ranging) is found. For example, if the absolute distance obtained by FMCW ranging , The difference between the relative distance measurement value and the relative distance measurement value within a certain distance range is a linear relationship. Then the relative error correction coefficient can be obtained by linear fitting, or the absolute distance obtained by FMCW ranging. , If the difference between the relative distance measurement value and the relative distance measurement value within a certain distance range is a fixed error value, then the absolute error correction parameter calculated by the error model can be used; Based on the absolute error correction parameter or relative error correction coefficient, the absolute distance , Corrections are performed respectively. Specifically, the relative distance measurement value is multiplied by the relative error correction coefficient to obtain the absolute distance obtained by FMCW ranging, or the relative distance measurement value is subtracted or added with the absolute error correction parameter to obtain the absolute distance obtained by FMCW ranging.

[0034] In practical applications, the error models of FMCW ranging and relative distance measurement may not be globally consistent, that is, the error characteristics of the two may be different in different ranging ranges. Therefore, the error model can be subdivided into multiple different ranging intervals, and different correction factors can be used for different intervals. The FMCW ranging results can be continuously corrected by dynamic error compensation algorithms (such as Kalman filtering or other adaptive filtering methods).

[0035] Among them, the error analysis method and data fitting method involved can all adopt the existing technical principles in the field, and will not be elaborated here.

[0036] For example, the loop unit 4 uses 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 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 photodetector 6.

[0037] The fiber collimation unit 5 adopts a fiber collimator with a PC-type interface, a lens with transmission and reflection functions, or a combination structure of an optical fiber and a collimator. The present invention does not limit the selection of the fiber collimation unit 5, and only requires that the fiber collimation unit 5 can realize the transmission and reflection functions of the laser signal, and the laser signal is reflected at the end face of the fiber collimation unit 5 to form a reference light. After the object light passes through the fiber collimation unit 5, it is transmitted to the photodetector 6 through the loop unit 4 with the reference light, so that the optical path difference between the reference light and the object light is closer to the distance to be measured, reducing the measurement error.

[0038] System Example 2 like Figure 2 As shown, in system embodiment 2, an electro-optic modulator 13 is arranged between the pulse laser 9 and the optical fiber coupler 10, and the electro-optic modulator 13 is used to modulate the signal of the frequency calibration laser.

[0039] The pulse laser 9 may be a mode-locked laser. Existing low-repetition-rate mode-locked lasers have limited modulation methods and cannot flexibly achieve rapid adjustment and selective control of frequency, which limits their scope of application in complex application scenarios. By introducing the electro-optic modulator 13, the existing high-repetition-rate mode-locked laser can select and adjust the frequency to reduce the system's dependence on the physical cavity length, thereby achieving miniaturization, integration, and higher anti-interference capabilities.

[0040] The arbitrary waveform generator 1 is also used to generate a synchronous electrical signal and transmit it to the data collector 7. The synchronous electrical signal has the same frequency as the frequency modulation signal.

[0041] Method Example 1: 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 be a mode-locked laser. The method specifically comprises the following steps: 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: in: is the frequency of the nth spectral line of the optical frequency comb, 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. 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 ; n Is a positive integer.

[0042] The frequency of the laser signal of the semiconductor laser is , , , ; in, represents the initial frequency of the laser, is the frequency modulation rate, that is, the slope of the rising edge of the triangle wave, is the frequency modulation bandwidth. After mixing the mode-locked laser with 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 to be ignored, so the difference frequency signal expression is: It can be seen that Figure 8 As shown, Figure 8 The medium frequency modulated laser is the semiconductor laser in this application. Taking the laser as an example, the difference frequency signal presents a periodic distribution and is discretely distributed. The mode-locked laser in the system selects a mode-locked laser with a repetition rate of about 8MHz, and the filter selects a bandpass filter with a center bandwidth of 2MHz and a filter bandwidth of 1MHz. After the difference frequency signal passes through the filter, it will only have an output when the difference frequency signal frequency is 1.5-2.5MHz. Therefore, there will be an output only when the modulated spectrum line is close to or far away from the mode-locked laser spectrum line, and each spectrum line corresponds to a pair of output signals. Through the data at the output end of the bandpass filter, the frequency modulation rate of the modulated spectrum line at different times can be obtained by the interval between each output and the data width of a single output, so as to obtain the nonlinearity of the frequency modulation and the influence on the ranging.

[0043] Assuming the FM bandwidth is 800MHz, there are 200 outputs in one FM cycle. The distance between two outputs is used as a variable. After fitting, a curve reflecting the FM linearity can be obtained to correct the ranging result. For phase ranging, since the triangle wave is synchronously output at the arbitrary waveform generator, the triangle wave waveform can be used to determine the frequency spectrum line corresponding to the measurement time point. When the FM spectrum line is ideal, the obtained beat frequency signal should be a sine wave except for the first and last segments, that is, Figure 4 The frequency difference in is the constant part.

[0044] 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 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 calculated, and the frequency modulation rate of the semiconductor laser frequency modulation signal at different times is obtained, thereby obtaining the nonlinear amount of frequency modulation.

[0045] For short-distance measurement, the beat frequency conversion time is very short. When processing, the beat frequency signal can be truncated to obtain a complete sine wave part. By matching the measured modulated spectrum line with the actual sine wave, the measured sine wave signal can be corrected, making the obtained signal closer to the theoretical signal and the obtained phase more accurate.

[0046] The ideal frequency of a semiconductor laser is: , , , in, represents the initial frequency of the laser, The frequency modulation rate, that is, the slope of the rising edge of the triangle wave, is the FM bandwidth; After calibration, the actual curve detected is: , , , in, is the nonlinear part of the actual laser frequency. After mixing, the beat 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 ] , the phase correction of the beat frequency signal 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 time with lower accuracy obtained first is The corrected image is brought in to obtain a more precise time interval, and then a more accurate ranging result is obtained by iteration.

[0047] In the embodiment of the high-precision laser ranging method based on pulse laser calibration, the specific calibration scheme is as follows: Firstly, the frequency modulation curve of the single-frequency laser is obtained by using the mode-locked laser and the single-frequency laser; The signals emitted by the mode-locked laser and the semiconductor laser are coupled, and the frequency of the mode-locked laser can be expressed as in, It is the optical frequency comb n The frequency of the spectral line, 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. 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 .

[0048] , Both can be obtained through measurement and are known quantities, and the frequency of the semiconductor laser can be calibrated by using this known quantity.

[0049] For the rising edge period of frequency modulation, the frequency expression of the known semiconductor laser is: , Then the difference frequency between the mode-locked laser and the semiconductor laser is 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, the difference frequency has no negative value part, the value range of the first spectral line of the difference frequency between the single-frequency signal and the mode-locked laser is The corresponding physical meaning is that if this part of the spectrum is observed, it reflects the frequency difference between the single spectrum line and the nearest mode-locked laser spectrum line. Therefore, by observing this part of the spectrum, the movement of the single spectrum line can be obtained, and the modulated spectrum line can be obtained. Or use a bandpass filter (passband in ), and the output is obtained each time a single spectrum line passes through a certain frequency band. The physical meaning is that there will be output when a single spectrum line reaches and leaves a certain modulated spectrum line. The obtained pulse pair can be used to fit the modulated spectrum line.

[0050] After obtaining the FM curve, the correct single-frequency FM curve is corrected to ; For phase distance measurement, the beat frequency image is needed for phase calculation, so consider the time axis of nonlinear frequency modulation t Corrected to an ideal linear timeline ,satisfy The beat frequency signal is mapped into a new beat frequency image, which corresponds to the correct linear time axis and eliminates nonlinearity.

[0051] The frequency-time relationship of a nonlinear signal is: By establishing a time mapping function , the nonlinear time axis Convert to linear timeline .satisfy: Need to pass Inverse , that is, solving This equation can be solved numerically: 1. Sample Generation The curve is constructed based on the experimental data of known mode-locked lasers and single-frequency lasers. : 1. In time Generate a set of discrete sample points on the axis, and according to the curve Generating the corresponding frequency values ​​generates a set of discrete sample points: 2. Calculate the ideal time corresponding to discrete time points 2. Interpolation function Using discrete points Constructing the interpolation function The commonly used methods are: Linear interpolation: Suitable for simple cases with a small number of points.

[0052] Spline interpolation: suitable for situations with a large number of points and requirements for smoothness.

[0053] The interpolation function is in the form of: Then the measured beat frequency signal is mapped according to time Resampling or interpolation is performed, and then the corrected phase can be obtained.

[0054] This embodiment uses a mode-locked laser for frequency calibration, which can avoid additional complex reference optical paths, is easy to integrate, and has volume advantages. Using a mode-locked laser for frequency calibration has the advantage of real-time monitoring, and the obtained frequency information can not only be used to correct the frequency in frequency ranging, but also to correct the obtained beat frequency information, so that the obtained phase information is more accurate. 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 by the frequency modulation line, which can adapt to different application scenarios.

[0055] Method Example 2: 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 comprises the following steps: First, we discuss the frequency spectrum characteristics of pulsed lasers. The Fourier transform of periodic pulses 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: Fourier transform of a single pulse: Assume that the duration of a single pulse is The Fourier transform of a single rectangular pulse is: in, ; Periodic pulse train: If the pulse is periodic Repeated, the entire pulse train can be viewed as a single pulse and a periodic impulse function In Fourier transform, time domain convolution is equal to frequency domain product, which will cause the pulse spectrum to become discrete spectral lines with a spectral line interval equal to the repetition frequency. ; Spectral characteristics: The frequency domain signal after Fourier transform appears as The envelope shape of each frequency component is determined by the Fourier transform of a single pulse (e.g., the function).

[0056] 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. It can be seen that the spectrum of the pulse signal can also be used for frequency correction of ranging in theory.

[0057] The subsequent principle is consistent with the previous mode-locked laser calibration, but 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.

[0058] In some situations where it is inconvenient to use a mode-locked laser, or where the cost and complexity of a mode-locked laser need to be considered, a general pulsed laser and its above-mentioned principles can be used.

[0059] Based on the above method and principle, the following steps are performed in the processor after receiving each signal: Step 1: Pass the square wave obtained from the arbitrary waveform generator through the comparator, and change the high level of the square wave to 1 and the low level to 0; Step 2: Multiply the square wave obtained in step 1 by the beat frequency signal obtained by photodetector 1 to obtain the rising edge of the triangular wave; then invert the square wave obtained in step 1 and multiply it by the beat frequency signal obtained by photodetector 1 to obtain the falling edge of the triangular wave: Step 3: According to the slope of the triangle wave of the arbitrary waveform generator, the rising and falling edges of the obtained triangle wave can be filtered to remove the linear component; 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: The following steps for the rising and falling edges are processed in the same way, taking the rising edge signal as an example: Step 5: Use the rising edge pulse signal obtained in step 4 to draw a scatter plot, and determine the time node corresponding to the pulse signal according to the half-height full width of the pulse signal during processing; Step 6: According to the ideal frequency of semiconductor laser , , , , and after calibration, the actual curve detected is , , , ,in is the nonlinear part of the actual laser frequency. Ideally, the scatter plot obtained in step 5 should be a straight line. The scatter plot fitted curve drawn in step 5 is obtained. Expression of Step 7: Repeat steps 5 and 6 for the falling edge signal; Step 8: According to the beat frequency signal obtained in step 4, cut off the small non-sinusoidal wave part at the beginning and end of each cycle, and then calculate its frequency using the zero crossing method, that is, the frequency is obtained by the time interval between two zero points; Step 9: Substitute the beat signal frequency and time delay according to the beat frequency formula , get the time delay , the corrected beat signal frequency is {f}_{b}\left ( {t} \right )=\alpha \tau +\left [ {{f}_{n}\left ( {t} \right )-{f}_{n}\left ( {t-\tau} \right )} \right ] , get the new frequency and new time delay again and iterate, set the number of iterations and iteration conditions according to the target accuracy, and finally get the updated frequency and time delay ; Step 10: Delay the obtained time Arrive , the corrected phase term can be obtained.

[0060] In addition, the repetition rate of the mode-locked laser can be reduced by performing a rectangular modulation with a lower repetition rate through an electro-optical modulator.

[0061] After modulation, the mode-locked laser waveform is obtained as Figure 6 shown.

[0062] Depend on Figure 6 It can be seen that when using an electro-optic modulator for frequency selection, the switching time of the electro-optic modulator must be less than the repetition rate period of the mode-locked laser. The opening time of the electro-optic modulator is usually in the range of picoseconds to nanoseconds, depending on the material and application. The repetition rate of the mode-locked laser is generally in the GHz range, and the repetition rate period is in the nanosecond range, so it is feasible. Adjust the switching time of the electro-optic modulator so that there is only one pulse when it is turned on: Step 1: Determine the opening time of the electro-optic modulator according to the repetition rate of the mode-locked laser, and ensure that the opening time of the electro-optic modulator is less than the repetition rate period; Step 2: Determine the switching frequency of the electro-optic modulator based on the frequency modulation bandwidth of the semiconductor laser. It is expected that a better effect will be achieved when the frequency of the electro-optic modulator is below 2% of the frequency modulation bandwidth, that is, the repetition rate of the mode-locked laser is reduced.

[0063] Calibration by reducing the repetition rate can avoid the weak anti-interference ability and insufficient modulation flexibility of long-cavity long-mode-locked lasers, and the structure of mode-locked lasers has the advantages of low cost and small size. In terms of signal processing methods, since the frequency modulation nonlinearity curve has been obtained, the data can be post-processed after measurement, and the information can also be negatively fed back to change the modulation current to remove the frequency modulation nonlinearity; and the processing process is relatively simple.

[0064] The principles and implementation methods of the present invention are described in this article using specific examples. The description of the above embodiments is only used to help understand the method and core idea of ​​the present invention. At the same time, for those skilled in the art, according to the idea of ​​the present invention, there will be changes in the specific implementation methods and application scope. In summary, the content of this specification should not be understood as limiting 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 ranging unit transmits a laser signal to the target for ranging; the photoelectric detector 1 (6) obtains the ranging 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 1 (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.

2. The high-precision laser ranging system based on pulse laser calibration according to claim 1, characterized in that: 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 1 (6); the laser signal is transmitted to the target via the optical fiber collimation unit (5) for distance measurement.

3. The high-precision laser ranging system based on pulse laser calibration according to claim 2, 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 the 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).

4. The high-precision laser ranging system based on pulse laser calibration according to claim 3, 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.

5. 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.

6. 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.

7. 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.

8. 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 7, 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: in, is the repetition frequency, which is equal to the repetition rate of the laser pulses; 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 ; 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: ， ， ， in, represents the initial frequency of the laser, is the frequency modulation rate, that is, the slope of the rising edge of the triangle wave, is the FM bandwidth, is the frequency modulation period; After calibration, the actual curve detected is: ， ， ， in, is the nonlinear part of the actual laser frequency. After mixing, the beat 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 ] , the phase correction of the beat frequency signal 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 time with lower accuracy obtained first is The frequency image of the post-corrected beat signal is brought in to obtain a more precise time interval, and then a more accurate ranging result is obtained by iteration.

9. The high-precision laser ranging method based on pulse laser calibration according to claim 8, 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: in: It is the optical frequency comb n The frequency of the spectral line, 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. 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 .

10. The high-precision laser ranging method based on pulse laser calibration according to claim 8, 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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