A nonlinear calibration system and method for a frequency modulated continuous wave lidar light source

Abstract

This invention relates to a nonlinear calibration system and method for a frequency-modulated continuous wave lidar source. The system includes: a driving module for generating a square wave signal based on sampling points of the signal to be calibrated, and applying the square wave signal to a laser; a beat frequency module including a coupling unit and a detection unit, wherein the coupling unit converts the optical signal generated by the laser into an optical signal similar to two DC signals of different amplitudes, and beats the optical signal to obtain a difference frequency optical signal containing the square wave corresponding to different amplitudes; the detection unit converts the difference frequency optical signal into an electrical signal; and a calibration module for calculating the frequency difference of the difference frequency optical signal based on the electrical signal, comparing the frequency difference with a set ideal frequency difference, and adaptively filtering the voltage value of the square wave signal if the error requirement is not met, and applying the adaptively filtered voltage value as the amplitude of the new square wave signal to the laser. This invention can obtain more accurate beat frequency information while reducing calibration difficulty.

Description

Technical Field

[0001] This invention relates to the field of frequency modulated continuous wave lidar technology, and in particular to a nonlinear calibration system and method for a frequency modulated continuous wave lidar light source. Background Technology

[0002] Frequency Modulated Continuous Wave (FMCW) works by modulating a transmitted signal with a continuous frequency signal in the time domain. The modulated signal is transmitted to the target and bounces back; the returned signal is also a frequency-modulated signal. By measuring the frequency difference between the echo signal and the transmitted signal, and then using known parameters such as the modulation period, modulation bandwidth, and modulation time, the time difference between the two can be obtained, allowing the calculation of information such as the distance to the target.

[0003] The ranging and velocity measurement principle of lidar is based on the linearly continuously modulated laser output from a laser. Therefore, on-chip FMCW lidar requires a reliable, highly linear frequency-sweeping light source. Even small nonlinearities can affect the accuracy and resolution of ranging and velocity measurement in FMCW lidar. FMCW lidar typically uses triangular or sawtooth current to modulate the laser, thereby obtaining a laser whose frequency changes linearly with time. However, semiconductor lasers exhibit nonlinear frequency response to injected current, and temperature changes in the laser also affect its frequency response. Therefore, designing an on-chip lidar system capable of generating linearly continuously modulated waves and a corresponding signal generation and frequency stabilization system is essential. Summary of the Invention

[0004] The technical problem to be solved by the present invention is to provide a nonlinear calibration system and method for frequency-modulated continuous wave lidar light sources, which can obtain more accurate beat frequency information while reducing calibration difficulty.

[0005] The technical solution adopted by this invention to solve its technical problem is: to provide a nonlinear calibration system for a frequency-modulated continuous wave lidar source, comprising:

[0006] The driving module is used to generate a square wave signal based on the sampling points of the signal to be calibrated, and to apply the square wave signal to the laser. It is also used to output the calibrated signal to the laser.

[0007] The beat frequency module includes a coupling section and a detection section. The coupling section is used to convert the optical signal generated by the laser into an optical signal similar to two DC signals of different amplitudes, and beat the optical signal similar to the two DC signals of different amplitudes to obtain a difference frequency optical signal containing different amplitudes of square waves. The detection section is used to convert the difference frequency optical signal into an electrical signal.

[0008] The calibration module is used to calculate the frequency difference of the difference frequency optical signal based on the electrical signal, compare the frequency difference with the set ideal frequency difference, and adaptively filter the voltage value of the square wave signal if the error requirement is not met, and apply the adaptively filtered voltage value as the amplitude of the new square wave signal to the laser.

[0009] The driving module includes:

[0010] The preset point determination unit is used to take equally spaced points on the amplitude of the triangular wave signal to be calibrated, which has a determined frequency and amplitude.

[0011] The square wave generation unit is used to generate a corresponding square wave signal by taking the amplitude of selected adjacent points as the two amplitudes of the square wave signal, setting the period of the square wave signal, and making the square wave signal act on the laser.

[0012] The coupling section of the beat frequency module includes a first coupler and a second coupler arranged sequentially. The first output end of the first coupler is connected to the first input end of the second coupler through a first optical fiber, and the second output end of the first coupler is connected to the second input end of the second coupler through a second optical fiber. The first optical fiber and the second optical fiber have different lengths, which allows the optical signal generated by the laser to be converted into an optical signal similar to that generated by two DC signals of different amplitudes. The second coupler is used to beat the optical signal generated by the two DC signals of different amplitudes to obtain a difference frequency optical signal containing different amplitudes of square waves.

[0013] The detection unit of the beat frequency module includes:

[0014] A balanced detector is used to convert the difference frequency optical signal into an electrical signal in the form of a cosine with the frequency difference.

[0015] The signal selection unit is used to select the time-domain value of the beat frequency signal corresponding to the middle part of the period of the square wave signal;

[0016] An extraction unit is used to extract the corresponding electrical signal from the cosine form electrical signal containing the frequency difference based on the time domain value.

[0017] The calibration module includes:

[0018] The calculation unit is used to perform a fast Fourier transform on the electrical signal output by the detection unit to obtain the frequency difference of the difference frequency optical signal.

[0019] The comparison unit is used to compare the frequency difference with the set ideal frequency difference;

[0020] An adaptive filtering unit is used to adaptively filter the voltage value of the square wave signal when the error requirement is not met. When performing adaptive filtering, the ideal value of the adaptive filtering is set to the square wave amplitude difference * (1 - ideal frequency difference / frequency difference), and the step size of the adaptive filtering is set to (adaptive filtering updated voltage amplitude - input low voltage value) * ideal frequency difference / (adaptive filtering updated frequency difference)^2.

[0021] The technical solution adopted by this invention to solve its technical problem is: to provide a nonlinear calibration method for a frequency-modulated continuous wave lidar source, comprising the following steps:

[0022] Step (1): Generate a square wave signal based on the sampling points of the signal to be calibrated, and apply the square wave signal to the laser;

[0023] Step (2) converts the optical signal generated by the laser into an optical signal generated by two DC signals with different amplitudes, and beats the optical signal generated by the two DC signals with different amplitudes to obtain a difference frequency optical signal containing different amplitudes of square waves.

[0024] Step (3): Convert the difference frequency optical signal into an electrical signal, and obtain the frequency difference value of the difference frequency optical signal based on the electrical signal;

[0025] Step (4) compares the frequency difference with the set ideal frequency difference, and performs adaptive filtering on the voltage value of the square wave signal if the error requirement is not met. The voltage value after adaptive filtering is used as the amplitude of the new square wave signal and applied to the laser. Then, return to step (2) until the error requirement is met.

[0026] Step (1) specifically includes:

[0027] Take equally spaced points on the amplitude of the triangular wave signal to be calibrated, given a fixed frequency and amplitude.

[0028] The amplitudes of selected adjacent points are used as the two amplitudes of the square wave signal. The period of the square wave signal is set to generate the corresponding square wave signal, and the square wave signal is applied to the laser.

[0029] In step (2), the optical signal generated by the laser is split into two paths by the first optical coupler, and the optical signal generated by the laser is converted into optical signals similar to two DC signals with different amplitudes by two optical fibers of different lengths. The optical signals similar to two DC signals with different amplitudes are beat-frequency generated by the second optical coupler to obtain the difference frequency optical signal containing different amplitudes of square waves.

[0030] Step (3) specifically includes:

[0031] The difference frequency optical signal is converted into an electrical signal in cosine form containing the frequency difference;

[0032] Select the time-domain value of the beat frequency signal corresponding to the middle part of the period of the square wave signal;

[0033] Based on the time-domain value, the corresponding electrical signal is extracted from the cosine form electrical signal containing the frequency difference;

[0034] The frequency difference of the difference frequency optical signal is obtained by performing a fast Fourier transform on the extracted electrical signal.

[0035] When performing adaptive filtering in step (4), the ideal value of adaptive filtering is set to square wave amplitude difference * (1 - ideal frequency difference / frequency difference), and the step size of adaptive filtering is set to (adaptive filtering updated voltage amplitude - input low voltage value) * ideal frequency difference / (adaptive filtering updated frequency difference)^2.

[0036] Beneficial effects

[0037] By adopting the above-mentioned technical solution, this invention has the following advantages and positive effects compared with the prior art: Based on the principle of DDS, this invention stretches each stepped signal into a square wave signal. The amplitude in the DDS memory corresponds to the amplitude at the preset point. Then, by time delay, the square wave signal is superimposed into a DC signal with two similar amplitudes. This point-by-point calibration method reduces the difficulty of frequency detection, thereby obtaining more accurate beat frequency information. At the same time, an improved adaptive calibration algorithm based on the principle of adaptive filter is used to replace the original iterative algorithm, reducing computational complexity and making the calibration results more accurate. This provides a new algorithm calibration approach and also provides conditions for the application of this algorithm on other platforms. Attached Figure Description

[0038] Figure 1 This is a schematic diagram showing how a square wave signal is superimposed into two similar DC signals by time delay;

[0039] Figure 2 This is a block diagram of the calibration system according to an embodiment of the present invention;

[0040] Figure 3 This is a flowchart of the calibration process according to an embodiment of the present invention;

[0041] Figure 4 This is a schematic diagram illustrating the generation of optical signals similar to two DC signals of different amplitudes in an embodiment of the present invention;

[0042] Figure 5 This is a schematic diagram of the electrical signal obtained by the balanced detector in an embodiment of the present invention;

[0043] Figure 6This is a schematic diagram of the adaptive filtering principle in an embodiment of the present invention. Detailed Implementation

[0044] The present invention will be further illustrated below with reference to specific embodiments. It should be understood that these embodiments are for illustrative purposes only and are not intended to limit the scope of the invention. Furthermore, it should be understood that after reading the teachings of this invention, those skilled in the art can make various alterations or modifications to the invention, and these equivalent forms also fall within the scope defined by the appended claims.

[0045] The present invention relates to a nonlinear calibration system for a frequency-modulated continuous wave lidar source, comprising: a driving module for generating a square wave signal based on sampling points of a signal to be calibrated, and applying the square wave signal to a laser; a beat frequency module, comprising a coupling unit and a detection unit, wherein the coupling unit is used to convert the optical signal generated by the laser into an optical signal similar to two DC signals of different amplitudes, and beat the optical signal similar to the two DC signals of different amplitudes to obtain a difference frequency optical signal containing different amplitudes of the square wave; the detection unit is used to convert the difference frequency optical signal into an electrical signal; and a calibration module for calculating the frequency difference of the difference frequency optical signal based on the electrical signal, comparing the frequency difference with a set ideal frequency difference, and performing adaptive filtering if the error requirement is not met, and applying the voltage value after adaptive filtering as the amplitude of the new square wave signal to the laser.

[0046] The driving module includes: a preset point determination module, used to select points at equal intervals in amplitude of the triangular wave signal to be calibrated with a determined frequency and amplitude; and a square wave generation unit, used to use the amplitudes of the selected adjacent points as the two amplitudes of the square wave signal, set the period of the square wave signal, generate the corresponding square wave signal, and make the square wave signal act on the laser.

[0047] The coupling section of the beat frequency module includes a first coupler and a second coupler arranged sequentially. The first output end of the first coupler is connected to the first input end of the second coupler through a first optical fiber, and the second output end of the first coupler is connected to the second input end of the second coupler through a second optical fiber. The first optical fiber and the second optical fiber have different lengths, which allows the optical signal generated by the laser to be converted into an optical signal similar to that generated by two DC signals of different amplitudes. The second coupler is used to beat the optical signal generated by the two DC signals of different amplitudes to obtain a difference frequency optical signal containing different amplitudes of square waves.

[0048] The detection unit of the beat frequency module includes: a balanced detector for converting the difference frequency optical signal into an electrical signal in the form of a cosine with a frequency difference; a signal selection unit for selecting the time domain value of the beat frequency signal corresponding to the middle part of the period of the square wave signal; and an extraction unit for extracting the corresponding electrical signal from the electrical signal in the form of a cosine with a frequency difference based on the time domain value.

[0049] The calibration module can be implemented via a host computer and includes: a calculation unit for performing a fast Fourier transform on the electrical signal output by the detector to obtain the frequency difference of the difference frequency optical signal; a comparison unit for comparing the frequency difference with the set ideal frequency difference; and an adaptive filtering unit for adaptively filtering the voltage value of the square wave signal when the error requirements are not met. During adaptive filtering, the ideal value of the adaptive filter is set to square wave amplitude difference * (1 - ideal frequency difference / frequency difference), and the step size of the adaptive filter is set to (updated voltage amplitude of the adaptive filter - low input voltage value) * ideal frequency difference / (updated frequency difference of the adaptive filter)^2.

[0050] Based on the principle of DDS (Direct Digital Frequency Synthesis), when an arbitrary waveform generator generates an analog signal, it first executes the signal amplitude stored in each memory location step by step with an address counter, generating a stepped periodic signal. This signal is then passed through a low-pass filter to obtain a smooth and continuous analog signal. This implementation utilizes the principle of DDS to stretch each stepped signal into a square wave signal. The amplitude in the DDS memory corresponds to the amplitude at a preset point. Then, through time delay, the square wave signals are superimposed to form a DC signal with two similar amplitudes (e.g., ...). Figure 1 As shown, the signal is fed into a 2×2 3dB coupler for beat frequency measurement. In this way, under ideal conditions, any interval can be taken as the beat frequency signal. The frequency of the beat frequency signal can be obtained by performing a Fast Fourier Transform, and then calibration is performed using adaptive filtering based on the relationship between the measured value and the ideal value.

[0051] The present invention will be further illustrated by a specific embodiment below.

[0052] In this embodiment, the entire calibration system consists of an arbitrary waveform generator, a laser controller, a temperature controller, a current-tunable laser, a coupler, a delay fiber, a balanced detector, an oscilloscope, and a host computer (e.g., Figure 2 As shown), Figure 3 As shown, the specific calibration process is as follows:

[0053] The first step is to take equally spaced points on the amplitude of the triangular wave signal to be calibrated, given a predetermined amplitude and period. Simultaneously, the single-mode range corresponding to the laser and the frequency modulation frequency are determined, and the ideal value is calculated.

[0054] The second step involves selecting the amplitude values ​​corresponding to adjacent points of the triangular wave signal to be calibrated as the two amplitude values ​​of the square wave signal. Simultaneously, a suitable square wave period is set to satisfy both the laser's frequency response and the driver's output limit. In this embodiment, the square wave period is set to 10MHz. The pre-set square wave signal is then generated using an arbitrary waveform generator and applied to the laser.

[0055] The third step involves applying the generated square wave signal to the laser. The resulting optical signal is then split into two paths via a 1×2 coupler. One path is a time-delay fiber, which is 15m out of phase with the other fiber, corresponding to a time delay of 50ns, which perfectly matches the period of the square wave signal. This allows the generation of optical signals similar to those produced by two DC signals of different amplitudes (e.g.,...). Figure 4 (As shown).

[0056] The fourth step involves sending the optical signals generated by two DC signals of similar different amplitudes into a 2×2 3dB coupler for beat frequency testing, resulting in a difference frequency optical signal containing different amplitudes of square waves.

[0057] The fifth step involves sending the difference-frequency optical signal into a balanced detector to obtain a cosine-form electrical signal containing the frequency difference. The form of the electrical signal is I∝P1P2cos[(ω1-ω2)t](e.g., ...). Figure 5 (As shown).

[0058] The sixth step is to input the electrical signal obtained from the balanced detector into the oscilloscope, select the time domain value of the beat frequency signal corresponding to the middle 50% of the period of the square wave signal, and then use the host computer to read the selected corresponding electrical signal part. Finally, perform a fast Fourier transform on the selected electrical signal part to find the frequency difference of the difference frequency signal.

[0059] The seventh step involves comparing the frequency difference with the set ideal frequency difference. If the error requirement is not met, a calibration algorithm is designed for the square wave voltage value. In this embodiment, the calibration algorithm is based on the principle of the LMS adaptive filter (e.g., ...). Figure 6 As shown, during adaptive filtering, the ideal value for adaptive filtering is set to the square wave amplitude difference * (1 - ideal frequency difference / actual frequency difference), and the step size for adaptive filtering is set to (adaptive filtering updated voltage amplitude - input low voltage value) * ideal frequency difference / (adaptive filtering updated frequency difference)^2. After completing one adaptive filtering cycle, the updated voltage value is used as the amplitude of the new square wave signal and applied to the laser, then the process returns to step three.

[0060] If the updated beat frequency signal frequency difference meets the error requirement compared to the ideal frequency difference, the updated square wave amplitude is recorded. If the updated beat frequency signal frequency difference still has a large error compared to the ideal signal, adaptive filtering calibration is performed again until the error meets the requirement, completing the calibration of the first group of points. Then, the above method is used to calibrate each subsequent group of points sequentially. After calibrating all points, the arbitrary waveform generator's arbitrary wave function is used to set a 1kHz signal to modulate the calibrated points into a signal, which is then used as the final calibrated signal to power the laser to obtain a linearly frequency-modulated signal.

[0061] It is not difficult to see that this invention, based on the principle of DDS, elongates each stepped signal into a square wave signal. The amplitude in the DDS memory corresponds to the amplitude at a preset point. Then, through time delay, the square wave signal is superimposed into a DC signal with two similar amplitudes. This point-by-point calibration method reduces the difficulty of frequency detection, thereby obtaining more accurate beat frequency information. At the same time, an improved adaptive calibration algorithm based on the principle of adaptive filters is used to replace the original iterative algorithm, reducing computational complexity and making the calibration results more accurate. This provides a new algorithm calibration approach and also provides conditions for the application of this algorithm on other platforms.

Claims

1. A nonlinear calibration system for a frequency-modulated continuous wave lidar source, characterized in that, include: The driving module is used to generate a square wave signal based on the sampling points of the signal to be calibrated, and to apply the square wave signal to the laser. The driving module includes: The preset point determination unit is used to take equally spaced points on the amplitude of the triangular wave signal to be calibrated, which has a determined frequency and amplitude. A square wave generation unit is used to generate a corresponding square wave signal by taking the amplitudes of selected adjacent points as the two amplitudes of the square wave signal, setting the period of the square wave signal, and making the square wave signal act on the laser. The beat frequency module includes a coupling section and a detection section. The coupling section is used to convert the optical signal generated by the laser into an optical signal similar to two DC signals of different amplitudes, and beat the optical signal similar to the two DC signals of different amplitudes to obtain a difference frequency optical signal containing different amplitudes of square waves. The detection section is used to convert the difference frequency optical signal into an electrical signal. A calibration module is used to calculate the frequency difference of the difference-frequency optical signal based on the electrical signal, compare the frequency difference with a set ideal frequency difference, and adaptively filter the voltage value of the square wave signal if the error requirement is not met. The adaptively filtered voltage value is then used as the amplitude of a new square wave signal and applied to the laser. The calibration module includes: The calculation unit is used to perform a fast Fourier transform on the electrical signal output by the detection unit to obtain the frequency difference of the difference frequency optical signal. The comparison unit is used to compare the frequency difference with the set ideal frequency difference; An adaptive filtering unit is used to adaptively filter the voltage value of the square wave signal when the error requirement is not met. When performing adaptive filtering, the ideal value of the adaptive filtering is set to the square wave amplitude difference * (1 - ideal frequency difference / frequency difference), and the step size of the adaptive filtering is set to (updated voltage amplitude after adaptive filtering - low input voltage value) * ideal frequency difference / (updated frequency difference after adaptive filtering)^2.

2. The nonlinear calibration system for a frequency-modulated continuous wave lidar source according to claim 1, characterized in that, The coupling section of the beat frequency module includes a first coupler and a second coupler arranged sequentially. The first output end of the first coupler is connected to the first input end of the second coupler through a first optical fiber, and the second output end of the first coupler is connected to the second input end of the second coupler through a second optical fiber. The first optical fiber and the second optical fiber have different lengths, which allows the optical signal generated by the laser to be converted into an optical signal similar to that generated by two DC signals of different amplitudes. The second coupler is used to beat the optical signal generated by the two DC signals of different amplitudes to obtain a difference frequency optical signal containing different amplitudes of square waves.

3. The nonlinear calibration system for a frequency-modulated continuous wave lidar source according to claim 1, characterized in that, The detection unit of the beat frequency module includes: A balanced detector is used to convert the difference frequency optical signal into an electrical signal in the form of a cosine with the frequency difference. The signal selection unit is used to select the time-domain value of the beat frequency signal corresponding to the middle part of the period of the square wave signal; An extraction unit is used to extract the corresponding electrical signal from the cosine form electrical signal containing the frequency difference based on the time domain value.

4. A nonlinear calibration method for a frequency-modulated continuous wave lidar source, characterized in that, Includes the following steps: Step (1) involves generating a square wave signal based on the sampling points of the signal to be calibrated, and applying the square wave signal to the laser; step (1) specifically includes: Take equally spaced points on the amplitude of the triangular wave signal to be calibrated, given a fixed frequency and amplitude. The amplitudes of selected adjacent points are used as the two amplitudes of the square wave signal. The period of the square wave signal is set to generate the corresponding square wave signal, and the square wave signal is applied to the laser. Step (2) converts the optical signal generated by the laser into an optical signal generated by two DC signals with similar amplitudes, and beats the optical signal generated by the two DC signals with similar amplitudes to obtain a difference frequency optical signal containing different amplitudes of square waves. Step (3): Convert the difference frequency optical signal into an electrical signal, and obtain the frequency difference value of the difference frequency optical signal based on the electrical signal; Step (4): Compare the frequency difference with the set ideal frequency difference, and if the error requirement is not met, perform adaptive filtering on the voltage value of the square wave signal. Apply the voltage value after adaptive filtering as the amplitude of the new square wave signal to the laser, and return to step (2) until the error requirement is met. When performing adaptive filtering in step (4), set the ideal value of adaptive filtering to square wave amplitude difference * (1 - ideal frequency difference / frequency difference), and set the step size of adaptive filtering to (voltage amplitude after adaptive filtering - low input voltage value) * ideal frequency difference / (frequency difference after adaptive filtering)^2.

5. The nonlinear calibration method for a frequency-modulated continuous wave lidar source according to claim 4, characterized in that, In step (2), the optical signal generated by the laser is split into two paths by the first optical coupler, and the optical signal generated by the laser is converted into optical signals similar to two DC signals with different amplitudes by two optical fibers of different lengths. The optical signals similar to two DC signals with different amplitudes are beat-frequency generated by the second optical coupler to obtain the difference frequency optical signal containing different amplitudes of square waves.

6. The nonlinear calibration method for a frequency-modulated continuous wave lidar source according to claim 4, characterized in that, Step (3) specifically includes: The difference frequency optical signal is converted into an electrical signal in cosine form containing the frequency difference; Select the time-domain value of the beat frequency signal corresponding to the middle part of the period of the square wave signal; Based on the time-domain value, the corresponding electrical signal is extracted from the cosine form electrical signal containing the frequency difference; The frequency difference of the difference frequency optical signal is obtained by performing a fast Fourier transform on the extracted electrical signal.

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