A FMCW laser ranging light source nonlinear correction system and method

Abstract

The present invention discloses a FMCW laser ranging light source nonlinear correction system and method, which belongs to the fields of laser radar, laser ranging, laser three-dimensional scanning, etc., and solves the problem that the existing technology cannot accurately extract the target position, thereby affecting the detection and identification of the target. The present invention first initializes the semiconductor laser LD, SOA and FPGA; then the sawtooth wave or triangle wave light source signal amplified by the SOA is collected by a photodetector PD and transmitted to the FPGA main control unit for processing, and after processing, the power feedback control signal is output to the SOA drive controller; the beat frequency signal generated by the Mach-Zehnder interferometer in the system is collected by PD and transmitted to the FPGA main control unit for processing, and after processing, the iterative sawtooth wave or triangle wave modulation signal is output to correct the LD nonlinearity. The present invention is used for application scenarios such as high-precision laser ranging, laser radar and three-dimensional imaging.

Description

Technical Field

[0001] A FMCW laser ranging light source nonlinear correction system and method are used in high-precision laser ranging, laser radar and three-dimensional imaging application scenarios, effectively eliminating the power imbalance and frequency modulation nonlinearity problems caused by large-scale frequency scanning, thereby reducing the measurement error introduced by laser tuning and improving the ranging accuracy. The system and method belong to the field of laser radar, laser ranging and laser three-dimensional scanning. Background Art

[0002] Currently, in the field of laser ranging, the pulse method and the phase method are commonly used methods. However, the measurement resolution of the pulse method is limited by the bandwidth of electronic devices, and the resolution is only in the millimeter order. The phase method has a 2π winding ambiguity problem. Frequency modulated continuous wave laser absolute ranging can effectively solve the 2π winding ambiguity problem, and the accuracy can reach the micron order. In practical applications, FMCW laser ranging usually requires ideal linear frequency sweeping to invert the distance. However, when the frequency sweeping coverage is expanded, the linearity is obviously distorted in a narrow frequency range due to the strong nonlinearity and thermal effect of the laser itself, and the power changes synchronously with the laser tuning, which leads to the distortion of the ranging result. This is due to the nonlinear characteristics of the frequency modulation curve of the laser. The frequency of the beat signal changes with time, and the spectrum peak area corresponding to the beat signal is seriously expanded, resulting in a large error when extracting the beat frequency, thereby reducing the measurement accuracy. When the laser works in a high-bandwidth, high-speed frequency sweeping condition, its nonlinearity is more significant, so that the corresponding frequency of the target cannot be accurately extracted, affecting the detection and identification of the target by the laser radar, and limiting the light source in practical applications. In order to solve the problem of low measurement accuracy in the existing FMCW technology-based laser radar signal measurement method for distance measurement, a system solution and a nonlinear correction method are proposed.

[0003] On the other hand, the laser uses linear frequency modulation to measure distance, and the ranging signal output by it must have optical power fluctuations, which affect the accuracy and distance resolution of ranging. Therefore, the optical power stabilization control system based on SOA studied in this invention aims to stabilize the optical power of the modulated output optical signal without changing the modulation information carried by the modulated signal, thereby improving the measurement accuracy and distance resolution of the FMCW system.

[0004] In the selection of laser light sources, the current typical swept-frequency light sources mostly use external cavity grating mechanical modulation. Although the frequency modulation range is large, its complex mechanical structure is not conducive to integration. Summary of the invention

[0005] In view of the above-mentioned research problems, the purpose of the present invention is to provide a FMCW laser ranging light source nonlinear correction system and method, so as to solve the problem that when the laser in the prior art works in the case of high bandwidth and high-speed frequency sweeping, its nonlinearity is more significant, so that the target position and the corresponding frequency cannot be accurately extracted, which affects the detection and identification of the target. When the frequency modulation form is used for ranging, the output ranging signal must have optical power fluctuations, and the optical power fluctuations affect the ranging accuracy and distance resolution, and the frequency sweeping speed, coherence and reliability of the external cavity grating mechanical modulation are limited by the complex mechanical structure.

[0006] In order to achieve the above object, the present invention adopts the following technical solution:

[0007] A FMCW laser ranging light source nonlinear correction system comprises a semiconductor laser LD, an isolator ISO, an optical attenuator VOA and an optical amplifier SOA which are sequentially connected to the output end of the semiconductor laser LD, the output end of the optical amplifier SOA is connected to an 80:10:10 coupler, the first port of the coupler outputs 80% of the light as a measurement path signal and is connected to a circulator;

[0008] It also includes a photodetector PD and a spectrum analyzer which are sequentially connected to the output end of the circulator;

[0009] The second port and the third port of the coupler respectively output light of 10% of the reference path signal;

[0010] The second port is connected to the photodetector PD and the FPGA system in sequence;

[0011] The third port is connected in sequence to a Mach-Zehnder interferometer composed of a 50:50 first coupler, a time-delay optical fiber and a 50:50 second coupler, a photodetector PD and an FPGA system;

[0012] The output end of the FPGA system is connected to the SOA drive controller, and the output end of the SOA drive controller is connected to the semiconductor laser LD and the optical amplifier SOA respectively;

[0013] The Mach-Zehnder interferometer divides the optical signal from one signal into two signals through a 50:50 first coupler. After one signal passes through a delay optical fiber with a fixed length difference, it is combined with the other signal into one signal through a 50:50 second coupler.

[0014] The present invention also provides a nonlinear correction method for a FMCW laser ranging light source, comprising:

[0015] Step 1: First, initialize the semiconductor laser LD, optical amplifier SOA and FPGA system;

[0016] Step 2: After initialization, the modulated optical signal output by the optical amplifier SOA in the system is collected by the photodetector PD and transmitted to the FPGA main control unit of the FPGA system for processing, and after processing, a feedback control signal is output to the SOA drive controller;

[0017] Step 3: If the optical power output from the optical amplifier SOA is stable within a given small interval, execute step 4, otherwise repeat step 2;

[0018] Step 4: The beat frequency signal generated by the Mach-Zehnder interferometer in the system is collected by a photodetector PD and transmitted to the FPGA main control unit of the FPGA system for processing, and the processed output iterative sawtooth wave or triangle wave periodic signal is used to correct the nonlinearity in the swept frequency light source of the semiconductor laser LD.

[0019] Further, the step 1 is specifically as follows:

[0020] Step 1.1: Set the initial temperature of the optical amplifier SOA to 25°C, the initial current to 350mA, and the initial DC signal and store the DC signal in the ROM of the FPGA system as the initial control signal. The initial control signal is output through the DA1 port of the FPGA system, and the DA1 port is connected to the modulation port of the SOA drive controller. The DC signal is processed by the system to obtain a feedback control signal.

[0021] Step 1.2: Set the initial temperature of the semiconductor laser LD to 25°C, the initial current to 350mA, and the initial sawtooth wave or triangle wave periodic signal, store the initial sawtooth wave or triangle wave periodic signal in the ROM of the FPGA system, and output the initial sawtooth wave or triangle wave periodic signal through the DA2 port of the FPGA system, and the DA2 port is connected to the modulation port of the laser diode drive controller of the semiconductor laser LD.

[0022] Further, the step 2 is specifically as follows:

[0023] Step 2.1: After initialization, the optical power output by the optical amplifier SOA in the system is collected by the photodetector PD. The electrical signal after power conversion by the photodetector PD is connected to the AD module input port of the FPGA system. The AD module input port then transmits the collected electrical signal to the FPGA main control unit of the FPGA system.

[0024] Step 2.2: The electrical signal transmitted to the FPGA main control unit is processed by the equalization algorithm, and the processed output result is transmitted to the DA1 port, which then outputs the result to the modulation port of the SOA drive controller to perform real-time control of the optical power in the optical path.

[0025] Furthermore, in step 2.2, the electrical signal of the FPGA main control unit is processed by the equalization algorithm as follows: firstly, open-loop control is performed by the pre-correction algorithm, and then closed-loop control is performed by the incremental PID negative feedback algorithm;

[0026] The pre-correction algorithm is as follows:

[0027] By testing the amplification characteristics of the optical amplifier SOA, the functional relationship between the characteristics of the optical amplifier SOA and the modulation current is measured, which is denoted as K(i1). K(i1) represents the amplification factor when the modulation current output from the optical amplifier SOA to the modulation port of the SOA driving controller is i1.

[0028] The optical power P before entering the optical amplifier SOA in And the actual output optical power P out The sizes are:

[0029]

[0030] Where P(t) represents the optical power output by the semiconductor laser LD at time t, σ is the attenuation coefficient of the optical path before entering the optical amplifier SOA, and P out (t), P in (t), K(i1(t)) are the optical power P output by the optical amplifier SOA at time t. out , the optical power P of the input optical amplifier SOA in and amplification factor, i1(t) represents the pre-corrected current output to the modulation port of the SOA driver controller at time t, i.e., the modulation current;

[0031] Ideally, there are

[0032] P out (t)≡P s (1-2)

[0033] Among them, P s It represents the ideal power, that is, when the power is stable, the power size does not change and is a constant value;

[0034] The formula for obtaining the pre-corrected current i1(t) output to the modulation port of the SOA drive controller at time t is as follows:

[0035]

[0036] Get the pre-correction current i1(t), write the pre-correction current i1(t) of the whole cycle into the dual-port RAM of the FPGA system at one time, the FPGA system reads the pre-correction current i1(t), outputs it to the DA module, and performs the next incremental PID negative feedback algorithm;

[0037] In the incremental PID negative feedback algorithm, the control object is the optical amplifier SOA, and the controlled quantity is the optical power. The formula is as follows:

[0038]

[0039] Where Δi represents the change of the pre-correction current i1(t), i.e., the negative feedback current, and e(k) is t k The power P(t k ) and ideal power P s The difference, t k represents the sampling time of the kth discrete sampling, A, B, C are three adjustable fixed parameters, P(t k ) is the size of the electrical signal transmitted to the FPGA main control unit;

[0040] The re-collected optical power signal is calculated by the incremental PID algorithm to obtain the negative feedback current. After the negative feedback current is synchronized with the pre-correction current signal, the two are added together through the adder inside the FPGA system. After addition, the result is output to the modulation port of the SOA drive controller through the DA2 port of the DA module to stabilize the optical power.

[0041] Further, the step 4 is specifically as follows:

[0042] Step 4.1: The beat frequency signal generated by the Mach-Zehnder interferometer composed of a 50:50 first coupler, a delay optical fiber and a 50:50 second coupler is collected by a photodetector PD, and the electrical signal after power conversion by the photodetector PD is connected to the AD module input port of the FPGA system, and the AD module input port then transmits the collected electrical signal to the FPGA main control unit of the FPGA system;

[0043] Step 4.2: The FPGA main control unit processes the electrical signal through a nonlinear iterative correction algorithm, and transmits the processed output result to the DA2 port, which then outputs the result to the modulation port of the laser diode drive controller of the semiconductor laser LD to correct the nonlinearity in the swept frequency light source of the semiconductor laser LD.

[0044] Furthermore, in step 4.2, the FPGA main control unit processes the electrical signal through a nonlinear iterative correction algorithm as follows:

[0045] The instantaneous frequency of the laser of the semiconductor laser LD during the scanning period is expressed as:

[0046] ω(t)=ω0+K LD [i2(t)]·i2(t) (2-1)

[0047] Among them, ω0 is the initial optical frequency of the semiconductor laser LD, i2(t) is the modulation current of the modulation port of the laser diode driver controller of the semiconductor laser LD at time t, K LD [i2(t)] refers to the gain of the modulation current i2(t) or the frequency modulation coefficient of the modulation current i2(t) in the nonlinear frequency modulation response of the semiconductor laser LD;

[0048] According to formula (2-1), the relationship between the instantaneous frequency of the laser of the semiconductor laser LD and the beat signal frequency collected by the photodetector PD is:

[0049]

[0050] Among them, ω PD It refers to the beat signal frequency collected by the photodetector PD, ω(t) is the instantaneous frequency of the laser output by the semiconductor laser LD, and τ represents the delay of the Mach-Zehnder interferometer MZI. From the above two formulas (2-1) and (2-2), the following formula is obtained:

[0051]

[0052] Among them, ω PD (t) represents the beat signal frequency of the photodetector PD at time t,

[0053] is the frequency sweep rate of the semiconductor laser LD, F dist (i2) represents a nonlinear function, i2 represents a modulation current of a modulation port of a laser diode driving controller of a semiconductor laser LD;

[0054] From formula (2-3), we can get the nonlinear function F dist (i2), as shown below:

[0055]

[0056] Ideally, there are

[0057] ω PD (t)≡ω desired (2-5)

[0058] Among them, ω desired It represents the ideal frequency, that is, when the signal is stable, the frequency remains unchanged and is a constant value.

[0059] Then we have:

[0060]

[0061] The scanning nonlinearity of the semiconductor laser LD is reduced by the current obtained from formula (2-6), but Fdist (i2) is unknown, therefore, the beat frequency signal collected by the photodetector PD under the initial sawtooth wave or triangle wave periodic signal modulation is substituted into formula (2-7) to solve F dist (i2), and then substitute it into formula (2-8) to get a new current to reduce the laser frequency sweep nonlinearity, repeat this process, that is, iterate n times, until the frequency sweep light output by the semiconductor laser LD approaches the ideal linearity, that is, the iterative equation group of the current is obtained through formula (2-4) and formula (2-6), as shown in formula (2-7) and (2-8):

[0062]

[0063]

[0064] Substituting formula (2-7) into (2-8), the final iterative equation is as follows:

[0065]

[0066] Further, iterate ω PD (t) n-1 The specific steps are:

[0067] Step 4.2.1: Before the first iteration, the interference signal generated by the semiconductor laser LD is collected by the photodetector PD to obtain AD data, and the AD data is passed through a bandpass filter to obtain signal 1, where the AD data is the electrical signal input to the FPGA main control unit;

[0068] Step 4.2.2: Pass signal 1 through the phase detector filter and the delay device to obtain signal 2 and signal 3;

[0069] Step 4.2.3: Input signal 2 and signal 3 into the inverse tangent module to obtain phase signal 4;

[0070] Step 4.2.4: Input the phase signal 4 into the differentiator to obtain the frequency signal 5;

[0071] Step 4.2.5: Input the frequency signal 5 into a low-pass filter to obtain a frequency signal 6 after noise reduction;

[0072] Step 4.2.6: According to formula (2-9), the frequency signal 6 is input into the iterator to obtain the DA modulation signal after one iteration, that is, the processed iterative signal. The iterative signal generates a beat frequency signal through the FMCW system. The beat frequency signal is collected by the photodetector PD and used as the input signal of the next iteration of the photodetector PD. After iterating n-1 times, the photodetector PD collects ω PD (t) n-1 .

[0073] Compared with the prior art, the present invention has the following beneficial effects:

[0074] 1. The system solution in the present invention includes both a power balancing system and a nonlinear correction system. The frequency sweeping light source adopts a current modulation method, which has obvious advantages such as simple and compact structure, high cost performance, fast frequency modulation speed, and the ability to accurately extract the target position and corresponding frequency. It is an ideal way to realize a linear frequency sweeping light source;

[0075] Second, the present invention builds an optical power stabilization control system based on an optical amplifier SOA for the optical power fluctuation in frequency modulation, aiming to stabilize the optical power of the modulated output optical signal without changing the modulation information carried by the modulated signal. The power fluctuation is reduced from 2.52 to 0.12, thereby improving the measurement accuracy and distance resolution of the FMCW system;

[0076] 3. The present invention has built a dual-path frequency-modulated continuous-wave laser light source correction system for frequency-modulated nonlinearity, mathematically derived and established a suitable mathematical model based on the interference principle of dual-path lasers and the generation mechanism of nonlinearity, and simulated and verified the power balance and nonlinear correction in MATLAB, proving that multiple iterations and power open-closed loop control have a significant improvement on the light source;

[0077] 4. The correction effect of the iterative algorithm in the present invention has significantly improved the frequency modulation nonlinearity of the current-tuned semiconductor laser. In terms of instantaneous frequency acquisition, it is equivalent to adding multiple digital filters to the FPGA system for denoising, reducing the performance requirements and cost of the photodetector PD, effectively suppressing noise, and making the sampling results clearer and more accurate. Repeatability tests have been carried out, proving that the iterative correction algorithm has good repeatability and stability, which not only simplifies the system but also improves the system accuracy. BRIEF DESCRIPTION OF THE DRAWINGS

[0078] Figure 1 It is a schematic diagram of the system of the present invention;

[0079] Figure 2 It is a schematic diagram of iteration in the present invention;

[0080] Figure 3 This is a diagram showing the effect of the electrical signal of the FPGA main control unit before being processed by the equalization algorithm in the present invention;

[0081] Figure 4 This is a diagram showing the effect of the electrical signal of the FPGA main control unit in the present invention being processed by an equalization algorithm;

[0082] Figure 5 This is a diagram showing the effect of the electrical signal of the FPGA main control unit before passing through the nonlinear iterative correction algorithm in the present invention;

[0083] Figure 6 This is a diagram showing the effect of the electrical signal of the FPGA main control unit in the present invention being iterated once by the nonlinear iterative correction algorithm;

[0084] Figure 7 This is a diagram showing the effect of the electrical signal of the FPGA main control unit in the present invention being iterated twice by a nonlinear iterative correction algorithm;

[0085] Figure 8 It is a spectrum diagram of the electrical signal of the FPGA main control unit in the present invention before it is iterated by the nonlinear iterative correction algorithm;

[0086] Fig. 9 It is a spectrum diagram of the electrical signal of the FPGA main control unit in the present invention after iterating twice through the nonlinear iterative correction algorithm. DETAILED DESCRIPTION

[0087] The present invention will be further described below in conjunction with the accompanying drawings and specific implementation methods.

[0088] Currently, most of the LiDARs on the market use "Time of Flight - TOF" technology. This direct detection method leads to poor anti-interference and short detection distance, and it is difficult to meet the requirements of LiDARs above L3 level for autonomous driving. The purpose of this application proposal is to propose a high-precision frequency modulated continuous wave (FMCW) LiDAR solution. Frequency modulated continuous wave coherent detection has many advantages over TOF LiDAR, but when we input a linear modulation signal, nonlinear frequency sweeping will occur, and there will be ranging errors caused by power imbalance. In order to solve the problem of spectrum broadening of the measurement signal and decreased signal-to-noise ratio caused by nonlinear frequency sweeping in existing methods, which seriously affects the ranging accuracy, the solution first built a dual-path frequency modulated continuous wave laser light source correction system, and established a suitable mathematical model based on the interference principle of dual-path lasers and the nonlinear generation mechanism. The power balancing and nonlinear correction were simulated and verified in MATLAB, which confirmed that multiple iterations and power open-closed loop control have a great improvement on the light source. Finally, with the experiment as the core, it was verified that the correction effect of the iterative algorithm has significantly improved the frequency modulation nonlinearity of the current-tuned semiconductor laser. In terms of instantaneous frequency acquisition, the cost of the photodetector was reduced, and multiple digital filters based on the FPGA platform were added for denoising, which effectively suppressed the noise and made the sampling results clearer and more accurate. Repeatability tests were also carried out, proving that the iterative correction algorithm has good repeatability and stability, which not only simplifies the system but also improves the system accuracy.

[0089] A FMCW laser ranging light source nonlinear correction system comprises a semiconductor laser LD, an isolator ISO, an optical attenuator VOA and an optical amplifier SOA which are sequentially connected to the output end of the semiconductor laser LD, the output end of the optical amplifier SOA is connected to an 80:10:10 coupler, the first port of the coupler outputs 80% of the light as a measurement path signal and is connected to a circulator;

[0090] It also includes a photodetector PD and a spectrum analyzer which are sequentially connected to the output end of the circulator;

[0091] The second port and the third port of the coupler respectively output light of 10% of the reference path signal;

[0092] The second port is connected to the photodetector PD and the FPGA system in sequence;

[0093] The third port is connected in sequence to a Mach-Zehnder interferometer composed of a 50:50 first coupler, a time-delay optical fiber and a 50:50 second coupler, a photodetector PD and an FPGA system;

[0094] The output end of the FPGA system is connected to the SOA drive controller, and the output end of the SOA drive controller is connected to the semiconductor laser LD and the optical amplifier SOA respectively;

[0095] The Mach-Zehnder interferometer divides the optical signal from one signal into two signals through a 50:50 first coupler. After one signal passes through a delay optical fiber with a fixed length difference, it is combined with the other signal into one signal through a 50:50 second coupler.

[0096] A nonlinear correction method for a FMCW laser ranging light source, comprising:

[0097] Step 1: First, initialize the semiconductor laser LD, optical amplifier SOA and FPGA system;

[0098] Specifically:

[0099] Step 1.1: Set the initial temperature of the optical amplifier SOA to 25°C, the initial current to 350mA, and the initial DC signal and store the DC signal in the ROM of the FPGA system as the initial control signal. The initial control signal is output through the DA1 port of the FPGA system, and the DA1 port is connected to the modulation port of the SOA drive controller. The DC signal is processed by the system to obtain a feedback control signal.

[0100] Step 1.2: Set the initial temperature of the semiconductor laser LD to 25°C, the initial current to 350mA, and the initial sawtooth wave or triangle wave periodic signal, store the initial sawtooth wave or triangle wave periodic signal in the ROM of the FPGA system, and output the initial sawtooth wave or triangle wave periodic signal through the DA2 port of the FPGA system, and the DA2 port is connected to the modulation port of the laser diode drive controller of the semiconductor laser LD, wherein the semiconductor laser LD can be a 1550nm semiconductor laser LD.

[0101] Step 2: After initialization, the modulated optical signal output by the optical amplifier SOA in the system is collected by the photodetector PD and transmitted to the FPGA main control unit of the FPGA system for processing, and after processing, a feedback control signal is output to the SOA drive controller;

[0102] Specifically:

[0103] Step 2.1: After initialization, the optical power output by the optical amplifier SOA in the system is collected by the photodetector PD. The electrical signal after power conversion by the photodetector PD is connected to the AD module input port of the FPGA system. The AD module input port then transmits the collected electrical signal to the FPGA main control unit of the FPGA system.

[0104] Step 2.2: The electrical signal transmitted to the FPGA main control unit is processed by the equalization algorithm, and the processed output result is transmitted to the DA1 port, which then outputs the result to the modulation port of the SOA drive controller to perform real-time control of the optical power in the optical path.

[0105] The electrical signal of the FPGA main control unit is processed by the equalization algorithm as follows: firstly, open-loop control is performed by the pre-correction algorithm, and then closed-loop control is performed by the incremental PID negative feedback algorithm;

[0106] The pre-correction algorithm is as follows:

[0107] By testing the amplification characteristics of the optical amplifier SOA, the functional relationship between the characteristics of the optical amplifier SOA and the modulation current is measured, which is denoted as K(i1). K(i1) represents the amplification factor when the modulation current output from the optical amplifier SOA to the modulation port of the SOA driving controller is i1.

[0108] The optical power P before entering the optical amplifier SOA in And the actual output optical power P out The sizes are:

[0109]

[0110] Where P(t) represents the optical power output by the semiconductor laser LD at time t, σ is the attenuation coefficient of the optical path before entering the optical amplifier SOA, and P out (t), P in (t), K(i1(t)) are the optical power P output by the optical amplifier SOA at time t. out , the optical power P of the input optical amplifier SOA in and amplification factor, i1(t) represents the pre-corrected current output to the modulation port of the SOA driver controller at time t, i.e., the modulation current;

[0111] Ideally, there are

[0112] P out (t)≡P s (1-2)

[0113] Among them, P s Indicates ideal power, that is, when the power is stable, the power size remains unchanged and is a constant value:

[0114] The formula for obtaining the pre-corrected current i1(t) output to the modulation port of the SOA drive controller at time t is as follows:

[0115]

[0116] Get the pre-correction current i1(t), write the pre-correction current i1(t) of the whole cycle into the dual-port RAM of the FPGA system at one time, the FPGA system reads the pre-correction current i1(t), outputs it to the DA module, and performs the next incremental PID negative feedback algorithm;

[0117] In the incremental PID negative feedback algorithm, the control object is the optical amplifier SOA, and the controlled quantity is the optical power. The formula is as follows:

[0118]

[0119] Where Δi represents the change of the pre-correction current i1(t), i.e., the negative feedback current, and e(k) is t k The power P(t k ) and ideal power P s The difference, t k represents the sampling time of the kth discrete sampling, A, B, C are three adjustable fixed parameters, P(t k ) is the size of the electrical signal transmitted to the FPGA main control unit;

[0120] The re-collected optical power signal is calculated by the incremental PID algorithm to obtain the negative feedback current. After the negative feedback current is synchronized with the pre-correction current signal, the two are added together through the adder inside the FPGA system. After addition, the result is output to the modulation port of the SOA drive controller through the DA2 port of the DA module to stabilize the optical power.

[0121] Step 3: If the optical power output from the optical amplifier SOA is stable within a given small interval, execute step 4, otherwise repeat step 2;

[0122] Step 4: The beat frequency signal generated by the Mach-Zehnder interferometer in the system is collected by a photodetector PD and transmitted to the FPGA main control unit of the FPGA system for processing, and the processed output iterative sawtooth wave or triangle wave periodic signal is used to correct the nonlinearity in the swept frequency light source of the semiconductor laser LD.

[0123] Specifically:

[0124] Step 4.1: The beat frequency signal generated by the Mach-Zehnder interferometer composed of a 50:50 first coupler, a delay optical fiber and a 50:50 second coupler is collected by a photodetector PD, and the electrical signal after power conversion by the photodetector PD is connected to the AD module input port of the FPGA system, and the AD module input port then transmits the collected electrical signal to the FPGA main control unit of the FPGA system;

[0125] Step 4.2: The FPGA main control unit processes the electrical signal through a nonlinear iterative correction algorithm, and transmits the processed output result to the DA2 port, which then outputs the result to the modulation port of the laser diode drive controller of the semiconductor laser LD to correct the nonlinearity in the swept frequency light source of the semiconductor laser LD.

[0126] The FPGA main control unit processes the electrical signal through a nonlinear iterative correction algorithm as follows:

[0127] The instantaneous frequency of the laser of the semiconductor laser LD during the scanning period is expressed as:

[0128] ω(t)=ω0+K LD [i2(t)]·i2(t) (2-1)

[0129] Among them, ω is the initial optical frequency of the semiconductor laser LD, i2(t) is the modulation current of the modulation port of the laser diode driver controller of the semiconductor laser LD at time t, K LD [i2(t)] refers to the gain of the modulation current i2(t) or the frequency modulation coefficient of the modulation current i2(t) in the nonlinear frequency modulation response of the semiconductor laser LD;

[0130] According to formula (2-1), the relationship between the instantaneous frequency of the laser of the semiconductor laser LD and the beat signal frequency collected by the photodetector PD is:

[0131]

[0132] Among them, ω PD It refers to the beat signal frequency collected by the photodetector PD, ω(t) is the instantaneous frequency of the laser output by the semiconductor laser LD, and τ represents the delay of the Mach-Zehnder interferometer MZI. From the above two formulas (2-1) and (2-2), the following formula is obtained:

[0133]

[0134] Among them, ω PD (t) represents the beat signal frequency of the photodetector PD at time t,

[0135] is the frequency sweep rate of the semiconductor laser LD, F dist (i2) represents a nonlinear function, i2 represents a modulation current of a modulation port of a laser diode driving controller of a semiconductor laser LD;

[0136] From formula (2-3), we can get the nonlinear function F dist (i2), as shown below:

[0137]

[0138] Ideally, there are

[0139] ω PD (t)≡ω desired (2-5)

[0140] Among them, ω desired It represents the ideal frequency, that is, when the signal is stable, the frequency remains unchanged and is a constant value.

[0141] Then we have:

[0142]

[0143] The scanning nonlinearity of the semiconductor laser LD is reduced by the current obtained from formula (2-6), but F dist (i2) is unknown, therefore, the beat frequency signal collected by the photodetector PD under the initial sawtooth wave or triangle wave periodic signal modulation is substituted into formula (2-7) to solve F dist(i2), and then substitute it into formula (2-8) to get a new current to reduce the laser frequency sweep nonlinearity, repeat this process, that is, iterate n times, until the frequency sweep light output by the semiconductor laser LD approaches the ideal linearity, that is, the iterative equation group of the current is obtained through formula (2-4) and formula (2-6), as shown in formula (2-7) and (2-8):

[0144]

[0145]

[0146] Substituting formula (2-7) into (2-8), the final iterative equation is as follows:

[0147]

[0148] Get ω PD (t) n-1 The specific steps are:

[0149] Step 4.2.1: Before the first iteration, the interference signal generated by the semiconductor laser LD is collected by the photodetector PD to obtain AD data, and the AD data is passed through a bandpass filter to obtain signal 1, where the AD data is the electrical signal input to the FPGA main control unit;

[0150] Step 4.2.2: Pass signal 1 through the phase detector filter and the delay device to obtain signal 2 and signal 3;

[0151] Step 4.2.3: Input signal 2 and signal 3 into the inverse tangent module to obtain phase signal 4;

[0152] Step 4.2.4: Input the phase signal 4 into the differentiator to obtain the frequency signal 5;

[0153] Step 4.2.5: Input the frequency signal 5 into a low-pass filter to obtain a frequency signal 6 after noise reduction;

[0154] Step 4.2.6: According to formula (2-9), the frequency signal 6 is input into the iterator to obtain the DA modulation signal after one iteration, that is, the processed iterative signal. The iterative signal generates a beat frequency signal through the FMCW system. The beat frequency signal is collected by the photodetector PD and used as the input signal of the next iteration of the photodetector PD. After iterating n-1 times, the photodetector PD collects ω PD (t) n-1 .

[0155] The laser used in the present invention is a semiconductor laser LD, and the change of the output light wavelength is achieved by changing the intensity of the injected current, thereby changing the frequency of the output signal. Compared with other types of lasers, it has the advantages of simple and compact structure, high cost performance, and fast frequency modulation speed.

[0156] The above are only representative embodiments of the present invention in many specific application scopes, and do not constitute any limitation on the protection scope of the present invention. Any technical solutions formed by transformation or equivalent replacement fall within the protection scope of the present invention.

Claims

1. A FMCW laser ranging light source nonlinear correction system, characterized in that: It includes a semiconductor laser LD, an isolator ISO, an optical attenuator VOA and an optical amplifier SOA which are sequentially connected to the output end of the semiconductor laser LD, the output end of the optical amplifier SOA is connected to an 80:10:10 coupler, the first port of the coupler outputs 80% of the light as the measurement path signal and is connected to a circulator; It also includes a photodetector PD and a spectrum analyzer which are sequentially connected to the output end of the circulator; The second port and the third port of the coupler respectively output light of 10% of the reference path signal; The second port is connected to the photodetector PD and the FPGA system in sequence; The third port is connected in sequence to a Mach-Zehnder interferometer consisting of a 50:50 first coupler, a time-delay optical fiber and a 50:50 second coupler, a photodetector PD and an FPGA system; The output end of the FPGA system is connected to the SOA drive controller, and the output end of the SOA drive controller is connected to the semiconductor laser LD and the optical amplifier SOA respectively; Among them, the Mach-Zehnder interferometer divides the optical signal from one signal into two signals through a 50:50 first coupler. After one signal passes through a delay optical fiber with a fixed length difference, it is combined with the other signal into one signal through a 50:50 second coupler.

2. A correction method for a FMCW laser ranging light source nonlinear correction system according to claim 1, characterized in that: include: Step 1: First, initialize the semiconductor laser LD, optical amplifier SOA and FPGA system; Step 2: After initialization, the modulated optical signal output by the optical amplifier SOA in the system is collected by the photodetector PD and transmitted to the FPGA main control unit of the FPGA system for processing, and after processing, a feedback control signal is output to the SOA drive controller; Step 3: If the optical power output from the optical amplifier SOA is stable within a given small interval, execute step 4, otherwise repeat step 2; Step 4: The beat frequency signal generated by the Mach-Zehnder interferometer in the system is collected by a photodetector PD and transmitted to the FPGA main control unit of the FPGA system for processing, and the processed output iterative sawtooth wave or triangle wave periodic signal is used to correct the nonlinearity in the swept frequency light source of the semiconductor laser LD.

3. The calibration method according to claim 2, characterized in that: The step 1 is specifically as follows: Step 1.1: Set the initial temperature of the optical amplifier SOA to 25°C, the initial current to 350mA, and the initial DC signal and store the DC signal in the ROM of the FPGA system as the initial control signal. The initial control signal is output through the DA1 port of the FPGA system, and the DA1 port is connected to the modulation port of the SOA drive controller. The DC signal is processed by the system to obtain a feedback control signal. Step 1.2: Set the initial temperature of the semiconductor laser LD to 25°C, the initial current to 350mA, and the initial sawtooth wave or triangle wave periodic signal, store the initial sawtooth wave or triangle wave periodic signal in the ROM of the FPGA system, and output the initial sawtooth wave or triangle wave periodic signal through the DA2 port of the FPGA system, and the DA2 port is connected to the modulation port of the laser diode drive controller of the semiconductor laser LD.

4. The calibration method according to claim 3, characterized in that: The step 2 is specifically as follows: Step 2.1: After initialization, the optical power output by the optical amplifier SOA in the system is collected by the photodetector PD. The electrical signal after power conversion by the photodetector PD is connected to the AD module input port of the FPGA system. The AD module input port then transmits the collected electrical signal to the FPGA main control unit of the FPGA system. Step 2.2: The electrical signal transmitted to the FPGA main control unit is processed by the equalization algorithm, and the processed output result is transmitted to the DA1 port, which then outputs the result to the modulation port of the SOA drive controller to perform real-time control of the optical power in the optical path.

5. The calibration method according to claim 4, characterized in that: In the step 2.2, the electrical signal of the FPGA main control unit is processed by the equalization algorithm as follows: firstly, open-loop control is performed by the pre-correction algorithm, and then closed-loop control is performed by the incremental PID negative feedback algorithm; The pre-correction algorithm is as follows: By testing the amplification characteristics of the optical amplifier SOA, the functional relationship between the characteristics of the optical amplifier SOA and the modulation current is measured, which is denoted as K(i1). K(i1) represents the amplification factor when the modulation current output from the optical amplifier SOA to the modulation port of the SOA driving controller is i1. The optical power P before entering the optical amplifier SOA in And the actual output optical power P out The sizes are: Where P(t) represents the optical power output by the semiconductor laser LD at time t, σ is the attenuation coefficient of the optical path before entering the optical amplifier SOA, and P out (t), P in (t), K(i1(t)) are the optical power P output by the optical amplifier SOA at time t. out , the optical power P of the input optical amplifier SOA in and amplification factor, i1(t) represents the pre-corrected current output to the modulation port of the SOA driver controller at time t, i.e., the modulation current; Ideally, there are P out (t)≡P s (1-2) Among them, P s It represents the ideal power, that is, when the power is stable, the power size does not change and is a constant value; The formula for obtaining the pre-corrected current i1(t) output to the modulation port of the SOA drive controller at time t is as follows: Get the pre-correction current i1(t), write the pre-correction current i1(t) of the whole cycle into the dual-port RAM of the FPGA system at one time, the FPGA system reads the pre-correction current i1(t), outputs it to the DA module, and performs the next incremental PID negative feedback algorithm; In the incremental PID negative feedback algorithm, the control object is the optical amplifier SOA, and the controlled quantity is the optical power. The formula is as follows: Where Δi represents the change of the pre-correction current i1(t), i.e., the negative feedback current, and e(k) is t k The power P(t k ) and ideal power P s The difference, t k represents the sampling time of the kth discrete sampling, A, B, C are three adjustable fixed parameters, P(t k ) is the size of the electrical signal transmitted to the FPGA main control unit; The re-collected optical power signal is calculated by the incremental PID algorithm to obtain the negative feedback current. After the negative feedback current is synchronized with the pre-correction current signal, the two are added together through the adder inside the FPGA system. After addition, the result is output to the modulation port of the SOA drive controller through the DA2 port of the DA module to stabilize the optical power.

6. The calibration method according to claim 5, characterized in that: The step 4 is specifically as follows: Step 4.1: The beat frequency signal generated by the Mach-Zehnder interferometer composed of a 50:50 first coupler, a delay fiber and a 50:50 second coupler is collected by a photodetector PD, and the electrical signal after power conversion by the photodetector PD is connected to the AD module input port of the FPGA system, and the AD module input port then transmits the collected electrical signal to the FPGA main control unit of the FPGA system; Step 4.2: The FPGA main control unit processes the electrical signal through a nonlinear iterative correction algorithm, and transmits the processed output result to the DA2 port, which then outputs the result to the modulation port of the laser diode drive controller of the semiconductor laser LD to correct the nonlinearity in the swept frequency light source of the semiconductor laser LD.

7. The calibration method according to claim 6, characterized in that: In step 4.2, the FPGA main control unit processes the electrical signal through a nonlinear iterative correction algorithm as follows: The instantaneous frequency of the laser of the semiconductor laser LD during the scanning period is expressed as: ω(t)=ω0+K LD [i2(t)]·i2(t) (2-1) Among them, ω0 is the initial optical frequency of the semiconductor laser LD, i2(t) is the modulation current of the modulation port of the laser diode driver controller of the semiconductor laser LD at time t, K LD [i2(t)] refers to the gain of the modulation current i2(t) or the frequency modulation coefficient of the modulation current i2(t) in the nonlinear frequency modulation response of the semiconductor laser LD; According to formula (2-1), the relationship between the instantaneous frequency of the laser of the semiconductor laser LD and the beat signal frequency collected by the photodetector PD is: Among them, ω PD It refers to the beat signal frequency collected by the photodetector PD, ω(t) is the instantaneous frequency of the laser output by the semiconductor laser LD, and τ represents the delay of the Mach-Zehnder interferometer MZI. From the above two formulas (2-1) and (2-2), the following formula is obtained: Among them, ω PD (t) represents the beat signal frequency of the photodetector PD at time t, is the frequency sweep rate of the semiconductor laser LD, F dist (i2) represents a nonlinear function, i2 represents a modulation current of a modulation port of a laser diode driving controller of a semiconductor laser LD; From formula (2-3), we can get the nonlinear function F dist (i2), as shown below: Ideally, there are oh PD (t)≡ω desired (2-5) Among them, ω desired It represents the ideal frequency, that is, when the signal is stable, the frequency remains unchanged and is a constant value. Then we have: The scanning nonlinearity of the semiconductor laser LD is reduced by the current obtained from formula (2-6), but F dist (i2) is unknown, therefore, the beat frequency signal collected by the photodetector PD under the initial sawtooth wave or triangle wave periodic signal modulation is substituted into formula (2-7) to solve F dist (i2), and then substitute it into formula (2-8) to get a new current to reduce the laser frequency sweep nonlinearity, repeat this process, that is, iterate n times, until the frequency sweep light output by the semiconductor laser LD approaches the ideal linearity, that is, the iterative equation group of the current is obtained through formula (2-4) and formula (2-6), as shown in formula (2-7) and (2-8): Substituting formula (2-7) into (2-8), the final iterative equation is as follows:

8. The calibration method according to claim 7, characterized in that: Iteration ω PD (t) n-1 The specific steps are: Step 4.2.1: Before the first iteration, the interference signal generated by the semiconductor laser LD is collected by the photodetector PD to obtain AD data, and the AD data is passed through a bandpass filter to obtain signal 1, where the AD data is the electrical signal input to the FPGA main control unit; Step 4.2.2: Pass signal 1 through the phase detector filter and the delay device to obtain signal 2 and signal 3; Step 4.2.3: Input signal 2 and signal 3 into the inverse tangent module to obtain phase signal 4; Step 4.2.4: Input the phase signal 4 into the differentiator to obtain the frequency signal 5; Step 4.2.5: Input the frequency signal 5 into a low-pass filter to obtain a frequency signal 6 after noise reduction; Step 4.2.6: According to formula (2-9), the frequency signal 6 is input into the iterator to obtain the DA modulation signal after one iteration, that is, the processed iterative signal. The iterative signal generates a beat frequency signal through the FMCW system. The beat frequency signal is collected by the photodetector PD and used as the input signal of the next iteration of the photodetector PD. After iterating n-1 times, the photodetector PD collects ω PD (t) n-1 .

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