调频连续波相干激光雷达的多脉冲频域相干累积研究

Multi-Pulse Coherent Accumulation in Frequency Domain Based on Frequency-Modulation Continuous-Wave Coherent Lidar

针对相干激光雷达在低空、远距离、高动态目标探测等应用场景中存在的弱信号探测问题,基于调频连续波相干激光雷达,采用多脉冲频域相干累积方法提高激光雷达弱回波信号的信噪比。结合调频连续波理论,分析了信号幅度起伏和相位起伏对多脉冲频域相干累积信噪比效果的影响,实验结果表明,经过M脉冲周期的频域相干累积,微弱回波信号对数信噪比可提升10lg M dB,并且信号幅度起伏、相位起伏可导致多脉冲相干累积信噪比低于理想相干累积信噪比,对于均值分别为2.25×10^(-4)、1.65×10^(-4)和相对起伏幅度分别为0.69、0.93的回波信号而言,相位补偿后的回波信号在50脉冲周期内相干累积信噪比的误差因子最大为0.57%、0.3%,该多脉冲频域相干累积方法对相干激光雷达在远距离目标探测、微弱信号接收方面的应用具有重要意义。

Objective When coherent lidar detects a target at a long distance,the laser beam is affected by atmospheric absorption,scattering,turbulence,diffuse scattering of target and nonideal detection.Consequently,the noise exceeds the signal,severely influencing the detection ability for long-range targets.For the weak signal of low-speed moving targets,coherent lidar usually uses the pulse coherent accumulation method and can receive lower-signal power,which greatly improves its sensitivity.Range and doppler migrations easily occur in the coherent accumulation of high-speed manoeuvring targets,resulting in phase inconsistency of the echo signal,energy dispersion in the frequency domain,and limited coherence accumulation time,which affects the multi-pulse coherent accumulation.In this case,microwave radars use Hough transform,sequence shift gene algorithm,acceleration blind estimation to achieve long-time coherent accumulation of the echo signals of high-speed moving targets.Noncoherent lidar achieves coherent accumulation of multiple pulses for high-speed moving targets using pulse shift and time-resolution reduction.Meanwhile,the range resolution of the system is also reduced.However,the processing method for the weak signals of high-speed moving targets using coherent lidar has not been reported.Improving the detection performance of high dynamic targets in long-distance and high-dynamic targets is a challenge.In addition,the amplitude and phase fluctuations of the signals considerably affect multi-pulse coherent accumulation;thus,investigating their effects is necessary.Methods The theory of multi-pulse coherent accumulation in frequency domains based on frequency modulation continuous wave(FMCW)coherent lidar was confirmed through a laboratory verification experiment.The experiment was conducted through the following steps.First,fast Fourier transform and multi-pulse non-coherent accumulation were used to find the signal frequency point.Then,the phase of the signal frequency point in each pulse was extracted,and phase matching was performed for each pulse.Finally,multi-pulse period signals were selected for coherent accumulation and to improve R_(SN).In addition,the theory under amplitude and phase fluctuations and speeds was established,and its performance was confirmed through simulations.The relevant theory was used to compare the experimental results and errors after multi-pulse coherent accumulation in the frequency domain based on FMCW coherent lidar with phase matching.Results and Discussions After coherent accumulation in the frequency domain of 1〜50 pulse periods,the error factor of coherent accumulations in the frequency domain of multiple pulses gradually decreased for echo signals with the same amplitude fluctuation mean value and variance of 0.02,0.52 and 1.02(Fig.2).With amplitude fluctuations,the R_(SN)of coherent accumulation of multi-pulse in the frequency domain was lower than that of ideal coherent accumulation,but the effect of poor coherent accumulation of multi-pulse was improved by increasing the number of coherent accumulation cycles.R_(SN)with variances of 0.52,1.02,1.52,2.02 and 2.52 was lower than that of the ideal case(variance=0),and the variance of the phase fluctuation was higher.These results show that the R_(SN)of echo signals is lower than the ideal R_(SN).Furthermore,the R_(SN)error-factor distribution of multi-pulse coherent accumulation in the frequency domain of echo signals with phase-fluctuation mean value of 0 and variances of 0,0.52,1.02,1.52,2.02 and 2.52 was obtained(Fig.5).When the coherent accumulation period was 30,the maximum coherent accumulation R_(SN)errors of echo signals with variances of 0.52,1.02,1.52,2.02 and 2.52 were 0,1.29,4.51,6.538,11.59 and 15.81 dB,respectively.The greater the amplitude fluctuation,the greater the coherent accumulation R_(SN)error.In addition,multi-pulse coherent accumulation in the frequency domain based on FMCW coherent lidar was independent of the target speed(Fig.6).For actual moving targets,the R_(SN)of FMCW coherent lidar increased logarithmically(Figs.11 and 14),and their maximum error factors in a 50-pulse period were 0.57%and 0.3%at pair mean values of 2.25 × 10^(-4) and 1.65 ×10^(-4) and relative amplitude fluctuations of 0.69 and 0.93,respectively.Conclusions In this study,we verified the theory of multi-pulse coherent accumulation in the frequency domain based on FMCW coherent lidar through laboratory experiments.For echo signals with fixed pulse repetition rates,the R_(SN) of beat frequency signals after M-cycle coherent accumulation in the frequency domain was 10lg M dB higher than the log R_(SN) of the original signal,and the weak R_(SN) of the echo signal increased exponentially.Under the same pulse accumulation period and target detection,for the frequency modulated continuous wave whose pulse repetition frequency meets m times,the R_(SN) of the single-cycle high repetition frequency single-pulse echo signals was -10lg M dB higher than that of low-repetition frequency signals.With the amplitude and phase fluctuations of signals,the multi-pulse coherent accumulation R_(SN) was lower than the ideal coherent accumulation R_(SN),and the pair mean values were 2.25 × 10^(-4) and 1.65 × 10^(-4) with relative amplitude fluctuations of 0.69 and 0.93,respectively,and the maximum error factors of the multi-pulse coherent accumulation R_(SN) of the echo signal in the 50-pulse period were 0.57%and 0.3%,respectively,which are attributed to the amplitude fluctuation of the echo signal,linear frequency modulation power and fluctuations.