Research On Echo Optical Phase-locked Technology For Coherent Lidar

Lidar is a new system radar that uses laser technology to realize target detection.It uses the laser as the radiation source to extend the working band of the radar to the light wave range,so it has the characteristics of high detection accuracy,high resolution,strong anti-interference,and so on.Compared with microwave radar,lidar is smaller in size and lighter in weight,which is more suitable for space applications.Moreover,due to its shorter working wavelength,the Doppler frequency shift in the echo signal is large,which is more sensitive to low-speed motion.Although it has the above advantages,due to the limitation of the laser technology development,the lidar range is not as far as microwave radar,limiting the application of lidar in long-distance detection.In addition,due to the strong interference of sunlight,more background noise is introduced into the light in free-space transmission,which causes the echo signal to become extremely weak.Therefore,the main technical development direction that can effectively improve the operating range of lidar is to use optical coherent detection system.To reduce the influence of noise and avoid the excessive power consumption caused by high-speed AD and DSP devices,the intermediate frequency bandwidth of coherent lidar is generally on the order of 100 MHz.However,due to the short wavelength of the laser,Doppler frequency shift of several hundred MHz or even several GHz can be easily introduced by the high-speed moving target.Even if the coherent lidar receives laser scattering echo,the beat signal obtained exceeds the bandwidth of the photodetector and the AD sampler.In order to solve this problem,researchers propose a post-processing method to separate and compensate the Doppler frequency shift caused by target translation,but the premise of this method is that the beat signal can be detected.There is also a method based on the frequency shift of heterodyne detection to compensate Doppler frequency shift,but the frequency shift generated by acousto-optic frequency shifter is fixed,and the compensation range is small.The compensation range of the variable frequency shift scheme constructed by two lasers is large,but the target motion needs to be predicted,and the system flexibility is poor.In addition,researchers also suppress the Doppler frequency shift by building a dual-frequency Doppler-lidar system.This method utilizes a microwave beat frequency generated by two different optical frequencies as the carrier of the Doppler effect to reduce the Doppler frequency shift.Nevertheless,this scheme has high requirements for the light source,challenging to adjust the optical path collimation and low coherence efficiency.Further,some scholars use a method to detect the high-speed moving target by using the heterodyne detection of the pulse echo comb laser and the continuous-wave signal-frequency local laser,which has a wide range of velocity measurement.However,its energy utilization is low when detecting high-speed moving targets.The optical phase-locked loop allows the slave laser to be phase synchronized to the master laser,which can be used in the echo receiving and processing of coherent lidar.Specifically,the optical phase-locked loop realizes the phase synchronize between the local oscillator light and the received echo light by tuning the local oscillator laser,so the beat signal with absolute but adjustable frequency offset can be obtained by the heterodyne optical phase-locked loop.Therefore,this paper proposes to use the optical phase-locked loop technology at the system receiver.At this time,the tunable local oscillator light frequency can track echo light frequency in real-time,which can adaptively eliminate the influence of atmospheric disturbance,laser frequency drift,large Doppler frequency shift,and other factors on coherent detection,and adjust the beat frequency to a smaller intermediate frequency bandwidth to achieve target detection.The main features and innovations of this paper are as follows:1)this paper focuses on the problem of moving target detection by coherent lidar based on an optical phase-locked loop.Through theoretical research and simulation analysis of the optical phase-locked loop,a scheme of using the optical phase-locked loop at the coherent lidar receiver is proposed,and the system feasibility is verified by simulation.On this basis,we built the experimental platform and optimized the loop through system debugging,which improves the loop performance and lays the foundation for the follow-up research.2)we research the frequency compensation ability of optical phase-locked loop at the receiver of coherent lidar.The spinning disc simulates the moving targets with different speeds.Then an experimental platform for coherent lidar to detect targets with different moving speeds is built,which verifies that the system can adaptively compensate the Doppler frequency shift caused by translation.Meanwhile,the loop control voltage can monitor the loop motion state,and the velocity can be estimated according to the compensated frequency shift.Aiming at the translation compensation problem of high-speed target,a frequency compensation scheme based on dual-loop tuning is proposed,which realizes the compensation of large frequency difference by regulating the loop tuning mode by FPGA.The experimental results show that this system can effectively detect the echo signal of the high-speed moving target by using the photodetector with small bandwidth,which verifies the scheme effectiveness.3)because the micro-Doppler frequency shift and variable Doppler frequency shift can be frequency aliased,we propose and design an optical phase-locked loop working in carrier tracking mode,which can compensate the Doppler frequency shift without compensating the micro-Doppler frequency shift,thus realizing the recognition of complex moving targets.Based on the principle of lidar vibration measurement,we establish the scheme model through detailed theoretical analysis.The soundbox simulates the vibration source,and the vibration detection of complex moving targets is completed,verifying the scheme feasibility.4)based on the above research,the integrated ranging and communication system based on optical phase-locked loop is extended and derived.We propose a method through frequency hopping modulation,which can greatly improve the antiinterference and confidentiality of the system.In addition,the frequency shift keying based on pseudorandom coding has strong anti-crosstalk ability,and overcomes the distance ambiguity problem of lidar ranging.Meanwhile,the ranging accuracy and resolution can be improved by smaller symbol width.In the laboratory,we preliminarily demonstrate the scheme effectiveness through experiments.Due to limitation of the experimental device performance,the range resolution needs to be improved,which can be further studied.This paper profoundly studies the coherent lidar receiver system based on an optical phase-locked loop.The feasibility and effectiveness of the proposed scheme are fully proved by theoretical derivation,system behavioral simulation,and experimental verification.Further exploits the potential of coherent lidar provides a novel technical way to expand the velocity measurement range and enhance the micro-Doppler detection sensitivity of complex moving targets.In addition,it also provides an effective research idea for the follow-up research of the integrated ranging and communication system.