| In traditional lidar,the time-of-flight(TOF)of an ultra-short and unmodulated light pulse reflected from a target is used to measure distance.Meanwhile,the distance difference between adjacent pulses is used to measure velocity.In order to achieve high accuracy and sensitivity in remote measurements,many TOF lidar systems use ultra-short pulsed lasers with low repetition and extremely high peak power.The system has three main defects.First of all,the high peak power laser pulse will gradually damage the optical devices and shorten the lifetime of the system.Secondly,the system is not desirable in cases when safety is of great concern.Thirdly,the system can only measure the average velocity in the differential time,and the real-time performance is poor.Although the peak power problem has been improved by photon counting lidar developed on TOF,it is still higher than the coherent detection system,and it is also mainly used in ranging,so it is difficult to measure velocity in real time.The frequency-modulated continuous wave(FMCW)scheme based on modulated pulse can overcome the defect of high peak power to a certain extent,and can also be used for real-time ranging and velocity measurement.However,in the case of optical mixing,this scheme obtains the target distance through the frequency of the electrical beat signal of the echo and the local oscillator,so it can only use the overlap part of the local oscillator and the echo in a sweep cycle.As the target distance increases,the overlap time is reduced,which results in lower signal energy utilization efficiency.Pulse-compression lidar does not have above limitations.In this scheme,the local oscillator is unmodulated.It obtains the full sweep electric signal by beating the echo light and the local oscillator light,and then compacts the signal into narrow pulses in the time domain through matched filtering.Through the location of narrow pulses,the target distance and velocity are obtained.In this scheme,the peak power requirement is not high,usually m W or W class.It can measure the speed of target in real-time and the energy efficiency is not limited by the overlap time.At present,almost all the reports about pulse-compresssion coherent lidar are based on single-band modulation.In single-band modulation scheme,in order to ensure the speed measurement ability,upper sweep frequency and lower sweep frequency are usually required,namely triangular sweep frequency.The normalized sidelobe of triangular sweep frequency is about 10 d B higher than the upper sweep sidelobe,which limits the system sensitivity,that is,limits the signal-to-noise ratio,and then the measurement accuracy,measurement distance.To solve this problem,a pulse-compresssion coherent lidar system based on dual-band modulation is proposed in this paper.The system has the ability of distance measurement and velocity measurement only by the up-sweep frequency,which avoids the high sidelobe problem in the triangular sweep frequency and improves the sensitivity of the system.Firstly,the basic principle of pulse-compression coherent lidar is introduced in detail.This paper make theory and simulation analysis of range and speed measuring principle,and the analysis of influence of various parameters such as pulse length,sampling rate of compression and the phase difference,and the establishment of Hilbert transform signal processing scheme,and the simulation analysis on comparing the normalized sidelobe of triangular frequency sweep and up-sweep frequency,to prove that the sidelobesof up-sweep is about 10 d B lower than triangle sweep.Secondly,an experimental platform of pulse-compression lidar based on dual-band modulation is built when simulating the moving target with a rotating disc,which proves that the system can measure the speed of the moving target in real-time only by up-sweep frequency.Based on the principle of electro-optic modulation and the built lidar system,the principle of ranging and velocity measurement of moving target based on dual-band modulation pulse-compression coherent lidar is deduced.Then we complete the ranging and speed measurement experiment based on the turntable: detailed analysis of the system's signal-to-noise ratio,detection threshold calculation method,through several measurements to take standard deviation to measure the measurement accuracy,the results show that the theoretical value is not much different with real value,and the test measurement of the system sensitivity.Finally,the influence of velocity on compression gain is simulated and analyzed.Then,aiming at the decrease of detection probability caused by the random phase difference between the echo and the local oscillator when the target is stationary,a 90° optical hybrid was added to solve the problem.The white painted wall at the end of the corridor was taken as the target to carry out the experimental verification of the improved system.Through the combination of I and Q signals,the problem is basically solved,and the detection probability and ranging accuracy are greatly improved.The improved system can measure both moving and stationary targets.Then,the system was optimized to determine the output mode,light spot radius,sampling equipment resolution etl.The sensitivity was improved by adding Windows to the signal to reduce sidelobe.After optimization,the system can measure buildings at about 856 m and 5726 m in the city,which verifies the remote measurement capability of the system.The influence of the target micro-vibration is verified by simulation and experiment.Since the micro-vibration target can also be regarded as a moving target in theory,the ability of the system to measure distance and velocity of long-distance moving target is also verified to a certain extent.Finally,the experimental verification of 3D scanning imaging capability of the laser radar system based on dual-band modulation pulse-compression is completed.Scanning and imaging the common buildings in cities with distance ranging from 856 to 987m is demonstrated,which proves the imaging ability of the system on the whole and details of buildings.To sum up,the proposed pulse-compression coherent lidar system based on dual-band modulation is capable of ranging and velocity real-time measurement for long distance hard targets in free space.At the same time,it has the capability of 3D scanning and imaging of the target.It provides a complete technical solution and theoretical support for related military and civil application scenarios. |
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