Research On High Precision Lidar Ranging Technology Based On Optical Frequency Modulation

In the 1960s and 1970s,the emergence of the world's first laser makes the use of laser target detection a new means of ranging.Compared with the traditional microwave radar,lidar has the advantages of good monochromaticity,strong directivity and high ranging accuracy,so it is more and more widely used in military,construction,aerospace,medical and other fields.Common laser ranging methods,such as pulse laser ranging,whose characteristics are commercially mature,long distance detection and centimeter-level ranging accuracy;the phase laser ranging method can detect medium and short range targets and achieve ranging accuracy of millimeter scale,but its nonfuzzy distance is small,and it needs to be detected with the help of multiple optical rulers.In recent years,with the development of science and technology,people have put higher requirements on the ranging accuracy and anti-interference performance of the laser ranging technology.Lidar based on optical frequency modulation(OPF)is a laser ranging method which uses a large bandwidth linearly modulated laser as the light source and calculates the target distance by measuring the frequency of the interference signal.The ranging precision and resolution of this method are inversely proportional to the modulation bandwidth of the laser.With the development of laser modulation technology,the modulation bandwidth of the laser optical frequency can reach up to terahertz,which generated the advantages of high ranging precision and resolution.In addition,it also has the characteristics of no blind ranging area and strong antibackground light ability,so the ranging method has a very important application value in location and high-precision instrument manufacturing and surface detection.Due to the low control accuracy of the DC(direct current)motor inside the laser,temperature change,PZT nonlinear creep and hysteresis effect,it is difficult to realize the ideal optical frequency linear modulation in the actual working process.The nonlinear modulation of laser will cause the spectrum of the system interference signal to be widened seriously,which will lead to the decrease of the ranging resolution and accuracy.In order to solve this problem,it is a good choice to use dual-interference optical path system.In the system,one is the refernece interferometer and the other is the measurement interferometer.Since the two interferometers are simultaneously affected by the nonlinear modulation of the light source,the instantaneous frequency of the two interference signals is proportional.The influence of optical frequency modulation nonlinearity on distance measurement can be eliminated by regarding the signal of reference interference signal as the external clock of the system.The main features and innovations of this paper are as follows:In order to solve the nonlinear effect of optical frequency modulation,it is necessary to ensure that the frequency of the reference signal and the measurement interference signal meet The Nyquist sampling law.This means that the optical path difference(OPD)of the refernece interferometer needs to reach more than four times of the target distance,and the OPD of the reference interferometer needs to reach several hundred meters for the distance of the target within 100 meters.To reduce the limitation to the OPD of reference interferometer,a dual-interference optical path ranging system based on a frequency multiplier is proposed in this paper.The frequency multiplier is used to improve the instantaneous frequency of the reference interference signal,and the OPD of the reference interferometer is effectively down by a quarter.In order to eliminate the effect of laser accidental mode hopping on the ranging process,a method based on phase splicing is proposed.The key of using the reference signal as the external clock is to extract the equal phase interval points of the signal.When the laser mode hopping occurs,the reference signal will have a strong intensity noise in the mode hopping section,which has a great influence on the extraction of resampling time points.To eliminate the influence of the distorted part in the resampling signal,the positioning,phase unrolling and splicing of the laser mode hopping are completed in turn.Finally,CZT is played and then the target distance information without the influence of the laser mode hopping is obtained.In dual interference optical path systems,the reference interferometer is usually composed of fiber Mach-Zehnder interferometer,while the measurement interference optical path is in free space.The dispersion phenomenon of large bandwidth optical modulated laser in fiber propagation will lead to the dispersion mismatch between the reference interferometer and the measurement interferometer,resulting in the decrease of the target range resolution and the increase of the range error.In this paper,a theoretical model is established for the effect of optical fiber reference interferometer on the target distance,and a dispersion mismatch compensation method based on resampling of modulated signals is proposed according to the phase characteristics of the resampling signal affected by the dispersion.This method is not limited by the change of target distance and laser modulation bandwidth.Finally,simulation and experiment show the effectiveness of this method.In this paper,the high precision lidar ranging method based on optical frequency modulation is studied,and the key technologies in the ranging process are learned.In this paper,a lidar ranging system based on optical frequency modulation is established on the basis of the above key technology research methods,which provides theoretical support and experimental reference for the realization of high-precision measurement of short-range target distance.