Research On Lidar Control And Data Acquisition System

Lidar uses laser as a carrier to detect the target.It has high resolution and strong anti-interference capability,so it has been widely used in atmospheric wind field detec-tion,geographical mapping,traffic speed measurement,biomedicine and other fields.In the field of atmospheric wind field detection,lidar mainly uses the Doppler effect of light to detect the change of laser frequency after the interaction between laser and atmospheric particles,and finally obtains the wind field information through inverse algorithm according to the detection results.In terms of detection mode,there are two ways to measure Doppler frequency shift:direct detection and coherent detection.In this paper,different lidar control and acquisition systems are designed for these two detection methods.Using a single detector to realize the detection of wind lidar in the upper atmo-sphere is the simplest signal detection mode,but the disadvantage of single detector is that it would limit the dynamic range of wind Lidar.When the detection area is 30km,the linear dynamic range of electronic system is required to be 5～6 orders of magni-tude.In this case,it is difficult to satisfy the demand by using analog sampling or photon counting alone.In order to extend the dynamic range of wind lidar based on single de-tector,we developed a set of wind lidar readout electronics in the upper atmosphere based on single detector which has large dynamic range.The system of the detector used analog measurement and photon counting measurement at the same time,namely,analog sampling is adopted to measure the strong echo signals and photon counting is adopted to measure the weak echo signals.At the same time,the calibration laser energy is obtained by using the integral channel to improve the accuracy of lidar calibration.After signal acquisition,data of analog sampling channel and photon counting channel are splicing,and finally transmitted to the upper computer.The results of electronics test show that the linear dynamic range of readout electronics system can reach 5～6 or-ders of magnitude through the combination of two channel signals,which can meet the design requirements.In addition,the high integration of this electronic readout system can meet the demand of miniaturization of current wind lidar.For the coherent detection method,the purpose is to measure the extremely weak laser echo signal.In order to imporve the ability of digital receiver to detect extremely weak signal,a balanced detector system with high SNR and a data acquisition system are needed.The photodiode is the main detection component of a balanced detector system.The variance of junction capacitance of different photodiodes will cause the degradation of SNR.Based on the linear relationship between junction capacitance of photodiode and reverse voltage in the double logarithmic coordinates,a balanced detector system is designed to have the ability to adjust the reverse voltage.An optimization of the circuit performance can be achieved by adjusting the junction capacitance which significantly affects the circuit noise and signal bandwidth in the design of the high bandwidth TIA which is sensitive to parasitic capacitance and component error.A simplified optical system is built to test the performance of the designed balanced detector system.The results show that the SNR of the detector system is approximately 10%higher than that of the commercial product,which can further increase the detection distance of the wind lidar by approximately 1.5km for the detection limit of 10km.The innovation points of this paper are as follows:1.A new 2μm balanced detector is proposed,which greatly improves the signal amplitude and signal-to-noise ratio of the detector system,and solves the problem of in-sufficient detection distance caused by low signal-to-noise ratio of the balanced detector used by the current 2μm coherent wind lidar;2.A design scheme of divider gate for protecting the PMT is presented,which realizes the control of the output signal of the PMT,and protects the PMT from the strong light signal in practical application;3.By combining analog sampling and photon counting,the detection range of direct detection lidar is improved,and the corresponding data acquisition system is designed to realize the integrated design of detection and acquisition.