| Lidar is a kind of active detection technology,which can acquire the threedimensional spatial information of the target accurately and quickly.The detection technology of Lidar is mainly divided into direct detection and coherent detection.Because of the high reliability and simple implementation of direct detection,most lidar systems adopt direct detection.However,the direct detection method is suitable for strong light detection,which needs to receive more signal than noise,especially for photon counting lidar based on single photon detector,the filter bandwidth of spectral filtering should be as narrow as possible to reduce the false alarm probability during detection.However,the general lidar usually works in the daytime,which will be affected by strong solar background radiation noise.Therefore,In order to achieve the suppression of background radiation noise,we commonly use narrowband filter to carry out spectral filtering.However,when the line width of the laser and the filter bandwidth of the received filter are very narrow,due to the changes of the external environment such as force and heat,their central wavelength will no longer match,at this time,our filtering effect will be greatly affected.Therefore,in order to maintain the stability and excellent detection performance of laser radar under long working time,wavelength matching under narrow bandwidth filtering is an urgent problem to be solved.In order to solve the wavelength matching problem under the narrow band filter of lidar,this paper proposes two schemes to realize the wavelength matching of laser radar.One is to use tunable lasers in conjunction with narrow-band filters,and the other is to use narrow-band tunable filter.By comparing the advantages and disadvantages of the two,the wavelength matching in lidar system is realized by using tunable filter with narrow bandwidth.In this paper,several mainstream tunable filters are investigated.By comparing their spectral filter bandwidth,tunable range and tuning mode,the reflective volume grating is selected as the key device of tunable filter design.The reason is that the filtering bandwidth of reflective volume grating is very narrow,which can theoretically reach 20 pm,which is exactly what our lidar system needs.Moreover,it is tuning mode is very simple,which can be realized by changing the angle of incident light.Therefore,it is suitable for the design of tunable filter.Based on the analysis of spectral filtering characteristics,angle characteristics and symmetrical spatial structure characteristics of reflective volume grating,a design scheme of automatic matching system of laser radar transceiver wavelength based on narrowband optical filtering of volume grating is proposed.Based on the derivation and simulation analysis of the basic theory of volume grating,a volume grating with a filter bandwidth of 100 pm is designed,and the experimental test of the physical object after processing is carried out,and the corresponding comparative analysis is made with the theory.Subsequently,the key devices used in the system were theoretically analyzed and selected according to the wavelength matching resolution index.Finally,the whole laser radar transceiver wavelength matching system was actually built in the laboratory.Finally,the wavelength matching accuracy is better than 2.9pm under the conditions of fixed laser wavelength and tunable laser wavelength.At this point,the intensity of the received signal is 99.97 percent of that of a perfect match.Therefore,this paper designs the system of laser radar can be implemented under the narrow filter bandwidth and automatic matching of wavelength,avoid the influence caused by the mismatch between the receiving filter and the transmitting laser due to wavelength drift when affected by the environment,It further improves the background light suppression capability and stable and reliable performance of lidar,especially Fraunhofer lidar,and reduces the complexity of subsequent lidar data processing. |
