衍射光学系统激光雷达接收波束展宽及作用距离分析

Analysis of Receiving Beam Broadening and Detection Range of LiDAR Based on Diffractive Optical System

对基于衍射光学系统的激光雷达的波束展宽方法和作用距离进行了分析,介绍了实验样机的研制情况。根据实际激光雷达的成像特点并结合宽幅成像的需求,提出了离焦扩束、加柱面镜扩束以及基于衍射镜的波长变化扩束3种接收波束展宽方法,进行了仿真计算并给出了实验样机的部分测试结果。给出了扩束情况下激光雷达的作用距离表达式,同时讨论了模数转换(AD)采样量化对接收信号采样的影响,明确了接收扩束产生的增益下降可由电子学放大器来弥补的观点,并结合实际数据给出了验证结果和分析结果。

Objective To ensure a detection range,the receiving telescope of lidar should adopt a larger aperture to receive more echo energy.When receiving with a singleelement detector,increasing the receiving aperture typically reduces the receiving beam width.Array detectors are commonly used to receive wide fieldofview echo signals to achieve a wide receiving beam and form a large observation width.Considering an optical system with a 100 mm aperture and 480 mm focal length as an example,it is assumed that a singleelement detector with a photosensitive surface size of 9.5μm is used for reception.For a central wavelength of 1.55μm,the corresponding receiving beam width is approximately 20μrad,which is close to the diffraction limit.The size of the detector array required to cover a receiving fieldofview of 3 mrad is 150×150,resulting in a sharp increase in the number of LiDAR channels with a coherent detection system and an extremely complex technical implementation of the system.The size of photosensitive surface of the singleelement detector increases to approximately 1 mm.In principle,one channel can realize the reception of a wide fieldofview laser echo signal.Methods Receiving beam broadening can realize the function of a detector with a large photosensitive surface.Receiving beam broadening can be realized at the primary lens or the feed,and the method of realizing receiving beam broadening at the feed can include the common aperture function of the primary lens.Following references[5-6]and the feed beam expansion method,three receiving beam broadening methods,including defocus beam expansion,cylindrical lens beam expansion,and wavelengthconversion beam expansion based on a membrane diffractive lens,are proposed.A simulation calculation was performed.Some verification results are provided in combination with the development of the actual system.In addition,considering the problem of receiving gain reduction caused by beam expansion,the working distance equation of LiDAR in the case of beam expansion is also provided in this paper,and the analysis and verification are conducted based on experimental data.Results and Discussions Based on the three beam expansion methods proposed in this study,the experimental prototype expanded the transmitted beam to 5 mrad in the elevated direction under the condition of beam expansion by a cylindrical lens(Fig.6).Simultaneously,in the defocus beam expanding mode,to realize coherent detection of the laser echo signal based on the allfiber optical path,a fiber collimator with a highorder phase beam expander is used to bring the fieldofview of the 3 mrad laser echo signal into a singlemode polarizationmaintaining fiber with a core diameter of 9.5μm(Fig.3).The echo signal is further received by the optical fiber collimator array with a highorder phase spherical lens,the overlapping fieldofview of 0.5 mrad is realized,and the total receiving fieldofview of the system is 5.5 mrad(Fig.4).In the case of beam expansion,the chimney at 1.1 km and the highreflectivity target at 5.4 km are detected.After multipulse coherent accumulation and selffocusing processing,the signaltonoise ratio is greater than 30 dB(Figs.12 and 13),which satisfies the requirements of highresolution imaging.Conclusions The LiDAR prototype developed in this study utilized the lightweight and thin characteristics of the membrane diffractive lens to realize the lightweight and large aperture of the receiving system.Combined with the receiving beam expander system,the targets at 1.1 and 5.4 km were detected using the receiving beam expander system.These results indicate that the proposed method is effective.Under the condition of a large receiving aperture,combined with transceiver beam broadening,it can not only reduce the number of receiving channels but also ensure the imaging resolution and longdistance detection signaltonoise ratio,as well as obtain a large instantaneous observation width.It is of great significance to continue relevant research,which is expected to meet the requirements of longdistance,widerange,highresolution imaging.The decrease in the receiving gain caused by the receiving beam expansion can be compensated by adding amplifiers to the electronics.At present,electronic amplifiers with a gain of 40‒50 dB are very common,and a reasonable design of the system parameters is of great significance.