【作者】 李彦超；

【导师】 王春晖；

【作者基本信息】 哈尔滨工业大学 ， 物理电子学， 2012， 博士

【摘要】 “晴空”大气矢量风场测量一直是微波气象雷达发展的盲点。随着激光雷达技术和外差探测技术的迅速发展，基于多普勒效应的相干测风激光雷达已经成为大气矢量风场、大气湍流和局域风切变等测量的主要手段之一。相干测风激光雷达具有高分辨率和高精度的优点，而且由于激光与大气物质相互作用的特殊物理机制，它还具有极强的晴空探测能力，与微波气象雷达互补，可以构成“无缝隙”的大气遥感系统。因此，国内外各高端科研机构纷纷投入大量人力和物力进行研究和开发。目前，2μm相干测风激光雷达是国际测风激光雷达研究热点之一。本文围绕其所涉及的科学问题和关键技术，研究总结了大气后向散射信号的外差探测理论模型，提出了自由空间与光纤复合相干光学系统设计方法，研究了极微弱宽带中频信号提取与正交数字化处理的关键技术，最后搭建了一套2μm相干测风激光雷达外差探测实验系统，进行了实验验证。本论文主要研究内容包括以下几个方面：（1）系统地研究大气后向散射气象回波信号的外差探测理论，以此指导相干测风激光雷达系统设计。在考虑大气折射率湍流影响的条件下，利用菲涅尔衍射理论和光场的路径积分表达式，建立了相干测风激光雷达系统的外差效率的理论模型，分别给出了探测平面、接收平面和目标平面上外差效率的表达式，得到了相干探测激光雷达方程；根据相干激光雷达风速测量原理，给出了三维矢量风速的反演方法；建立了信噪比随本振光功率变化的理论模型，仿真结果表明：最佳本振光功率约为0.268mW；研究了三种相位失配和偏振态失配对外差效率的影响，针对于不同的情况给出了响应的仿真结果；探讨了高灵敏度平衡外差探测新方法，结果表明：可以明显克服本振光散粒噪声和提高外差探测的信噪比。（2）研究空间光与光纤高效耦合型相干光学系统设计方法，建立了一套收发合置相干光学系统。依据收发合置光学天线发射和接收光束的截断比理论，给出外差接收信号随截断比变化的数学表达式，研究结果表明：最佳光束截断比约为1.75；理论推导天线效率与外差效率之间的数学关系，仿真计算在弱湍流Cn²=10^-15条件下光学天线效率约为0.51、在强湍流Cn²=10^-13条件下约为0.03；利用ZEMAX软件优化设计与实现一套收发合置离轴卡塞格伦光学天线系统和预扩束系统，达到衍射极限水平；设计并实现一种耦合效率约为83%的2μm单模保偏光纤耦合器、2μm单模保偏光纤合束器、2μm单模保偏光纤分束器以及2μm收发偏振隔离光开关（即偏振分光棱镜+λ/4波片），克服光束在自由空间传输产生的退偏及本振光泄露问题。（3）研究极微弱宽带气象回波中频信号提取与正交数字解调技术。讨论了多脉冲积累法提取极微弱信号的方法，即对多脉冲气象回波信号进行中频直接采样、数字信号相干积累及功率谱非相干积累；针对于105MHz载频和30MHz带宽的中频信号，重点讨论正交混频低通滤波和奇偶抽取多相滤波的两种数字化处理算法，在64阶原型等纹波FIR滤波器设计基础上，以响应带宽和镜频抑制比为评价参数，比较分析7种FIR滤波器设计方案，从中优选出适合宽带信号数字处理算法。仿真结果表明：在回波信噪比为-13.24dB条件下，进行8ms的相参积累，信噪比增益可达19dB以上；先进行8ms短时相参积累，再进行非相参积累，当积累总时间超过20ms时，信噪比增益将达到18dB以上；1/2抽取64阶原型滤波器的多相滤波方案，不仅具有近似64阶原型FIR滤波器的镜频抑制比，而且数据输出速率减半，接收带宽约为采样率的25%以内，满足30MHz带宽中频信号的数字处理要求。（4）建立2μm相干激光测风雷达外差探测实验系统，验证上述理论模型、光学设计和极微弱信号提取算法的正确性。外差探测系统中激光发射机采用了波长2μm、单脉冲能量为2mJ、重复频率为100Hz、脉宽为300ns的固体Ho:YAG脉冲激光器；相干光学系统采用了150mm离轴卡塞格伦光学天线、偏振分束棱镜、/4波片、2μm单模保偏光纤耦合器等；接收机采用全光纤平衡式InGaAs光电探测器（响应带宽150MHz）和自研的中频信号采集卡。实验结果表明：设计加工的光学天线系统的波像差为0.052λ，预扩束系统的发散角为0.075mrad，达到了衍射极限水平；完成的2μm单模保偏光纤合束器、分束器和耦合器，不仅实现了外差探测系统偏振匹配，而且还实现了相位匹配及偏振态匹配；采用自研的2μm平衡式光电探测器检测微弱的外差信号，分别得到了16m处和96m处孔径为200mm合作目标的外差信号，信噪比分别为45.16dB和43.91dB。更多还原

【Abstract】 The measurement of Atmospheric wind vector field clean-air has been a blindspot of microwave radar for a long time. As the development of lidar technologyand heterodyne technology, coherent wind lidar which based on Doppler effect hasbecame one of the effective means to study on Atmospheric wind vector field,atmospheric turbulence and local windshear.This method has many advantages,such as high accuracy, high resolution. Due to the special physical mechanis m ofthe interaction between laser and atmospheric material, coherent wind lidar has highdetection capability for clear-air and could make up for the shortage of microwaveradar, to form a seamless atmospheric remotely sensed system, and high endscientific research institutions abroad spend plenty of Manpower And MaterialResources on it one after another.At present,2μm coherent wind lidar is one of focus in international wind lidarresearch. this paper focuses on the involved scientific issues and key technologies,then establishes the theoretical mode of backscatting signal of the atmospheric,proposes a method for coherent optical system consist of free space optical antennaand optic fiber receiver, solved the key technology of the very weak bandintermediate frequency（IF） signa l extracting and orthogonality digital signalprocessing, finally builds a2μm coherent lidar heterodyne experimental system andverified the feasibility by experiment. The main contents of this paper inc lude thefollowing aspects:Firstly, heterodyne detection theory of atmospheric backscatting signal isdiscussed, and it has been the designing principles of coherent wind lidar system. Inthe condition of considering the effect of atmospheric turbulence of refractive index,based on Fresnel diffraction theory and the path integral formulation of the lightfie ld, a theoretical model of the heterodyne efficiency for coherent lidar system isestablished, and express the heterodyne efficiency expressions of the detector plane,the receiver plane and target plane, and the equation of coherent lidar has been got.At the same time, according to the wind speed measurement principle of coherentlidar, express the inversion of three-dimensional wind speed. Established atheoretical model of this signal to noise ratio changes with optical local oscillatorpower and get the optimum optical local oscillator power is0.268mW bysimulation.Focused on the study of the influence of polarization mismatch and three phasemismatch to the heterodyne effic iency and get the simulation results for different circumstances. Discussed high sensitivity balanced heterodyne detection theory. Theresults showed that: the local oscillator optical shot noise is overcome better andsignal to noise ratio（SNR） of heterodyne detection is improved obviously.Secondly, coherent optical system consists of free space optical antenna andoptic fiber receiver system is discussed. According to the theory of truncated ratio ofmonostatic optical antenna and receive beams, a mathematical expression of theheterodyne received signal changes with truncation ratio is given, the results showthat: the optimum truncation ratio of monostatic optical antenna system is1.75, andit also provides a theoretical basis for the calculation of the antenna effic iency,obtained antenna effic iency under the conditions of weak turbulence(Cn²=10^-15) is0.51and under the conditions of strong turbulence(Cn²=10^-13) is0.03by simulation.Then we using the optical design software ZEMAX to designed a set of monostatic,off-axis Cassegrain coherent optical antenna system and beam expander system,which attached to the diffraction limit. At the same time, designed2μm single-modepolarization maintaining fiber coupler with coupling efficiency of83%,2μmsingle-mode polarization maintaining fiber combiners,2μm single-modepolarization maintaining fiber beam splitter and2μm transmitter-receiverpolarization isolation optical switch that is polarizing beamsplitter+/4wave plate.This device overcomes the depolarization phenomenon caused by the beamtransmission in free space and the leakage of local oscilla tor light impact on theheterodyne signal.Thirdly, worked on the extraction of very weak signals and processingorthogonal digital signals of weather echo signal. Discussed the multi-pulseintegration method for extremely weak echo IF signal which is sampling IF signaldirectly, coherent accumulating of digital signals and non-coherent accumulating ofpower spectrum.Countered the IF signal with carrier frequency of105MHz and bandwidth of30MHz, discussed two digital processing arithmetic such as low-pass filtering oforthogonal frequency mixing and polyphones filter method of even-odd extraction.Based on the designing of64-orders prototype FIR filter, using responsivebandwidth and image rejection ratio as evaluation parameters, compared seven FIRfilter, choosing the better one which is suitable for the bandwidth.Simulated result shows that: when the SNR of echo signa l is-13.24dB, SNRgains is above19dB by the coherent integration time above8ms. First short-timecoherent integration in8ms then non-coherent integration, SNR gains above18dBwhen the accumulated total time is more than20ms. Poly-phase filter of1/2extraction64-orders prototype filter not only has a image rejection ratio which is similar to the64-orders prototype FIR filter, but also has a half data output rate,receiving bandwidth is low than25%of the sampling rate, which is fit for the digitalprocessing of30MHz IF signal.Fourthly,2μm heterodyne experimental system for coherent wind lidar isestablished, verify the correctness of theoretical mode, optical designing and thearithmetic of very weak signa l extraction. The solid Ho:YAG pulsed laser as anheterodyne detection system detect light, the laser parameters are as follows:2μmwavelength, single pulse energy2mJ, repetition frequency of100Hz, the pulse widthis300ns. Coherent optical system consist on off-axis Cassegrain optical antennawith150mm, PBS,/4wave plate,2μm single-mode polarization maintainingfiber devise, etc. The receiver is2μm balanced InGaAs photodetector with150MHzresponsive bandwidth and self-research IF digital signal processing board.The results showed that: when the wave aberration of the optical antennasystem is0.052λ and the divergence of beam expander is0.075mrad, thus can meetthe diffraction limit of heterodyne detection system. Used of optical passivesingle-mode polarization maintaining fiber devices such as2μm single-modepolarization maintaining fiber combiners, optical fiber beamsplitter and coupler. Notonly to meet the polarization matching requirements of heterodyne detection system,but also to meet the phase matching requirements. The self-research2μm balancedphotodetector is used to detect weak heterodyne signal which is high sensitivity,wide bandwidth, and fast response. The heterodyne signal for the cooperationobjectives with aperture of200mm from the16m and96m is obtained, also signalto noise ratio of the heterodyne signal as45.16dB and43.91dB respectively.更多还原