Study On Heterodyne Method And Key Technologies For2-micrometer Wind Lidar

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.