Study On High-Spatial-Resolution Coherent Doppler Wind LiDAR Technique

Wind distribution is an important parameter in scientific researches and engineering applications.In fields like aerodynamics,aviation safety,and wind power generation,it is practically needed to measure the wind profiles in real time and high resolution.Especially,the detection of small aircraft wake and small-scale atmospheric turbulence puts forward higher requirements for the spatial resolution.As an active remote sensing technology,coherent Doppler wind LiDAR has advantages in accuracy,robustness,and background light tolerance,making it an important solution of the highresolution wind measurement.Regarding the demand of high-spatial-resolution wind detection and the laser power limitation under current material and hardware technology conditions,this thesis carries out the study on high-resolution coherent Doppler wind LiDAR.By proposing two types of spatial resolution enhancement techniques and corresponding optimizations,the pulse duration and spatial resolution are decoupled.The main contents are as follows:(1)A spatial resolution enhancement technique based on differential correlation pair(DCP)is proposed.Using pulse pairs composed of common and probing pulses and matched window functions,the influence of the common parts can be eliminated in the difference of the cross-correlation,so as to enhance the spatial resolution.In the correlation domain,the signal’s spectral width is no longer only dependent on the width of the probing pulse,thus avoiding the accuracy degeneration caused by the spectral broadening at high spatial resolution.(2)To reduce the gain difference of the amplification caused by the different intensity envelopes of the odd and even pulses,a pulse pair generation scheme based on phase shift keying(PSK)is designed.A single Mach-Zehnder modulator is used to realize pulse shaping and binary PSK of the probing pulses in the DCP method.This scheme reduces the difficulty in the pulse pair generation,improves the detection efficiency,and enhances the stability of the system.In the experiment,the wind profiles detection with spatial/temporal resolution of 3 m/0.1 s in a range of 800 m is realized.(3)A spatial resolution enhancement technique based on pseudo-random phase modulation(PRPM)is proposed.With the help of the spectrum spread and despread characteristics of the pseudo-random code,the spatial resolution is decoupled with the pulse duration.Under a certain pulse duration,the spatial resolution is improved by increasing the modulation rate,and the disadvantages of signal energy reduction and spectral broadening caused by narrow time-domain pulse are avoided.(4)To solve the high bandwidth requirement problem introduced by the rectangular baseband signal in the PRPM technique,a pseudo-random modulation scheme based on continuous phase modulation(CPM)is designed.By mapping the pseudo-random binary sequence into different transition states rather than phase levels,a significant modulation efficiency improvement can be achieved,reducing the bandwidth requirement to only 1/10 of the original scheme.In the experiment,wind field remote sensing with sub-meter and sub-second spatial and temporal resolutions(0.9m/0.5s)is realized.(5)To improve the robustness of the PRPM technique,a pseudo-random modulation scheme based on phase shift keying is designed.In a complex environment,the halfwave voltage drift of phase modulators would lead to the deterioration of the signal-tonoise ratio.To solve this problem,a Mach-Zehnder modulator is used to realize the pulse shaping and pseudo-random phase shift keying modulation simultaneously.By measuring the intensity changes of the modulated pulses,the working point drift of the modulator can be monitored,thus improving the stability of the system.