| In the applications of local area wind sensing, pulsed coherent lidar systems for atmospheric wind measurements have demonstrated their efficacy in terms of real time, high spatial and temporal resolutions even in the clear atmosphere. Nowadays, coherent wind lidars are widely employed for measurements of wind profiles, warnings of wind shear as well as detections of aircraft wake vortices, which play an importment role in a variety of areas, such as weather forecast, wind energy industry, aerospace industry, military field, and so on. As the rapid progress of erbium-doped fiber amplifier (EDFA) and with the implement of large-mode area (LMA) fiber in EDFA to avoid non-linear optical effects, such as stimulated Brillouin scattering (SBS), the pulse energy and average power generated by EDFA are greatly increased. Coherent wind lidars based on1.55μm all-fiber laser are attracting more and more attentions because of compactness, eye-safety, flexibility and long lifetime without maintenance.A coherent wind lidar system based on a1.55μm all-fiber laser is developed according to the wind measurement requirements for the insurance of the aviation safety and the increase of the wind power efficiency. The performance specifications of the developed lidar system are summarized as follow,(1) detection range:3km;(2) range resolution:60m;(3) wind velocity accuracy:0.5m/s;(4) wind velocity range:±30m/s;(5) time resolution:1s. Experiments demonstration that all specifications meet the design parameters. Specifically, the effective measurable range of more than3.6km is realized.In the design procedure of the lidar system, in order to avoid the tremendously complex integral calculation of system carrier-to-noise ratio (CNR) at detector plane, a concept of backscattered propagated local oscillator (BPLO) is employed to simplify the CNR mathematical calculting at target plane. That is to say, the local (reference) beam at the detector plane propagates backwards through space and time to the target (aerosols) plane at the same path as the aerosols backscattering signal propagates from the target plane to the detector plane. The coherent lidar system design is to maximize the antenna efficiency, to which the system CNR is proportion. The optimal truncation ratio of the telescope is calculated. The result shows that the optimal antenna efficiency of0.422is obtained when the truncation ratio is0.823, on the condition of transmitted Gaussian beam and uniform BPLO. With the optimal truncation ratio, the effect of the aperture of the telescope on the system CNR of the lidar is analyzed. The design parameters of the telescope are optimized. According to the lidar system design demand and the laser parameters, the optical telescope is optimized as follow,(1) beam expanding ratio is14and (2) telescope aperture is64.3mm.Retrieving wind velocity from weak aerosol backscattering signals is a key problem in coherent lidar. The Cramer-Rao lower bound (CRLB) of the maximum likelihood (ML) discrete spectral peak (DSP) estimation is discussed based on the statistical model of the covariance matrix of zero mean complex Gaussian random process of the backscattering signal. The CRLBs both from the ML DSP and Fisher information matrix are compared. On the condition of the covariance matrix statistical model of the backscattering signals in coherent wind lidar, the performance of the ML DSP estimation is examined by employing the computer Monte Carlo simulations, and the probability density function of the estimations of the wind velocity is researched as well. The effects of CNR, the accumulation number of the laser pulse as well as pulse width of the outgoing laser on ML DSP wind velocity estimations are illustrated, respectively. The calculation and simulation results show that,(1) the CRLB of the ML DSP is lower than the exact CRLB from Fisher information matrix;(2) both of the fractions of the "bad" estimations are0, and the standard deviations of the "good" estimations are0.62m/s and0.50m/s, respectively, on the condition of the SNR of-20dB,100laser pulses accumulation and on the condition of the SNR of-30dB,10000laser pulses accumulation. Based on the Monte Carlo simulations, the detection probability of99.99%and wind velocity measurement accuracy of0.22m/s are calculated theoretically among the range of3600m on the condition of1000pulses accumulation.To analyze the spectrums of aerosols backscattering signal, a friendly graphical user interface (GUI) program is developed by employing Microsoft Visual Studio C++6.0integrated development environment (IDE). Such spectrums are illustrated with range resolution of60m and pulse accumulations of1000,3000,5000and10000, respectively. Doppler frequency shift, spectrum width as well as system CNR are retrieved from those spectrums. CNR from experiments are validated to be coincident with theoretical calculation between the300m and2700m. Among the detection range of3600m, CNRs both from experiments and calculations are higher than-37dB, guaranteeing that the CRLB of the wind measurement accuracy is better than0.1m/s with the ML DSP retrieval algorithm. Regarding to the calibration of the lidar system, both rotating and non-moving hard targets are employed. The calibration results by the rotating hard target are list as follow,(1) the coefficient of determination of the linear fitting is0.999;(2) the slope is1.002;(3) velocity standard deviation is0.48m/s. And calibration results by the non-moving hard target are list as follow,(1) detection probability is100.0%;(2) velocity standard deviation is0.06m/s.A parallel wind velocity retrieval algorithm is proposed to solve the problems of huge volume data and time-consuming calculation in the coherent wind lidar system. For this purpose, a digital signal processor card integrating four Texas Instruments (TI) TMS320C6678processor with eight cores in each processor is employed. The digital data from analog-to-digital converter (ADC) is allocated equally to each processor core and processed in single instruction multiply data (SIMD) way. On the condition of10000pulses accumulation, the time resolution of1s is realized. Continuous measurements of line of sight (LOS) wind velocity in real time is reached. And measurements by plan position indicator (PPI) scanning mode is demonstrated as well. |
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