Spaceborne lidar for ocean vertical profile detection requires a narrow linewidth pulsed laser source with a wavelength of 486.134 nm, which coincides with the H-β Fraunhofer absorption line. An injection-seeded optical parametric oscillator (OPO) is developed to meet this requirement. The 355 nm ultraviolet (UV) laser obtained from a 1064.4 nm Nd:YAG nonplanar ring oscillator (NPRO) after multi-stage cascade amplification and frequency tripling is used as the pump source for the OPO. The OPO is a single-resonant three-mirror ring cavity structure with a critical phase-matched β-barium borate (BBO) as the nonlinear crystal. A 486.134 nm continuous-wave (CW) single-frequency laser obtained by frequency doubling of a 972.268 nm laser diode (LD) with a linewidth of less than 100 kHz is injected into the OPO as a seed source. With an incident 355 nm pump energy of 58.4 mJ, a signal pulse energy of 23.8 mJ and pulse width of 5.05 ns is obtained, with the pump to signal conversion efficiency of 40.7%. The central wavelength of the signal pulse is 486.134 nm, and the corresponding spectral linewidth is 0.017 nm, with the measured wavelength stability over 30 minutes of less than 11 MHz.
Based on the time-domain simulation model of coherent wind measurement, it is deduced that the detected atmospheric backscattered spectrum is a two-dimensional convolution of the real atmospheric information, and the instrument point spread function. The spatial resolution can be improved by applying the image deconvolution algorithm to the inversion of the Coherent Wind Lidar (CWL) signal. The effect of the deconvolution algorithm in improving spatial resolution is compared in simulation and experiment. The results show that the spatial resolution can be improved effectively by using the deconvolution algorithm when the transmitted pulse waveform is known.
In this paper, the effects of turbulence intensity and transmit-receive matching angle residuals on detection performance of spaceborne coherent wind lidar were studied and analyzed. The antenna efficiency equation was derived and simulated in the target plane by Monte Carlo and Backpropagated local oscillator (BPLO) methods. Normalized CNR was defined as a measure. The antenna aperture corresponding to the maximum normalized CNR was considered optimal. We simulated the optimal aperture with different mismatch angles of 0 µrad, 2 µrad, 4 µrad, and 6 µrad under weak, intermediate, and strong turbulence intensities respectively. From the simulation results, it is concluded that:as the turbulence intensity and angular residual increase, the coherence length and the optimal antenna aperture decreases. Besides, under strong turbulence, the effect of mismatch angle on the normalized CNR is weakened and the appropriate range of antenna aperture is narrow. The optimal antenna aperture is about 400 mm under the condition of weak or intermediate turbulence.
The characteristics and capability of a homemade all-fiber 1.54-μm pulsed coherent Doppler lidar (CDL) were validated in field experiments by comparing the detection results with a collocated lidar and sounding balloons. With the range gate of 30 m and temporal resolution of 16 s at velocity–azimuth display mode, the detection capability of the CDL ranged from 0.1 to 5 km, and the time sequence and height position of this CDL were calibrated by the collocated lidar. In the intercomparison experiments with sounding balloons, the discrepancy of 30-s averaged measurement results of horizontal wind speed and wind direction was nearly 0.7 m / s and 5.3 deg, respectively. The good agreement achieved in such a short averaged time period was a convincing case of intercomparison experiments between CDL and sounding balloon. The CDL system demonstrated good reliability and operational stability in field experiments.
Airborne integrated path differential absorption (IPDA) lidar system is an important instrument to verify the performance and data inversion methods of future space-borne lidar systems for atmospheric CO2 measurement. A ground vertical path validation experiment of atmospheric CO2 measurement by an airborne double-pulsed 1.57-μm IPDA lidar has been implemented. The experiment was carried out and temperature, pressure and humidity profiles of Local Meteorological Station at almost the same time are adopted. Backscattering signals from clouds at altitudes of nearly 5 km were received. To avoid the influence of stray light from mirrors, the energy monitoring signal was delayed through the 200 m multimode fiber. But it is interfered by the aerosol scattering echo signals. Inversely, considering the stray light as monitoring signal, the inversion result of XCO2 is pretty good. Six methods are studied and compared to reduce the bias and improve the CO2 column-averaged dry-air mixing ratio (XCO2) accuracy. The “PIM, AVD” and “PIM, AVX” methods are more effective when clouds are acted as hard target. The mean value of lidar measured XCO2 calculated by “PIM, AVD” and “PIM, AVX” methods is 409.63 ppm. The average value of in-situ instrument UGGA is 411.05 ppm over the same period. The bias between IPDA lidar and UGGA is -1.42 ppm. With averaging 148 shots, the standard deviation of XCO2 of the IPDA lidar system is 3.68 ppm.
The high spectral resolution lidar (HSRL) technique employs a narrow spectral filter to separate the aerosol and molecular scattering components from the echo signals and therefore can retrieve the aerosol optical properties and lidar ratio (i.e., the extinction-to-backscatter ratio) profiles directly, which is different from the traditional Mie lidar with assumed lidar ratio. Accurate aerosol profiles measurement are useful for air quality monitoring. In this paper, a spaceborne HSRL lidar system simulation model based iodine vapor cell filter was presented. According to three different atmosphere aerosol distribution models and the uncertainties of atmosphere temperature and pressure, the signal to noise ratio (SNR) and the relative errors profiles of the backscattering coefficients of this lidar was simulated theoretically in daytime and nighttime. The result shows that the errors of aerosol backscattering coefficients are smaller in the aerosols dense area than in the sparse area. As altitude increases, the relative error of backscattering coefficient is increased. The relative backscattering coefficient error is within 16.5% below 5 km with 30 m range resolution and 10 km horizontal resolution.
An all‐fiber pulsed coherent Doppler LIDAR (CDL) system is described. It uses a fiber laser as a light source at a 1.54‐μm wavelength, producing 200 μJ pulses at 10 kHz. The local oscillator signal is mixed with the backscattered light (of different frequency) in the fiber. The atmospheric wind speed is determined through the fast Fourier transform applied to the difference frequency signal acquired by an analog‐to‐digital converter card. This system was used to measure the atmospheric wind above the upper‐air meteorological observatory in Rongcheng (37.10°N, 122.25°E) of China between January 7 and 19, 2015. The CDL data are compared with sounding‐ and pilot‐balloon measurements to assess the CDL performance. The results show that the correlation coefficient of the different wind‐speed measurements is 0.93 and their discrepancy 0.64 m/s; the correlation coefficient for wind‐direction values is 0.92 and their discrepancy 5.8 deg. A time serial of the wind field, which benefits the understanding of atmospheric dynamics, is presented after the comparisons between data from CDL and balloons. The CDL system has a compact structure and demonstrates good stability, reliability, and a potential for application to wind‐field measurements in the atmospheric boundary layer.
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