日盲紫外域拉曼激光雷达探测大气水汽技术研究

拉曼激光雷达通过探测与水汽浓度相关的大气水汽振动拉曼散射回波信号，可实现大气水汽混合比廓线的探测。然而由于振动拉曼信号非常微弱，在白天测量时振动拉曼散射光谱会淹没在太阳背景光中，多在夜间测量。为实现大气水汽的全天时测量，设计开发一套日盲紫外波段拉曼激光雷达系统。该系统选择Nd∶YAG脉冲激光器的四倍频输出-266.0 nm日盲紫外波段作为拉曼激光雷达系统的激励波长，采用镀高增益介质膜的牛顿式望远镜作为接收器，同时利用二向色镜和超窄带干涉滤光片设计高效率的高光谱分光系统，实现了大气氧气、氮气和水汽振动拉曼散射回波信号277.5，283.6和294.6 nm的精细提取。计算仿真结果表明，臭氧吸收对日盲紫外域拉曼激光雷达探测存在一定的影响，主要是探测距离的影响；氮气通道不受白天太阳背景光噪声的影响；水汽通道存在少量太阳背景光噪声，对系统探测距离略有影响。而系统信噪比计算结果表明，设计的日盲紫外域拉曼激光雷达系统可实现白天3.5 km大气水汽的探测。实际进行水汽探测时，可利用氮气和氧气通道反演出臭氧浓度廓线，修正臭氧对发射波长、各通道拉曼散射波长的吸收，进一步提升系统的探测能力和探测精度。

Research of Solar-Blind Ultraviolet Raman Lidar for Water Vapor Measurement Technology

Raman Lidar can detect water vapor mixing ratio by atmospheric water vapor vibration Raman scattering echo signal associated with the water vapor concentration. However, vibration Raman scattering spectra would drown in the sun background light due to the weak Raman scattering echo signal, therefore the measured time is usually at night. All radiation below 300nm known at solar-blind ultraviolet band is absorbed by the ozone layer in the stratosphere. The shorter the wavelength is, the stronger the energy is. To realize the detection of atmospheric water vapor at daytime and night time, a Raman Lidar is developed at a solar-blind ultraviolet band. The system consists of the laser, telescope, photoelectric acquisition and signal processing part. Briefly, the forth harmonic output (ultraviolet 266 nm) of an externally triggered, 10 Hz repetition rate, Nd∶YAG laser is employed as the transmitter. The bore sight assembly uses a turning prism controlled by a New Focus actuator. With 400 mm diameter, 0.5 m rad field of view, a telescope forms the main part of the receiving optics. To obtain signals with fine separation and high efficient extraction, three dichroic mirrors separate out the detection channels by reflecting light with longer wavelengths while transmitting light with shorter wavelengths, a combination of narrow bandwidth (FWHM=1 nm) interference filters is employed to filter the backscattered signal. The rejection rate of the Mie-Rayleigh scattering signals reaches to 10-7. Before reaching the photomultiplier tube (PMT) in each channel, a plano-convex lens is employed to focus the backscattered signal on the front face of the PMT. The backscattered radiation is collected and analyzed at four wavelengths of interest, 266.0 nm for the elastic scattering, 277.6, 283.6 and 294.5 nm for the Raman scattering of O₂, N₂ and H₂O molecules, respectively. The four PMTs output signals are then input into a multi-channel digitizer to record the backscattered signal, which is used to retrieve the water vapor profile. We use the standard atmospheric scattering models and aerosol extinction coefficients, set system of the sampling interval to 80 ns, cumulative average pulse number to 36 000, the signal-to-noise ratios of atmospheric water vapor measurement are simulated. The simulation results show that there exists influence on ozone absorbing mainly at the Solar-blind Ultraviolet Raman Lidar detection range. The signal to noise ratio simulation results show that the measurement height of the designed Solar-blind Ultraviolet Raman Lidar system can be up to 3.5 km during the daytime measurement. The optimal parameters of Lidar system are obtained based on the detailed analysis and the discussion of the SNR of echo signals. It is concluded that this new solar-blind ultraviolet band Raman Lidar system has the advantage of measuring the water vapor in the daytime without the influence of solar background radiation.