水汽含量是大气最基本的物理参量之一, 大气水汽垂直分布结构对于大气过程的研究十分有意义。 差分吸收激光雷达可以昼夜获取高精度、 高距离分辨率的大气水汽垂直分布廓线, 是最有潜力的探测手段。 国际上已经发展出几种类型的差分吸收激光雷达, 对它们的发展路径做一梳理, 理清发展脉络, 具有有益的参考价值。 其中, 稍早时期水汽差分吸收激光雷达工作在4ν振动吸收带720～730 nm频域, 以Alexandrite为主流的激光器或者Nd∶YAG/ruby固体激光器泵浦的染料激光器作为发射光源, 光电倍增管仍然可以在这个波段担任探测器, 代表性的仪器是法国的机载LEANDRE Ⅱ。 此后发展的820 nm波段的水汽差分吸收激光雷达, 以钛宝石激光器或钛宝石光放大器为发射机, 以硅的雪崩二极管作为探测器, 紧跟前置放大和数据的AD采集器, 如德国Hohenheim大学的车载扫描激光雷达, 可以获得对流层300～4 000 m之间水汽两维或三维分布结构; 德国Institutfür Meteorologie und Klimaforschung所建立的差分吸收激光雷达可以探测3～12 km高度之间大气的水汽垂直分布。 720和820 nm波段水汽吸收截面较小, 更适合于地基或车载的对流层水汽廓线探测。 而水汽3ν振动谱935 nm区域吸收截面较大, 是为了空间探测大气对流层上、 平流层下相对干燥区域的水汽分布而准备的, 且可以安排多个探测波长, 和一个参考波长, 它们对水汽的吸收截面大小呈梯度分布, 以应对空间对地观测时不同高度大气水汽浓度的差别。 基于种子注入的光参量振荡器或Nd∶YGG全固态激光器的935 nm差分吸收激光雷达, 以德国Deutsches Zentrumfür Luft- und Raumfahrt的研究最为成功, 推动了欧洲空间局立项发展空间水汽差分吸收激光雷达WALES(Water Vapour Lidar Experiment in Space) , 测量从地球表面到平流层下、 高垂直分辨率和高精度水汽浓度分布。 机载多波长水汽差分吸收激光雷达1999年建立起来, 担当空间WALES任务的模拟器, 2006年完成了机载飞行试验。 以823～830 nm分布布拉格反射半导体激光器和半导体光放大器为核心、 采用雪崩二极管盖格光子计数技术的微脉冲差分吸收激光雷达, 是差分吸收激光雷达面向商业化、 可普及的方向迈出的重要一步, 目前已经发展到第四代产品。 发射机激光工作波长的长期稳定十分重要而棘手, 以窄带连续波种子激光注入脉冲激光器的谐振腔锁定其的腔长, 种子激光的波长以水汽的多通道光吸收池为参照标准, 或以高精度波长计为误差获取手段, 通过负反馈进行主动稳频; 其次, 需要仔细考虑大气对激光的后向散射光谱线型, 显然Rayleigh后向散射光的多普勒展宽与水汽吸收光谱线宽度可以比拟, 所以其吸收截面σon和σoff必需加以修正; 水汽的空间垂直分布梯度大, 因此差分吸收激光雷达应该实行分通道探测。

Water vapor is one of the basic atmospheric parameters, and the vertical structure of the atmosphere is of great importance to process studies. Differential absorption lidar is the techniques which provide high resolution and accuracy for water vapor profiles daytime and nighttime, and that is the most potential instrument. Differential absorption lidar (DIAL) operates in the 720~730 nm region of the 4ν overtone vibrational bands of H2O where previous using tunable dye, or Alexandrite ring laser injection seeded, however, photomultiplier acts as detector. The represent is airborne lidar LEANDRE II. And the DIAL transmitter is based on an injection-seeded, Ti:Sapphire laser or Ti:Sapphire amplifier operated at 820 nm, Si-APD act as detector. University Hohenheim mobile lidar can perform the measurements of the 3-dimensional structure of the water vapor field from 300 m to 4 km altitude. The high-power DIAL at the Schneefernerhaus research station has successfully demonstrated its measurement capabilities of vertical structure of water vapor from 3 to 12 km above sea level. The development of an OPO at 935 nm in the spectral region of the 3ν overtone vibrational band of H2O was stimulated by the need to develop an airborne water vapor DIAL with high measurement sensitivity at tropopause height, particularly in case of very dry air from the lower stratosphere. In this 935 nm wavelength range, the line strengths of suitable water vapor absorption lines are more than a magnitude higher than near 720 nm or 830nm. Based on single-frequency, a diode-pumped Nd∶YGG laser system or optical parametric oscillator emitting at 935 nm, differential absorption lidar has recently been developed to space borne measures water vapor profile of upper troposphere/lower stratosphere (UTLS) region. DBR diode laser and semiconductor optical amplifier as transmitter, APD as Geiger counter, micro-pulse DIAL for measuring water vapor in the lower troposphere has been developed and validated at field campaigns, and the fourth generation product has been constructed and tested. The application required a single-frequency laser transmitter operating at near infrared region of the water vapor absorption spectrum, capable of being on/off wavelength seeded and locked to a reference laser source for DIAL measurements. The system is based on extended-cavity diode lasers and distributed-feedback lasers. It is achieved by locking the laser wavelength to a water vapor absorption line using compact water-vapor reference cells. or the wavemeter readout for frequencies of laser1 or laser 2 counts as the error signal, product of the error signal and the PID adds as a correction to the applied voltage on the piezo controllers and injection current to tune more stably and smoothly frequencies of the diode laser. Second, precise knowledge of spectral properties of water vapor absorption, laser emission, and atmospheric scattering is necessary. However, to get high accuracy one has to consider methods of treating the problem of Doppler-broadened Rayleigh back scattering and correcting absorption section of water vapor in DIAL experiments. At last, the backscatter signal in the near-field channel rapidly drops to a level at which the results are affected by noise and electromagnetic interference. So the matching of near- and far-field channels in the lower part of the operating range with both detection channels is necessary.