Compact Fiber-Based Water Vapor Lidar

水汽是大气中最重要的微量气体之一。 它不仅是云和降水形成的核心，而且影响太阳和地球辐射的传输，是与二氧化碳相同意义上的“温室”气体。 与二氧化碳不同，水蒸气在时间和空间上变化很大;一个持续的挑战是设计一种准确可靠的方法，通过某种形式的遥感来测量水蒸气的垂直分布。 到目前为止，唯一的方法，可以做到这一点与可接受的垂直分辨率是基于激光雷达技术，无论是拉曼激光雷达或差分吸收激光雷达（DIAL）。 DIAL技术依赖于水蒸气的吸收系数随波长的变化。 窄带相干光以两个波长在大气中传输，这两个波长很接近，但选择一个波长时吸收最大，另一个波长时吸收最小。 光被大气分子和气溶胶后向散射，并被激光雷达接收器收集。 在两个波长处，来自给定范围的信号强度的差异取决于对该范围的双向积分衰减，并且可以用于推断作为沿路径沿着位置的函数的水蒸气的浓度。DIAL系统往往是大的、重的和昂贵的，因为它们采用的精密光学设备需要紧密的气候控制和振动屏蔽。 本项目的目的是根据光纤技术的最新进展开发一种新型的DIAL系统。 变送器是系统的独特之处。 它由半导体激光器主振荡器和光纤功率放大器组成。 其结果是一个更便宜，更轻，更简单的激光雷达，产生更大的光谱纯度比传统的激光雷达基于散装光学传输波。 该概念已被证明与原型模型在940 nm的水蒸气波段操作。 该项目的目标是通过将发射功率按比例提高约30分贝来开发原型的实地版本，并通过与其他遥感和现场观测进行比较来评价该仪器。

Water vapor is one of the most important trace gases in the atmosphere. Not only is it central to the formation of clouds and precipitation, but it affects the transfer of both solar and terrestrial radiation and is a "greenhouse" gas in the same sense as carbon dioxide. Unlike CO2, water vapor is highly variable in time and space; a continuing challenge has been to devise an accurate and reliable way of measuring the vertical profile of water vapor by some form of remote sensing. To date the only methods that can do this with acceptable vertical resolution are based on lidar techniques, either Raman lidar or differential absorption lidar (DIAL). The DIAL technique depends on the variation of the absorption coefficient of water vapor with wavelength. Narrow-band coherent light is transmitted through the atmosphere at two wavelengths that are close together, but chosen so that one is at an absorption maximum and the other at a minimum. The light is backscattered by atmospheric molecules and aerosols and collected at the lidar receiver. The difference in signal intensity from a given range at the two wavelengths depends on the two-way integrated attenuation to the range and may be used to infer the concentration of water vapor as a function of position along the path. DIAL systems tend to be large, heavy, and expensive because the delicate optical equipment they employ requires close climate control and shielding from vibration. The purpose of this project is to develop a novel DIAL system based on recent advances in fiber-optics technology. The transmitter is the unique aspect of the system. It consists of a laser diode master oscillator and a fiber power amplifier. The result is a cheaper, lighter, and simpler lidar that produces a transmitted wave with greater spectral purity than a conventional lidar based on bulk optics. The concept has been proven with a prototype model operating in the 940 nm water vapor band. The goal of this project is to develop a fieldable version of the prototype by scaling the transmitted power upwards by about 30 dB and to evaluate the instrument by comparison with other remote sensing and in situ observations.