Measuring weak water vapour absorption using a supercontinuum source (MASS)

质量是一项概念验证建议，旨在评估将超连续光源(SCLS)耦合到傅里叶变换光谱仪(FTS)以实现高精度分子光谱的潜力。拟议的试点应用是对大气条件下1.6微米近红外窗口中水蒸气的弱(连续)吸收进行新的测量，如果成功，这将导致我们对这种吸收性质的理解取得重大进展。这项工作与其他波长的改进测量，以及对光谱学、大气和行星科学以及环境监测中具有重要意义的其他气体和颗粒的大范围弱吸收的表征具有更广泛的相关性。它与“水”、“监测”和“气候系统建模的基础”分主题直接相关。它结合了雷丁大学的大气科学专业知识和STFC卢瑟福·阿普尔顿实验室分子光谱设施的实验室分子光谱专业知识。SCLS提供了独特的优势，可以极大地促进使用FTS的高分辨率精密光谱领域的发展。它们可以取代传统使用的白炽灯。特点包括：(1)亮度：白炽灯在传输宽带光方面效率很低。超小型发光二极管是更明亮的光源(与传统光源相比，波长为1.6微米的光源亮度约为三个数量级)，从而保持了宽带特性。如果假设探测器受限的FTS具有所需的动态范围，则亮度优势直接转化为记录光谱的信噪比改善。(2)空间相干性：与灯不同，SCL是类似于激光的空间相干源。这种根本的差异带来了在精密分子光谱学中可以利用的独特的传播优势。提高光谱仪灵敏度的一种方法是增加样品与光之间的相互作用长度。对于非相干光，例如由灯发出的光，通过次数通常限制为大约15次(在多路通过单元中)。由于更好的传播特性，来自超连续激光系统的相干光可以更有效地折叠，可能通过40-200次。(3)超连续谱的噪声：由于超连续谱产生的非线性效应的复杂性，超连续谱的噪声特性仍在研究中。它们基本上是噪声源，当源驱动参数优化时，它们仍然可以接近低噪声相干源的理想实现。前人的工作表明，为了实现探测器的限幅光谱并充分利用SCLS的功率优势，源功率参考是必要的。本文提出的新系统的应用是对水蒸气引起的近红外连续吸收的改进表征。在大气科学中，水蒸气的非结构连续吸收在水蒸气的旋转和振动-旋转带之间的“窗口”中是最重要的。这些窗口还被广泛用于遥感，以确定地球表面、云和大气气溶胶的性质，对于这些性质，水蒸气吸收起到了干扰的作用，必须消除这种干扰，才能可靠地恢复感兴趣的性质。这里的重点是1.6微米窗口，因为最近使用各种技术进行的近室温吸收测量在数量级上不一致，而且对温度相关性的了解有限。

MASS is a proof-of-concept proposal to assess the potential of coupling a super-continuum light source (SCLS) to a Fourier Transform Spectrometer (FTS) to achieve high-precision molecular spectroscopy. The proposed pilot application is to make new measurements of the weak (continuum) absorption of water vapour in the 1.6 micron near-infrared window in atmospheric conditions, which, if successful, would lead to a major advance in our understanding of the nature of that absorption. The work has a much wider relevance to improved measurements at other wavelength and to the characterisation of a wide range of weak absorption by other gases and particulates of importance in spectroscopy, atmospheric and planetary science and environmental monitoring.The work falls in the "Environment" priority area of the Global Challenges call. It is of direct relevance to the "water", "monitoring" and "underpinning of climate system modelling" sub-themes. It links atmospheric science expertise at the University of Reading and the laboratory-based molecular spectroscopy expertise at the STFC Rutherford Appleton Laboratory Molecular Spectroscopy Facility.SCLS are a new compact way of generating "white" light. SCLS offers unique advantages that can strongly benefit the field of high resolution precision spectroscopy using FTS. They could replace the traditionally-used incandescent lamps. Features include:(1) Brightness: Incandescent lamps are rather inefficient in delivering broadband light. SCLS are far brighter sources (about three orders of magnitude at wavelengths of 1.6 micron compared to traditional lamps) which maintain a broadband nature. If a detector-limited FTS with the required dynamic range is assumed, the brightness benefits turns directly into a signal to noise ratio improvement on the recorded spectra.(2) Spatial coherence: Unlike lamps, SCLS are spatially coherent sources in a similar way to laser light. This fundamental difference brings unique propagation benefits to be exploited in precision molecular spectroscopy. One way to improve spectrometer sensitivity consists of increasing the interaction length between the sample and light. With incoherent light such as emitted by lamps, the number of passes remains limited to typically ~15 passes (in a muli-pass cell). Owing to far better propagation characteristics, coherent light from SCLS can be more effectively folded with passes of 40-200 possible. (3) Noise of SCLS: Due to the complexity of non-linear effects producing the super continuum, noise properties of SCLS are still being researched. They are fundamentally noisy sources that can nevertheless approach the ideal realization of a low-noise coherent source when the source driving parameters are optimized. Previous work suggests that source power referencing is necessary to achieve detector limited spectroscopy and fully benefit from the SCLS power advantage.The application of the new system proposed here is the improved characterisation of the near-infrared continuum absorption due to water vapour. The unstructured continuum absorption due to water vapour is of most importance in atmospheric science in the "windows" between the rotation and vibration-rotation bands of water vapour. These windows are also widely used in remote sensing to derive the properties of the Earth's surface, clouds and atmospheric aerosols, for which water vapour absorption acts as an interference that must be removed for robust retrieval of the properties of interest. The focus here is on the 1.6 micron window as recent near-room-temperature absorption measurements using a variety of techniques disagree by orders of magnitude, and there is limited understanding of the temperature dependence.