FTIR USING ASYNCHRONOUS FEMTOSECOND OPOS: A NEW PARADIGM FOR HIGH-RESOLUTION FREE-SPACE MID-INFRARED SPECTROSCOPY

这项关于EPSRC-NPL博士后研究伙伴关系的提案针对的是确定的呼叫主题，即碳捕获和储存以及其他气候变化应用中的光学遥感光子技术，并将与NPL环境测量小组负责人Tom Gardiner博士合作进行。NPL的环境测量小组使用其独特的红外微分吸收激光雷达(DIAL)系统，对填埋场、石化烟囱等工业设施的空气排放进行远程监测，这样的定量监测是政府遵守《京都议定书》要求的二氧化碳和甲烷排放减少的重要程序--通过对污染者征收碳税。虽然红外Dial是一项成熟的技术，但它已经达到了其探测器技术的极限，而且由于无法同时测量不同的气体而受到严重限制，这是该技术中使用两个窄线宽激光波长的直接后果。Dial无法在一次测量中获得真实的光谱信息，因此无法利用FTIR中存在的复杂分析工具从吸收光谱中提取多种物种的浓度。将FTIR的带宽和光谱分辨率与DIAL的遥感设施相结合的系统将增强和扩展NPL监测与气候变化有关的温室气体排放的技术能力。我们建议通过以下方式满足这一要求：(A)我们在飞秒(FS)FTIR光谱方面的世界领先工作(我们是第一个演示该技术的团体)(B)我们在中红外OPO频率梳方面的世界领先专业知识(再次，我们是第一个演示飞秒OPO频率梳的小组)，通常FTIR光谱是在热源(球星等)下进行的，这些热源产生的红外光束准直度很差，与远距离自由空间传输不兼容。相比之下，飞秒光学振荡器的输出具有很宽的红外带宽，接近热发射器的输出，但关键是具有激光的空间相干性，允许长距离的自由空间传输。考虑到远距离实施干涉技术的实际挑战，我们提出了一种新的技术方法，来实现一个健壮的、不动部件的FTIR系统，我们将在NPL的特丁顿站点分阶段地从台式演示器发展到自由空间现场试验。该概念基于用两个异步的、来自相同OPOS的相位相干中红外脉冲序列。独一无二的是，这种方法很健壮，有可能在几毫秒内获得高分辨率(&lt；0.01 cm-1)的光谱。它提供精确的距离选通检测，传播距离可能为100‘S米，也许还会进一步使用与时间相关的红外单光子计数检测(该项目将在赫里奥特-瓦特获得这项技术)。与Dial相比，该技术提供了更好的光谱分辨率和同时探测多种气体的能力。此外，它是全固态的，提供了比Dial更紧凑和更高效的解决方案，并且避免了使用染料激光技术，消除了相关的致癌危险。它将把传感扩展到没有合适拨号线路的新化学物种，并与外差/射频锁定检测技术兼容，以改善信号：噪声性能。

This proposal for an EPSRC-NPL Postdoctoral Research Partnership addresses the call theme identified as, Photonic Technologies for Optical Remote Sensing in Carbon Capture and Storage and Other Climate Change Applications, and will be conducted in partnership with Dr Tom Gardiner, head of NPL's Environmental Measurement Group.Using their unique infrared differential absorption LIDAR (DIAL) system, NPL's Environmental Measurement Group carries out remote monitoring for the qualification of airborne emissions from industrial installations such as land-fill repositories, petrochemical stacks, cement factories etc. Such quantitative monitoring is an essential procedure in the government's compliance - via carbon taxes imposed on polluters - with the reductions in CO2 and CH4 emissions mandated by the Kyoto Protocol.While being a mature technique, infrared DIAL has reached the limits of its detector technology, and moreover is seriously constrained by its inability to simultaneously measure different gases, a direct consequence of the use of two narrow-linewidth laser wavelengths in the technique. DIAL lacks the ability to obtain true spectroscopic information in a single measurement, and therefore cannot benefit from the sophisticated analysis tools that exist in FTIR to extract the concentrations of multiple species from an absorption spectrum. A system combining the bandwidth and spectral resolution of FTIR, with the remote-sensing facility of DIAL would enhance and extend NPL's technical capability to monitor the emissions of greenhouse gases implicated in climate change.We propose to address this requirement by combining...(a) our world-leading work in femtosecond (fs) FTIR spectroscopy (we were the first group to demonstrate the technique)(b) our world-leading expertise in mid-infrared OPO frequency combs (again, we were the first group to demonstrate a fs OPO frequency comb)Normally FTIR spectroscopy is carried out with thermal sources (globars etc) that produce poorly collimated IR beams that are incompatible with free-space propagation over extended distances. By contrast, the output from a fs OPO has a broad IR bandwidth, approaching that of a thermal emitter, but - critically - has the spatial coherence of a laser, permitting free-space propagation over long distances. A fs OPO therefore provides a route to implementing free-space FTIR across several hundred cm-1 in a single measurement - sufficient to probe the characteristic absorption lines of many chemical species in a single measurement.Conscious of the practical challenges of implementing an interferometric technique over long distances, we propose a novel technical approach, to implement a robust, no-moving-parts FTIR system, which we will progress, in stages, from a bench-top demonstrator to a free-space field-trial at NPL's Teddington site.The concept is based around replacing a mechanically-scanned optical delay line with two asynchronous, phase-coherent mid-IR pulse sequences derived from identical OPOs. Uniquely, this approach is robust, and has the potential to acquire a high (< 0.01 cm-1) resolution spectrum in only a few milliseconds. It offers precisely range-gated detection over propagation distance of potentially 100's of metres, and perhaps further using infrared time-correlated single-photon-counting detection (a technique which the project will have access to at Heriot-Watt).In comparison with DIAL, the technique offers greater spectral discrimination and the ability to simultaneously probe for multiple gases. Furthermore, it is all-solid-state, providing a potentially more compact and efficient solution than DIAL, and avoids the use of dye laser technology, eliminating the associated carccarcinogen hazard. It will extend sensing to new chemical species where suitable DIAL lines are unavailable, and is compatible with heterodyne / RF lock-in detection techniques for improved signal:noise performance.