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

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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.