Spectroscopic measurement of atmospheric trace gases using time-tagged photon detection

Optical spectroscopy has a rich heritage in the measurement of atmospheric gases and changes to atmospheric composition. Famous early examples include the initial observation of polar stratospheric ozone depletion by measurement of the attenuation of sunlight at characteristic wavelengths due to absorption by the overlying ozone column. Nowadays, global measurements of ozone (and other species) are updated daily from satellite observations using essentially the same spectroscopic principles. There are main two reasons why spectroscopic methods at visible, UV or IR wavelengths are such valuable tools for atmospheric applications. (i) selectivity - within the very complex and sometimes rapidly changing mixture that comprises our atmosphere, it is often possible to target one compound by choosing a particular wavelength that is selectively absorbed by that compound. Alternatively, observations are performed over a sufficiently broad range of wavelengths that distinct features in the compound's absorption spectrum unambiguously identify its presence in the atmospheric sample. (ii) sensitivity - the chemically most important trace gases are typically present at mixing ratios in the range 10-9 to 10-12, thus requiring extremely sensitive detection methods. The observation of an emission signal following excitation of the target compound is a particularly sensitive approach, but can only be applied to a small number of compounds that fluoresce. Absorption methods are much more widely applicable and can often be used to directly quantify the concentration of an absorber without complex calibration procedures. But the light generally needs to travel a long distance through the sample for highly dilute species to be detectable. One way to do this is to fold the light many times inside an optical cavity formed by specialist high reflectivity mirrors. These cavity-based instruments are particularly useful for field observations because they provide in situ gas measurements at a well defined location, an important constraint for very reactive species that are too short-lived to be evenly mixed though the atmosphere. This proposal's aim is to build a new, highly sensitive broadband absorption spectrometer by combining the sensitivity & selectivity advantages of an existing field-tested broadband cavity instrument with innovative detector technology arising from space research. Time-tagged photon imaging offers a unique capability for a multi-wavelength phase-shift version of a technique called 'broadband cavity enhanced absorption spectroscopy' (BBCEAS). The detector tags each detected photon with a three-dimensional x,y,t coordinate which is used to identify whether the photon has passed through the cavity or is a reference signal from the light source (x co-ordinate), and the wavelength of each photon along the other (y) dispersion axis. The time coordinate, t, identifies the photon arrival time. Phase shift and attenuation as a function of wavelength, for both cavity output and the light source, are determined by time-histogramming the imaged spectra. This provides access to a key quantity (the number of times the lights passes back & forth within the cavity) that cannot be measured directly by conventional BBCEAS instruments which consequently require a separate calibration. Our combination of technologies offers an instrument able to be calibrated by a simple procedure not requiring technical input or calibration gases, and which therefore could be automated. Proof of this concept offers the realistic possibility of developing a ubiquitous instrument, with the high performance characteristic of spectroscopic methods, and capable of autonomous operation for remote monitoring of atmospheric pollutants, or even diverse applications such as breath monitoring in healthcare scenarios such as medical research or clinical diagnostics.