Multi-species sensing using multi-mode absorption spectroscopy with mid-infrared interband cascade lasers

Multi-mode absorption spectroscopy, MUMAS,
^1,2,3
is a technique that allows simultaneous multi-species gas sensing with compact, robust, simple and relatively inexpensive laser sources. It provides wider spectral coverage than is afforded by traditional TDLAS, (tunable diode laser absorption spectroscopy), is simpler and less expensive than fs-comb methods since it does not require complex laser systems nor high resolution dispersive optics or complex electronics required for heterodyning methods. A MUMAS signal is generated by measuring the temporal variation in total transmitted intensity as the modes are scanned in frequency over the inter-mode frequency interval, Δν
_mode
. A change in transmitted intensity, relative to the incident intensity, is recorded whenever any of the modes, during their scan across Δν
_mode
, comes into resonance with an absorption line in the spectrum of the gas. The signal, therefore, consists of a superposition of single mode scans, one for each mode lying within the spectral range of the laser output, Δν
_band
. The resulting signal is characteristic of the absorbing species probed by the particular multi-mode laser used. The signal can also be modelled if the spectral locations of the absorption lines are available from a suitable database such as HITRAN,
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and the laser mode parameters are also known. By fitting the modelled MUMAS signature to the experimental data, using the gas concentrations as fit parameters, the absolute and relative concentrations can be determined.