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Title: Multi-mode absorption spectroscopy for multi-species and multi-parameter sensing
Author: O'Hagan, Seamus
ISNI:       0000 0004 6499 6193
Awarding Body: University of Oxford
Current Institution: University of Oxford
Date of Award: 2017
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The extension of Multi-mode Absorption Spectroscopy (MUMAS) to the infra-red spectral region for multi-species gas sensing is reported. A computationally efficient, theoretical model for analysis of MUMAS spectra is presented that avoids approximations used in previous work and treats arbitrary and time-dependent spectral intensity envelopes, thus facilitating the use of commercially available Interband Cascade Lasers (ICLs) and Quantum Cascade Lasers (QCLs). The first use of an ICL for MUMAS is reported using a multi-mode device operating at 3.7 μm to detect CH4 transitions over a range of 30 nm. Mode-linewidths are measured using the pressure-dependent widths of an isolated absorption feature in HCl. Multi- species sensing is demonstrated by measurement of partial pressures of CH4, C2H2 and H2CO in a low-pressure mixture with uncertainties of around 10%. Detection of CH4 in N2 at 1 bar is demonstrated using a shorter-cavity ICL to resolve spectral features in pressure-broadened and congested spectra. The first use of a QCL for MUMAS is reported using a commercially available device operating at 5.3 μm to detect multiple absorption transitions of NO at a partial pressure of 2.79 μbar in N2 buffer gas. The revised model is shown to enable good fits to MUMAS data by accounting for the time-variation of the spectral intensity profile during frequency scanning. Individual mode-linewidths are derived from fits to pressure- dependent MUMAS spectra and features from background interferences due to H2O in laboratory air are distinguished from those of the target species, NO. Data obtained at scan rates up to 10 kHz demonstrate the potential for achieving short measurement times. The development of a balanced ratiometric detection scheme for MUMAS with commercially available multi-mode lasers operating at 1.5 μm is reported for simultaneous detection of CO and CO2 showing improved SNR performance over previous direct transmission methods and suitability for a compact field-employable instrument. In addition, MUMAS spectra of CO2 are used to derive gas temperatures with an uncertainty of 3.2% in the range 300 - 700 K.
Supervisor: Ewart, Paul Sponsor: Engineering and Physical Sciences Research Council
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: Physics ; Spectroscopy ; Lasers ; MUMAS ; Laser noise canceller ; Interband cascade lasers ; Quantum cascade lasers