Multimode absorption spectroscopy of CO and CO2 gas mixtures

Thesis

The development of multimode absorption spectroscopy (MUMAS) for multi-species detec- tion and its potential for process control or environmental monitoring is reported. The simultaneous detection of CO and CO2 is demonstrated in a proof-of-principle experiment for applications in industrially relevant gas species monitoring. The technique of MUMAS is extended to the near infrared in order to detect these and other industrially relevant species.

A laser was designed and constructed to emit a multimode spectrum in the region of 1.57um to take advantage of the spectral overlap of the second vibrational overtone of CO and the combination band 3ν1 + ν3 of CO2. The laser consisted of a semi-confocal cavity employing an Er:Yb glass chip as the gain medium. The laser was pumped by a 1W laser diode at 980nm and emitted up to 30mW in a bandwidth of 180GHz. The laser emitted between 6-10 modes depending upon the selective cavity length. Mode spacings varied between 18GHz to 33GHz with an individual mode linewidth of less than 8MHz. The laser modes were simultaneously scanned using a piezo-electric transducer (PZT) in order to modulate the cavity length at frequencies between 1Hz and 10Hz.

A system for linearizing the MUMAS spectra with respect to frequency was devised based on a transmission spectra of a confocal Fabry-Perot etalon. Refinements to the MUMAS fitting code were developed to improve the computational efficiency. An initial demonstration of MUMAS on a known gas mixture of CO and CO2 was per- formed. The ratio of CO:CO2 concentrations in the gas mixture was measured with an accuracy of 0.4% which was within the supplier’s quoted uncertainty.

MUMAS is then applied to the detection of CO and CO2 concentrations in exhaust gas produced by a 1.3 litre 4-cylinder turbo-charged spark ignition engine. Relative and absolute concentrations were derived from MUMAS signals and values compared to measurements using a 4-gas analyser. Concentrations of CO and CO2 were measured using MUMAS to a precision of 0.17% and 0.23% respectively compared to less than 0.1% for the 4-gas analyser. Ratios of CO and CO2 were determined with a precision of 0.28 using MUMAS compared to 0.11 with the 4-gas analyser. The detection limit of CO was found to be 1486ppm in these circumstances.

Finally a discussion is presented of potential improvements arising from wavelength mod- ulation spectroscopy and cavity enhancement techniques.

Authors: Thompson, A; Alexander William John Thompson

• Publication date: 2013
• DOI: 10.5287/ora-nby68xr4v
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: Oxford University, UK
• Language: English
• Keywords: gas sensing; spectroscopy; exhaust gas
• Subjects: Atomic and laser physics