IEEE Photonics Journal (Jan 2018)

A New RAM Normalized 1<inline-formula><tex-math notation="LaTeX">$f$</tex-math></inline-formula>-WMS Technique for the Measurement of Gas Parameters in Harsh Environments and a Comparison With <inline-formula><tex-math notation="LaTeX">${2f\!/\!1f}$</tex-math></inline-formula>

  • Abhishek Upadhyay,
  • Michael Lengden,
  • David Wilson,
  • Gordon Samuel Humphries,
  • Andrew P. Crayford,
  • Daniel G. Pugh,
  • Mark P. Johnson,
  • George Stewart,
  • Walter Johnstone

DOI
https://doi.org/10.1109/jphot.2018.2883548
Journal volume & issue
Vol. 10, no. 6
pp. 1 – 11

Abstract

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A calibration-free first harmonic (1$f$) wavelength modulation spectroscopy (1$f$-WMS) technique for gas species parameter measurement is demonstrated. In this technique, the total magnitude of the 1$f$-WMS signal is normalized by a component of the 1$f$ residual amplitude modulation signal. This method preserves the advantages of the traditional $nf\!/\!1f$-WMS ($n\geq 2$) technique, such as the immunity to the non-absorbing systematic losses and the accurate recovery of gas parameters, without the requirement for non-absorbing regions for normalization at high pressure or high modulation index values (m-values). The proposed technique only requires the 1$f$ signal, which has the largest magnitude of all the harmonics signals, and, therefore, fundamentally has a higher sensitivity to the $nf\!/\!1f$ technique. Furthermore, since only the 1$f$-WMS signal is used, the technique is less complex in terms of signal processing and data acquisition. This paper also shows a comparison of the proposed technique and 2$f$/1$f$ for measuring CO$_2$ in the exhaust of a combuster. The data highlight how nonlinearities in the optical detection system as a function of frequency have a considerable effect on the recovered $2f\!/\!1f$ spectra, causing variation in the recovered gas concentrations. This effect is not seen in the methodology proposed in this paper.

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