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>

A calibration-free first harmonic (1<inline-formula><tex-math notation="LaTeX">$f$</tex-math></inline-formula>) wavelength modulation spectroscopy (1<inline-formula><tex-math notation="LaTeX">$f$</tex-math></inline-formula>-WMS) technique for gas species parameter measurement is demonstrated. In this technique, the total magnitude of the 1<inline-formula><tex-math notation="LaTeX">$f$</tex-math></inline-formula>-WMS signal is normalized by a component of the 1<inline-formula><tex-math notation="LaTeX">$f$</tex-math></inline-formula> residual amplitude modulation signal. This method preserves the advantages of the traditional <inline-formula><tex-math notation="LaTeX">$nf\!/\!1f$</tex-math></inline-formula>-WMS (<inline-formula><tex-math notation="LaTeX">$n\geq 2$</tex-math></inline-formula>) 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 (<italic>m</italic>-values). The proposed technique only requires the 1<inline-formula><tex-math notation="LaTeX">$f$</tex-math></inline-formula> signal, which has the largest magnitude of all the harmonics signals, and, therefore, fundamentally has a higher sensitivity to the <inline-formula><tex-math notation="LaTeX">$nf\!/\!1f$</tex-math></inline-formula> technique. Furthermore, since only the 1<inline-formula><tex-math notation="LaTeX">$f$</tex-math></inline-formula>-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<inline-formula><tex-math notation="LaTeX">$f$</tex-math></inline-formula>/1<inline-formula><tex-math notation="LaTeX">$f$</tex-math></inline-formula> for measuring CO<inline-formula><tex-math notation="LaTeX">$_2$</tex-math></inline-formula> 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 <inline-formula><tex-math notation="LaTeX">$2f\!/\!1f$</tex-math></inline-formula> spectra, causing variation in the recovered gas concentrations. This effect is not seen in the methodology proposed in this paper.