Abstract

The dependence of Doppler broadened noise-immune cavity-enhanced optical heterodyne molecular spectrometry (NICE-OHMS) on the modulation index, $\beta$, has been investigated experimentally on ${\mathrm{C}}_2{\mathrm{H}}_2$ and ${\mathrm{CO}}_2$, both in the absence and the presence of optical saturation. It is shown that the maximum signals are obtained for $\beta$ that produce more than one pair of sidebands: in the Doppler limit and for the prevailing conditions (unsaturated transition and the pertinent modulation frequency and Doppler widths) around 1 and 1.4 for the dispersion and absorption detection phases, respectively. The results verify predictions given in an accompanying work. It is also shown that there is no substantial broadening of the NICE-OHMS signal for $\beta <1$. The use of $\beta$ of unity has yielded a Db-NICE-OHMS detection sensitivity of $4.9\times {10}^{-12}{\mathrm{cm}}^{-1}{\mathrm{Hz}}^{-1/2}$, which is the lowest (best) value so far achieved for NICE-OHMS based on a tunable laser. The number of sidebands that needs to be included in fits of the line-shape function to obtain good accuracy has been assessed. It is concluded that it is enough to consider three pairs of sidebands whenever the systematic errors in a concentration assessment should be below 1% when $\beta <2$ are used and $<1‰$ for $\beta <1.5$.

© 2014 Optical Society of America

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