High resolution ultra-sensitive trace gas detection by use of cavity-position-modulated sub-Doppler NICE-OHMS - Application to detection of acetylene in human breath
A sensitive high resolution sub-Doppler detecting spectrometer, based on noise-immune cavity-enhanced optical heterodyne molecular spectrometry (NICE-OHMS), for trace gas detection of species whose transitions have severe spectral overlap with abundant concomitant species is presented. It is designed around a NICE-OHMS instrumentation utilizing balanced detection that provides shot-noise limited Doppler-broadened detection. By synchronous dithering the positions of the two cavity mirrors, the effect of residual etalons between the cavity and other surface in the system could be reduced. An Allan deviation of the absorption coefficient of coefficient of $2.2 \times 10^{-13}$ $\text{cm}^{-1}$ at 60 s, which, for the targeted transition in $\text{C}_{2}\text{H}_{2}$, corresponds to a $3σ$ detection sensitivity of 130 ppt, is demonstrated. It is shown that despite significant spectral interference from $\text{CO}_{2}$ at the targeted transition, which precludes Db detection of $\text{C}_{2}\text{H}_{2}$, acetylene could be detected in exhaled breath of healthy smokers.