High-resolution trace gas detection by sub-Doppler noise-immune cavity-enhanced optical heterodyne molecular spectrometry: application to detection of acetylene in human breath.

Gang Zhao,Florian M Schmidt,Weiguang Ma,Ove Axner,Thomas Hausmaninger

doi:10.1364/oe.27.017940

Abstract

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 (Db) detection. By synchronous dithering the positions of the two cavity mirrors, the effect of residual etalons between the cavity and other surfaces in the system could be reduced. An Allan deviation of the absorption coefficient of 2.2 × 10-13 cm-1 at 60 s, which, for the targeted transition in C2H2, corresponds to a 3σ detection sensitivity of 130 ppt, is demonstrated. It is shown that despite significant spectral interference from CO2 at the targeted transition, which precludes Db detection of C2H2, acetylene could be detected in exhaled breath of healthy smokers.

Highlights

Laser absorption spectroscopy (LAS) is a powerful technique for trace gas detection that is often characterized by high sensitivity, selectivity and accuracy [1]
The high intracavity power in resonant cavities, paired with the extraordinary long interaction lengths, open up for sD detection for weak molecular transitions in the NIR range. Despite this potential, it has been found that the ability of the aforementioned cavity enhanced detection techniques to utilize sD spectroscopy for trace gas detection is often restricted because of technical reasons; cavity enhanced absorption spectroscopy (CEAS) is limited by the frequency-to-amplitude noise conversion [23], cavity ring down spectroscopy (CRDS) is, in its simplest realizations, restricted by the frequency stability of the system and the control of the intracavity buildup power affecting the degree of saturation, while off-axis integrated cavity output spectroscopy (OA-ICOS) is hampered by its reduced output intensity [24]
The output of the laser is passed through a fiber-coupled acoustic optic modulator, a fiber-coupled polarizer (f-POL), and a fiber-coupled electro-optic modulator, the latter with a proton exchanged waveguide to minimize the generation of RAM [66] before it is emitted into free space by the use of a fiber-coupled collimator (f-C)

Summary

Introduction

Laser absorption spectroscopy (LAS) is a powerful technique for trace gas detection that is often characterized by high sensitivity, selectivity and accuracy [1]. The high intracavity power in resonant cavities, paired with the extraordinary long interaction lengths, open up for sD detection for weak molecular transitions in the NIR range (including those that possess small dipole moments) Despite this potential, it has been found that the ability of the aforementioned cavity enhanced detection techniques to utilize sD spectroscopy for trace gas detection is often restricted because of technical reasons; CEAS is limited by the frequency-to-amplitude noise conversion [23], CRDS is, in its simplest realizations, restricted by the frequency stability of the system and the control of the intracavity buildup power affecting the degree of saturation, while OA-ICOS is hampered by its reduced output intensity [24]. An averaging of the signal could efficiently be used to reduce the effect of the remaining etalons in the background signal

System evaluation – reduction of influence of background signals

Findings

Summary and conclusions