Shot-noise-limited dual-beam detector for atmospheric trace-gas monitoringwith near-infrared diode lasers

A dual-beam detector is used to measure atmospheric trace species by differ ential absorption spectroscopy with commercial near-infrared InGaAs laser d iodes. It is implemented on the Spectrometre a Diodes Laser Accordables, a balloonborne tunable diode laser spectrometer devoted to the in situ monito ring of CH4 and H2O. The dual-beam detector is made of simple analogical su btractor circuits combined with InGaAs photodiodes. The detection strategy consists in taking the balanced analogical difference between the reference and the sample signals detected at the input and the output of an open opt ical multipass cell to apply the full dynamic range of the measurements (16 digits) to the weak molecular absorption information. The obtained sensiti vity approaches the shot-noise limit. With a 56-m optical cell, the detecti on limit obtained when the spectra is recorded within 8 ms is similar to 10 (-4) (expressed in absorbance units). The design and performances of both a simple substractor and an upgraded feedback substractor circuit are discus sed with regard to atmospheric in situ CH, absorption spectra measured in t he 1.653-mu m region. Mixing ratios are obtained from the absorption spectr a by application of a nonlinear least-squares fit to the full molecular lin e shape in conjunction with in situ P and T measurements. (C) 2000 Optical Society of America. OCIS codes: 300.6340, 300.6380, 300.6260, 280.3420, 010 .1280, 300.1030.