High-sensitivity detection of trace gases using sweep integration and tunable diode lasers

A tunable diode laser spectrometer capable of high-sensitivity detection of trace gases over a wide range of pressures is described. Infrared absorptions of 10(-3)% over path lengths of up to 200 m have been observed in low-pressure gases. In preliminary measurements at atmospheric pressure we have detected differential absorptions of 10(-3)% over path lengths of a few meters. Data are acquired by rapidly sweeping the laser wavelength over ~0.4 cm(-1) and summing the results of many sweeps using a signal averager. To increase sensitivity and allow efficient use of the digital signal averager, an analog circuit is used to null the intensity modulations which arise from wavelength-modulating the diode laser. This sweep integration technique allows direct determination of line shapes and linewidths from the data and improves the repeatability of the recorded spectra.