On-line multicomponent trace-gas analysis with a broadly tunable pulsed difference-frequency laser spectrometer

The design and application of a novel automated room-temperature laser spectrometer are reported. The compact instrument is based on difference-frequency generation in bulk LiNbO3. The instrument employs a tunable cw external-cavity diode laser (795-825 nm) and a pulsed diode-pumped Nd:YAG laser (1064 nm). The generated mid-IR nanosecond pulses of 50-mu W peak power and 6.5-kHz repetition rate, continuously tunable from 3.16 to 3.67 mu m, are coupled into a 36-m multipass cell for spectroscopic studies. On-line measurements of methane are performed at concentrations between 200 ppb (parts in 10(9) by mole fraction) and approximate to 1%, demonstrating a large dynamic range of 7 orders of magnitude. Furthermore computer-controlled multicomponent analysis of a mixture containing five trace gases and water vapor with an overall response time of 90 s at an averaging time of only approximate to 30 s is reported. A minimum detectable absorption coefficient of 1.1 x 10(-7) cm-l has been achieved in an averaging time of 60 s, enabling detection limits in the ppb range for many important trace gases, such as CH4, C2H6, H2CO, NO2, N2O, HCl, HBr, CO, and OCS. (C) 1999 Optical Society of America.