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 LiNbO<sub>3</sub>. 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-μW peak power and 6.5-kHz repetition rate, continuously tunable from 3.16 to 3.67 μm, 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<sup>9</sup> by mole fraction) and ≈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 ≈30 s is reported. A minimum detectable absorption coefficient of 1.1 × 10<sup>−7</sup> cm<sup>−1</sup> has been achieved in an averaging time of 60 s, enabling detection limits in the ppb range for many important trace gases, such as CH<sub>4</sub>, C<sub>2</sub>H<sub>6</sub>, H<sub>2</sub>CO, NO<sub>2</sub>, N<sub>2</sub>O, HCl, HBr, CO, and OCS.
© 1999 Optical Society of America
(010.1120) Atmospheric and oceanic optics : Air pollution monitoring
(120.6200) Instrumentation, measurement, and metrology : Spectrometers and spectroscopic instrumentation
(190.2620) Nonlinear optics : Harmonic generation and mixing
(280.3420) Remote sensing and sensors : Laser sensors
(300.6340) Spectroscopy : Spectroscopy, infrared
Michael Seiter and Markus W. Sigrist, "On-line Multicomponent Trace-Gas Analysis With a Broadly Tunable Pulsed Difference-Frequency Laser Spectrometer," Appl. Opt. 38, 4691-4698 (1999)