Trace gas detection using near-infrared diode lasers

Visible/near-infrared diode lasers are well-suited for use as spectroscopic light sources in detection of a wide variety of gases by measurement of optical absorption. The high spectral resolution of these devices permits the selective detection of targeted species, while their characteristics of room temperature operation and compatibility with fiber optics make them attractive for instrument development. The nominal range of available wavelengths includes absorption bands of numerous gas species that need to be monitored to meet various industrial objectives. While the molecular absorption bands in this spectral region consist mostly of relatively weak overtone or combination bands (some electronic bands are accessible at the shorter wavelengths), detection sensitivities of about 1 ppm-meter or better can be achieved for many gases using frequency or wavelength modulation techniques that permit routine measurement of small optical absorbances. Subppm measurement sensitivities are easily achieved using simple optical multipass designs if necessary. Diode laser-based instruments for measurement of some of these species in process control or environmental monitoring applications are now commercially available. The paper describes recent progress in development of diode laser-based trace gas measurement by presenting two examples: measurement of trace moisture contamination in high purity process gases and perimeter monitoring of a toxic gas in a refinery environment.