Diode Lasers for Industrial Spectroscopy

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 relatively low cost, room temperature operation, and compatibility with fiber optics make them attractive for instrument development. The nominal range of available wavelengths (presently ~0.63 −~2.0 μm for commercially available devices that operate at room temperature) includes absorption bands of numerous gas species that need to be monitored to meet various industrial objectives (e.g. process control, emissions monitoring, toxic gas detection, etc.) 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.1-2 Sub-ppm measurement sensitivities are easily achieved using simple optical multipass designs if necessary. A partial list of industrially significant gases that may be measured by this approach includes oxygen, water vapor, methane, acetylene, carbon monoxide, carbon dioxide, hydrogen halides, ammonia, hydrogen sulfide, and nitrogen oxides (both NO and NO2). Diode laser- based instruments for measurement of some of these species in process control or environmental monitoring applications are now commercially available.