Laser Sources and Sensitivity Calculations for a Near-Infrared Dial System

The applications of DIAL systems operating in the near ultraviolet and visible spectral regions are limited to the small number of species that absorb in those regions. However, the infrared spectral region offers the possibility of detecting a much larger number of gases, including most of the hydrocarbons. Although many DIAL systems have been constructed for use in the mid-infrared (at 10 μm in particular), relatively few attempts1-4 have been made to use the near-infrared despite the large number of strong absorption lines, particularly around 3 μm. The design of a direct-detection DIAL system to operate in this region involves a considerable number of differences from DIAL systems operating in other spectral regions. We consider here: the spectroscopy of target gases, possible laser sources, and the sensitivity and accuracy that might be expected from such a system, with particular consideration to its use for measuring hydrocarbon concentrations at the ppm level in the atmosphere.