Standoff and Miniature Chemical Vapor Detectors Based on Tunable Diode Laser Absorption Spectroscopy

Trace gas sensing and analysis by tunable diode laser absorption spectroscopy (TDLAS) has become a robust and reliable technology accepted for industrial process monitoring and control, quality assurance, environmental sensing, plant safety, and infrastructure security. Sensors incorporating well-packaged wavelength-stabilized near-IR (1.2-2.0 ¿m) laser sources sense over a dozen toxic or industrially-important gases. Recently developed mid-IR lasers, particularly quantum cascade devices spanning wavelengths of 3-12 ¿m, can sense in real-time sub-parts per million concentrations of many hydrocarbons.A large emerging application for TDLAS is standoff sensing of chemical vapors, e.g., leaks from natural gas pipelines. Employing a 10-mW DFB laser, the eye-safe, battery-powered, 6-lb handheld remote methane leak detector illuminates a noncooperative topographic surface and analyzes returned scattered light to deduce the presence of excess methane. For aerial surveying, replacing the handheld transceiver with a large-aperture telescope and adding an erbium-doped fiber amplifier to the laser transmitter extends the standoff distance to 3000 m. By selecting a laser source having an appropriate wavelength, the standoff TDLAS tool detects trace concentrations of nonmethane hazardous gases, including several high-priority toxic industrial compounds and emissions from illicit chemical production laboratories. This paper also describes concepts for miniature integrated optic TDLAS sensors that combine a laser source, sampling section, and detector on a monolithic semiconductor materials system substrate. Such chip-scale low-power integrated optic gas-phase chemical sensors may enable low-cost mass production, so that many hundreds or thousands of such sensors can be distributed cost-effectively over a wide area of interest and communicate via wireless networks.