Network Reference - Differential Absorption Lidar

Every atom, ion or molecule exhibits a unique characteristic when interacts with electromagnetic radiation. The basic methodology behind spectrometers such as differential absorption lidar (DIAL) involves the illumination of molecules, atoms or ions of a sample with coherent light. The electromagnetic radiation absorbed or emitted by them, stems from the changes in their rotational, vibrational and electronic energies, is then measured and interpreted. Based on these phenomena, a number of absorption spectroscopic instruments have been successfully developed in the past decade. The primary spectral range employed has been in the near-infrared region (~800nm to 2μm) because of the availability and simple operating behavior of high optical quality tunable diode lasers (TDLs). However, only a limited number of molecular species have absorption features in the near-infrared. Furthermore, many species have strong fundamental absorption bands (as well as some overtone) and strong combination rotational-vibrational bands in the mid-infrared regime of between 2μm and 5μm including virtually every hydrocarbon with their C-H stretch bands in the 2μm to 4μm. In fact, molecular absorption line strengths located in the mid-infrared region can be as much as four orders of magnitude greater than in the near-infrared region.

NAL Research Corporation is developing a portable tunable DIAL that allows the measurements of toxic chemicals in the atmosphere emitted during open-burn/open-detonation events. In contrast to nonlaser-based methods, NAL Research's tunable DIAL technique provides several significant advantages. These include the ability to measure in near real-time multiple atmospheric trace gases with an order of magnitude increase in wave-number resolution, single-site operation without the need of a retro-reflector, ability to easily map a large area and increased range due to the high-intensity, well-focused laser beam. The innovation of the NAL Research's DIAL is the implementation a compact solid-state laser and a quasi-phase matched (QPM) optical parametric oscillator (OPO) consisted of a periodically poled (PP) nonlinear crystal to produce radiation tunable over the spectral range from 3μm to 5μm. This broadly tunable mid-infrared laser source is well suited to probe a large number of toxic chemicals.