A dual CO(2) laser-based differential absorption lidar (DIAL) system has been constructed and demonstrated for range-resolved mapping of chemical vapor plumes. The system acquires high range resolution through the use of plasma-shutter pulse clippers that extinguish the nitrogen tail of the CO(2)-laser output. Aprogrammable servomotor-driven scanner allows full hemispherical coverage of the interrogated field. A high-speed direct-detection receiver subsystem is used to gather, process, and display vapor-concentration data in near real time. Data demonstrating range-resolved detection of low concentrations of chemical plumes from ranges of 1 to 2 km are presented. In the column-content detection mode, trace levels of secondary vapors from various organophosphate liquids were monitored. Detection of an SF(6) vapor plume released 16 km from the DIAL system is also adduced.
An open path diode laser sensor was constructed with near-infrared diode lasers and two-tone frequency-modulation spectroscopy. The sensor incorporates several novel features (such as digital signal-processing algorithms, a computerized line-locking routine, and discontinuous wavelength scanning) that are important in a field instrument. The sensor was used to monitor oxygen, water, and carbon dioxide in the near-infrared spectral range. For oxygen, an absorbance detection sensitivity of 2 × 10(-6) in a 10-Hz bandwidth was demonstrated with a GaALAs laser at 760.56 nm. The stability of the sensor was 0.1% over a period of 10 h when an absorbance of 6 × 10(-3) was monitored.
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