PPLN-OPO-based backscatter absorption gas imaging (BAGI) system and its application to the visualization of fugitive gas emissions

摘要

Abstract: We report on a laser active imager suitable for thevisualization of natural gas leaks and volatile organiccompounds emitted by oil refineries. The describedbackscatter-absorption gas-imaging (BAGI) systememploys a raster scanner in conjunction with a tunablecontinuous wave (cw) laser source. The imager createsreal-time video imagery of a scene, while illuminatingit with infrared laser light at a wavelength that isabsorbed by the gas to be detected. Thus, gas plumesthat otherwise cannot be seen by the human eye appearin BAGI images as dark clouds. In order to produce thehigh intensity infrared light that is needed to imagenatural gas and refinery by-products, we used anonlinear frequency-conversion technique that employsthe quasi-phase-matched crystal periodically poledLiNbO$- 3$/. The crystal serves as the active medium ina cw optical parametric oscillator (OPO) that is pumpedby a diode-pumped Nd:YAG laser. The output frequencieswere selected to coincide with absorption features ofgeneral aliphatic species (2935 and 2968 cm$+$MIN@1$/),aromatics, such as benzene and toluene (3033cm$+$MIN@1$/), and methane (3018 cm$+$MIN@1$/). Thecrystal was engineered to cover the desired spectralrange using a fan-out design. This allows tuning of theOPO between 2832 and 3145 cm$+$MIN@1$/ in idlerwavelength by simply translating the crystal at a fixedtemperature. Presented data demonstrate the performanceof this system for imaging species of interest atrelevant concentrations and ranges up to about 30 m.!7