Explosive vapor detection via mid-infrared laser spectroscopy

Accurate explosives detection would be an effective defense / deterrent against bomb attacks. Chemical vapor sensing has the potential to be both specific and sensitive. The work described here involves the use of mid-infrared laser spectroscopy to detect the presence of nitro-group based explosive vapors. Two different singly-resonant Silver Gallium Selenide OPOs were examined for use as the spectroscopy source: one was pumped with a 1.319 mu m Nd:YAG laser, and the other was pumped by a 1.57 mu m KTP/Nd:YAG OPO-laser. Both OPOs can be angle-tuned over wavebands of interest, producing coherent mid-infrared idler (non-resonated) output power at 5-8 mu m, as well as near-infrared outputs. Current results indicate that output energies of similar to 300 mu J in the mid-infrared can be generated from 10 mJ of pump energy. The OPO outputs can be used to detect chemical vapors using a sample-reference (differential) absorbance experiment. Both OPO outputs are passed through a cell containing the vapor to be measured. The mid-infrared output is attenuated by the strong absorption of the vapors, while the near-infrared output, being unabsorbed, serves as a reference. The ratio measurement has a shot-noise limited accuracy better than one part per million absorbance.