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Laser Photoacoustic Spectroscopy (L-PAS)

The U.S. military has a pressing requirement for compact, rapid, reliable, and cost-effective chemical agent point sensors. This need is made clear by the growing specter of the potential use of weapons of mass destruction by either global terrorist organizations, or unfriendly nations. This is the motivation behind DARPA's Laser Photoacoustic Spectroscopy (L-PAS) program. L-PAS seeks to develop revolutionary technologies for the early detection and accurate identification of a wide variety of lethal chemical agents and explosives or combinations thereof.

L-PAS is a well known technique for performing very sensitive detection of trace chemicals by directly measuring the amount of light absorbed by the sample.

L-PAS is a well known technique for performing very sensitive detection of trace chemicals by directly measuring the amount of light absorbed by the sample. The technique involves collecting a sample of ambient air into a cell, where it is then probed by a laser beam which has a small-signal modulating frequency superimposed on the optical carrier. If the laser wavelength is tuned to an absorption feature of the species present in the cell, the photon energy will couple to the vibrational and rotational modes of the molecule and result in localized heating. This in turn causes an acoustic wave to be generated at the frequency of the superimposed signal on the laser. The magnitude of the acoustic wave is proportional to the incident laser power, the total number of molecules within the optical path, and the absorption cross-section of the sample species. The sound waves are in turn detected by sensitive microphones, where the acoustic signal is converted back into a voltage for subsequent amplification and signal processing.

The program will be conducted in three phases. Phase I will be a baseline effort of 12 months, and will focus on the development of tunable infrared (IR) laser sources. Phases II and III will be conducted as options on the baseline contract. Phase II will be 12 months and conclude with the demonstration of a continuously tunable laser sources in the appropriate portions of the IR. Phase III will be 24 months and will focus on the construction, testing, and demonstration of the L-PAS chemical sensor.