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23 May 2011 Picosecond laser pulses improves sensitivity in standoff explosive detection
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Portendo has in collaboration with FOI, the Swedish Defence Research Agency, developed a world-leading technique of trace detection of explosives at standoff distance using Raman spectroscopy. The technology is further developed in order to enhance the sensitivity of the method and to be able to extend the field of applications. Raman scattering is a well-established technique able to detect substances down to single micrograms at standoff distances, however, one of the obstacles limiting the detection possibilities is interfering fluorescence, originating either from the substance itself or from the surrounding material. One main challenge for this technology is thus to either omit the excitation of the fluorescent process altogether or to be able to separate the two processes and only detect the Raman signal. Due to the difference in the temporal behavior of the two processes - Raman scattering occurs in the order of femtoseconds while fluorescence typically has a lifetime in the order of nanoseconds - one way to theoretically separate them is to limit the measurement to as short time as possible, cutting off most of the emitted fluorescence. The improvement depends on how much of the fluorescence can be omitted without decreasing the Raman signal. Experimentally, we have verified this improvement in signal to noise ratio when using a laser with picosecond pulses instead of nanosecond pulses, which has resulted in an improvement in SNR of up to 7 times for bulk ANFO. These results verify the predicted signal enhancement and suggest higher sensitivity for standoff detection in future systems.
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Madeleine Åkeson, Markus Nordberg, Anneli Ehlerding, Lars-Erik Nilsson, Henric Östmark, and Pierre Strömbeck "Picosecond laser pulses improves sensitivity in standoff explosive detection", Proc. SPIE 8017, Detection and Sensing of Mines, Explosive Objects, and Obscured Targets XVI, 80171C (23 May 2011);

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