Investigation of standoff explosives detection via photothermal/photoacoustic interferometry
2011
Progress in standoff detection of surface-bound explosives residue using photothermal and photoacoustic (PT/PA)
imaging and spectroscopy has been reported recently. Photothermal/photoacoustic interferometry (PTI), a variation of
the aforementioned techniques, is a candidate for standoff detection as a result of its non-contact and non-destructive
approach. In PTI, the transient PT/PA hydrodynamic response produced by impulsive infra-red laser excitation(s) are
detected by an overlapping focused probe laser beam. The return back-scattered/reflected probe laser beam is collected
and coupled into a single-mode optical fiber. The PT/PA-induced perturbation on the return probe laser, in the form of
phase or amplitude modulation or both, is extracted interferometrically. The resulting quadrature signals are digitized
and processed to recover the minute PT/PA dynamics above background noise. Characteristic spectra for materials can
be obtained by quantifying the PT response as a function of excitation(s) wavelength. The CW probe laser, operating in
the 1550 nm range, and the constituents of the coherent detection system are commercial off-the-shelf components. A
commercially available and continuously tunable quantum cascade laser (QCL) with output pulse energies up to 50 nJ
was employed to generate the PT/PA spectra in the 8.8-10.2 μm range. PTI detected absorption spectra were collected
for HMX, RDX, and PETN, with the probe laser system positioned 5 meters away from the explosives targets. In
addition, PTI measurements of the stimulated Raman (SR) spectra of ammonium nitrate and 2,4,6-trinitrotoluene
obtained using a near-IR OPO laser are described. We believe this is the first-ever application of photothermal
techniques to the measurement of the SR effect on solid explosive materials at meaningful standoff distances.
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