Rejection of fluorescence from Raman spectra of explosives by picosecond optical Kerr gating

This paper describes how optical Kerr gating can be used for effective rejection of fluorescence from Raman spectra of explosives and explosives precursors. Several explosives are highly fluorescent, and this method enables Raman detection of explosives materials that would else be complicated or impossible to identify. Where electronic cameras (intensified charge-coupled devices, ICCDs) have showed not yet to be sufficiently fast to be used for rejection of this fluorescence, Kerr gating is here proved to be an efficient alternative, demonstrated by measurements on plastic explosives. Results were obtained using a gating time of ~30 ps. The Kerr gate was driven by the fundamental mode of an Nd:YAG laser, at 1064 nm, with pulses of ~8 mJ, 50 Hz and 30 ps. CS2 was used as a Kerr medium and Glan polarizing prisms were important features of the system. Raman spectra were obtained using a 532 nm probe wavelength, from the same Nd:YAG laser being frequency doubled, with a ~2 mJ pulse energy. Gating times of ~30 ps were thus achieved, with a fluorescence rejection factor of more than 1300, for the first time revealing detailed characteristics in Raman spectra from highly fluorescent PETN based plastic explosive.