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There are currently lots of research activities concerning explosive hazards detection, and stand-off detection of explosives is in main focus. The reason for this interest is the occurrence of terrorist attacks on the civilian society involving Improvised Explosives Devices (IED). Laser-based spectroscopies are the only currently viable techniques that can be utilized to detect trace amounts of explosives at stand-off distances. In particular, Raman Spectroscopy (RS) has been shown to be effective for stand-off detection and has the ability to both detect and identify explosive materials. Raman spectroscopy is virtually instantaneous, non-destructive in nature and provides high selectivity. The traditional Raman spectrometer utilizes continuous lasers and CCDs to detection the scattering signal, which greatly limits the application of Raman spectroscopy in the stand-off detection of explosive hazards due to the weak signal, strong background fluorescence, ambient light interference, and long analysis time. Time-gated Raman spectroscopies are based on ultrafast pulsed lasers and time-resolved single-photon detection techniques. Through the time-gated method, the Raman signal intensity can be greatly improved, and the influence of fluorescence and environmental light can be effectively suppressed. In this work, the time-gated Raman system utilizing frequency-doubled Nd:YAG lasers at 532 nm excitation was developed. The Cassegrain telescope was coupled to the Raman spectrometer using a fiber optics cable, and notch filter was used to reject Rayleigh scattering light. The Raman scattered light was collected by a telescope and then transferred via fiber optic to spectrometer and finally directed into Intensified CMOS (ICMOS) detector. The applications of time-gated Raman spectroscopy in stand-off detection of hazardous explosives have been performed. The Raman spectra of DNT, TNT, RDX and NaNO3 at a stand-off distance of 50 m have been identified with a detection limit of 1 mg/cm2.
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