Eye-safe UV stand-off Raman spectroscopy for the ranged detection of explosives in the field

The number of worldwide terrorist attacks has increased annually since 2004, with the most recent collections of data showing that a record number of terrorist attacks occurred in 2012 with a corresponding increase in fatalities and injuries. Raman spectroscopy is one technique that has been proven to be highly effective for the ranged detection of explosives and other dangerous materials. To accurately discriminate the substance over large distances, a high‐signal level is necessary, and typically, this requires the use of lasers with high‐output powers or pulse energies. Unfortunately, in the many practical Raman laser systems used for ranged detection of substances, the laser irradiance or radiant exposure used is often many orders of magnitude greater than what is safe for the eye or the skin in order to increase the detection range to practical distances. As a result, this risk of injury to unprotected personnel in the vicinity of the laser limits the use of the stand‐off Raman detection technique in the field.

In this project, we considered the wavelength dependence of the MPEs of both the eye and the skin to laser radiation, to identify spectral regions where both organs are more resilient to injury. We experimentally compared the use of 532‐nm and 266‐nm nanosecond pulsed laser radiation for stand‐off Raman signal detection of Teflon and determine, for the equipment used, what the maximum operational range is for each wavelength while maintaining emission to below the relevant MPE. It is shown that safe ultraviolet pulse energies can be more than three orders of magnitude greater than equivalent safe visible pulse energies. Coupling this to the 16‐fold increase in Raman signal obtained in the ultraviolet at 266 nm over that at 532 nm results in a 131 times larger detection range for the eye‐safe 266‐nm system over an equivalent eye‐safe 532‐nm laser system. For the Raman system described here, this translates to a maximum range of 42 m for detecting Teflon with a 266‐nm laser emitting a 100‐mm diameter beam of 23.5‐mJ nanosecond pulses.