Conventional and standoff pulsed laser-Raman-echelle-time-gated (PRET) system

Time-gated Raman system has been assembled using a pulsed laser and high-resolution echelle spectrograph. Echelle spectrograph provides broad spectral band-pass of 0–8500 cm�1 in a single scan with a resolution of 1.75 cm�1 for 532 nm at an extremely small slit width of 10 μm. Advantages of gated mode detection over continuous-wave mode detection are improved signal-to-noise ratio, reduced background signal, and unwanted fluorescent emission. Feasibility of echelle spectrograph for Raman measurements has been tested for both conventional and standoff distance of 5 m with all the external illumination sources on. The echelle system provides high-quality Raman signals with an extremely low inherent bandwidth in solid and liquid samples, (~8 cm�1 ) for 478 cm�1 fundamental band of sulfur. Intensity ratio of e2 fundamental to a1 fundamental of conventional and remote Raman measurements indicates that the spectral features remain the same in both measurements. The advantages of ‘time-gated’ Raman compared with continuous-wave mode detection, like low background even under extraneous illumination and lowered fluorescence, are also illustrated with spectrum of sulfur taken in the two modes.