Simultaneous measurement of spectra at multiple ranges using a single spectrometer.

We have designed and built an instrument having the capability to measure and display spectra at multiple ranges near simultaneously in real time. An excitation laser beam is oriented parallel to and offset from the axis of the light collection optics. The image of the laser beam is then displaced with range. Multiple optical fibers collect the displaced images at different ranges. The output ends of these fibers are positioned vertically along the input slit of a spectrometer that disperses the light from each fiber along different rows of the spectrometer's two-dimensional detector array. The detector array rows then give an immediate visual comparison of spectra at different ranges. A small prototype of this system covering a range from 3 to 13 m has been built. It has been successfully tested using containers holding two distinct fluorescent dyes. Numerical simulations indicate that the technique can be extended to longer-range systems.

Remote Raman spectra of benzene obtained from 217 meters using a single 532 nm laser pulse.

This report describes a mobile Raman lidar system that has been developed for spectral measurements of samples located remotely at ranges of hundreds of meters. The performance of this system has been quantitatively verified in a lidar calibration experiment using a hard target of standardized reflectance. A new record in detection range was achieved for remote Raman systems using 532 nm laser excitation. Specifically, Raman spectra of liquid benzene were measured with an integration time corresponding to a single 532 nm laser pulse at a distance of 217 meters. The single-shot Raman spectra at 217 meters demonstrated high signal-to-noise ratio and good resolution sufficient for the unambiguous identification of the samples of interest. The transmitter consists of a 20 Hz Nd:YAG laser emitting at 532 nm and 1064 nm and a 178 mm telescope through the use of which allows the system to produce a focused beam at the target location. The receiver consists of a large custom telescope (609 mm aperture) and a Czerny-Turner monochromator equipped with two fast photomultiplier tubes.

Pulsed remote Raman system for daytime measurements of mineral spectra.

A remote Raman system has been developed utilizing a 532nm pulsed laser and gated intensified charged couple device (ICCD) detector in the oblique geometry. When the system is set for 50m sample distance it is capable of measuring Raman spectra of minerals located at distances in the range of 10-65m from the telescope. Both daytime and nighttime operations are feasible and the spectra of minerals can be measured in a short period of time, of the order of a few seconds. In oblique geometry, measured sampling depth is more than 30m, during which the system maintains very high performance without any adjustments. Much longer sampling depth (0.1-120m) has been observed when the system is configured in the coaxial geometry. Clear advantages of using a gated detection mode over the continuous (CW) mode of operation in reducing the background signal and eliminating long-lived fluorescence signals from the Raman spectra are presented. The performance of the pulsed Raman system is demonstrated by measuring spectra of Raman standards including benzene (C(6)H(6)) and naphthalene (C(10)H(8)), a low Raman cross section silicate mineral muscovite (KAl(2)(Si(3)Al)O(10)(OH)(2)), and a medium Raman cross section mineral calcite (CaCO(3)).