In its simplest description, a
remote Raman measurement analyzes the spectral features of scattered
laser light from an analyte of interest from a standoff distance.
This is simply another method to identify analytes based on their
vibrational structure (not unlike infrared spectroscopy). In this
case, a small fraction of the scattered laser light is shifted in
frequency owing to the vibrational structure of the molecule.
Raman spectrometer systems usually employ a sharp, high-extinction edge
filter to block the Rayleigh (i.e. elastic) scattered light from the
spectrometer entrance. Typically grating-based spectrometers are
used for the spectral measurement. The strengths of Raman
spectroscopy include its suitability for in situ measurement (i.e. no
sample preparation is necessary), very small analyte thicknesses can be
detected, the spectral range is selectable by the laser wavelength (and
thus can be moved away from strong fluorescence), and potentially high
spectral resolution. Weaknesses include relatively weak
scattering cross sections. These weaknesses may be overcome
through careful design of an efficient optical system.
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| Figure 1: Remote Raman Measurement Setup |
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OPTRA has completed an SBIR funded by the
Department of Energy to design and build a telescope for short range
Raman lidar measurements of organic contaminants. During this
program we obtained a two-frequency helium cadmium laser (λ = 325 and
442 nm) from Midwest Laser, an Omega OD6 Raman edge filter which cuts
on at 330 nm, and an Ocean Optics USB2000 grating spectrometer and
performed a remote Raman measurement of acetone with these COTS
components and our telescope. We used an Avantes DH-2000
Deuterium lamp to calibrate the measurement.
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| Figure 2: Measured Raman Spectra of Acetone |
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