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Raman System Overview

Raman Schematic 
 
Raman systems have three major hardware components: an excitation laser, a spectral dispersing element and a detector. These components are shown in a block diagram in Figure 4, together with a series of lenses and the sample surface. In portable and handheld instruments, these components are miniaturized and fiber-coupled. The lasers in today’s handheld and portable instrumentation are frequency-stabilized solid state devices, which are small, inexpensive and remarkably long-lived. The spectrometers are small grating units, with the Raman scattered light from the sample being transmitted to them through optical fibers. Every component of the current generation of Raman spectrometers have been optimized and ruggedized for field use.

Detailed system component choice is driven primarily by performance. Lasers for miniaturized applications are generally diode or diode-pumped devices. Chief wavelengths of interest, primarily based upon price, ease of availability and technical maturity, are 532 nm (frequency-doubled diode pumped Nd:YAG), 785 nm and 905 nm (semiconductor laser) and 1064 nm (diode pumped Nd:YAG). The choice of wavelength is based upon the optimization of a few key tradeoffs. The first is that Raman excitation cross-sections (and therefore Raman signals) are a function of wavelength, proportional to (ν0- νvib)4, where ν0 is the frequency of the excitation laser (1/λ0) and νvib is the frequency of the vibrational mode being excited. For example, scattering cross-sections at 532 nm are about 6 times larger than at 785 nm. The competing factor that must be considered in choosing the wavelength for an application is the likelihood that fluorescence will be present in the system under measurement. Fluorescence is more common as the wavelength of excitation light decreases, and often interferes significantly when excitation light is in the UV region.

Once the scattered light has been dispersed by a grating in a miniature spectrometer, chosen specifically for the application at hand, it is detected with a CCD sensor. Raw signal counts are converted into useful information by a software system that uses the Raman spectrum to identify or match materials. These units are robust, sensitive and very technologically mature, characteristics that result in analyzers that work well, give useful data and are simple to use.

Besides the hardware components of any Raman system, data collection and analysis algorithms are necessary to permit the instruments to be used by non-experts. In a research system, these algorithms may simply present the spectrum to the user. In systems for use outside of research, they tend to do comparisons between a sample and items in a spectral database. These comparisons are based on a variety of mathematical approaches, from calculation of “hit quality index” (HQI) to sophisticated multivariate techniques like soft independent modeling of class analogy (SIMCA).

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