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Raman Spectrometer System Overview

Raman systems have three major hardware components:

  1. an excitation laser
  2. a spectral dispersing element 
  3. a detector

These components are shown in the diagram at right. The diagram also shows a series of lenses and the sample surface. In portable Raman spectrometers 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. They're small, inexpensive and remarkably long-lived. The spectrometers are small grating units. The Raman-scattered light from the sample transmits to the spectrometers through optical fibers. TSI designs every component of the current generation of Raman spectrometers for optimal performance and durability in field use.

Raman spectrometer LASER components and details

Performance drives the components chosen for the Raman spectrometers. Lasers for miniaturized applications are generally diode or diode-pumped devices. The choice of laser wavelengths primarily reflects price, ease of availability and technical maturity. Standard wavelengths include:

  • 532 nm (frequency-doubled diode pumped Nd:YAG)
  • 785 nm and 905 nm (semiconductor laser)
  • 1064 nm (diode pumped Nd:YAG)

The choice of wavelength depends on the optimization of a few key trade-offs. 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. Software using the Raman spectrum to identify or match materials converts raw signal counts into useful data. These units are robust, sensitive and very technologically mature. These qualities lead to analyzers that work well, give useful data, and are simple to use.

In addition to hardware, data collection and analysis algorithms 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 a “hit quality index” (HQI) to sophisticated multivariate techniques like soft independent modeling of class analogy (SIMCA).

View TSI's complete range of Raman spectrometer products.

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