Editorial Feature

What are the Optical Components in a Raman Spectrometer?

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Raman spectroscopy has become a widespread, routine tool for the identification and quantitative analysis of chemical compounds. Part of its appeal is the versatile nature of Raman spectroscopy, as it can be used for samples in any state of matter, gas, liquid or solid, and the wealth of structural and chemical information that can be extracted from the resulting Raman spectrum, that also provides an identifiable chemical ‘fingerprint’ for the species being measured.

Inside a Raman spectrometer, there are a number of optical components that determine the overall performance and specifications of the instrument. Modern Raman spectrometers typically use a laser as the excitation source as narrow spectral bandwidth is crucial for achieving a high instrument resolution.

To perform a Raman measurement, the laser light from the excitation source is directed through the spectrometer by a series of mirrors and to the sample. The resulting Raman signal is then dispersed by a diffraction grating onto a detector, often a CCD chip, where the signal can be recorded. However, the inclusion of additional optics such as filters can help improve the quality of the spectra obtained with the Raman instrument. This article will describe some of the key optical components in a Raman spectrometer and their role.

Strategic Filtering

One of the biggest challenges in recording Raman spectra is the issue of Rayleigh scattering and contributions from unwanted fluorescence. Rayleigh scattering refers to the elastically scattered light that undergoes no energy transfer with the sample. This means that the Rayleigh scattering signal appears at the same wavelength as the excitation wavelength and can be several orders of magnitude greater in intensity than the Raman signal of interest.

As it can be challenging or impossible to remove the Rayleigh scattering signal through post-processing of the recorded spectra, it is often desirable to find an experimental solution to the problem. One possibility is to include a notch filter placed after the sample and before the detector that blocks the wavelengths at which the Rayleigh scattering occurs.

Ideally, such filters will block a relatively narrow range of wavelengths so that if the signal of interest only has a small Stokes or anti-Stokes shift (the name for the wavelength change of the Raman signal versus the incident light) then the Raman signal of interest can be detected. This means that the filters must also have a sharp onset while still allowing broad transmission in all other spectral regions but the advantage is both the higher and lower wavelength Stokes and anti-Stokes signals can be measured simultaneously. Where simultaneous measurement is not required, an edge filter which blocks all wavelengths above or below a certain value can often lead to higher Raman signal levels.

Laser line filters may also be used to reduce the bandwidth of the laser source, helping to ensure even the most closely spaced Raman features be resolved.

Diffraction Gratings

The diffraction grating plays a very important role in a Raman spectrometer. The diffraction grating is used to spatially separate the different wavelengths of the Raman signal coming from the sample. This is necessary as CCD-type detectors cannot distinguish between different wavelength components inherently. The amount of spatial dispersion of the signal onto the detector determines the energy resolution and the total energy range that can be detected in a single experiment. For example, a large dispersion may be desirable where very high energy resolution is needed to distinguish between closely separated peaks but this will also limit the energy window of the spectrum that can be detected.

Ideally, the specifications of the grating will be well-matched to the detector used. CCD arrays with fewer pixels and smaller spatial coverage will require gratings with lower dispersion. The quality of the gratings is also very important as small imperfections can lead to stray light and resulting artifacts in measurements.

Minimizing Losses

The other key optical component in a Raman spectrometer is the mirrors. As the Raman effect is so weak, it is important that the mirrors have maximum reflectance over the desired wavelength region to minimize losses. This wavelength range may need to be quite broad for instruments where the excitation wavelength can be changed. The mirrors must have sufficient damage thresholds to withstand the input laser power and good quality coatings can also help reduce the amount of scattered light within the instrument.

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Rebecca Ingle, Ph.D

Written by

Rebecca Ingle, Ph.D

Dr. Rebecca Ingle is a researcher in the field of ultrafast spectroscopy, where she specializes in using X-ray and optical spectroscopies to track precisely what happens during light-triggered chemical reactions.

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