Nondispersive Raman Spectrometers

A different approach to liquid crystal filters led to the development of a nondispersive Raman spectrometer used commercially in a Raman microscope... 

Figure 5.2. Schematic of a nondispersive, FT-Raman spectrometer. A single detector monitors photons with all Raman shifts, after each has been modulated by a multiplexer such as an interferometer. Raman spectrum is obtained by Fourier transformation of the detector output (interferogram).

At the time of this writing, the Raman spectrometer market is approximately split between dispersive (spectrograph/CCD) and nondispersive (FT-Raman) instruments. Both types have their pros and cons, which enter into a selection for a given application. Several generalizations are listed in Table 5.3. These... 

Both dispersive and nondispersive spectrometers can exhibit preferential transmission of light depending on its polarization. The efficiency of diffraction gratings and the polarization sensitivity of the beamsplitter can cause errors in the observed depolarization ratio, depending on several variables such as experimental geometry and Raman shift region. For this reason, it is often important to place a polarization scrambler between the sample and any polarization-sensitive components of the spectrometer other than the polarization analyzer itself. In addition, it is good practice to measure p for a few known systems to verify accuracy of the apparatus. 

Most implementations of filter-based nondispersive spectrometers occur in Raman microscopy and imaging, where their large clear aperture is particularly valuable. These applications are discussed further in Chapter 11, but the nature and performance of tunable filters as wavelength analyzers are outlined here. 

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