Notch filters, Raman spectroscopy

Fig. 12 Measurement setup for Raman spectroscopy. NF is a notch filter for 785 nm and LF is a long-path filter...

The most common implementation of Raman spectroscopy is the combination of a standard spectrograph coupled to a CCD array detector. The suppression of any of the popular excitation wavelengths (see Table 8.2) is achieved by using either so-called notch or razor-edge filters. 

Figure 8.5 Typical experimental set-up for Raman spectroscopy, using a fibre-coupled spectrograph and a 20-array CCD detector. Instead of the ambient-air interaction volume, a cell can be used. Performance curves for both notch and razor-edge filters are shown in the lower part of the figure (the shading towards shorter wavelengths indicates the change of transmission with tilt angle)...

The range of applications of Raman spectroscopy has also been extended by several important recent developments, such as Raman microscopy, which makes it possible to study extremely small samples. One can also analyze the surface of an extended inhomogeneous sample to obtain very high spatial resolution, or scan across a surface using fiber optics. It is also possible to use specially developed interference filters or holographic notch filters in certain applications as an alternative to a dispersing spectrometer, provided that one suppresses the fluorescence that would otherwise interfere with the measurements. 

B Yang, M Morris, H Owen. Holographic notch filter for low-wavenumber stokes and anti-Stokes Raman spectroscopy. Appl Spectrosc 45 1533-1536, 1991. 

NJ Everall, H Owen, J Slater. Application of an integrated Raman system for remote process analysis using a fiber optic probe, holographic spectrograph, holographic notch filter and CCD detector. In N-T Yu, X-Y Li, eds. Proceedings of the XIVth International Conference on Raman Spectroscopy. Chichester Wiley, 1994, pp 1110-1111. 

Volume holographic optical elements developed for military uses, are successfully applied for Raman spectroscopy a so-called notch filter reflects a laser line virtually completely, its intensity is reduced by >6 orders of magnitude, with a transmission of about 70% at the other wavelengths. 

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