What types of laser (Rayleigh) filtering are used?

There are two main classes of filter used for Raman spectroscopy.

  • Optical filters – these optical components are placed in the Raman beam path, and are used to selectively block the laser line (Rayleigh scatter) whilst allowing the Raman scattered light through to the spectrometer and detector.  Each laser wavelength requires an individual filter.  There are two main types of filter used, both of which can be used without user intervention or optimisation:

    • Edge.  An edge filter is a long pass optical filter which absorbs all wavelengths up to a certain point, and then transmits with high efficiency all wavelengths above this point.  As an example, a 532 nm edge filter will absorb all light up to 534 nm or above (e.g., 70 cm-1 or above) including the laser emission.  Above 534 nm it will transmit light, allowing detection of the Raman spectrum (stretching from 70 cm-1 up to 3500 cm-1 or above).  The edge filters have an ultra steep edge between the absorbing and transmitting spectral region, and offer excellent blocking of the laser line.  The advantage of the edge filter is that it is environmentally stable with a near infinite lifetime.
    • Holographic notch.  A notch filter has a sharp, discrete absorption which for Raman is chosen to coincide with a specific laser wavelength.  Typically the absorption will be a few nanometers wide (corresponding to a few hundred cm-1).  As an example, for a 532 nm laser a notch filter will be chosen with a central absorption at 532 nm.  The laser line will be absorbed, but the Raman spectrum above will be transmitted.  Unlike the edge filter, the notch does have a finite lifetime, and will degrade with time.  The advantage of the notch is that it allows measurements to be made for both the Stokes and anti-Stokes Raman scattering, which is useful for certain specialised measurements.

  • Tunable filtering spectrometer – with a triple monochromator instrument, it is possible to work in a ‘double subtractive, single spectrograph’ mode.  The first two monochromators are used as an integral pair, which firstly disperse both the Rayleigh (laser) and Raman scattered light, physically block the Rayleigh, and then recombine the light.  The third spectrometer then acts in the normal way to disperse the Raman scattered light with subsequent detection.  The advantage of using a triple spectrometer in this way is that the laser filter is infinitely variable, and the one instrument can be used to work with any number of laser sources.  In addition, the filtering performance is excellent, and allows Raman analysis down to 4-5 cm-1.  However, such an instrument does require rather more expertise to operate in comparison with the more standard filter based single monochromator systems – as such it would rarely be considered for routine analysis.
Back to FAQs