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Filter Types for Raman Spectroscopy Applications

Raman microscope

In Raman spectroscopy, an intense laser beam is used to excite a sample and the Raman "finger print" is measured by a dispersive or Fourier Transform spectrometer. Optical filters are used to prevent the undesired light from reaching the spectrometer and drowning out the relatively weak Raman signal.

There are four basic types of filters to choose from: a long wave pass (LWP) edge filter, a short wave pass (SWP) edge filter, a notch filter, and a laser line filter (each shown below). Laser line filters transmit only the laser and block all other light, while notch filter block only the laser line while passing both long and shorter wavelengths.  By using these two filters together, both Stokes and Anti-Stokes Raman scattering can be measured simultaneously.

An edge filter can provide a superior alternative, as they offer the narrowest transition to see Raman signals extremely close to the laser line. For more details on choosing between edge filters and notch filters, see Edge Filters vs. Notch Filters for Raman instrumentation.

The examples below show how different filter types can be used in a Raman system.  The blue lines represent the filter transmission spectra, the green lines represent the laser spectrum, and the red lines represent the Raman signal.

 

 

 

    Laser Transmitting Filters   Laser Blocking Filters
           
    SWP Edge Filter   LWP Edge Filter
Filters for only Stokes measurements    shor wave pass edge filter for Stokes measurement    
           
    LWP Edge Filters   SWP Edge Filter
Filters for only Anti-Stokes measurements   long wave pass filter for anti-Stokes measurements  
           
    Laser-line Filter   Notch Filters
Filters for simultaneous Stokes and Anti-Stokes measurements   laser line filter for simultaneous Stokes and Anti-Stokes measurements   notch filter for simultaneous Stokes and Anti-Stokes measurements

 

Semrock stocks MaxLine™ laser-line filters for laser transmission and RazorEdge® LWP filters and StopLine® notch filters for laser blocking, all as standard catalog products. 

 

Raman Raman-Spectroscopy Raman-filters Stokes Anti-stokes