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|
|LWP Edge Filters||SWP Edge Filter|
|Filters for only Anti-Stokes measurements|
|Laser-line Filter||Notch Filters|
|Filters for simultaneous Stokes and Anti-Stokes measurements|