Optical Filters

Specifying Optical Filters for Raman Spectroscopy

Raman spectroscopy allows the detection and identification of molecules through their unique vibrational and rotational energy level structure.

Direct detection of a molecule without any chemical alteration can be achieved opposed to fluorescence methods, which require the addition of a separate fluorescing molecule as a ‘tag’ attached to the molecule of interest. However the scattered Raman signal (as a percentage of the excitation power) is several orders-of-magnitude weaker than the corresponding fluorescence signal. Lasers are used as excitation source and very sensitive detectors (mostly spectrometers) to detect the very faint signal. To achieve an optimum S/N ratio excellent optical filters are essential to block the very intense laser light while still allowing high transmission of the slightly wavelength-shifted Raman scattered signal.

Beside the most popular laser at 785 nm, an increasing number of compact, afford-able, and high-power UV lasers have become available (Nd:YAG lasers at 266 nm, NeCu laser at 248.6 nm), ultra-sensitive UV Raman spectroscopy has become a more common technique. Optical filters are now also available in the UV range for the most popular UV lasers.

(Images with courtesy of Semrock Inc., USA).

Although UV lasers tend to excite strong autofluorescence, it typically occurs only at wavelengths above about 300 nm, independent of the UV laser wavelength.

There are four basic types of filters to choose from:

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  • Long wave pass (LWP) edge filter: it blocks the laser line and transmits the Stokes signal.
  • Short wave pass (SWP) edge filter, it blocks the laser line and transmits the Anti-Stokes signals.
  • The laser line filter resp. laser clean-up filter transmits only the laser and blocks all other light.
  • The notch filter blocks only the laser line while passing both long and shorter wavelengths.
The examples 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.

As far as different options are available how can the user decide between use of an edge filter or a notch filter to block the laser line in a Raman system?

The RazorEdge® edge filters and the StopLine® notch filters from Semrock Inc. e.g. are both available with OD>6 blocking of the laser line. The main benefit of the RazorEdge® filter is the very narrow transition width, allowing users to measure even the smallest Raman shifts. Although the StopLine® notch filters do not transit from blocking to transmission as close to the laser line, they are tunable from 0° to 14°, or 1 % of the laser line to allow users to shift the bandwidth in response to variable laser lines. The RazorEdge® filters are tunable as well, from 0° to 8°, or 0.3 % of the laser wavelength. In addition to the greater ability to angle tune, the notch filters` two passbands allow users to measure Stokes and Anti-Stokes shifts without changing filters.

Comparison of emission filter transmission between LWP Edge Filter and Notch filter.

Many high-volume Raman systems are designed to be used for a specific application in which only one or several Raman lines are important. The only demands are high blocking of the laser and high transmission of the Raman signal, but edge steepness is not always important, therefore cost effective EdgeBasic™ filters complete the filter portfolio with a low price Raman filter series.

AHF analysentechnik AG has a long time experience in specifying optical filters. In cooperation with excellent filter manufactures filter set-ups for people working in research or routine projects can be optimized to cost-efficiency and maximum signal/noise efficiency.

Expression of thanks
The author thanks Semrock Inc., USA, for providing technical information.

Dr. Ingrid Feuerbacher, AHF analysentechnik AG

Author:
Dr. Ingrid Feuerbacher
AHF analysentechnik AG
Kohlplattenweg 18
72074 Tübingen
Tel. 07071 970901-0
E-Mail: info@ahf.de

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