Optical Filter Design Considerations for Handheld Raman Spectroscopy

Light Notes

Optical filter design is crucial in benchtop Raman spectroscopy instruments and recently for handheld Raman spectroscopy instruments, which are becoming increasingly popular for rapid forensic screening, historical investigations and pharmaceutical quality control.1 While many of the optical components in a handheld Raman spectroscopy instrument are similar to those in a benchtop instrument, with handheld Raman spectroscopy there are additional considerations that need to be factored in such as the stability and size of the optical components.

One way of miniaturizing Raman spectroscopy equipment is to remove as many moving parts as possible and use monolithic optical mounts and custom-sized optics that are designed to be the smallest size necessary for the instrument. All of this avoids the need for bulky actuators and motorized translation stages and minimizes the space used by optical mounts in the Raman spectroscopy instrument.

Additional considerations for handheld Raman spectroscopy instruments may also be the temperature and environmental stability of optical components as the instrumentation may be used in a real-world environment with little or no environmental controls.

Iridian Spectral Filters specialize in developing custom optical components, including a range of filters for Raman spectroscopy. They can help create filters that are both the right dimensions and have the right optical properties for your instrumentation needs.

Raman Filter Properties

Many different types of filters are used in Raman spectroscopy instruments, including laser line, notch, and long and short wave pass filters. The two main applications of spectral filters in Raman spectroscopy are to remove any residual beam transmission from the typically very intense laser source and select either the Stokes or anti-Stokes Raman signal.

The right choice of filters can enhance the performance of a handheld Raman spectroscopy instrument so careful consideration needs to be given to the desired range of properties. One of the most important is the operating wavelength range, or the cut-on and cut-off wavelengths.

Notch and bandpass filters will selectively block or allow a given bandwidth centered around a particular wavelength. For laser line filters, the central wavelength will generally be matched to the laser. For narrowband laser sources, the notch filter blocking range may only need to be a few nanometers wide to cover the entire laser spectrum but a high optical density (OD) will be crucial as the laser intensity is likely to be several orders of magnitude greater than the signal contribution in Raman spectroscopy.

Another key property of Raman spectroscopy filters is edge steepness. The edge steepness is defined as the spectral width on the slope on the edge of the filter. Particularly for long and short-pass filters where there is sometimes little wavelength separation between the elastic scattering lines and Stokes or anti-Stokes to be detected, a steep edge prevents the loss of any desired signals while blocking the region of unwanted light.

Other considerations may be the angle of incidence of operation of the optical components in Raman spectroscopy, the polarization purity and the beam properties of the light. Many filters become less efficient when the laser beam becomes more cone-shaped and so it may be necessary to correct for optical aberrations in the light path to achieve the best performance.

References and Further Reading

  1. Sorak, D., Herberholz, L., Iwascek, S., Altinpinar, S., Pfeifer, F., & Siesler, H. W. (2012). New developments and applications of handheld raman, mid-infrared, and near-infrared spectrometers. Applied Spectroscopy Reviews, 47(2), 83–115. https://doi.org/10.1080/05704928.2011.625748

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