We report the first results of ultra-low frequency Stokes and anti-Stokes Raman spectra at 785nm showing clearly
resolved frequency shifts down to 10cm-1 from the excitation line, using commercially available ultra-narrow band notch
and ASE suppression filters, and a single stage spectrometer. Near infra-red (NIR) wavelengths are of particular interest
for Raman spectroscopy due to the reduced fluorescence observed for most materials. Previously reported attempts to
produce ultra-low frequency Raman spectra at 785nm with volume holographic notch filters were largely unsuccessful,
due to the fact that these ultra-narrow line notch filters and the wavelength of the laser must be very well matched to be
effective. Otherwise, if the filters have any manufacturing errors or the laser wavelength is unstable, insufficient
suppression of the Rayleigh scattered light will allow it to overwhelm the Raman signal.
Recent improvements in both notch and ASE filters, wavelength-stabilized lasers, and optical system design have
enabled low-frequency Raman spectra to be successfully taken at 785nm for several typical materials. Two ultra-narrow
line notch filters formed as volume holographic gratings (VHGs) in glass with individually measured optical densities of
4.5 were used to block the Rayleigh scattered light from a matched VHG wavelength stabilized laser. Five discrete peaks
below 100cm-1 were simultaneously observed for sulfur in both the Stokes and anti-Stokes regions at 28, 44, 52, 62, and
83cm-1. With no degradation in filter performance over time and extremely narrow spectral transition widths of less than
10cm-1, this relatively simple system is able to make ultra-low frequency Stokes and anti-Stokes Raman measurements at
a fraction of the size and cost of traditional triple monochromator systems.