Volume Bragg gratings as ultra-narrow and multiband optical filters

High efficiency volume Bragg gratings (VBGs) in photo-thermo-refractive (PTR) glass provide unmatched optical filtering capabilities with optical densities as high as 50 dB and linewidths as narrow as 1 cm^-1. In this work we review recent advances in VBG technologies that enabled key improvements of high efficiency grating properties and led to development of unique VBG based optical filters for Raman spectroscopy and other applications. Such narrow band notch and bandpass filters make ultra-low frequency Raman measurements possible with single stage spectrometers, therefore, largely improving optical throughput of high end Raman instruments while reducing complexity of the measurements. In this work we also present novel volume multiplexed ultra-narrow band VBG filters with high reflection at multiple wavelengths. Such multiband holographic optical elements are formed by overlapping of several high efficiency VBGs in a single glass plate. Raman spectra obtained with multiband VBG filters and single stage spectrometers, show unmatched capability of the filters to provide simultaneous access to Stokes and anti-Stokes Raman modes with frequencies as low as 5 cm^-1 at different wavelengths.