A Bragg tunable filter is a filter that exploits Bragg gratings in order to extract a small bandwidth of wavelengths out of a polychromatic input. As stated by Bragg’s law, θ determines which wavelength is diffracted. Hence, by tuning the angle of the grating, we can scan the output wavelength over hundreds of nanometers (see figure 1 (a)). Since Bragg gratings are dispersive, their output is divergent. Therefore, a second pass in the grating is essential in order to recombine the diffracted beam and cancel out this divergence. The second pass helps reduces the bandwidth and provides an output parallel to the input beam. This technology allows for the detection of a whole image at one wavelength [1].
In order to create a volume Bragg grating, a photo-thermo-reflective (PTR) glass is exposed to ultra-violet laser radiation at 325 nm. The PTR glass is placed in a sea of interference which will induce a migration of ions. This will generate a variation in the electronic density over the whole material. The variation of the charge distribution gives rise to a variation in the refractive index. When the radiation stops this variation persists and the glass is then exposed to high temperature in order to accentuate this modulation. Other materials can be used to produce volume hologram but PTR glasses offer unpolarised output (in transmission) and are highly resistant, this is why they are the most popular. For more details on the fabrication method see [2].
[1] S. Marcet, M. Verhaegen, S. Blais-Ouellette, and R. Martel, Raman Spectroscopy Hyperspectral Imager Based on Bragg Tunable Filters.
[2] A. L. Glebov, O. Mokhun, A. Rapaport, S. Vergnole, V. Smirnov, L. B. Glebov, Volume Bragg Gratings as Ultra-Narrow and Multiband Optical Filters, Invited Paper, Proc. of SPIE Vol. 8428 84280C-1, doi: 10.1117/12.923575.