Note:
Historically, a Bragg filter was a colloidal dispersion of spheres in a regular close packed array, which selectively diffracts/reflects wavelengths that satisfy the Bragg diffraction criterion for the lattice but transmits all other wavelengths. Now largely superseded by Volume Bragg Gratings, where photo-induced modulation of the refractive index of the filter medium is used to form a volume phase hologram with reflecting planes separated by a well-defined spacing. In each case, angle tuning is used to match the laser wavelength to the Bragg diffraction condition. The filter can be manufactured either as a notch filter, which transmits wavelengths either side of the laser line, or as an edge filter, which only transmits wavelengths longer than the laser line (i.e., Raman light subject to Stokes scattering).