WHAT IS A VOLUME BRAGG GRATING?
A volume Bragg grating (VBG) is a diffraction grating in which there is a periodic modulation of the refractive index through the entire volume of a photosensitive material. As shown in figure 1, this modulation can be oriented either to transmit (a) or reflect (b) the incident beam. VBGs can be fully described by the following parameters (see Fig. 1 a): the thickness of the grating, the refractive index of the photo-termo-refractive glass it's made of (n0), the period (⋀) of the grating (or spatial frequency f = 1/⋀), the angle (θ) between the incident beam and the normal of the entrance surface (N), and the inclination of the Bragg planes (φ) defined as the angle between the normal (N) and the grating vector (Kg).
Like shown in Fig. 1 (a), the incoming collimated light is diffracted by the volume holographic filter, and only a small fraction of the spectrum is affected. In order to select which particular wavelength will be diffracted, the angle of the filter is adjusted to meet Bragg’s condition: λB=2n0Λcos(θ+φ), where λB is the diffracted wavelength. Like shown in Fig 1 (a), for transmission gratings, φ = π/2 (Bragg planes are perpendicular to the entrance surface). In this case, the Bragg condition becomes: λB=2n0Λsin(θ). As mentioned, this condition is valid for transmission gratings and has to be altered for reflection gratings where Bragg planes are parallel to the entrance surface. For reflection gratings, φ = 0 and the Bragg condition becomes: λB=2n0Λcos(θ). If the beam does not meet the Bragg condition, it passes through the filter undiffracted.