What is 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.
Bragg Tunable Filter
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 .
Fabrication Method: How is it created?
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 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. **