 A volume hologram is usually made by exposing a photo-thermo-refractive glass to an interference pattern from an ultraviolet laser.It is also possible to make volume holograms in nonphotosensitive glass by exposing it to femtosecond laser pulses 

In the case of a simple Bragg reflector the wavelength selectivity formula_1 can be roughly estimated by formula_2, where formula_3 is the vacuum wavelength of the reading light, formula_4 is the period length of the grating and formula_5 is the thickness of the grating.The assumption is just that the grating is not too strong, i.e., that the full length of the grating is used for light diffraction.