Volume holograms are holograms where the thickness of the recording material is much larger than the light wavelength used for recording.
In this case diffraction of light from the hologram is possible only as Bragg diffraction, i.e., the light has to have the right wavelength (color) and the wave must have the right shape (beam direction, wavefront profile).
Volume holograms were first treated by H. Kogelnik in 1969[1] by the so-called "coupled-wave theory".
For volume phase holograms it is possible to diffract 100% of the incoming reference light into the signal wave, i.e., full diffraction of light can be achieved.
Volume absorption holograms show much lower efficiencies.
H. Kogelnik provides analytical solutions for transmission as well as for reflection conditions.
A good text-book description of the theory of volume holograms can be found in a book from J. Goodman.
[2] A volume hologram is usually made by exposing a photo-thermo-refractive glass to an interference pattern from an ultraviolet laser.
[citation needed] It is also possible to make volume holograms in nonphotosensitive glass by exposing it to femtosecond laser pulses.
[3] In the case of a simple Bragg reflector the wavelength selectivity
is the vacuum wavelength of the reading light,
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.
Considering that because of the Bragg condition the simple relation
is the modulated refractive index in the material (not the base index) at this wavelength, one sees that for typical values (
, showing the extraordinary wavelength selectivity of such volume holograms.
In the case of a simple grating in the transmission geometry the angular selectivity
, showing the impressive angular selectivity of volume holograms.
The Bragg selectivity makes volume holograms very important.