Volume hologram is a very popular type of transmission hologram. Volume holography has been mainly considered as a high-density data storage technology. This is a three-dimensional hologram created by recording the interference pattern of two mutually coherent light beams. In this type of hologram, the angle of difference between object beam and reference beam is from 90 degrees to 180 degrees. Volume holograms are used in various applications. Because of certain unique properties, volume holograms are used widely in various spectroscopic and imaging applications. Volume hologram is also known as thick hologram.

Properties of Volume Holograms

The three dimensional nature of volume holograms has some unique properties , such as follows:

• High diffraction efficiency which is as close to 100% at a single wavelength.
• High angle
• Spectral selectivity
• The ability of multiplexing many holograms in the same volume.

Recording Process of Volume Hologram

Volume hologram is recorded by using two coherent beams. One beam is called signal beam and the other is called reference beam. A strong diffraction is generated when the reconstruction beam probing the volume hologram is identical to the original reference beam with respect to the wavefront profile and propagating direction. The diffraction preserves identical wavefront profile and propagates along the signal beam propagating direction. This type of volume hologram is called Bragg matched. A volume hologram has two degenerate Bragg-phase-matching dimensions. This type of hologram provides the capability of volume holographic imaging. Due to the highly spatial and wavelength Bragg selectivity, the basic advantage of volume hologram is that a large number of holograms can be stored and read out selectively in the same volume.

Thick and Thin Holograms

Volume holograms are thick holograms. Very thin holograms have little depth to their object upon reconstruction. Embossed holograms are examples of thin holograms. Thick holograms can replay or reconstruct the image with considerable projection or depth. A hologram is considered to be thick if the thickness of the recording medium is more than the spacing between the interference fringes. Otherwise, it is a thin hologram.

The distance between interference fringes depend on a number of factors, like the wavelength of light used and the density of particles in the emulsion of the film plate. These interference fringes recorded on the film are called Bragg planes. They go all the way through the medium, but we see them only where they meet the surface.

In a reflection hologram, the reference beam and the object beam strike the plate from different sides, opposite to each other. The Bragg planes cut the the medium at very shallow angles.


On the other hand, in a transmission hologram, the reference beam and the object beam strike the plate from the same side. The Bragg planes slice through emulsion at much sharper angles.

Advantages of Volume Holograms

Some of the advantages of volume holograms are as follows:

• Higher light efficiency
• More design freedom
• Lower building cost
• Parallelism of information readout
• Large storage capacity
• A powerful tool for optical information processing.
• Used in extracting and processing complex signal wavefront as a color.
• For imaging application, volume holograms are recorded to process information from unknown complex incident wavefronts.
• Volume holograms can selectively extract specific information from the input, and project them into one or multiple detectors because of their extremely spatial and color selectivity of Bragg matching.

Applications of Volume Holograms

• Food packaging
• Microelectronic packaging
• Electrical packaging
• Optic packaging
• Isotopic labeling