2 Technical Description

The difference between holography and photography is best understood by considering what a photograph actually is: it is a point-to-point recording of the intensity of light rays that make up an image. Each point on the photograph records just one thing, the intensity (i.e. the square of the amplitude of the electric field) of the light wave that illuminates that particular point. In the case of a colour photograph, slightly more information is recorded (in effect the image is recorded three times viewed through three different colour filters), which allows a limited reconstruction of the wavelength of the light, and thus its colour.

However, the light which makes up a real scene is not only specified by its amplitude and wavelength, but also by its phase. In a photograph, the phase of the light from the original scene is lost. In a hologram, both the amplitude and the phase of the light (usually at one particular wavelength) are recorded. When reconstructed, the resulting light field is identical to that which emanated from the original scene, giving a perfect three-dimensional image (albeit, in most cases, a monochromatic one, though colour holograms are possible).

Hologram recording process

To produce a recording of the phase of the light wave at each point in an image, holography uses a reference beam which is combined with the light from the scene or object (the object beam). Optical interference between the reference beam and the object beam, due to the superposition of the light waves, produces a series of intensity fringes that can be recorded on standard photographic film. These fringes form a type of diffraction grating on the film.

Hologram reconstuction process

Once the film is processed, if illuminated once again with the reference beam, diffraction from the fringe pattern on the film reconstructs the original object beam in both intensity and phase. Because both the phase and intensity are reproduced, the image appears three-dimensional; the viewer can move their viewpoint and see the image rotate exactly as the original object would.

Because of the need for interference between the reference and object beams, holography typically uses a laser to produce them. The light from the laser is split into two beams, one forming the reference beam, and one illuminating the object to form the object beam. A laser is used because the coherence of the beams allows interference to take place, although early holograms were made before the invention of the laser, and used other (much less convenient) coherent light sources such as mercury-arc lamps.

The coherence length of the beam determines the maximum depth the image can have. A laser will typically have a coherence length of several meters, ample for a deep hologram. Small pen laser pointers tend to have a smaller coherence length and are considered too small to do holography. That has been shown to be incorrect, and people have successfully made small holograms with laser pens. Large analogue holograms cannot be made with laser pens due to their lower power (typically 1mw to 5mw). Digital holography does not suffer from this problem.

Other applications of holograms include metrology and optical computing.

3 Holography in Art

Salvador Dalí claims to have been the first to employ holography artistically. He was certainly the first and most notorious Catalan surrealist to do so, but the 1972 New York exhibit of Dali holograms had been preceded by "the first holographic art exhibition [which] was held at the Cranbrook Academy of Art in Michigan in 1968. The second took place at the Finch College gallery in New York in 1970 and attracted national media attention." (source: http://www.holophile.com/history.htm ). A vastly entertaining account of a 1973 Dali holography project (with Alice Cooper as subject) can be found here: http://www.alicecoopertrivia.pwp.blueyonder.co.uk/people/p-dali.php

4 Holographic Data Storage

Holography can be applied to a variety of uses that other than recording images. Holographic data storage is a technique that can store information at high density inside crystals (à la HAL 9000) or photopolymers. As current storage techniques such as DVD reach the upper limit of possible data density (due to the diffraction limited size of the writing beams), holographic storage has the potential to become the next generation of storage media. The advantage of this type of data storage is that the volume of the recording media is used instead of just the surface. This 3D aspect allows for a phenomenon known as Bragg volume selectivity to be utilised, whereby many information laden holograms can be superimposed or multiplexed in the same volume of medium. It is necessary to 'Bragg detune each hologram recorded with respect to its neighbours. This can be achieved by a number of methods, e.g. rotation of the media with respect the recording object and reference beams or changing the wavelength or phase of the recording laser beams for each hologram.

Like other media, holographic media is divided into write once (where the storage medium undergoes some irreversible change), and rewritable media (where the change is reversible). Rewritable holographic storage can be achieved via the photorefractive effect in crystals:

• Mutually coherent light from two sources creates an interference pattern in the media. These two sources are called the reference beam and the signal beam.
• Where there is constructive interference the light is bright and electrons can be promoted from the valence band to the conduction band of the material. The positively charged atoms they leave are called holes and they must be immobile in rewritable holographic materials. Where there is destructive interference, there is less light and few electrons are promoted.
• Electrons in the conduction band are free to move in the material. They will experience two opposing forces that determine how they move. The first force is the coulomb force between the electrons and the positive holes that they have been promoted from. This force encourages the electrons to stay put or move back to where they came from. The second is the pseudo-force of diffusion that encourages them to move to areas where electrons are less dense. If the coulomb forces are not too strong, the electrons will move into the dark areas.
• Beginning immediately after being promoted, there is a chance that a given electron will recombine with a hole and move back into the valence band. The faster the rate of recombination, the fewer the number of electrons that will have the chance to move into the dark areas. This rate will affect the strength of the hologram.
• After some electrons have moved into the dark areas and recombined with holes there, there is a permanent space charge field between the electrons that moved to the dark spots and the holes in the bright spots. This leads to a change in the index of refraction due to the electro-optic effect.

When the information is to be retrieved or read out from the hologram, only the reference beam is necessary. The beam is sent into the material in exactly the same way as when the hologram was written. As a result of the index changes in the material that were created during writing, the beam splits into two parts. One of these parts recreates the signal beam where the information is stored. Something like a CCD camera can be used to convert this information a more usable form.

Holograms can theoretically store equal to one bit per cubic block the size of the wavelength of light in writing. For example, light from a helium-neon laser is red, 632nm wavelength light. Using light of this wavelength, one square inch of holographic storage would be able to hold 1.63×10²² bits which is about 2,048,383,000 terabytes.

5 External links

• An article describing how to make holograms with a laser diode
• Forum on amateur holography
• How Holographic Memory Work - http://computer.howstuffworks.com/holographic-memory.htm
• How Holographic Environment Work - http://computer.howstuffworks.com/holographic-environment.htm

Optics

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