This article first appeared in Personal Computer World magazine, October 1996.
IN THE FILM "The Man Who Fell To Earth", an alien visits our planet on a mission to save his dehydrated world. To raise the vast sums of money needed to build a rescue ship he rapidly patents hundreds of out-of-this-world inventions based on own planet's highly advanced technology. One gadget stores hours of hi-fidelity stereo music, encoded in the molecular structure of a metallic ball the size of a malteser. Such technology may soon be within our own, terrestrial, grasp.
The last decade has seen extraordinary advances in data storage. One new idea offers huge increases in speed and capacity, throwing in massively parallel super-fast data transfer for good measure. Enormous amounts of data are stored as interference holograms in a single crystal the size of a sugar cube, and there are no moving parts.
When two beams of light overlap, the crests and troughs of their waves interact to produce patterns of interference. You can see this effect in the beautiful colours on the surface of a soap bubble, and in the supernumerary arcs of a rainbow. The use of interference for storing and reproducing images is not a new idea. It was first demonstrated in 1810 by Thomas Seebeck, and in 1908 the French physicist Gabriel-Jonas Lippman was awarded the Nobel prize for his work on 'interference heliography', a method of colour photography. But accurate control of interference was impossible without extremely pure light, and it was not until the invention of the laser that the science of holography became practical.
To record a hologram of an object, light from a laser is split into two beams. One beam, the 'signal', is reflected off the object onto photographic film, where it meets the other 'reference' beam, which has travelled direct from the laser. The two beams interfere, and the resulting pattern of light and shade is recorded on the film. If the image of the interference pattern is subsequently illuminated by the reference beam, the signal beam is re-created. Because the interference pattern represents the intersection of two three-dimensional wavefronts, the reconstructed signal beam is a true three-dimensional image of the original object.
Digital holography works on a similar principle, as illustrated in the diagram. As before, laser light is split into two beams, the 'signal', which will be used to encode the binary data, and the 'reference'. The signal beam passes through a 'spatial light modulator', which displays a 'page' of digital data as a pattern of black and white squares, rather like a crossword grid. The light takes on the image of the data, and the beam is then focussed onto the holographic storage medium, typically a crystal of lithium niobate. At the focus point, the signal beam meets the reference beam, and an interference pattern forms. The pattern very slightly distorts the molecular structure of the crystal, and the data is recorded.
To read the data back, the reference beam is again fired at the crystal, and when it meets the stored interference pattern, the original signal beam is regenerated. Focusing the signal beam onto an array of charged-coupled devices reveals the complete page of data.
The set-up is very delicate. In order to successfully decode the hologram, the reference beam must be aligned at precisely the same angle used to record it. Although this means that the relative positioning of the beam and crystal must be finely controlled, it also implies that many different holograms can be stored in the same crystal, by recording each one at a slightly different angle. This idea, known as 'angle multiplexing' has been used by researchers at the California Institute of Technology to store 5000 high-resolution images simultaneously in a single one-cubic-centimetre crystal.
The technique can be taken a step further, using 'spatial multiplexing', with thousands of crystals arranged on the surface of a disk. Each crystal holds many angle-multiplexed holograms, as before, but the overall storage capacity is vastly increased by optically selecting each crystal for reading/writing. Experimental versions have been built, employing sheets of hologram-sensitive polymer film.
With the rapid increases in the speed and storage capacity of traditional magnetic media, exotic technologies like holography might appear to be of only academic interest. However, in August last year the Californian company Holoplex announced the first commercial product based on this technology, capable of storing and matching 1000 fingerprints. Although the overall data capacity is only 300Mb, half that of a conventional CD, all the data can be read in parallel in one second.
Government, industry and academia are taking digital holography very seriously. In 1995 a five-year programme was established in the USA to the tune of $32 million, with 50% of the funding coming from the US Defense Department. The key players include IBM, Kodak, Rockwell, and several universities, and the goal is to build a system which can store 128 GBytes with a transfer rate of 1 Gbit per second.
It is likely that the prospect of holographic storage with 1000 times the capacity and speed of today's CDs will usher in whole new applications of interactive video on demand, massive image libraries, and, quite possibly, little musical maltesers.
Toby Howard teaches at the University of Manchester.