Data
DATA STORAGE 43
have the high storage capabilities associated with the conven- tional optical techniques without the problems stated above.
The advantages to be gained from the use of holography in optical memories are many: first, each hologram, if properly illuminated, will project a real image which can be detected without the use of imaging optics. Each hologram can be considered as storing jointly an array of data and possessing the· optical properties of a high powered magnifying lens.
Second, a large number of holograms can be stored in such a way that the real image from each hologram will appear in a prescribed position. For the case where each hologram is read out separately, only one detector array is required and the need for mechanical transport of the stored data is elimi- nated. Third, image resolution can be as good as the diffrac- tion limit imposed by the size of the hologram. The aberrations that occur with magnifying lenses can be avoided in hologram imaging. Fourth, since the information stored in the hologram is not localised but can be focussed, diffused and distributed, the hologram record is relatively insensitive to dust or scratches. Distribution of stored data implies re- dundancy and, with it, tolerance in registering a hologram with the readout beam. Fifth, the form of hologram need bear no relation to the form of stored data. Thus, the data can be stored at high density, while the readout can have a relatively low density. Sixth, the hologram may be formed on non-ab- sorbing material without the necessity of using complex imaging technique. Seventh, holograms may be superim- posed unambiguously by forming them in thick media, thus increasing the capacity of the memory. Eighth, the informa- tion stored can be three dimensional; phase and colour infor- mation can be stored as well.
These are compelling reasons to study the possibility of using holographic techniques for optical memory and optical data processing. However, full use of these advantages re- quires a careful systems design, since various holographic
44 HOLOGRAPHY
More information can be stored in a single hologram than possible by optical memory systems
parameters must be carefully controlled to realize optimum performance.
A holographic memory with a rapid data access capability can be designed by using a block data transfer configuration.
In this case, the memory input format is a planar array containing 10,000 to one million bits of information. The input data which may originate from a number of primary sensors, may conceivably be incompatible with such a block format. It is therefore necessary to consider a buffer that organises and temporarily stores the information for forming the block input. This information can then be rapidly trans- ferred to the optical memory for storage. The input data will
•
DATA STORAGE 45
be given out by a sensor which generates electrical signals that are led to optical memory.
The first step is to prepare the data blocks of appropriate size for recording in the holographic storage system. To do this, some means for spatially modulating (or manipulating) a coherent beam of light with the information, must be pro- vided. In prinCiple, continuous (or analog) modulation is possible but it is more likely that a binary (or digital) coding will be used. In the final design, the input format will be a planar array of bright and dark areas with the presence of a bright spot in a bit storage location indicating a '1' and the absence indicating a '0'.
A number of materials can be used as a data composer component. Some possible configurations ,.re as follows:
When using ferromagnetic materials, the data block is composed by scanning a focussed light beam over the film surface. The light modulation is controlled by the data to be stored: a '1' would turn the beam on and a '0' would not. The intensity of the focussed spot is adjusted to locally raise the temperature of the ferromagnetic film to above 360°C Celsius (Curie Point), thus altering the sense of the original magnet- isation. Consequently, at these points, the polarization of a linearly polarised light beam transmitted through or re- flected from the film is rotated relative to the remainder of the film. A polarization analyser is oriented to pass light from the regions that were magnetically altered by the scanned light beam. The result is a spatially modulated light wave, which forms the block of data to be stored.
The resolution of ferromagnetic (manganese-bismuth:
Mn-Bi) films can be as high as 1000 lines/mm, so that spot sizes could approach a few microns. Since the available input area will be of the order of 2-3cm2for an input format of106 bits, each bit will occupy a 25 x 25 micron region.
46 HOLOGRAPHY
The Mn-Bi (manganese-bismuth) film can be erased and used to prepare a subsequent block of data by applying a high magnetic field over the entire input area.
Photochromic materials change transmittance during an exposure and thus can also be used to form the data blocks for input to the hologram memory. Real-time operation is then possible, because this change in transmittance can be reversed by illuminating the photochromic material with light of a different wavelength. A modulating scanning beam records information on the photochromic material by gener- ating transparent and opaque dots that are fed as input to the optical memory. Since resolutions are more than 3000 lines/mm, one million bits of information could be recorded in a 230mm2 area only.
Beam Deflection
The laser beam deflecting systems may have to operate at several wavelengths and over a wide range of beam intensi- ties. For either the data composer or the holographic storage media, the number of randomly accessible positions will be in the 10,000 to 1,000,000 range over a two-dimensional for- mat. Laser beam deflection techniques for this application will involve either electro-optic or acousto-optic approaches·
or both.
A beam deflection system may also be required if, for example, a photochromic data storage material is used, and local erasing is accomplished by absorption of ultraviolet energy.
Holographic Storage Format
Two basic types of hologram fotmats are normally used for optical memory: Fourier transform hologram and Fres- nel hologram. The former has the advantage of requiring less space in the recording medium, and it generates less aberra- tion in the readout wave front.
DATA STORAGE 47
A computer-generated hologram
A scheme for storage or retrieval of hologram arrays is shown below. An angularly deflected beam acts as both reference and read out beams. The retrieved data are pro- jected towards the input data array but a beam splitter directs
a part of energy to output detectors.
Light Sources
One or more monochromatic light sources (the sources emitting light of only one colour or frequency) are required to store and retrieve the blocks or data in the hologram memory. A separate source might be needed, if an optical modulating scheme is used to compose data blocks. Another light source might be required for the erasing function. The data readout illumination will probably be derived from the source used to record the corresponding data block, but a
48 HOLOGRAPHY
means of adjusting the power level of the readout beam may
be needed. '
The specific characteristics required of these light sources depend on other system parameters, such as the sensitivities anq efficiencies of the data composer, the beam deflecting devices, the hologram material and the output detector. If wavelength sensitivity is used for holographic storage on a thick medium, an electrically tunable laser would be desir- able so that the laser lines be sufficiently spaced in wave- length. The degree of temporal and spatial coherence required to permit full use of the space bandwidth product and dynamic range available from any storage material must also be considered. Several other factors involved in deter- mining a source suitable for the optical system, include the electrical (or chemical) to optical conversion efficiency, and wavelength and intensity stability.