Seminar on Holographic Memory

Ram Pothuraju


Holographic memory is developing technology that has promised to revolutionalise the storage systems. It can store data upto 1 Tb in a sugar cube sized crystal. Data from more than 1000 CDs can fit into a holographic memory System. Most of the computer hard drives available today can hold only 10 to 40 GB of data, a small fraction of what holographic memory system can hold. Conventional memories use only the surface to store the data. But holographic data storage systems use the volume to store data. It has more advantages than conventional storage systems. It is based on the principle of holography.

Scientist Pieter J. van Heerden first proposed the idea of holographic (three-dimensional) storage in the early 1960s. A hologramis a block or sheet of photosensitive material which records the interference of two light sources. To create a hologram, laser light is first split into two beams, a source beam and a reference beam. The source beam is then manipulated and sent into the photosensitive material.
Description of Holographic Memory


The components of Holographic data storage system is composed of

» Blue-green argon laser
» Beam splitters to spilt the laser beam
» Lithium-niobate crystal or photopolymer
» Charge coupled device camera

They can be classified into three sections namely recording medium, optical recording system and photodetector array. The laser is used because it provides monochromatic light. Only the interference pattern produced by the monochromatic beam of light is stable in time. Lithium niobate crystal is used as photosensitive material on which hologram is recorded. It has certain optical characteristics that make it behave as photosensitive material. CCD camera detects the information in the light, converts to digital information and forwards it to computer.


When data is recorded in a holographic medium, certain factors can lead to erroneously recorded data. One major factor is the electronic noise generated by laser beams. When a laser beam is split up ( for example, through a SLM ), the generated light bleeds into places where light was meant to be blocked out. Areas where zero light is desired might have minuscule amounts of laser light present which mutates its bit representation.

For example, if too much light gets recorded into this zero area representing a binary 0, an erroneous change to a binary 1 might occur. Changes in both the quality of the laser beam and recording material are being researched, but these improvements must take into consideration the cost-effectiveness of a holographic memory system. These limitations to current laser beam and photosensitive technology are some of the main factors for the delay of practical holographic memory systems.

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