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Holographic Storage Memory - An Introduction

As demand for high-speed (in terms of access time) storage devices, which can store a huge volume of information is increasing, holographic storage technique has emerged as one of the best method to fulfill these requirements...
During past few decades, right from the day first processor was developed; the computation speed of processors has increased approximately twice every 18 months. Today, the processors we are using have a tremendous computation power and can perform an operation within a fraction of a second, which would have required time more than few decades to hundred thousand years.

However, we are unable to utilize this tremendous processing speed of the processors, as the storage devices and other peripherals, which work with these processors, have very less speed as compared to the speed of the processor. Even though continuous efforts are being put in improvements of response time of these devices e.g. storage devices, input/output devices etc. the speed of these devices is not compatible to that of the speed of the processors. This problem results in the degradation in utilization of the processor speed, as the processor has to communicate with the peripheral devices and because of the lower speed of these devices, the processor's speed ultimately gets reduced.

Devices that provide huge data storage area and have faster response are in demand today. To meet the current demand in market, devices with large storage capacity are introduced, Compact Disks are capable of storing more than 700 MB to 800 MB of information, Digital Versatile Disc (DVD) offer storage space for more than 15 GB of information, common hard disks used in PCs provide data storage space in form of 20, 40, 60, 80 GB etc sizes. These devices satisfy the demand at certain extent, but as demand for more storage capacity devices which have faster response rate is increasing, scientists are now trying to develop new storage devices based on optical storage methods, these devices will provide facility to store data in three-dimension form. The storage devices available in market today, use surface storage techniques.

Storage devices based on optical storage techniques, are considered to allow storage of very huge volume of data nearly 1 Terabyte of information (1000 Gigabytes) inside a 1*1*1 cm sugar-cube-sized crystal. These devices will also have a faster response (access rate) as compared to the storage devices used nowadays.

Let us discuss in brief, kind of setup used to create a holographic storage and store information in three-dimension. The basic components that are needed to construct holographic device are: blue-green Argon laser, a beam splitter which will be used to split the laser beam, LCD panel (spatial light modulator), mirrors to direct the laser beams, lenses to focus the laser beams, a photopolymer (e.g., Lithium-niobate crystal), and Charge-Coupled device (CCD) camera etc.

At first when a blue-green argon laser is fired, a beam splitter is used to divide the laser into two beams. One of the two beams is called the Object or Signal beam, this beam goes straight and bounces off the mirror and travels through the Liquid Crystal Display, which shows pages of raw binary data as clear and dark boxes. The signal beam then carries the information from the page of binary code around the light-sensitive Lithium-Niobate crystal (or a photopolymer crystal). The second beam takes a separate path to the crystal. When both the beams meet, an interference pattern is created. The pattern created by the interference of the signal beam and the second beam that traveled via different path to the crystal, is stored in a particular area in the crystal, the data stored this way is stored as a hologram.

When retrieving the information back from the crystal where it is stored, the reference beam is shined into the crystal at exactly the same angle at which it entered to store that page of data. The crystal diffracts the reference beam allowing the original page to be reconstructed. This reconstructed page is then projected onto the charge-coupled device (CCD) camera, which interprets and forwards the digital information to a computer.

A Polaroid scientist Peter J. Van Heerden first proposed the idea of holographic memory storage in early 1960s. The holographic storage system can store thousands of pages containing millions of bits of information in a very small area of the crystal and as no moving part or no magnetic field are used in this technique, the access of the stored data is many folds faster than normal storage devices available today, a very large amount of data can be accessed at a time from the same. In addition, the data stored in holographic storage will be much reliable and there will hardly be a defect in the information stored (which will be negligible) as compared to the normal storage devices where a small defect on the surface storage causes loss of whole information.
By Nilesh Parekh
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