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ABSTRACT
Holographic memory is developing technology that has promised to revolutionalise the storage systems. It can store data up to 1 Tb in a sugar cube sized crystal. Scientist Pieter J. van Heerden first proposed the idea of holographic (three-dimensional) storage in the early 1960s. . Data from 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. more than 1000 CDs can fit into a holographic memory System..
INTRODUCTION:
Devices that use light to store and read data have been the backbone of data storage for nearly two decades. Compact discs revolutionized data storage in the early 1980s, allowing multi-megabytes of data to be stored on a disc that has a diameter of a mere 12 centimeters and a thickness of about 1.2 millimeters. In 1997, an improved version of the CD, called a digital versatile disc (DVD), was released.
CDs and DVDs are the primary data storage methods for music, software, personal computing and video. A CD can hold 783 megabytes of data. A double-sided, double-layer DVD can hold 15.9 GB of data. scientists are now working on a new optical storage method called holographic memory that wili go beneath the surface and use the volume of the recording medium for storage, instead of only the surface area. Three-dimensional data storage will be able to store conventional storage mediums meet today's storage needs, but storage technologies have to evolve to keep pace with increasing consumer demand. CDs, DVDs and magnetic storage all store bits of information on the surface of a recording medium. In order to increase storage capabilitiesre more information in a smaller space and offer faster data transfer times.
HOLOGRAPHY
A hologram is 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. Once inside this material, it intersects the reference beam and the resulting interference of laser light is recorded on the photosensitive material, resulting in a hologram. Once a hologram is recorded, it can be viewed with only the reference beam. The reference beam is projected into the hologram at the exact angle it was projected during recording. When this light hits the recorded diffraction pattern, the source beam is regenerated out of the refracted light. An exact copy of the source beam is sent out of the hologram and can be read by optical sensors.
APPLICATION TO BINARY
In order for holographic technology to be applied to computer systems, it must store data in a form that a computer can recognize. In current computer systems, this form is binary. In the previous section, it was mentioned that the source beam is manipulated. In common holograms, this manipulation ;s the creation of an optical image such as a ball or human face. In computer applications, this manipulation is in the form of bits. The next section explains the spatial light modulator, a device that converts laser light into binary data.
SPATIAL LIGHT MODULATOR (SLM)
A spatial light modulator is used for creating binary information out of laser light. The SLM is a 2D plane, consisting of pixels which can be turned on and off to create binary 1.s and O.s. An illustration of this is a window and a window shade. It is possible to pull the shade down over a window to block incoming sunlight. If sunlight is desired again, the shade can be raised. A spatial light modulator contains a two-dimensional array of windows which are only microns wide. These windows block some parts of the incoming laser light and let other parts go through. The resulting cross section of the laser beam is a two dimensional array of binary data, exactly the same as what was represented in the SLM. After the laser beam is manipulated, it is sent into the hologram to be recorded. This data is written into the hologram as page form. It is called this due to its representation as a two dimensional plane, or page of data. Spatial light modulator is a Liquid Crystal Display panel that consists of clear and dark areas corresponding to the binary information it represent.
Spatial light modulator is actually that device which makes holography applicable to computers. Sort is one of the important components of Holographic Data Storage System.
IMPLEMENTATION
The components of Holographic data storage system is composed of
• Blue-green argon laser
• Beam splitters to spilt the laser beam
• Mirrors to direct the laser beams
• LCD panel (spatial light modulator)
• Lenses to focus the laser beams
• Lithium-niobate crystal or photopolymer
• Charge coupled device camera
They can be classified into three sections namely recording medium, optical recording system and photo detector 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.
RECORDING OF DATA IN HOLOGRAPHIC MEMORY SYSTEM
When the blue-green argon laser is fired, a beam splitter creates two beams. One beam, called the object or signal beam, will go straight, bounce off one mirror and travel through a spatial-light modulator (SLM). An SLM is a Liquid crystal display (LCD) that shows pages of raw binary data as clear and dark boxes. The information from the page of binary code is carried by the signal beam around to the light-sensitive lithium-niobate crystal. Some systems use a photopolymer in place of the crystal. A second beam, called the reference beam, shoots out the side of the beam splitter and takes a separate path to the crystal. When the two beams meet, the interference pattern that is created stores the data carried by the signal beam in a specific area in the crystal - the data is stored as a hologram.