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, which enabled the storage of full-length movies on a single disc.
                                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, which is about eight hours of movies. These 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 capabilities, scientists are now working on a new optical storage method called holographic memory that will 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 more information in a smaller space and offer faster data transfer times. 


                               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.    For  example,  a  hologram  that  can  be  obtained from  a  toy  store  illustrates  this  idea.    Precise laser equipment is used at the factory to create the hologram.  A recording material which can recreate recorded images out of natural light is used so the  consumer  does  not  need  high-tech  equipment  to  view  the information  stored  in  the  hologram.    Natural light becomes the reference beam and human eyes become the optical sensors. 
                 Holography was invented in 1947 by the Hungarian-British physicist Dennis Gabor (1900-1979), who won a 1971 Nobel Prize for his invention.  

                         The future of holographic memory is very promising.   The page access  of  data  that  holographic memory creates will provide a window  into  next  generation  computing  by  adding  another dimension  to  stored  data.    Finding holograms in personal computers might be a bit longer off, however.  The large cost of high-tech optical equipment would make small-scale systems implemented with holographic memory impractical.
                 Holographic memory  will  most  likely  be  used  in  next  generation  super computers where cost is not as much  of  an  issue.    Current magnetic storage devices remain far more cost effective than any other medium on the market.    As computer systems evolve, it is not unreasonable to believe that magnetic storage will continue to do so.   As mentioned earlier, however, these improvements are not made on the conceptual level.  The current storage in a personal computer operates on the same principles used in the first magnetic data storage devices.  The parallel nature of holographic memory has many potential gains on serial storage methods.  However,  many  advances  in  optical  technology  and photosensitive  materials  need  to  be  made  before  we  find holograms in computer systems.

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