Holographic Versatile Disc

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.    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. 

Abstract

Currently data access times are extremely slow for magnetic disks when compared to the speed of execution of CPUs so that any improvement in data access speeds will greatly increase the capabilities of computers, especially with large data and multimedia files. Holographic memory is a technology that uses a three dimensional medium to store data and it can access such data a page at a time instead of sequentially, which leads to increases in storage density and access speed. Holographic data storage systems are very close to becoming economically feasible. Obstacles that limit holographic memory are hologram decay over time and with repeated accesses, slow recording rates, and data transfer rates that need to be increased. Photorefractive crystals and photopolymers have been used successfully in experimental holographic data storage systems.

Page-Level Parity Bits

Once error-free data is recorded into a hologram, methods which read data back out of it need to be error free as well.  Data in page format requires a new way to provide error control.  Current error control methods concentrate on a stream of bits.    Because  page data  is  in  the  form  of  a  two  dimensional  array,  error  correction needs  to  take  into  account  the  extra  dimension  of  bits.    When  a page  of  data  is  written  to  the  holographic  media,  the  page  is separated into smaller two dimensional arrays.  These sub sections are appended with an additional row and column of bits.    The added bits calculate the parity of each row and column of data.  An odd number of bits in a row or column create a parity bit of 1 and an even number of bits create a 0.   A parity bit where the row and column meet is also created which is called an overall parity bit.    The sub sections are rejoined and sent to the holographic medium for recording.

Holographic Versatile Disc (HVD)

Holographic recording technology records data on discs in the form of laser interference fringes, enabling discs the same size as today's DVDs to store more than one terabyte of data (200 times the capacity of a single layer DVD), with a transfer rate of over one gigabit per second (40 times the speed of DVD). This approach is rapidly gaining attention as a high-capacity, high-speed data storage technology for the age of broadband.

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, which enabled the storage of full-length movies on a single disc.

Challenges

During the retrieval of data the reference beam has to be focused at exactly the same angle at which it was projected during recording. A slight error can cause a wrong data page to be accessed.  It is difficult to obtain that much of accuracy. The crystal used as the photographic filament must have exact optical characteristics such as high diffraction efficiency, storage of data safely without any erasure and fast erasure on application of external stimulus light ultra violet rays.  With the repeated number of accesses the holograms will tend to decay. 

Conclusion

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.