More Information about Holographic Memory
Although conventional storage methods adapt to the growing needs of computer Systems, they are reaching their fundamental limits. Often improvements made to these storage methods decrease access times or reduce the size of stored bits, but the design of such systems is based on serial access, or reading in a one dimensional streams of bits. Conventional storage also relies on mechanical devices to retrieve data, such as the arm which passes over magnetic platters in a hard drive.
As computer systems continue to become faster, they will need a way to access larger amounts of data in shorter periods of time. This paper provides a description of holographic memory, a three-dimensional Data storage system which has fundamental advantages over conventional read/write memory systems. A brief overview of the properties of holograms will be presented first. This will cover the way in which data can be stored in a hologram with the diffraction of laser light.
Holographic memory is a technique that can store information at high density inside crystal or photopolymer. As current storage techniques such as DVD reach the upper limit of possible data density (due to the diffraction limited size of the writing beam), holographic storage has the potential to become the next generation of storage media. The advantage of this type of data storage is that the volume of the recording media is used instead of just the surface. This three-dimensional aspect allows for a phenomenon known as Bragg selectivity to be utilized, whereby many information laden holograms can be superimposed or multiplexed in the same volume of medium. It is necessary to Bragg detune each hologram recorded with respect to its neighbors.
This can be achieved by a number of methods, e.g. rotation of the media with respect to the recording object and reference beams or changing the wavelength or phase of the recording laser beams for each hologram
Holography method of reproducing a three-dimensional image of an object by means of light wave patterns recorded on a photographic plate or film. Holography is sometimes called lens less photography because no lenses are used to form the image. The plate or film with the recorded wave patterns is called a hologram. The light used to make a hologram must be coherent. (A coherent beam of light can be produced by a laser.) Before reaching the object, the beam is split into two parts; one (the reference beam) is recorded directly on the photographic plate and the other is reflected from the object to be photographed and is then recorded.
Holographic Versatile Disc (HVD) is an optical disc technology still in the research stage which would hold up to 3.9 terabyte (TB) of information. It employs a technique known as collinear holography, whereby two lasers, one red and one blue-green, are collimated in a single beam. . The blue-green laser reads data encoded as laser interference fringes from a holographic layer near the top of the disc while the red laser is used as the reference beam and to read servo information from a regular CD-style aluminum layer near the bottom. Servo information is used to monitor the position of the read head over the disc, similar to the head, track, and sector information on a conventional hard disc drive. On a CD or DVD this servo information is interspersed amongst the data.
These discs have the capacity to hold up to 3.9 terabyte (TB) of information, which is approximately 6,000 times the capacity of a CD-ROM, 830 times the capacity of a DVD, 160 times the capacity of single-layer Blu-ray-Discs, and about 8 times the capacity of standard computer hard drives as of 2007. The HVD also has a transfer rate of 1 gigabyte/s. Optware was expected to release a 200 GB disc in early June 2006 and Maxell in September 2006 with a capacity of 300 GB and transfer rate of 20 MB/s. Since, there have been no further news or products on market.
Advantages of HVD
1. Resistance to damage - If some parts of the medium are damaged, all information can still be obtained from other parts.
2. Efficient retrieval - All information can be retrieved from any part of the medium.
3. These discs have the capacity to hold up to 3.9 terabyte (TB) of information, which is approximately 6,000 times the capacity of a CD-ROM, 830 times the capacity of a DVD, 160 times the capacity of single-layer Blu-ray-Discs, and about 48 times the capacity of standard computer hard drives.
4. The HVD also has a transfer rate of 1 gigabit/s.
5. It is 3D in structure and store data in hologram.