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.
Imagine being able to record 100 movies on a disk the size of a CD, or one day recording the contents of the Library of Congress on such a disk. These are the promises of holographic data storage. Holographic memory offers the possibility of storing 1 terabyte (TB) of data in a sugar cube sized crystal. A terabyte of data equals 1,000 gigabytes, 1 million megabytes or 1 trillion bytes. Data from more than 1,000 CDs could fit on a holographic memory system. Most computer hard drives only hold 10 to 40 GB of data, a small fraction of what a holographic memory system might hold.
Holography enables storage densities that can far surpass the superparamagnetic and diffraction limits of traditional magnetic and optical recording. Holography can break through these density limits because it goes beyond the two-dimensional approaches of conventional storage technologies to write data in three dimensions. In addition, unlike conventional technologies which record data bit by bit, holography allows a million bits of data to be written and read out in single flashes of light, enabling data transfer rates as high as a billion bits per second (fast enough to transfer a DVD movie in about 30 seconds).
With its powerful combination of high storage densities and rapid data transfer rates, holography stands poised to become a compelling choice for next-generation storage needs.
Development of Holographic Memory
With its omnipresent computers, all connected via the Internet, the Information Age has led to an explosion of information available to users. The decreasing cost of storing data, and the increasing storage capacities of the same small device footprint, has been key enablers of this revolution. While current storage needs are being met, storage technologies must continue to improve in order to keep pace with the rapidly increasing demand.
However, both magnetic and conventional optical data storage technologies, where individual bits are stored as distinct magnetic or optical changes on the surface of a recording medium, are approaching physical limits beyond which individual bits may be too small or too difficult to store. Storing information throughout the volume of a medium, not just on its surface offers an intriguing high-capacity alternative. Holographic data storage is a volumetric approach which, although conceived decades ago, has made recent progress toward practicality with the appearance of lower-cost enabling technologies, significant results from longstanding research efforts, and progress in holographic recording materials.
Polaroid scientist Pieter J. Van Heerden first proposed the idea of holographic (three-dimensional) storage in the early 1960s. A decade later, scientists at RCA Laboratories demonstrated the technology by recording 500 holograms in an iron-doped lithium-niobate crystal, and 550 holograms of high-resolution images in a light-sensitive polymer material.
The lack of cheap parts and the advancement of magnetic and semiconductor memories placed the development of holographic data storage on hold. However, it is the recent availability of relatively low-cost components, such as liquid crystal displays for Spatial Light Modulators (SLMs) and solid-state camera chips from video camcorders for detector arrays, which has led to the current interest in creating practical holographic storage devices.
Exceptional possibilities of the topographic memory have interested many scientists of universities and industrial research laboratories. This interest long time ago poured into two research programs. The first of them is PRISM (Photorefractive Information Storage Material), which is targeted at searching of appropriate light-sensitive materials for storing holograms and investigation of their memorizing properties. The second program is HDSS (Holographic Data Storage System). Like PRISM, it includes fundamental investigations, and the same companies participate there. While PRISM is aimed at searching the appropriate media for storing holograms, HDSS is targeted at hardware development necessary for practical realization of holographic storage systems.
Figure 1: Laboratory simulation of Holographic Memory
Over the past decade, the Defense Advanced Research Projects Agency (DARPA) and high-tech giants IBM and Lucent's Bell Labs have led the resurgence of holographic memory development. A team of scientists from the IBM Research Division have been involved in exploring holographic data storage, partially as a partner in the DARPA-initiated consortia on holographic data storage systems (HDSS) and on photorefractive information storage materials (PRISM). Prototypes developed by Lucent and IBM differ slightly, but most holographic data storage systems (HDSS) are based on the same concept.
Here are the basic components that are needed to construct an HDSS:
- 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 (CCD) camera.
Figure 2: Construction of Holographic Memory device
Desktop Holographic Data Storage
After more than 30 years of research and development, a desktop holographic storage system (HDSS) is close at hand. There is still some fine tuning that must be done before such a high-density storage device can be marketed, but IBM researchers have suggested that they will have a small HDSS device ready as early as 2003. These early holographic data storage devices will have capacities of 125 GB and transfer rates of about 40 MB per second. Eventually, these devices could have storage capacities of 1 TB and data rates of more than 1 GB per second -- fast enough to transfer an entire DVD movie in 30 seconds. So why has it taken so long to develop an HDSS, and what is there left to do?
When the idea of an HDSS was first proposed, the components for constructing such a device were much larger and more expensive. For example, a laser for such a system in the 1960s would have been 6 feet long. Now, with the development of consumer electronics, a laser similar to those used in CD players could be used for the HDSS. LCDs weren't even developed until 1968, and the first ones were very expensive. Today, LCDs are much cheaper and more complex than those developed 30 years ago. Additionally, a CCD sensor wasn't available until the last decade. Almost the entire HDSS device can now be made from off-the-shelf components, which means that it could be mass-produced.
Although HDSS components are easier to come by today than they were in the 1960s, there are still some technical problems that need to be worked out. For example, if too many pages are stored in one crystal, the strength of each hologram is diminished. If there are too many holograms stored on a crystal, and the reference laser used to retrieve a hologram is not shined at the precise angle, a hologram will pick up a lot of background from the other holograms stored around it. It is also a challenge to align all of these components in a low-cost system.
Researchers are confident that technologies will be developed in the next two or three years to meet these challenges.
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