Working of Holographic Memory

How Holographic Memory Will Work

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.

In a holographic memory device, a laser beam is split in two, and the two resulting beams interact in a crystal medium to store a holographic recreation of a page of data.

CDs and DVDs are the primary data storage methods for music, software, personal computing and video. A CD can hold 783 megabytes of data, which is equivalent to about one hour and 15 minutes of music, but Sony has plans to release a 1.3-gigabyte (GB) high-capacity CD. 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. In this article, you will learn how a holographic storage system might be built in the next three or four years, and what it will take to make a desktop version of such a high-density storage system.

A Little Background

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.
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.
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.

The Basics

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
  

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.

Images courtesy Lucent Technologies These two diagrams show how information is stored and retrieved in a holographic data storage system.

An advantage of a holographic memory system is that an entire page of data can be retrieved quickly and at one time. In order to retrieve and reconstruct the holographic page of data stored in the crystal, the reference beam is shined into the crystal at exactly the same angle at which it entered to store that page of data. Each page of data is stored in a different area of the crystal, based on the angle at which the reference beam strikes it. During reconstruction, the beam will be diffracted by the crystal to allow the recreation of the original page that was stored. This reconstructed page is then projected onto the charge-coupled device (CCD) camera, which interprets and forwards the digital information to a computer.
The key component of any holographic data storage system is the angle at which the second reference beam is fired at the crystal to retrieve a page of data. It must match the original reference beam angle exactly. A difference of just a thousandth of a millimeter will result in failure to retrieve that page of data.

Desktop Holographic Data Storage

After more than 30 years of research and development, a desktop holographic storage system (HDSS) is close at hand. 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. With such technologies on the market, you will be able to purchase the first holographic memory players by the time "Star Wars: Episode II" is released on home 3-D discs. This DVD-like disc would have a capacity 27 times greater than the 4.7-GB DVDs available today, and the playing device would have data rates 25 times faster than today's fastest DVD players.