Optical Data Storage

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Almost from the invention of the laser, researchers were considering the possibilities of optical data storage. Throughout the 1960s and 1970s, a number of companies were at work on optical data storage systems, held back in large part by the cost and performance level of available lasers. In 1982, Sony Corporation revolutionized the music industry with the introduction of the compact disc (CD). CDROM (compact disk read-only memory) systems for computers quickly followed, expanding the capability of desktop computing. More recently, writable optical disks have been developed, and considerable ground has been gained in holographic data storage technology. The audio compact disc is 120 mm or 80 mm in diameter. The larger size holds about 80 minutes of recordings, while the smaller one, only sold as CD singles, hold about 20 minutes of audio. As of 2006, the CD remains the standard commercial recording system. As of 2004, around 30 billion discs (CDs and their associated extensions) are sold worldwide.

Other forms of optical data storage includes digital versatile disc, sometimes called digital video disc, (DVD). It is used for data storage including movies with video and audio. They are similar to CDs with respect to size but are encoded with a different format and with a much higher density. The four basic types of DVDs are: single sided/single layer, single sided/ dual (double) layer, double sided/single layer, and double sided/dual layer. DVD disc capacity for 12-cm discs is: for a single sided/single layer, 4.7 GB (giga-bytes); double sided/single layer, 9.4 GB; single sided/ double layer, 8.5 GB; and double sided/double layer, 17 GB. Eight-centimeter discs are also available in the following four types: single sided/single layer, 1.4 GB; double sided/single layer, 2.8 GB; single sided/double layer, 2.6 GB; and double sided/double layer, 5.2 GB.

Some of the more popular formats of DVD include: DVD-R (DVD recordable), DVD+R, DVDRW (DVD rewriteable), and DVD+RW. DVD also comes in high density DVD, sometimes called high-definition DVD): HD-DVD. It is used specifically for high-definition video and data storage. Some of the commercial products using DVD technology include video games by Sony PlayStation and Microsoft Xbox.

Optical data storage refers to any method of storing data using light. The most common method is optical disk, which offers a data density considerably higher than magnetic methods. Three of the early types of optical disks were: ROM, or read only memory; WORM, or write once, read many times; and MO, or magneto optical disk, a disk which, like magnetic computer disks, can be repeatedly written on and repeatedly erased. Later, other types developed, including: CD-R (compact disc-recordable) and CDRW (compact disc rewritable).

For ROM systems, information is recorded on a master disk by pulsed laser. The laser beam is varied or modulated, such that digital data is encoded in the pulses. The beam heats up and distorts a thermally sensitive layer on the master disk, recording a bit of data as a depression in the surface. The depressions are submicron in size, separated by grooves spaced 1.6 microns apart.

Once the master disk is created, copies can be produced quickly and cheaply (the cost of a CD is estimated at less than $1). Injection molded polycarbonate replicas made from the master disk are coated with aluminum to increase reflectivity, then sealed in protective plastic.

The data retrieval system consists of a low power (3 mW), continuous wave diode laser, a series of optics to focus and circularize the beam before it reaches the CD, more optics to check that the beam is reading the proper area of CD at the correct location, and a detector to decode the signal. The disk spins, and the read head containing the laser and optics scans across it. The beam is reflected from the depressions on the optical disk, and the detector reads variations in the intensity and polarization of the light. These variations are decoded and converted to an electrical signal. In the case of a music CD, the electrical signal is transmitted to a speaker, the speaker diaphragm vibrates, and the result is music.

CDs are capable of carrying prodigious amounts of information, over 600 megabytes on a single disk. In addition to the music and film formats, CD-ROMs bring extensive databases to the desktop computer, and the average user’s fingertips. World atlases, encyclopedias, and comprehensive periodical indexes are just a few of the CD-ROM products available.

Write once, read many, or WORM systems, are a bit more complicated than ROM systems. Though they have essentially the same optical system for data retrieval, for writing operations they require a more powerful laser and a modified storage disk.

Writable WORM disks are made of different material than consumer CD-ROMs. Typically, a thermally sensitive film is sandwiched between layers of glass or plastic. During the write phase, digital data is converted into an optical signal by varying or modulating a laser beam. The laser puts out about 30 mW of power, since it has to be capable of distorting the write layer. The tightly focused, modulated beam shines through the transparent glass or plastic and hits the thermally sensitive layer, heating it to create distortions that represent bits of data. These distortions are usually either bumps, depressions, or variations in opacity in the material that will make changes in the reflectivity of the surface. To read the disk, the laser/ read assembly is scanned over the surface at lower power, and a detector reads and decodes variations in the surface reflectivity to obtain the original signal.

Once recorded, the data cannot be rewritten, and short of destruction of the disk, cannot be erased. WORM disks are being used for archival purposes or in documentation-intensive applications such as insurance, banking, or government.

Magneto-optical disks (MODs) are rewritable, and operate differently than either ROM or WORM disks. Data is not recorded as distortions of a thermally sensitive layer within the disk. Rather, it is written using combined magnetic and optical techniques. Digital data consisting of 1s and 0s is encoded in the optical signal from the laser in the usual manner. Unlike the ROM or WORM disks, however, the MOD write layer is magnetically sensitive.

On a microscopic level, magnetic materials are made up of tiny regions known as domains. Each domain acts like a small magnet. In non-magnetic material, the magnetic poles of the domains are randomly aligned. In magnetic material, the poles tend to align in the same direction, creating the macroscopic magnetic poles and field of the magnet. In paramagnetic material, the poles of the domains are flexible, and can be preferentially aligned by an external magnet so that the material becomes magnetic.

Returning to the MOD system, the modulated laser beam heats up a small spot of the magnetic write layer to its Curie temperature, the temperature at which magnetic material can become paramagnetic. The magnetic pole of the domain is then aligned by an external magnet located on the read/write head. As soon as the laser moves on, the spot cools down and the domain remains preferentially aligned. To record a binary 1, the magnetic pole of the external magnet is oriented upward, forcing the domain’s north pole to point upward. To write a binary 0, the external magnet’s magnetic field is reversed, and the domain’s north pole points downward.

The MOD is erased by orienting the external magnet’s north pole downward and scanning the laser across the disk with uniform beam intensity, recording 0s over the whole disk. The MOD is read by scanning a laser over the spinning disk and evaluating the effect of the magnetic pole orientations on the reflected light.

Other avenues of rewritable disk technology have been developed. One type uses differences in the reflectivity of a material in its crystalline and amorphous, or non-crystalline state, to record data. The laser heats up a tiny bit of surface and the material there crystallizes, standing for a binary 1. Bits of surface, still in the amorphous state, stand for a binary 0. To read, the laser is scanned over the surface, and the detector decodes the signal from variations in the reflectivity. To erase, the laser is scanned in a continuous wave, to heat the material up just enough to return to the amorphous 0 state.

The optical data storage capacity curve is going up exponentially. Optical disk data density is driven by the minimum spot size of the tightly focused laser beam writing the data. The spot size is directly proportional to the wavelength of the laser. The shorter the wavelength means the smaller the spot and the more data that can fit on the disk. Diode lasers currently used in optical disk systems emit in the red region of the spectrum (780 nm). Devices that emit in the green (532 nm) and the blue regions have been developed. Researchers are working to increase lifetime, output power, and reliability to be on a par with the currently used CD lasers. Those improvements will follow shortly, and it is simply a matter of time before even higher density optical disks are available.

The biggest advantage of optical data storage is the quantity of data that can be recorded. With

KEY TERMS

Curie temperature —The temperature at which magnetic domains become randomized.

Digital data —Data that is encoded in binary form, as a series of 1s and 0s.

Magnetic domain —Microscopic regions of a magnetic material that behave as miniature magnets and control the material’s behavior. If they are preferentially oriented, the material is magnetic; if they are randomly oriented, the material is non-magnetic.

Modulation —Variation, especially of optical signals.

Paramagnetic —A material whose magnetic domains are random, and can be oriented by an external magnetic field.

MODs, desktop computers can have the storage capabilities of a mainframe. There are at present, however, some drawbacks to the technology. Optical disk systems are significantly slower than conventional magnetic storage systems. Researchers are presently working on ways to consolidate and lighten the somewhat cumbersome optical systems required for read/write operations, allowing quicker operation.

Other methods of optical data storage are being explored, particularly holographic data storage. A hologram is simply an image recorded using optical phase information that makes it appear three-dimensional. A pattern of 1s and 0s can be recorded as easily as a picture, and more quickly than the corresponding number of 1s and 0s can be sequentially stored. Though groups have demonstrated the feasibility of this approach, the development of a rewritable material capable of recording holograms and offering long term storage stability is still in its early stages. Significant electronic development is required as well. For the time being, optical disk technology seems to be securely in the forefront.

Despite minor drawbacks, optical data techniques are the technology of the future for data storage. The potential for great strides forward in performance clearly exists.

Resources

BOOKS

Beiser, Leo. Unified Optical Scanning Technology. New York: Wiley-Interscience, 2003.

Hasegawa, Fumio, and Haruo Hiki, eds. Content Production Technologies. West Sussex, UK: John Wiley & Sons, 2004.

Petkovic, Milan, and Willem Jonker. Content-based Retrieval: A Database Perspective. Boston, MA: Kluwer Academic Publishers, 2004.

PERIODICALS

John, P. “Applied Physics: Toward Diamond Lasers.” Science 292, no. 5523 (2001): 1847-1848.

Shen, Yuzhen. “Contributed Papers: Optical Data Storage.” Applied Optics 41, no. 10 (2002): 1998-2007.

Kristin Lewotsky