Optical Storage

Summary

Optical storage refers to a variety of technologies that are used to read and write data. It employs special materials that are selected for the way they interact with light (an optical, or visible, medium). Most optical storage devices being manufactured are digital, though some analog systems remain in use. Both the computer and entertainment industries offer numerous practical applications of optical storage devices. Common optical storage applications are compact disks (CDs), digital versatile disks (DVDs), and Blu-ray disks (BDs). A variety of data can be stored optically, including audio, video, text, and computer programs. Data are stored in binary form.

Definition and Basic Principles

Optical storage differs from other data storage technologies such as magnetic tape, which stores data as an electrical charge. Most optical storage is in the form of optical disks, which are flat and circular. They contain binary data in the form of microscopic pits, which are non-reflective and have a binary value of 0, and smooth areas, which are reflective and have a binary value of 1. Optical disks are both created and read with a laser beam. The disks are encoded in a continuous spiral running from the center of the disk to the perimeter. Some disks are multilayer: with these disks, after reaching the perimeter, another spiral track is etched back to the center.

The amount of data storage is dependent upon the wavelength of the laser beam. The shorter the wavelength, the greater the storage capacity (shorter-wavelength lasers can read a smaller pit on the disk surface). For example, the high-capacity Blu-ray disk uses short-wavelength blue light. Lasers can be used to create a master disk from which duplicates can be made by a stamping process. Another, less common form of optical storage is optical tape, which consists of a long, narrow strip of plastic upon which patterns can be written and from which the patterns can be read back.

Optical media is more durable than electromagnetic tape and is less vulnerable to environmental conditions. However, with early formats including, CDs and DVDs, the speed of data retrieval is considerably slower than that of a computer hard drive. The storage capacity of most optical disks is also significantly less than that of hard drives, though experimental formats continue to expand capacity.

Background and History

Optical storage originated in the nineteenth century. In 1839, English inventor John Benjamin Dancer produced microphotographs with a reduction ratio of 160:1. Microphotography progressed slowly for almost a century until microfilm began to be used commercially in the 1920s. Between 1927 and 1935, more than three million pages of books and manuscripts in the British Library were microfilmed by the Library of Congress. Newspaper preservation on film had its onset in 1935 when Kodak's Recordak division filmed and published the New York Times on thirty-five-millimeter (mm) microfilm reels.

Analog optical disks were developed in 1958 by American inventor David Paul Gregg, who patented the videodisk in 1961. In 1969, physicists at the Netherlands-based Royal Philips Electronics began experimenting with optical disks. Subsequently, Philips and the Music Corporation of America (MCA) joined forces to create the laser disk (Laserdisk), which was first introduced in the United States in 1978. Although the laser disk achieved greater popularity in Asia and Europe than it did in the United States, it never successfully competed with VHS tape.

In 1980, Philips partnered with Sony to develop the compact disk (CD) for the storage of music. A few years later, the CD had evolved into a compact disk read-only memory (CD-ROM) format, which, in addition to audio, could store computer programs, text, and video. In 1996, the digital versatile disk (DVD) format was first introduced by Toshiba in Japan; it first appeared in the United States in 1997; in Europe in 1998; and in Australia in 1999. Several different versions of the DVD would follow.

A format war between two higher-capacity data storage technologies emerged in 2006 when Sony's Blu-ray and Toshiba's HD DVD players became commercially available for the recording and playback of high-definition video. Two years later, Toshiba conceded to Sony; Blu-ray was based on newer technology and had a greater storage capacity. Other variants of optical storage were also introduced over the years, often for specialized use or in pursuit of greater capacity, but none proved as popular as CDs, DVDs, or Blu-ray.

How It Works

CDs, DVDs, and Blu-ray disks are produced with a diameter of 120 mm. The storage capacity is dependent upon the wavelength of the laser: the shorter the wavelength, the greater the storage capacity. CDs have a wavelength of 780 nanometers (nm), DVDs have a wavelength of 650 nm, and Blu-rays have a wavelength of 405 nm. Some disk drives can read data from a disk while others can both read and write data.

Optical System. In a disk reader, the optical system consists of pickup head (which houses a laser), a lens for guiding the laser beam, and photodiodes that detect the light reflection from the disk's surface. The photodiodes convert the light into an electrical signal. An optical disk drive contains two main servomechanisms: one maintains the correct distance between the lens and the disk and also ensures that the laser beam is focused on a small area of the disk; the other servomechanism moves the head along the disk's radius, keeping the beam on a continuous spiral data path.

The same servomechanism can be used to position the head for both reading and writing. A disk writer employs a laser with a significantly higher power output. It burns data onto the disk by heating an organic dye layer, which changes the dye's reflectivity. Higher writing speeds require a more powerful laser because of the decreased time the laser is focused on a specific point. Some disks are rewritable—they contain a crystalline metal alloy in their recording layer. Depending on the amount of power applied, the substance may be melted into a crystalline form or left in an amorphous form. This enables the creation of marks of varying reflectivity. The number of times the recording layer of a disk can be reliably switched between its crystalline and amorphous states is limited. Estimates range from 1,000 to 100,000 times, depending on the type of media. Some formats may employ defect-management schemes to verify data as it is written and skip over or relocate problems to a spare area of the disk.

Double-Layer Media. Double-layer (DL) media has up to twice the storage capacity of single-layer media. DL media have a polycarbonate first layer with a shallow groove; a first data layer, a semi-reflective layer; a second polycarbonate spacer layer with a deep groove; and a second data layer. The first groove spiral begins on the inner diameter and extends outward; the second groove starts on the outer diameter and extends inward. If data exists in the transition zone, a momentary hiccup of sound and/or video will occur as the playback head changes direction. Formats with more than two layers have also been produced.

Disk Replication. Many commercial optical disks are copy protected; in some cases, a limited number of copies can be made. Disks produced on home or business computers can be readily copied with inexpensive (or included) software. If two optical drives are available, a disk-to-disk copy can be made. If only one drive is available, the data is first stored on the computer's hard drive and then transferred to a blank disk placed in the same read-write drive. For copying larger numbers of disks, dedicated disk-duplication devices are available. The more expensive ones incorporate a robotic disk-handling system, which automates the process. Some products incorporate a label printer. Industrial processes are used for mass replication of more than 1,000 disks, such as DVDs, CDs, or computer programs. These disks are manufactured from a mold and are created via a series of industrial processes including pre-mastering, mastering, electroplating, injection molding, metallization, bonding, spin coating, printing, and advanced quality control.

Applications and Products

Numerous applications and products are focused on optical storage. Most applications are geared toward the computer and entertainment industries, though archival needs are another important field.

Optical Disks. Most optical disks are read-only; however, some are rewritable. They are used for the storage of data, computer programs, music, graphic images, and video games. Since the first CD was introduced in 1982, this technology has evolved markedly. Optical data storage has in large part supplanted storage on magnetic tape. Although optical storage media can degrade over time from environmental factors, they are much more durable than magnetic tape, which loses its magnetic charge over time. Magnetic tape is also subject to wear as it passes through the rollers and recording head. This is not the case for optical media, in which the only contact with the recording surface is the laser beam.

CDs are commonly used for the storage of music: A CD can hold an entire recorded album and supplanted cassette tapes and vinyl records, which are more easily subject to wear and degradation. (However, those formats, especially vinyl, have seen a resurgence due to a perceived superiority of their sound quality.) A limitation of the CD is its storage capacity, which is typically 700 megabytes (MB) of data (eighty minutes of music).

The DVD, which appeared in 1996, rapidly gained popularity and soon outpaced VHS tape for the storage of feature-length movies. The standard DVD can store 4.7 gigabytes (GB) of data in single-layer format and 8.5 GB in dual-layer format. The development of high-definition television fueled the development of higher-capacity storage media. The Blu-ray (BD) disk can store about six times the amount of data as a standard DVD: 25 GB of data in single-layer format and 50 GB of data in dual-layer format. An evolution of the BD disk is the 3-D format; the increased storage capacity of this medium allows for the playback of video in three dimensions.

Computer Applications. Many computers contain one or more optical drives, although tablets and some laptops do not due to size considerations and some computer makers have moved away from optical drives altogether. A computer's optical drive may be used to load computer programs stored on an optical disk onto a hard drive, for data storage and retrieval, and for playback of CDs and DVDs. If a computer's optical drive is write-capable, data from the computer can be written (burned) onto blank or rewritable optical disks. As the price of Blu-ray technology fell, some computers also began to include Blu-ray readers. Most internal drives for computers are designed to fit in a 5.25-inch drive bay and connect to their host via an interface. External drives can be added to a computer through various connectivity standards, such as universal serial bus (USB).

Entertainment Applications. Standalone optical disk players are a common component of home entertainment systems. Most play DVDs as well as CDs, though some are specific to one format. As with computer applications, the presence of Blu-ray devices increased as prices fell. Some devices load a single optical disk at a time; others load a magazine holding multiple optical disks, from which users can select using an interface on the device or a remote control. CD, DVD, and Blu-ray players contain audio and video outputs to interface with home entertainment systems. If attached to an audio-video receiver (AVR), surround-sound audio playback can be enjoyed. Many later generations of players also have internet connectivity, moving beyond their optical data reading capability to allow for online streaming content and other forms of interconnectivity with the entire entertainment system.

Many automobiles contain a CD player for playback of audio, and in the early twenty-first century in-vehicle DVD drives became more common. DVD video is typically displayed to backseat passengers for safety reasons, though the rise of dashboard touchscreens made front-seat viewing capability more common. Some vehicle navigation systems have incorporated optical drives as well to load route information. Portable standalone CD, DVD, and Blu-ray players are also available. These range from small devices that can be strapped on an arm to systems similar in size to a laptop computer.

Games. Although a wealth of games can be played on a computer, a number of devices in the marketplace are designed strictly for game playing. Most of these devices attach to a television set or monitor and are interactive; thus, the player can immerse oneself in the action. Some accommodate more than one player. Although earlier devices had proprietary cartridges for data storage, these were generally superseded by DVDs and BDs.

Data Backup and Archival Storage. While entertainment and other consumer applications are the most familiar applications of optical storage, the technology has also been explored for other means. Prominent among these is the archival storage of data. Microfilm is still held by some libraries to archive newspapers and other publications, though digital formats now dominate. CDs and DVDs have commonly been used as a method of backing up data from computers, though solid-state technology such as USB flash drives have become more popular as their capacity has grown and prices have fallen. Researchers continue to experiment with different formats of optical storage that provide greater capacity. However, the ultimate longevity of optical media is uncertain due to the relative youth of the technology.

Careers and Course Work

The optical-storage industry offers careers ranging from entry-level positions such as assemblers to high-level and highly technical positions, which require a scientific or engineering degree. Management positions are also available. The high-level positions require at least a bachelor's degree with course work in several fields related to laser technology: engineering, physics, computer science, mathematics, and robotics. Many positions require a master's or doctorate degree. Management positions are more accessible to individuals with a master of business administration (MBA) degree. Positions are available for individuals with scientific degrees in both the government and private sector. The ability to be a team player is often of value for these positions because ongoing research is often a collaborative effort. University research positions are also available; in this arena, the employee is expected to divide his or her time between research and teaching.

Technicians are needed in a variety of fields for equipment repair. Many of these positions require some training beyond high school at a community college or trade school. If a company employs a number of technicians, supervisory positions may be available.

Social Context and Future Prospects

Within decades after optical-storage devices appeared on the market, they and their playback systems became innumerable and ubiquitous. Standalone DVD and CD players remain common household items, though they saw some decline in popularity as computers became cheaper and therefore more widespread. In the United States and other highly developed countries, most households contain one or more computers with optical drives, often including a CD and DVD burner. Video game systems using optical disks are also extremely popular, though the games themselves have often stirred cultural debate over issues such as violence and addiction.

However, while optical storage technology rose to popularity extremely quickly, it faced serious challenges by the 2010s. In particular, the rise of digital downloads and then streaming media quickly eroded the consumer market for CDs, DVDs, BDs, and their respective players. In 2014 global revenue from downloaded and streaming music overtook CD sales for the first time, and streaming services such as Spotify especially continued to grow in popularity at the expense of physical disks. In 2016 DVDs in turn were surpassed in sales by streaming video content on services such as Netflix and Hulu, according to the Entertainment Retailers Association.

Meanwhile, the falling costs of solid-state hard-disk storage systems such as flash drives, which have desirable features such as fast retrieval speeds and no moving parts, changed the face of the computer industry. Online stores for downloading software and other applications largely replaced the model of buying software on CD or DVD and uploading it onto a computer's hard drive. Personal computer makers such as Apple began to phase out optical drives from laptops and other products altogether, while tablet computers without optical drives gained in popularity. Even some automobiles stopped including built-in CD players in favor of connectivity with smartphones and other internet-based multimedia systems.

Still, CDs, DVDs, and Blu-rays have all remained in use. High-capacity optical media are used not only for entertainment, such as high-definition movies, but also for data storage. Technology companies have developed various optical formats that exceed the capacity of Blu-ray, with only Ultra HD Blu-ray finding commercial usage as of 2021. Ultra HD Blu-ray was first introduced in 2016 and is basically an improved version of a regular Blue-ray. It can store more information and has numerous video enhancement features. As of 2020, DVDs remained the best-selling optical disks, accounting for 60 percent of the market. Blu-ray disks made up 32.9 percent of sales, while Ultra HD Blu-ray was at 7.1 percent. The technologies are often not economically viable, and they are often quickly superseded by other formats with even more potential. Among the optical formats researched have been Sony's Archival disk (AD), stacked volumetric optical disc (SVOD), and holographic data storage.

Bibliography

Bunzel, Tom. Easy Creating CDs and DVDs. 2d ed. Indianapolis: Que, 2005. A book and CD-ROM edition, which explains how to burn CDs and DVDs.

Geutskens, Yoeri. "The State of Ultra HD Blu-ray." Flat Panels HD, 15 Apr. 2021, www.flatpanelshd.com/focus.php?subaction=showfull&id=1618485949. Accessed 28 Feb. 2022.

McDonald, Paul. Video and DVD Industries. London: British Film Institute, 2008. Examines the business of video entertainment and details divisions of the video business. It outlines industry battles over incompatible formats, from the Betamax-VHS war to competing laser disk systems, alternatives such as video compact disk or Digital Video Express, and the introduction of HD DVD and Blu-ray high-definition systems.

Meinders, Erwin R. Optical Data Storage: Phase-Change Media and Recording. Springer, 2011.

"Optical Storage." Computer History Museum, 2018, www.computerhistory.org/revolution/memory-storage/8/262. Accessed 30 Aug. 2018.

Silva, Robert. "What Is Blu-ray?" Lifewire, 22 Sept. 2021, www.lifewire.com/what-is-blu-ray-1846537. Accessed 28 Feb. 2022.

Taylor, Jim, Mark R. Johnson, and Charles G. Crawford. DVD Demystified. 3d ed. New York: McGraw-Hill, 2006. Covers basic technology as well as production and authoring processes. Describes the vast variety of competing video, audio, and data formats and explains how DVD standards and specs dovetail or clash with related digital-media standards.

Taylor, Jim, et al. Blu-ray disk Demystified. New York: McGraw-Hill, 2009. Provides a detailed overview of Blu-ray technology, its uses, and its shortcomings.

Xu, Duanyi. Multi-Dimensional Optical Storage. Springer, 2016.