When it comes time to relax and watch a movie or a recording of your favorite TV show, lasers are probably the last thing on your mind. Without them, however, not only would you be unable to watch your favorite DVD’s, but they would never have been recorded in the first place.
That’s because in order to record or play a DVD, you need an optical disc drive that uses laser lightwithin or near the visible light spectrum as part of the process of reading or writing data to and from optical discs. Some drives can only read from discs, but recent drives are commonly both readers and recorders, also called burners or writers. Compact discs, DVDs, and Blu-ray discs are common types of optical media that can be read and recorded by such drives.
History of optical disc drive
Optical disc drives are an integral part of stand-alone consumer appliances such as CD players, DVD players and DVD recorders. Laser disc technology, using a transparent disc, was invented by David Paul Gregg and James Russell in 1958 and first patented in 1961. The Gregg patents were purchased by MCA in 1968. Laser Disc was first available on the market, in Atlanta, on December 15, 1978, two years after the introduction of the VHS VCR, and four years before the introduction of the CD, which is also based on laser disc technology. The first Laser Disc title marketed in North America was theMCA DiscoVision release of Jaws. By the early 2000s, however, Laser Disc was completely replaced by the DVD in the North American retail marketplace.
The role of the laser
The most important part of an optical disc drive is an optical path, placed in a pickup head and usually consisting of a semiconductor laser, a lens for guiding the laser beam, and photodiodes, capable of converting light into an electrical signal, detecting the light reflection from the disc’s surface. Initially, CD lasers with a wavelength of 780 nm, which is within infrared range, were used. For DVDs, the wavelength was reduced to 650 nm (red color), and the wavelength for Blu-ray Disc was reduced to 405 nm (violet color).
During both recording and reading, a servomechanism is used to maintain a correct distance between the lens and the disc, ensuring that the laser beam is focused on a small laser spot on the disc. A second servo moves a head along the disc’s radius, keeping the beam on a groove, or continuous spiral data path.
During the manufacturing process, the groove on read only media (ROM), made of pits, is pressed on a flat surface called land. Because the depth of the pits is approximately one-quarter to one-sixth of the laser’s wavelength, the reflected beam’s phase is shifted in relation to the incoming reading beam, causing mutual destructive interference and reducing the reflected beam’s intensity. This is detected by photo diodes that output electrical signals.
A recorder encodes, or burns, data onto a recordable disc called a blank by selectively heating parts of an organic dye layer with a laser. This changes the reflectivity of the dye, thereby creating marks that can be read like the pits and lands on pressed discs. For recordable discs, the process is permanent and the media can be written to only once. While the reading laser is usually not stronger than 5mW, the writing laser is considerably more powerful. The higher the writing speed, the less time a laser has to heat a point on the media, so its power has to increase proportionally. Lasers used in DVD burners often peak at about 200mW, either in a continuous wave or in pulses.
For rewritable DVD media, the laser is used to melt a crystalline metal alloy in the recording layer of the disc. Depending on the amount of power applied, the substance may be allowed to melt back into crystalline form or left in an amorphous form, enabling marks of varying reflectivity to be created.
Because of the microscopic width of the groove on a DVD, the laser beam used for recording and reading must be precisely controlled and positioned. This is accomplished through the use of lenses, windows and mirrors supplied by Esco Optics, a leading manufacturer of precision optics.