Traditional single layer (SL) writable media are produced with a spiral groove molded in the protective polycarbonate layer (not in the data recording layer), to lead and synchronize the speed of recording head. Both layers are accessible from the same side, but require the optics to change the laser's focus. Depending on the amount of power applied, the substance may be allowed to melt back (change the phase back) into crystalline form or left in an amorphous form, enabling marks of varying reflectivity to be created.ĭouble-sided media may be used, but they are not easily accessed with a standard drive, as they must be physically turned over to access the data on the other side.ĭouble layer (DL) media have two independent data layers separated by a semi-reflective layer. DVD burners' lasers often peak at about 200 mW, either in continuous wave and pulses, although some have been driven up to 400 mW before the diode fails.įor rewritable CD-RW, DVD-RW, DVD+RW, DVD-RAM, or BD-RE media, the laser is used to melt a crystalline metal alloy in the recording layer of the disc. The higher writing speed, the less time a laser has to heat a point on the media, thus its power has to increase proportionally. While the reading laser is usually not stronger than 5 mW, the writing laser is considerably more powerful. For recordable discs, the process is permanent and the media can be written to only once. This changes the reflectivity of the dye, thereby creating marks that can be read like the pits and lands on pressed discs. This is detected by photodiodes that output electrical signals.Ī recorder encodes (or burns) data onto a recordable CD-R, DVD-R, DVD+R, or BD-R disc (called a blank) by selectively heating parts of an organic dye layer with a laser. 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. On read only media (ROM), during the manufacturing process the groove, made of pits, is pressed on a flat surface, called land. The second servo moves a head along the disc's radius, keeping the beam on a groove, a continuous spiral data path.
Two main servomechanisms are used, the first one to maintain a correct distance between lens and disc, and ensure the laser beam is focused on a small laser spot on the disc. 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). Initially, CD lasers with a wavelength of 780 nm were used, being within infrared range. The most important part of an optical disc drive is an optical path, placed in a pickup head ( PUH), usually consisting of semiconductor laser, a lens for guiding the laser beam, and photodiodes detecting the light reflection from disc's surface. Optical drives-along with flash memory-have mostly displaced floppy disk drives and magnetic tape drives for this purpose because of the low cost of optical media and the near-ubiquity of optical drives in computers and consumer entertainment hardware.ĭisc recording is generally restricted to small-scale backup and distribution, being slower and more materially expensive per unit than the moulding process used to mass-manufacture pressed discs.
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They are also very commonly used in computers to read software and consumer media distributed in disc form, and to record discs for archival and data exchange. Optical disc drives are an integral part of stand-alone consumer appliances such as CD players, DVD players and DVD recorders. Compact discs, DVDs, and Blu-ray discs are common types of optical media which can be read and recorded by such drives. Recorders are sometimes called burners or writers. Some drives can only read from discs, but recent drives are commonly both readers and recorders. In computing, an optical disc drive ( ODD) is a disk drive that uses laser light or electromagnetic waves near the light spectrum as part of the process of reading or writing data to or from optical discs.
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