Optical disc drive
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 and writing data. It is a computer's peripheral device, that stores data on optical discs. Some drives can only read from discs, but commonly drives are both readers and recorders. Recorders are sometimes called burners or writers.
Common media and technology families include CD, DVD, and Blu-ray Disc. Standalone, non-computer, optical storage devices also exist, for example popular CD players, DVD players, and some DVD recorders, but those are not covered in this article. Optical disc drives are generally used for small-scale archival or data exchange, being slower and more materially expensive per unit than the moulding process used to mass-manufacture pressed discs. But they—along with flash memory—have displaced floppy disk drives and magnetic tape drives in most cases because of the low cost of optical media and the near-ubiquity of optical drives in computers and consumer entertainment hardware.
Laser and optics
The most important part of an optical disc drive is an optical path, placed in a pickup head (PUH),[1] usually consisting of semiconductor laser, a lense for guiding the laser beam, and photodiodes detecting the light reflection from disc's surface.[2]
Initially, CD lasers with a wavelength of 780 nm were used, being within infrared range. 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).
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. The second servo moves a head along the disc's radius, keeping the beam on a groove, a continuous spiral data path.
On read only media (ROM), during the manufacturing process the groove, 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 photodiodes that output electrical signals.
A 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[citation needed]. 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 5 mW, the writing laser is considerably more powerful. The higher writing speed, the less time a laser has to heat a point on the media, thus its power has to increase proportionally.[citation needed] DVD burner's laser often peaks at about 100 mW in continuous wave, and 225 mW pulsed.
For rewriteable 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. 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.
Double-sided media may be used, but they are not easily accessed with a standard drive, as they require to be physically reverted to access the data on the other side.
Double layer (DL) media have two independent data layers separated by a semi-reflective layer. Both layers are accessible from the same side, but require the optics to change the laser's focus. 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. Double-layered writable media have: a first polycarbonate layer with a (shallow) groove, a first data layer, a semi-reflective layer, a second (spacer) polycarbonate layer with another (deep) groove, and a second data layer. The first groove spiral usually starts on the inner edge and extends outerwards, while the second groove starts on the outer edge and extends inwards.
Some drives support Hewlett-Packard's LightScribe photothermal printing technology for labeling specially coated discs.
