Optical disc drive
An optical disc drive is a disk drive that utilizes electromagnetic waves or a laser light near the light spectrum in order to read and write data to or from optical discs. Some drives can only read from discs; however, most recent drives are generally both readers and recorders. Recorders are often times referred to as burners or writers. CDs, DVDs, and Blu-ray discs are familiar examples of optical media.
The laser and optics aspects of optical disc drives are very complex. The most important part of an optical disc drive is an optical path. It is usually placed in a pickup head and consists of semiconductor laser, a guiding lens for the laser beam, and photodiodes to detect the light reflection from disc’s surface. In addition, two main servomechanisms are used to maintain an accurate distance between lens and disc and to ensure that the laser beam is focused on a small laser spot on the disc. Read only media consists of pits, which make up the groove, which is pressed on a land or flat surface. Because the pits are significantly smaller than the laser’s wavelength, the reflected beam’s phase is shifted in relation to the incoming reading beam. This, in turn, causes shared destructive interference and reduces the reflected beam’s strength. A recorder burns data onto a recordable blank by using a laser to selectively heat parts of an organic dye layer. This changes the reflectivity of the dye, thus creating marks that can be read like the pits and lands on pressed discs. For recordable discs, the data is permanently burned. The writing laser is considerably more powerful than the recordable layer, and the higher writing speed, the less time a laser has to heat a point on the disc. For rewritable discs, the laser melts a crystalline metal alloy in the recording layer of the disc, thereby allowing the substance to melt back if necessary.
Optical drives have rotational mechanisms that are very different from those found in hard disc drives. The final methods keep a consistent number of revolutions per minute, which in turn produces a higher throughput, and is generally achievable at an outer disc area, as compared to inner area. In reference to loading mechanisms, current optical drives use either a tray-loading mechanism or a slot-loading mechanism. In a tray-loading mechanism, the disc is loaded onto a motorized or manually operated tray; however, in a slot-loading mechanism, the disc is slid into a slot and drawn in by motorized rollers. Additionally, some drive models have a top-loading mechanism where the drive lid is opened aloft and the disc is placed directly onto the spindle.
The majority of internal drives for personal computers, servers and workstations are designed to correspond with a standard 5.25 inch drive bay and connect to their host through an ATA or SATA interface. When the optical disc drive was initially developed, it was difficult to add to computer systems. Some computers were formatted to correspond with the 3.5 inch disks, and did not have a place for a large internal device. To solve this problem, a parallel port external drive was developed that connected a printer to a computer. Today, most optical drives are backwards compatible with their ancestors up to CD, although this is not required by standards.