Optical disk – a storage medium from which data is read and to which data is written by lasers. Optical disks can store much more data (up to six gigabytes) than most portable magnetic media. There are three basic types of optical disks:
CD-ROM – Like audio CDs, CD-ROMs come with data already encoded onto them. The data is permanent and can be read any number of times, but CD-ROMs cannot be modified. The CD-ROM drive’s nominal speed is the same as its transfer rate. Single-speed drives have a 150KBps transfer rate while the rate for 12X drives is 1.8 Mbps. It is expected in the future that manufacturers are likely to shift from CLV (constant linear velocity) to CAV (constant angular velocity). While CLV rotates at varying speeds, CAV moves the disk at one constant speed. While this may not sound like much of a change to most, the difference is that this method is easier on the spindle motor because it does not require the drive to change motor speed as often, resulting in an improvement in performance.
WORM – Stands for write-once, read-many. With a WORM disk drive, the disk can be read and reread but once it is recorded it cannot be changed. After that, the WORM disk behaves just like a CD-ROM. The WORM drive is a high-capacity storage device and is best for storing archives and other large amounts of unchanging information.
Erasable-Optical disks – can be erased and loaded with new data, just like magnetic disks. These are often referred to as EO (erasable optical) disks.
These technologies are not compatible with each other. Each requires a different type of disk and drive.
An optical disk drive reads and writes data onto the disk. The disk is read by means of laser, then a magnetic field in addition to the laser is employed to write data onto the optical disk. The disk is exposed to a magnet on the label side and to the laser on the other side. A laser is used for two primary reasons: to allow a tiny one-micron diameter spot to be heated by an optical lens and the laser has enough energy to instantaneously reach Curie temperature (Curie temperature is 300 degrees Celsius, the level at which the magnetic domain loses its characteristic as a magnet).
Construction of the optical disk – The optical disk is mostly made of polycarbonate. The poly-carbonate plate should always allow the laser beam to transmit completely through the disk without a problem. Resin is applied to the disk substrate to ensure that the disk is not harmed in any way during the process (including heat damage, damage by impact, etc.). On the poly-carbonate resin substrate are seven types of film. Reflective film improves the read process. Protective film protects recording film.
Both first and second dielectric film protect the magneto-optical film, which is used for recording. Protective film protects the polycarbonate surface, and polycarbonate resin is a transparent plate.