The storage media of most optical storage systems in production today are in the form of a rotating disk. In general the disks are preformatted using grooves and lands (tracks) to enable positioning an optical pickup and recording head to access information on the disk. A focused laser beam emanating from the optical head records information on the media as a change in the material characteristics.
To record a bit, the laser generates a small spot on the media that modulates the phase, intensity, polarization, or reflectivity of a readout optical beam; that beam is subsequently “read” by a detector in the optical head. Drive motors and servo systems rotate and position the disk media and the pickup head, thus controlling the position of the head with respect to data tracks on the disk. Additional peripheral electronics are used for control and for data acquisition, encoding, and decoding. As for all data storage systems, optical disk systems are characterized by their storage capacity, data transfer rate, access time, and cost.