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Erasable optical disks are physically much like WORM disks. The main difference between the two is in the coating used on the recording surface. There are two types of erasable optical disk: Phase-Change and Magneto-Optic.

Erasable optical disks that employ phase-change technology rely on coatings for the recording surface consisting of thin films of tellurium or selenium. These coatings have the ability to exist in two different optically detectable states, amorphous and crystalline, and will switch between the two when heated to two different temperatures. If a spot on the recording surface is heated to a low temperature with a laser with 8 milliwatts of power, the spot will crystallize. If it is heated to a higher temperature with a laser with 18 milliwatts of power, the spot will melt and when it cools will revitrify to the amorphous state.

To register data on the recording surface, the power of the laser scanning the disk is simply modulated between 8 and 18 milliwatts. If a crystallized spot is scanned with 8 milliwatts of power, it will remain crystallized, if it is scanned with 18 milliwatts of power it will switch to the amorphous state. Similarly, if an amorphous spot is scanned with l8 milliwatts of power, it will remain in the amorphous state, and if it is scanned with 8 milliwatts of power it will switch to the crystallized state.

Reading data from the disk is simply a matter of scanning the surface with a low power laser (1 milliwatt) and detecting the changes in reflectivity that exist on the recording surface.

Erasable optical disks that employ magneto-optic technology store data on the disk magnetically, but read and write it optically. The coating used on the recording surface is a rare-earth transition-metal alloy such as terbium iron cobalt, terbium iron, or gadolinium terbium iron. It has the property of allowing the polarity of its magnetization to be changed when it is heated to a certain temperature (150

C).

Recording data is a two-stage process requiring two passes of the disk head over a sector. The first pass serves to erase the contents of the sector. This is done by first placing a magnetic field with north-pole down in the vicinity of the spot upon which the laser focuses. As the sector is scanned, this spot will quickly heat to 150 C and then immediately cool. As it does, domains of north-pole down are recorded throughout the sector; a north-pole down domain represents a 0 bit. Once the sector has been erased by the first pass, data can be written by the second.

When the sector is scanned a second time, the applied magnetic field is reversed. By modulating the power of the laser, selected portions of the sector can be heated to the required temperature and have their magnetizations reversed to north-pole up; a north-pole up domain represents a 1 bit. The remaining unheated portions of the sector retain their north-pole down magnetization.

Recorded data is read from the disk in a single pass. Reading relies on a physical effect known as the

Kerr magneto-optic effect, which was discovered by Kerr (1877) and which causes light passing through

a magentic field to become elliptically polarized. This effect allows the magnetization of a spot or

domain of the disk surface to be detected optically. To read a disk sector, it is scanned with the laser in a lower power mode (1 milliwatt) than used for writing (8 milliwatts). If the magnetization of a domain being scanned is north-pole-up, the polarization of the light reflected from the surface will be rotated clockwise, if north-pole-down, counter-clockwise. The sequence of polarity changes is detected and interpreted to produce a bit stream.

The main stumbling block in the development of erasable magneto-optic disk technology was the chemical instability of the coating caused by its repeated heating and cooling during the write-erase cycle. This instability limited the number of cycles that could occur and was caused by the high temperature (600 C) required to change the magnetization of a domain. The development of newer coatings that require lower temperatures (150 C) solved this problem. Current erasable magneto-optic

disks now allow some ten million or more write-erase cycles.

There are performance differences between the two types of erasable technologies. The Kerr effect only causes about a 1 percent change in the polarization of the reflected light. This requires more optics and associated hardware in the disk head of a magneto-optic disk drive to detect such a small change. The reflectivity difference between the two states of phase-change type erasable optical disks is relatively large and much easier to detect so the disk head can be much simpler. The net result is that the seek performance of a magneto-optic disk drive will generally be poorer than that of a phase-change drive because of its more massive disk head.

Optics

The optical assemblies found in all types of optical disk drives are similar in nature and resemble those of a medium power microscope. The task of the assembly is to focus a beam of coherent light from a semiconductor laser on to a 1 micron size spot and provide a return path for reflected light to reach a photodetector. This is a difficult feat to accomplish as the requirements for economical mass production of disks and drives imply a certain degree of flexibility in their precision. As such, it cannot be

guaranteed that the disk platter will be perfectly flat or round, or that the hole for the drive's spindle will be perfectly centered. These imperfections can cause the outer edge of the disk to move up and down as much as 1 millimeter and the disk itself side-to-side as much as 60 micrometers (37 tracks) as it rotates several times a second. These motions cause the position of a track to vary with time in three dimensions and require the optical assemblies to be much more than a simple arrangement of lenses.

To follow the moving track, the objective lens is encased in a voice-coil like arrangement that allows it to be moved up and down to adjust its focus and from side to side to allow it to follow the wandering track. This radial flexibility gives most optical disk drives the ability to quickly access more than one track on the disk from a single position of the access mechanism, simply by adjusting the position of the objective lens. This viewing capability is usually limited to a window of some 10 to 20 tracks on either side of the current position of the access mechanism.