A magneto-optical drive is a kind of optical disc drive capable of writing and rewriting data upon a magneto-optical disc. Both 130 mm (5.25 in) and 90 mm (3.5 in) form factors exist. In 1983, just a year after the introduction of the compact disc, Kees Schouhamer Immink and Joseph Braat presented the first experiments with erasable magneto-optical compact discs during the 73rd AES Convention in Eindhoven. The technology was introduced commercially in 1985. Although optical, they normally appear as hard disk drives to an operating system and can be formatted with any file system. Magneto-optical drives were common in some countries, such as Japan, but have fallen into disuse.
Overview
Early drives are 130 mm and have the size of full-height 130 mm hard-drives (like in the IBM PC XT). 130 mm media looks similar to a CD-ROM enclosed in an old-style caddy, while 90 mm media is about the size of a regular 3-inch floppy disk, but twice the thickness. The cases provide dust resistance, and the drives themselves have slots constructed in such a way that they always appear to be closed. Original MO systems were WORM (write once, read many), and later systems were read/write.
The disc consists of a ferromagnetic material sealed beneath a plastic coating. The only physical contact is during recording when a magnetic head is brought into contact with the side of the disc opposite to the laser, similar to Floptical drives, but not the same. During reading, a laser projects a beam on the disk and, according to the magnetic state of the surface, the reflected light varies due to the magneto-optic Kerr effect. During recording, laser power is increased to heat the material to the Curie point in a single spot. This enables an electromagnet positioned on the opposite side of the disc to change the local magnetic polarization. The polarization is retained after the temperature drops.
Each write cycle requires both a pass to erase a region and another pass to write information. Both passes use the laser to heat the recording layer; the magnetic field is used to change the magnetic orientation of the recording layer. The electromagnet reverses polarity for writing, and the laser is pulsed to record spots of "1" over the erased region of "0". As a result of this two-pass process, it takes twice as long to write data as it does to read it.
In 1996, Direct Overwrite technology was introduced for 90 mm discs eliminating the initial erase pass when writing. This requires special media.
By default, magneto-optical drives verify information after writing it to the disc, and are able to immediately report any problems to the operating system. This means writing can actually take three times longer than reading, but it makes the media extremely reliable, unlike the CD-R or DVD-R media upon which data is written without any concurrent data integrity checking. Using a magneto-optical disc is much more like using a diskette drive than a CD-RW drive.
During a read cycle, the laser is operated at a lower power setting, emitting polarized light. The reflected light has a change in Kerr rotation and Kerr ellipticity which is measured by an analyzer and corresponds to either a logical 0 or 1.
The 130 mm drives have been available in capacities from 650 MB to 9.2 GB. However, this is split in half over both sides of the disk. The 2.6 GB disks, for example, have a formatted capacity of 1.2 GB per side. The 130 mm drives were always SCSI. The 90 mm discs had their entire capacity on one side, with no capability to flip them over. The 90 mm drives were produced in SCSI, IDE, and USB formats. Capacities range from 128 MB to 2.3 GB.
While they were never particularly popular with consumers (the main consumer market was the 90 mm drives), the 130 mm drives had some lasting service in corporate storage and retrieval. Optical libraries, such as the Hewlett Packard 40XT, were created to automate loading and storing of the disks. A self-contained unit holding 16 or more disks and connected by SCSI to a host computer, the library required specialized archival software to store indices of data, and select disks. Popular uses were for legal document storage and medical imaging, where high reliability, long life, and (at the time) high storage capacity were required. The optical libraries could also manually be used on a Windows 2000/XP machine by selecting and ejecting discs under the Computer Management icon's Removable Storage Service, but this is cumbersome in practice.
LIMDOW
Light Intensity Modulated Direct OverWrite (LIMDOW) technology used a different write technology, which improved on the performance levels of earlier magneto-optical devices.