Technical Aspects
Initially the drives were 130 mm and had the size of full-height 130 mm hard-drives (like in 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 1.44MB 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. Originally, MO discs were WORM (write once, read many) drives, but later read/write MO drives became available.
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. 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, the laser power is increased so it can heat the material up to the Curie point in a single spot. This allows an electromagnet positioned on the opposite side of the disc to change the local magnetic polarization, and the polarization is retained when 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 for actually changing 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 it takes twice as long to write data as it does to read it.
In 1996, a Direct Overwrite technology was introduced for 90 mm discs, to avoid 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 report any problems to the operating system immediately. This means that writing can actually take three times longer than reading, but it makes the media extremely reliable, unlike the CD-R or DVD-R technologies upon which data is written to media without any concurrent data integrity checking. Using a magneto-optical disc is a lot more like using a diskette drive than using a CD-RW.
In 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.
Progress in magneto-optical technology received a boost in the spring of 1997 with the launch of Plasmon’s DW260 drive. This used Light Intensity Modulated Direct OverWrite technology to achieve an increased level of performance over previous magneto-optical drives.
The 130mm drives were available in capacities from 650MB to 9.2GB. However, this was split in halves per the sides of the disk. The 2.6GB disks, for example, had a formatted capacity of 1.2GB per side. The 130mm drives were always SCSI. The 90mm discs had their entire capacity on one side with no capability to flip them over. The 90mm drives were available in SCSI, IDE, and USB formats. Capacities ranged from 128MB to 2.3GB.
While they were never particularly popular with consumers (the main consumer marketing being for the 90mm drives), the 130mm 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 indexes of data and select disks. Popular uses were for legal document storage and medical imaging, where high reliability, long life, and (for 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 was cumbersome in practice.
Read more about this topic: Magneto-optical Drive
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