Magneto-optical drive

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. The technology was introduced commercially in 1985. Although optical, they appear as hard disk drives to the operating system and can be formatted with any file system. Magneto-optical drives were common in some countries, such as Japan because of the success of the Sony MiniDisc, but have fallen into disuse.

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 3½-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. 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 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 of this two-pass process, it takes twice as long to write data as it does to read it.

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