This section describes the theory of operation of MO drives. It is quite technical, and it is certainly not necessary to understand this in order to use a magneto-optical drive.

Magneto-optical drives use, as the name suggests, both magnetic and optical effects to read and write data. A layer within the media contains magnetically sensitive elements. When this layer is heated to its "Curie point", about 200°C, the polarity of the magnetic elements can be changed by an external magnetic field from the drive head.

To write data, on normal media two disk revolutions are needed. On the first revolution, a magnetic field is applied and the laser heats up the target area of the disk to its Curie point. This causes the magnetic elements to all align themselves parallel with the magnetic field. This effectively erases the target area, recording all 0 bits. On the next revolution of the disk, the magnetic field is reversed, and the laser heats up only those areas which are to have 1s recorded.

Because the written magnetic polarity is "frozen into" the disk, MO disks are not susceptible to magnetic fields as magnetic media are.

Data is read from the disk in a purely optical way. The laser is used at a low power which does not heat the disk. Depending on the recorded magnetic polarity, the polarity of the laser light reflected from the disk is rotated a few degrees either way. This rotation is called the "Kerr effect". The drive detects this, and thus determines whether a 0 or 1 was read.

LIMDOW Media

Use of LIMDOW media can greatly improve write speeds. LIMDOW-capable drives can read and write to conventional MO disks too.

When a LIMDOW disk is inserted, the drive detects this and enables additional control over the laser. LIMDOW disks have a different composition to normal media, which makes it possible to rewrite data in a single revolution rather than the two required by conventional disks.

The drive records data by heating areas of the disk to their Curie point, as with conventional media. However, depending on the laser beam power, the magnetic element orients itself with either the external magnetic field or with a reference layer that is built into the disk. Thus data can be written in a single pass.

Data is read from LIMDOW media in the same way as from conventional media.