Magneto Optical ( MO ) Technology

Magneto Optical disk drives are a specific class of data storage device which uses a hybrid of Magnetic and Optical Technology. First developed at the end of the 1980s they use optical disk media which has an active magnetic layer.

The magnetic layer can only change state when the optical layer is heated by a high powered laser beyond the "Curie" point.

Within the mechanism of a Magneto Optical Drive the observer may recognise a similar carriage and laser arrangement to an Early CD rom unit, but in addition there will be a large mechanically fixed electro magnet which spans the radius of the disk area. The magnet is only used in the data writing or erase processes, it writes each individual bit as the high powered laser locates, focusses and heats the attached optical substrate. To simplify, the laser finds the disk location and the magnet writes the data.
See graphic below.

When reading data, the magneto optical drive uses only reflected low power laser light to determine zeros and ones. It does this by comparing the laser light and its reflection from the MO media. What is actually detected is the plane of polarisation, and whether it is rotated clockwise or anticlockwise. One state representing "1", the other "0". This rotation is interpreted and converted to readable data by the drive electonics.

Here we see diagram of the reading phase of an MO drive. Kerr Rotation is the effect where the phase of polarised light can be observed to be altered.

Erasing and Rewriting on an MO disk is a two pass process, because the relevent section must be first returned to the erased state before it can have data written. This is done by heating the area to be erased, while applying the negative magnetic charge. On the next pass, data can be written as normal.

In almost all cases, an MO drive can be seen by the operating system as a standard disk volume and treated as such. In this way the media can be formatted to any one of a number of formats. Using more recent operating systems the installation is plug and play, with no extra preperation. During the 1990s, software drivers and patches had to be installed to enable the operating system to see the optical drive. This is not an issue with contemporary operating systems like Windows XP, Vista, Windows 7 or Apple OS X, because they all have these drivers built in as an integral part of the operating system..