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From: cb2o+@andrew.cmu.edu (Charles Edward Batey, Jr.)
Newsgroups: comp.sys.mac
Subject: NeXT Optical Drive/Magneto-Optics
Message-ID: <4XJgQly00Uw9049VhI@andrew.cmu.edu>
Date: 15 Oct 88 03:34:09 GMT
Organization: Carnegie Mellon
Lines: 31

As I understand it, the operation of the NeXT optical drive is as follows:

First, the disc used in the drive is different than a normal optical disc,
or CD.  It uses the same plastic coating as a standard CD, however, the core
of the disc is made of aluminum coated with some magnetic material.
Second, the laser in the drive operates at two intensities.  When writing,
the high-intensity laser is used to heat the magnetic material up to the
Curie point (where the current magnetic polarization of the material can be
affected by a Magnetic field).  Then a magnetic field is applied at that point
on the disc, thus altering the magnetic polarization.  There are two
orientations of the magnetic field - one represents a zero and the other a
one.  Accordingly, there are two possible polarized states the magnetic
material can have.
When reading, the drive uses the low-intensity laser setting and reads
the disc by reflecting the laser off the disc, through a polarizer, and
into an analyzer that determines the intensity of the beam.   It seems
that the beam, when refracted through the material and reflected off of
the aluminum core, has a distinct direction of polarization, one for each
of the two states of the material.  Thus by passing the beam through a
polarizing filter, the differing intensities of the resulting light represent
either a one or a zero.

Also, when writing to the disc, it uses a three pass system.  Once to set all
of the bits needed in storing the information to zero (the erase pass), a
second time to set all of the one bits, and a third time to verify the code.

Hope this helps.

Ted Batey
Carnegie Mellon University
Pittsburgh, PA