Path: utzoo!utgpu!attcan!uunet!vsi!friedl
From: friedl@vsi.COM (Stephen J. Friedl)
Newsgroups: comp.arch
Subject: Re: The NeXT Problem
Summary: details on the optical drive
Keywords: Curie point, local heating
Message-ID: <896@vsi.COM>
Date: 22 Oct 88 20:52:57 GMT
References: <26435@ucbvax.BERKELEY.EDU> <7774@gryphon.CTS.COM> <1490@imagine.PAWL.RPI.EDU>
Organization: V-Systems, Inc. -- Santa Ana, CA
Lines: 62

In article <1490@imagine.PAWL.RPI.EDU>, brazil@pawl18.pawl.rpi.edu (Timothy E. Onders) writes:
> The Canon drive is not a true Optical drive. It is an optically enhanced
> magnetic storage device. Through the use of lasers, the tracks can be made as
> small as the width of the beam. The actual reading a writing is done with
> a magnetic field, much the same as it is done in a normal hard drive.

I'm not sure this is entirely correct.  Writing is indeed done by
a magnetic field, but reading is done by the laser.  The
following is taken from the latest _Byte_, entirely without
permission.  The _italics_ represented here are in the original
article:

     "The optical platter is composed of the same clear rigid
     polycarbonate material that's used in CD-ROMs.  Embedded
     within the platter is a layer of reflective aluminum backing
     that's overlaid with a magneto-optical substrate.   The
     platter rotates inside the cartridge at 3000 revolutions per
     minute, 10 times the rotation speed of a CD-ROM, and almost
     as fast as a hard disk drive.

     "How does the magneto-optical drive work?  A single laser
     performs both read and write operations.  To write data to
     the disk, the drive first applies a magnetic field to the
     platter.  The orientation of the magnetic field determines
     the data to be written to the platter -- either a 0 or a 1.
     The magnetic field is first oriented to write 0s at the
     start of what's called the _erase pass_.

     "The laser uses a high-power beam to heat a sector on the
     platter's substrate to its _Curie point_ -- the temperature
     at which the crystals in the substrate "forget" their
     previous orientation and reorient themselves to the
     surrounding magnetic field.  All the data in the target
     sector is thus erased to 0s.

     "Next, a magnetic field is oriented to write 1s in the
     _write pass_, and at every spot in the sector where a bit
     must be set to a 1, the laser again heats the substrate to
     the Curie point.  Finally, the sector is read in a _verify
     pass_ to check the accuracy of the data.

     "To read data off the platter, the drive removes the
     magnetic field, and the laser directs a low-intensity beam
     at the platter.  The beam travels through the substrate and
     is reflected off the aluminum backing.  However, in a
     phenomenon known as the Kerr effect, the crystal alignment
     in the magneto-optical substrate alters the polarization of
     the reflected beam.  The amount of beam polarization
     determines its intensity as it passes through a polarizing
     filter to a photodetector.  The beam intensity indicates
     whether a 1 or 0 was read at the spot on the platter."

--------

The _Byte_ article (Nov 1988) is pretty detailed.

     Steve

-- 
Steve Friedl    V-Systems, Inc.  +1 714 545 6442    3B2-kind-of-guy
friedl@vsi.com     {backbones}!vsi.com!friedl    attmail!vsi!friedl
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