Optical Storage Mediums

James Ng

The most common way of storing data in a computer is magnetic. We have hard
drives and floppy disks (soon making way to the CD-ROM), both of which can store
some amount of data. In a disk drive, a read/write head (usually a coil of
wire) passes over a spinning disk, generating an electrical current, which
defines a bit as either a 1 or a 0. There are limitations to this though, and
that is that we can only make the head so small, and the tracks and sectors so
close, before the drive starts to suffer from interference from nearby tracks
and sectors. What other option do we have to store massive amount of data? We
can use light. Light has its advantages. It is of a short wavelength, so we
can place tracks very close together, and the size of the track we use is
dependent only on one thing - the color of the light we use. An optical medium
typically involves some sort of laser, for laser light does not diverge, so we
can pinpoint it to a specific place on the disk. By moving the laser a little
bit, we can change tracks on a disk, and this movement is very small, usually
less than a hairÕs width. This allows one to store an immense amount of data on
one disk. The light does not touch the disk surface, thereby not creating
friction, which leads to wear, so the life of an average optical disk is far
longer than that of a magnetic medium. Also, it is impossible to ÒcrashÓ an
optical disk (in the same sense as crashing a hard drive), since there is a
protective layer covering the data areas, and that the ÒheadÓ of the drive can
be quite far away from the disk surface (a few millimeters compared to
micrometers for a hard drive). If this medium is so superior, then why is it
not standard equipment? It is. Most of the new computers have a CD-ROM drive
that comes with it. Also, it is only recently that prices have come low enough
to actually make them affordable. However, as the acronym states, one cannot
write to a CD-ROM disk (unless one gets a CD-Recordable disk and drive). There
are products however, that allows one to store and retrieve data on a optical
medium. Some of those products are shown in table 1. However, the cost of this
is quite high, so it doesnÕt usually make much sense for consumer use yet,
unless one loves to transfers 20 megabyte pictures between friends. One will
notice on the table that there are some items labled ÒMOÓ or magnet-optical.
This is a special type of drive and disk that get written by magnetic fields,
and read by lasers. The disk itself is based on magnetism, that affects the
reflective surface. Unlike floppy disks, to erase such a disk at room
temperature requires a very strong magnetic field, much stronger than what
ordinary disk erasers provide. To aid in writing to this MO disks, a high-power
laser heats up part of the disk to about 150 oC (or the Curie temperature),
which reduces the ability for the disk to withstand magnetic fields. Thus, the
disk is ready to be rewritten. The disk needs to passes to change the bits
though. The first pass ÒrenewsÓ the surface to what it was before it was used.
The second pass writes the new data on. The magnetic fields then alters the
crystal structure below it, thereby creating places in which the laser beam
would not reflect to the photodetector. Another type of recordable
medium, is the one-shot deal. The disk is shipped from the factory with nothing
on it. As you go and use it, a high-power laser turns the transparent layer
below the reflective layer opaque. The normal surface becomes the islands (on a
normal CD) and the opaque surface the pits (pits on a normal CD do not reflect
light back). These CDs, once recorded, cannot be re-recorded, unless saved in a
special format that allows a new table of contents to be used. These CDs are
the CD-Recordable, and the Photo CD. The Photo CD is in a format that allows
one to have a new table of contents, that tell where the pictures are. It is
this that distinguishes between Òsingle-sessionÓ drives (drives that con only
read photos recorded the first time the disk was used) and Òmulti-sessionÓ
drives (that can read all the photos on a Photo CD). To read an optical
medium, a low-power laser (one that cannot write