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Optical Storage Mediums


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 hair's 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 "crash" 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 "head" 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 doesn't 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 "MO" 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 "renews" 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
"single-session" drives (drives that con only read photos
recorded the first time the disk was used) and
"multi-session" drives (that can read all the photos on a
Photo CD). To read an optical medium, a low-power laser
(one that cannot write to the disk) is aimed at the disk,
and data is read back, by seeing if the laser light passes
to the photodetector. The photodetector returns signals
telling if there is or is not light bouncing back from the
disk. To illustrate this process, see Figure 1
Optical data storage is the future of storage technology.
However, it will take some time before prices are low
enough for the general public. Applications get bigger,
data files get bigger, games get bigger, etc. The humble
floppy disk, with its tiny 1.44 megabyte (actually, 1.40
megabytes... since disk companies like to call 1 megabyte
1,024,000 bytes, when it is actually 1,048,576 bytes, or
220 bytes) capacity will be no match for the latest and
greatest game, requiring 2+ gigabytes of space (and such
games to exist now... in 4 CD-ROMs), the hard drive will
reach its capacity, while the optical drives get smaller,
faster, and cheaper. The speed of optical drives today is
appalling to say the least. Also in the future would be
hard drives based on optical technology, since nowadays a
51/4 inch disk can contain as much as 1 gigabytes of data.
Optical drives, with their high-bit densities are in the
near future.


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