Optical Storage MediumsJames 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 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...Sources Used:UMI -
May 1992 BYTE MagazineTOM - June 1992 PC Magazine (64J2528)CD-ROMs - GrolierÕs
multimediaPrinted - Various BYTE, ComputerCraft, MacUser and MacWorld magazinesInternet -
Figure 1: http://www.byte.com/art\9502\img\411016E2.htm		Table 1:
http://www.byte.com/art\9502\img\411016Z2.htm
