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 hairOs 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 OcrashO 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 OheadO 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 ot 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 doesnOt usually make much sense for consumer use yet, nless one loves to transfers 20 megabyte pictures between friends. One will notice on the table that there are some items labled OMOO 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 aser 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 OrenewsO 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 Osingle-sessionO drives (drives that con only read photos recorded the first time the disk was used) and Omulti-sessionO 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.
