Optical Storage

Optical storage Compact Disk(CD) was introduced in 1983. It was actually developed for the replacement of audio tape. The CD is a non erasable disk. The huge commercial success of the CD lead for the development of low cost optical storage technology has revolutionized computer data storage. Following are the variety of optical storage:

CD-ROM (Compact Disk Read Only Memory)
The CD-ROM is a read only optical storage medium. They are capable of holding above 700 MB of data, 74 minutes of audio or video. Acessing data from a CD-ROM is quite a bit faster than floppy disk but considerably slower than a modern hard disk.
This disc is made of a polycarbonate wafer, 120 mm in diameter and 1.2 mm thick, with a 15 mm hole in the center. The wafer base is coated with metallic film, usually an aluminum alloy. The aluminum film is the portion of the disc that the CD-ROM drive reads for information. The aluminum film is the covered by a plastic polycarbonate coating that protects the underlying data. CD-ROMs are single sided. Reading the information back is a matter of reflecting a lower-powered laser off the aluminum film. A receiver (or light receptor, also called photo detector) notes where light is strongly reflected or where it is absent or defused. Diffused or absent light is caused by the pits made the CD. Strong reflection of the light indicates no pit that is called land. The individual pits on a CD are 0.12 microns deep and about 0.6 microns wide (one micron is equals to one-millionth of a meter). By the collection of reflected and diffused light microprocessors translate the light pattern back into data and sound.
CD-ROM data is recorded using a technique called constant linear velocity. So that the disk must spin faster while reading the inner track area, and slower while reading the outer track area.

CD-R(CD-Recordable)
The CD-R media is coated with a photosensitive organic dye that has the reflective properties. The unrecorded CD-R disc can be assumed as one long land. The photosensitive organic dye within a CD-R disc changes from a reflective state to a non reflective state when it is exposed to the laser recording beam of a CD-R drive. The reflective state acts as land and non reflective state acts as pit as in CD-ROM. The change of state is permanent, so we also call WORM (Write Once Read Many)
medium.

CD-RW(CD-Rewriteable)
Unlike CD-R, the change is not permanent in CD-RW. The active layer of a CD-RW disc is an Ag-In-Sb-Te (silver-indium-antimony-tellurium) alloy that in its original state, has a polycrystalline that makes it reflective.

When the CD-RW drive writes to the disk, the laser uses its highest power setting, known as Pwrite, to heat the active material to a temperature between 500 and 700 degrees Celsius, causing it to liquefy. In its liquid state, the molecules of the active material flow freely, losing their polycrystalline structure and taking on an amorphous state. When the material solidifies in this amorphous state, it loses its reflectivity. By selecting firing the laser, the drive leaves parts of the disc in polycarbonate state, forming the land, and parts in the amorphous state forming the pits.
To reverse the phase of a specific area on a disc, the laser operates at a lower power setting (Perase) and heats the active material to approximately 200 degree Celsius at which it reverts back from its amorphous to its crystalline state and becomes reflective again.

DVD(Digital Versatile Disc)
The improved form of CD-ROM is called Digital Versatile Disc. The DVD was introduced for the replacement of video tape (Video Home System or simply VHS). The DVD took video into digital age. It delivers movie with impressive picture quality and can be accesed randomly like audio. In DVD vast volume of data can be stored and used for storing game, education software, full length movie etc. DVD offers an initial storage a capacity of 4.7 GB of digital information on single sided, single layered disc the same diameter and thickness of CD-ROM. Bits are packed more closely on a DVD than CD. Today DVD's are available as DVD-R to write once and DVD-RW for read and write.