In every CD player a laser diode can be found reading data from indentations on a CD. These pits make up a stream of digital information, consisting of 1's and 0's. As a CD spins, the laser shines light onto the media, which reflects the beam back to an optical sensor. This sensor can determine the single bit of information -- either a 1 or a 0 -- the laser beam is shining on. With this information, the CD player can process the digital code into an analog signal which is output to headphones or speakers.

Magnified view of data stored on a compact disc. (MOS Magazine)

A more modern application skips the digital to analog conversion, and allows computer-readable data to be stored on the same medium.

The laser itself, while invisible to the human eye, puts out approximagely 5mW at 780nm. Some people's eyes may be sensitive enough to see a slightly red emission, although the power emitted in the infrared part of the spectrum is about 10,000 times stronger than that which is visible. Most people cannot detect these wavelengths, or any others above 700nm.

A more detailed explanation of how a CD player works is available online.

digital versatile disc

Another type of disc, the DVD, also contains indentations which are read by a laser beam. However, the type of semiconductor laser used in DVD players differs in wavelength from CD players. Where CDs are read by a 780nm laser, DVDs are read by a 635nm laser. This allows for proportionally smaller pits, a higher pit density, and more data on the same sized disc.

Magnified view of data stored on a Digital Versatile Disc (DVD). (MOS Magazine)

Advances in laser diode technology will allow optical storage density to increase in the future. Green semiconductor lasers are already appearing in portable pointing devices, but their cost is too high to allow for their implementation in optical storage. It seems likely that, as the popularity of these green lasers increases, cost will drop, and we will eventually see optical discs the size of a DVD which can store nearly one terabyte of data.