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laser diodeA semiconductor-based laser used to generate analog signals or digital pulses for transmission through optical fibers. Both laser diodes and LEDs (light-emitting diodes) are used for this purpose, but the laser diode generates a smaller beam that is easier to couple with the smaller core of singlemode fibers. Laser diodes are designed to emit light either from their edge or their surface, the latter providing a circular beam that couples better with the round core of the fiber.
Laser diodes work on the same principle as the bigger gas lasers. They function as an optical oscillator by stimulating a chain reaction of photon emission inside a tiny chamber. In edge-emitting lasers, the semiconductor waver is cleaved, and the inherent properties of the semiconductor create reflective ends that may or may not be enhanced with additional reflective films. With vertical cavity surface emitting lasers (VCSELs), the reflectivity has to be added.
The most common semiconductors used in laser diodes are compounds based on gallium arsenide (750 to 900 nm in the infrared), indium gallium arsenide phosphide (1200 to 1700 nm in the infrared) and gallium nitride (near 400 nm in the blue). See laser and solid state laser.
|Laser diodes such as these are all fabricated using semiconductor processes just like CPU and memory chips. Note that the VCSELs (vixels) produce a circular beam that couples well with a fiber.|
|A Tunable Laser Diode|
|Laser diodes also make great sensors. This laser diode from Jet Propulsion Labs is tunable to different frequencies in order to detect various gases both on this and other planets. (Image courtesy of JPL's Microdevices Laboratory; Robert M. Brown, photographer)|
LaserDiscAn early optical videodisc technology for movies and training. Introduced in 1978, Pioneer LaserDisc players came out two years later and became the choice for commercial use. Never widely used, by the 1990s, LaserDiscs were superseded by Video CDs and then DVDs.
LaserDiscs Were Analog
Based on LaserVision (combination of MCA Disco-Vision and Philips Video Long Play), the LaserDisc recorded an analog composite video signal on a continuous, spiraling track (see CLV). Each side of the 12" platter held one hour of video in 108,000 frames.
For interactive training and games, a circular track held one video frame (see CAV), and 54,000 frames provided 30 minutes of video per side. The first LaserDiscs recorded analog audio, but digital audio was later added, and newer players supported multiple language soundtracks. See CED.
|Pioneer LaserDisc Player|
|The LaserDisc platter looks like a monster next to a DVD. Nevertheless, in the 1980s, the LaserDisc was very high tech.|
long distanceSpanning a wide geographic area. In the U.S. telephone world, long distance refers to crossing state lines (interstate). See WAN.
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