ations over the years including ruby rod and helium-neon, semicon- ductor lasers became the norm shortly after their introduction in the early 1960s because of their low cost and high stability. Light-Emitting Diodes (LEDs) LEDs come in two varieties: surface-emitting LEDs and edge-emitting LEDs. Surface-emitting LEDs give off light at a wide angle and therefore do not lend themselves to the more coherent requirements of optical data systems because of the difficulty involved in focusing their emitted light into the core of the receiving fiber. Instead, they are often used as indi- cators and signaling devices. They are, however, quite inexpensive, and are therefore commonly found. Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. Transport Technologies Transport Technologies An alternative to the surface-emitting LED is the edge-emitting device. Edge emitters produce light at significantly narrower angles and have a smaller emitting area, which means that more of their emitted light can be focused into the core. They are typically faster devices than surface emitters, but do have a downside: They are temperature sensi- tive and, therefore, must be installed in environmentally controlled devices to ensure the stability of the transmitted signal. Laser Diodes Laser diodes represent the alternative to LEDs. A laser diode has a very small emitting surface, usually no larger than a few microns in diameter, which means that a great deal of the emitted light can be directed into the fiber. Because they represent a coherent source, the emission angle of a laser diode is extremely narrow. It is the fastest of the three devices. Optical Fiber When Peter Schultz, Donald Keck, and Robert Maurer began their work at Corning to create a low-loss optical fiber, they did SO using a newly crafted process called inside vapor deposition (IVD). Whereas most glass