comparison to an LED, it becomes possible and practical to couple the laser diode to thin-core, single-mode fiber. Because of their higher power and quicker pulse rise time and coher- ent light generation, laser diodes are better suited for fiber-optic trans- mission that requires higher transmission rates over long distances. For this reason the use of laser diodes and single-mode fiber is the preferred mechanism for communications carriers. SELECTION CRITERIA The selection of a laser diode depends on a number of factors, including the wavelength of the optical output and the output power, as well as the modulation rate of the device. In addition, the dimen- sions of the laser should generally be as small as possible. One parameter that is extremely important is the size of the emitter area; the smaller the emitter area, the easier it becomes to couple light into a single-mode fiber. Light Sources and Detectors While most laser diodes have an emitting area between 1 and 3 um that provides an efficient coupling into single-mode fiber, some lasers have much larger emitter areas. For example, very-high-power laser diodes may have an emitter area of approximately 100 um. Such diodes can be efficiently coupled only to multimode fibers that have a similar core area. Thus, the type of fiber to be used governs the acceptable laser diode emitter area. LASER DEVELOPMENTS Although lasers date to the 1960s, work continues at a fast pace to develop new types of devices more suitable for different applications, including optical communications Two types of lasers that warrant attention are vertical-cavity surface-emitting lasers and tunable lasers. VERTICAL-CAVITY SURFACE-EMITTING LASERS A vertical-cavity laser repre- sents a laser formed by the vertical stacking of high-reflection mirrors. To obtain a high level of reflectivity, as many as 30 individual semicon- ductor layers are used to form each mirror. Between the mirrors there are several light-emitting semiconductor layers that result in the emis- sion of light vertical