ration of coherent light. Chapter Four side effect of the pair of photon emissions is the fact that they are in phase with one another, which results in the generation of coherent light. This is illustrated in Figure 4.9b. The coherent light generated by a laser represents a very narrow beam. That beam is normally sharply monochromatic, which means that it occupies a single color or frequency; however, it can also be gener- ated with more than one monochromatic frequency. When the latter occurs, the use of filters enables a single frequency to be separated from multiple monochromatic emissions. THE LASER DIODE The semiconductor laser, commonly referred to as a laser diode, uses a p-n junction that generates light similar to the way an LED emits light when a voltage is applied and current flows. Howev- er, unlike an LED, a laser diode requires the recombination of photons to be confined across the junction area and reflected by the use of partial- ly reflecting surfaces that result in the formation of a cavity. Parallel mirrors are formed through the use of groups III and V compound semiconductors that can be cleaved along their planes. Those mirrors confine injected electrons and light generated by photons within a region where they interact to enhance the stimulated emission process. Thus, a key difference between an LED and a laser diode is the popula- tion inversion and optical feedback of the semiconductor laser. Through optical feedback a laser allows photons generated by recombination to stimulate additional electron-hole recombinations. By the use of higher levels of doping and an increased level of current, additional excited electrons result in a population inversion. SINGLE- VERSUS DOUBLE-HETEROJUNCTION LASER DIODE The actual junction in a single crystal between two dissimilar semiconductors is referred to as a heterojunction. This type of junction is required to con- fine light to the region of the p-n junction as well as to obtain the abili- ty to continuously operate at room temperatures. The use of 