iber is the laser. An acronym for light amplification by stimulated emission of radiation, a laser represents a coherent light emitter. Here the term "coherent" means that Chapter Four all emitted photons travel in the same wave pattern, similar to the phe- nomenon that occurs when a stone is dropped into a pond, generating a series of waves traveling in a circular direction away from the location where the stone hit the water. Several types of lasers have evolved since their discovery during the 1960s; the major difference is the energized substance that increases the intensity of light that passes through the device. That energized substance, illustrated in Figure 4.8, is referred to as an amplifying medi- um. That medium can be a solid, a liquid, or a gas. AMPLIFYING MEDIUM In the world of communications the amplifying medium is almost always a thin layer of semiconductor material sandwiched between other semiconductor layers. The resulting laser is referred to as a laser diode, and the amount by which the intensity of light is increased is referred to as the gain resulting from the amplify- ing medium. That gain depends on several factors, including the wave- length of incoming light, the length of the amplifying medium, and the degree to which the amplifying medium was energized. Thus, the gain can be considered to be variable. Because light amplification is an essen- tial function of a laser, let's focus on this process. LIGHT AMPLIFICATION As noted earlier in this chapter, the outer orbit of an atom contains electrons that can be easily raised to a higher energy state. Let's assume that a photon of light is absorbed by an atom in which a hole in the in-type semiconductor material is initially in a low energy state denoted by a 0. In comparison, we can raise the energy of an electron in the p-type semiconductor material to a higher energy state, which we can denote as 1. The electron will remain in this excited state for a very short period of time and then return to the lower state of 0 as it emits a photo