Page 104 - Optical Communications Essentials

Light Sources and Transmitters

94 Chapter Six

Figure 6.7. (a) Cross-section (not to scale) of a GaAlAs light emitter. Here x y to provide both carrier
confinement and optical guiding. (b) Energy band diagram showing the active region and the electron
and hole confinement barriers. (c) Variations in the refractive index of the layers.

Laser Action The term laser actually is an acronym for the phrase “light amplifica-
tion by stimulated emission of radiation.” So what is stimulated emission of radiation
and how does it result in light amplification? First, let us look at the possible photon
emission and absorption processes shown in Fig. 6.8 for a two-level atomic system. An
electron can move from an energy level E 1 in the valence band (called the ground
state) to a higher state (called an excited state) at energy level E 2 in the conduction
band either by being pumped externally to that level or by absorbing the energy hν 12
from a passing photon. The latter process is called stimulated absorption. In either
case, an excited electron then can return to the ground state either spontaneously or

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