ABSTRACT
The rapid expansion of the field of optical communications was made possible by the development of low-loss optical fibers and of light sources that are compatible with the spectral transmission characteristics and physical dimensions of these fibers. For the sake of completeness, one should briefly mention the polarization and coherence properties of optical sources since these play an important role in some modulation schemes. The physical characteristics of a light source that are important in optical communication systems are wavelength of maximum emission, spectral bandwidth, light-current characteristics, far-field pattern, dynamic response, polarization and coherence properties, and noise. The recombination time constant for stimulated processes in laser diodes is shorter than the spontaneous recombination time constant by about an order of magnitude; modulation frequencies in the low GHz range can be reached with them. In the absorption process, a photon raises the energy of an electron by the amount of its own energy; in the emission process, the opposite takes place.
