摘要： An optical source provides the electrical–optical signal conversion efficiently that enables the optical output to be effectively coupled and launched into the optical fiber. It is an active device that requires external power supply for operation in optical fiber communications. This chapter begins with the major requirements or desirable properties for the optical source in general. Light emitting diodes (LEDs) and injection laser diodes (ILDs) {which are simply known as laser diodes (LDs)} are the most popular semiconductor optical sources. LED is an incoherent optical source. It can support many propagation modes of light within its structure. This is the reason why is it employed as a multimode optical source. Whereas, ILD is a highly coherent optical source that has a very narrow spectrum and fast response time. Thus, it is mostly used as a single-mode optical source in single-mode propagation requirements. In this chapter the basic principle of operation of both these optical sources along with their major structures and configurations are described. For producing the light signal, the emission can be a spontaneous emission, as in the case of LED, or a stimulated emission as in the case of ILD. The spontaneous emission takes place when electrons are brought to a very high energy level, and an unstable state. The electrons will return spontaneously (within few picoseconds) to a stable state, and will consequently emit photons.. The optical wavelength is determined by the amount of energy the electron releases. A laser diode operates with stimulated emission in which the electrons enters and stays in a high-energy state for a few microseconds. Then it changes its state spontaneously. During this state, the photon stimulates so as to emit the energy in the form of another photon. Thus, the laser produces the light signal. Finally, a typical functional block schematic including features and operations of optical transmitter is covered to assess the utility of optical sources.