The phenomenal growth in Internet traffic has lead to a huge increase in demand for data transmission capacity on a worldwide level. As a result, wavelength division multiplexing (WDM) technology emerged, which makes it possible to transmit a large number of optical channels on a single optical fiber. An equally significant development occurred in optical networks, where switching and routing of signals takes place in the optical domain. This technology places special demands on the optical sources (lasers) used in the system. This text offers a description of the optical sources (equipment...