ABSTRACT
Great progress has been made since the 1960s. Today, lasers have become ubiquitous in our lives. As another important family of lasers, gas lasers were fast growing in the laser industry due to their simple pumping schemes and the wide availability of gain media. In gas lasers, the population inversion is typically created by electric discharge, which is relatively simple to construct and operate. A semiconductor is a material whose electrical properties are between those of a conductor and an insulator. By the late 1970s, semiconductor lasers are able to operate in CW mode at room temperature. In semiconductor lasers, cleaved facets can be used as mirrors; in addition, linear cavities formed using distributed Bragg reflectors are also common. Optical amplifiers are essentially "single-pass" lasers, or "lasers" without the two mirrors.
