The word laser is an acronym for light amplification by stimulated emission of radiation. However, the term laser generally refers to a laser oscillator, which generates laser light without an input light wave. A device that amplifies a laser beam by stimulated emission is called a laser amplifier. Laser light is generally highly collimated with a very small divergence and highly coherent in time and space. It also has a relatively narrow spectral linewidth and a high intensity in comparison with light generated from ordinary sources. Due to the process of stimulated emission, an optical wave amplified by a laser amplifier preserves most of the characteristics, including the frequency spectrum, the coherence, the polarization, the divergence, and the direction of propagation, of the input wave. In this chapter, we discuss the characteristics of laser amplifiers. Laser oscillators are discussed in Chapter 11. Optical fiber amplifiers are of particular interest in photonics applications. They are specifically discussed in Section 10.5. Semiconductor laser amplifiers are discussed in Chapter 13.

Optical transitions

Optical absorption and emission occur through the interaction of optical radiation with electrons in a material system that defines the energy levels of the electrons. Depending on the properties of a given material, electrons that interact with optical radiation can be either those bound to individual atoms or those residing in the energy-band structures of a material such as a semiconductor. In any event, the absorption or emission of a photon by an electron is associated with a resonant transition of the electron between a lower energy level |1〉 of energy E1 and an upper energy level |2〉 of energy E2, as illustrated in Fig. 10.1.