Birnbaum George. Optical Masers

New York, London: Academic Press Inc, 1964. — 306 p.The terms maser or optical maser and laser have been used interchangeably to describe the quantum electronic devices which generate coherent electromagnetic radiation in the infrared, visible, and ultraviolet regions. The words have evolved as acronyms of the phrases "microwave (or molecular) amplification by the stimulated emission of radiation," or "light amplification" by such processes. In this book the term optical maser or maser is generally used, although popular usage now seems to favor laser to describe devices in the infrared and optical regions and maser for those in the microwave and millimeter regions. Laser oscillation is marked by a dramatic narrowing of the spectral and angular distribution of the spontaneous emission radiation at a
particular level of excitation called the threshold. The specific process of laser oscillation designating the behavior of a laser above a well defined threshold is distinguished from the general process of stimulated emission. A final matter of terminology is the use of the word atom to denote in a generic sense the active species of the maser, which may in fact be an atom, ion, exciton, or molecule.