Optical Frequency Oscillators: Lasers
The name laser is obviously derived from maser; instead of microwave we have light wave amplification by stimulated emission of radiation. Although the physical principles are identical since they both deal with electromagnetic waves, the practical problems of creating the conditions to observe them are very different. In common usage the terms laser and maser are used to denote an oscillator rather than an amplifier. It is a remarkable fact that the realization of a laser does not involve any physical theory or indeed any practical technique that had not been known long before laser action was actually observed. The concept of stimulated emission was well established, there was abundant spectroscopic data on the wavelengths and line intensities of spectral lines, and the practice of light wave interferometry was well advanced. Furthermore the study of electrical discharges through rarefied gases and the theory of crystal optics had been studied since the nineteenth century. It is little wonder that as soon as Maiman reported laser action in a ruby crystal in 1960, there was an explosion in the number of published reports of laser action in different materials. Lasers are playing such a transformational role in advances in the ultimate shape of atomic clocks and gyroscopes, and hence of navigation in the future, that this entire chapter is devoted to them.