## Abstract

This chapter gives a simple theory of the laser using the classical electromagnetic theory of Chap. 1 in combination with Chap. 5’s discussion of the interaction of radiation with two-level atoms. We consider arrangements of two or three highly reflecting mirrors that form cavities as shown in Fig. 6-1. Light in these cavities leaks out (decays to its 1/*e* value) in a time *Q*/*v* where *v* is the frequency of the light and *Q* is the cavity quality factor (the higher the *Q* the lower the losses). An active gain medium is inserted between the mirrors to compensate for the losses. In the simple cases we consider in this chapter, the electromagnetic field builds up until it saturates the gain down to the cavity losses. Chapter 10 considers some more complicated cases. Chapter 7 discusses a related cavity problem in which the medium in the cavity is not a gain medium, i.e., it has dispersion and/or absorption. This nonlinear cavity problem can lead to two or more stable output intensities for a given input intensity, and hence belongs to a class of problems called *optical bistability*. In the present chapter we also see a bistable configuration that involves active media, namely the homogeneously broadened ring laser.

## Keywords

Gaussian Beam Ring Laser Optical Bistability Cavity Loss Spatial Hole Burning## Preview

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