Introduction to Saturated Optical Absorption

Saturated absorption techniques are being used increasingly often to measure optical resonances having such narrow bandwidths that they would otherwise be obscured by Doppler broadening. The principle underlying these techniques is reviewed using four simple thought or "gedanken" experiments. A rudimentary explanation is based on Bennett\´s " hole-burning" picture. Two configurations of typical saturated absorption experiments are described. One illustrates absorption outside and the other inside of a laser cavity. The intracavity technique is recommended for weak absorbers and when especially simple compact apparatus is needed. For most other experiments, the gaseous absorber is better located outside the laser cavity.