Introduction

A laser source is made of an amplifying medium (solid, liquid or gaseous) placed inside a resonant optical cavity (between mirrors, one of which is partially transmitting). Inside this cavity, light travels a certain amount of time and gets amplified each time it passes through the gain medium. Finally it escapes through the partially transmitting mirror. The geometrical characteristics of the whole system lead to a weak beam divergence (corresponding to spatial coherence) and a great spectral purity (temporal coherence).