Introduction

The previous section showed how to favour population inversion by choosing the right spectroscopic system and energy levels. However, population inversion is not enough to generate a laser effect. As stated previously, stimulated and spontaneous emissions are competing with each other. Thus, before becoming an amplifying medium, a laser medium pumped by an external energy source is first a “lamp” (spontaneous emission). It is the optical cavity that creates the conditions necessary for stimulated emission to become predominant over spontaneous emission. The cavity or resonator is composed of several mirrors that bounce the beam back and forth through the amplifying medium. There are two different types (Figure 7): linear cavities (light is reflected back and forth) and ring cavities (light circulates round and round). The first type will be studied here.

When the laser starts up, the “lamp-amplifying medium” emits spontaneously in all directions. However, a small part of the emission occurs along the axis of the laser cavity. These spontaneous photons can travel backwards and forwards. Thus, over time, thanks to the amplifying medium, the amount of light in the cavity increases considerably. The confinement of the light increases the probability of stimulated emission rather than spontaneous emission occurring. At the same time, the cavity acts as a filter due to the numerous round trips: only the wave perfectly perpendicular to the axis of the cavity will be propagated and certain frequencies will be favoured (the resonance frequencies of the cavity). In this way, the cavity produces a specific radiation.