Introduction

The previous section shows that laser radiation is a concentrated spatial and spectral beam of light. It is also possible to qualify the beam as a “temporal concentration” by considering that the emitted photons are condensed into short energetic pulses. As previously stated, the probability of obtaining stimulated emission increases with the number of excited-state atoms and incident photons. There are therefore two ways to favour stimulated emission: either by raising the number of excited-state atoms or by raising the number of incident photons. The first method to achieve a temporal concentration is to trigger the stimulated emission only when there is a large number of atoms in the upper level (Q-switching). In the second method, the photons in the optical cavity are condensed into a “packet” or “pulse” that will bounce back and forth between the mirrors (mode-locking).