The basic operating principle of lasers is the same across various types, except for free-electron lasers. Essential conditions for generating laser light include population inversion and gain exceeding loss. Therefore, the indispensable components of a laser are a pumping source and a working medium with metastable energy levels.
Pumping involves the working medium absorbing external energy and being excited to a higher energy state, creating and maintaining conditions for population inversion. Methods of pumping include optical, electrical, chemical, and nuclear. The presence of metastable energy levels in the working medium ensures that stimulated emission dominates, leading to light amplification.
Another common component in lasers is the resonant cavity. While not strictly necessary (see optical resonator), the resonant cavity ensures that photons within it have consistent frequency, phase, and direction, enhancing the laser's directionality and coherence. Additionally, the resonant cavity can effectively shorten the length of the working material and allow for mode selection by adjusting the cavity length. Therefore, most lasers are equipped with a resonant cavity.
