Laser cavity

Spontaneous vs stimulated emission

An atom in an excited state will eventually drop to its ground state and emit a photon spontaneously — at a random time, in a random direction, with a random phase. This is how a light bulb works. But if a photon of exactly the right energy passes by the excited atom, it can trigger the atom to emit immediately: same direction, same phase, same wavelength. This is stimulated emission, predicted by Einstein in 1917 and the mechanism behind every laser. The emitted photon is an exact clone of the triggering photon, which is why laser light is coherent.

Population inversion

For stimulated emission to dominate over absorption, more atoms must be in the excited state than in the ground state — a condition called population inversion. This is deeply unnatural: in thermal equilibrium, the ground state always has more population (Boltzmann distribution). A laser achieves inversion by continuously pumping energy into the gain medium, driving atoms from the ground state to the excited state faster than they decay. The pump energy slider controls this rate. Below the lasing threshold, losses exceed gain and only spontaneous emission occurs. Above it, stimulated emission cascades and the photon count explodes.

The optical cavity

Two mirrors facing each other form a Fabry–Pérot cavity. Photons bounce back and forth, passing through the gain medium on each trip. Each pass amplifies the light through stimulated emission. One mirror is slightly less reflective (the output coupler), allowing a fraction of light to escape as the laser beam. Higher reflectivity means more passes and more amplification, but also means less output power escapes. The balance between gain and loss determines whether the laser reaches threshold.