Laser Cavity
A gain medium between two mirrors, governed by the laser rate equations for the population inversion and the cavity photon number. The continuous-wave operating point is the exact steady state of those equations: below a sharp pump threshold the excited atoms glow but the cavity stays dark, and above it the inversion clamps at N = 1/(B tauC) while every extra pumped atom feeds the beam, so the output rises linearly with a sharp kink. The intracavity standing wave grows with the photon number and the beam leaves the partial output coupler. Arm the Q-switch and the cavity is spoiled while the pump piles up a large inversion; release it and the stored energy dumps as one giant pulse, integrated numerically.
WHAT TO TRY
- Watch the pump build the population inversion: below threshold the cavity stays dark, then once gain beats loss the photon number explodes and the beam switches on.
- See the standing wave between the mirrors: only modes that fit a whole number of half-wavelengths survive, the cavity resonance that sets the lasing frequency.
- Push past threshold: the output climbs linearly with pump and the inversion clamps, the defining behaviour of a laser above its turn-on point.