Laser Cavity

A gain medium between two mirrors, governed by the laser rate equations for the population inversion and the cavity photon number, integrated live. Below a sharp pump threshold the excited atoms glow but the cavity holds only a negligible incoherent trickle; above it the inversion clamps and every extra pumped atom feeds the beam, so the output rises linearly with a sharp kink. None of this is scripted: the threshold falls out of the steady state. Arm the Q-switch and the cavity is spoiled while the pump piles up a huge inversion; release it and the stored energy dumps as one giant pulse.