Superposition of N longitudinal cavity modes — random phases vs phase-locked ultrashort pulses
A mode-locked laser superposes N longitudinal modes with spacing Δν=c/2L. With random phases the output is CW noise; with locked phases E(t)=Σ exp(i[nΔωt+φ₀]) → a train of ultrashort pulses of width τ≈1/NΔν (femtoseconds for Ti:sapphire). Mode-locking is achieved via saturable absorbers (passive) or acousto-optic modulators (active).