Mode-locking: A laser cavity supports modes at ωₙ = ω₀ + n·Δω where Δω = πc/L (FSR). When phases are locked, E(t) = e^{iω₀t}·Σ Aₙe^{inΔωt} — a Fourier series giving a pulse train with repetition rate Δω. Pulse width τ ~ 1/(N·Δω): more modes → shorter pulses. Random phases give continuous-wave output. GDD (group-delay dispersion) chirps the pulse and broadens it.