A mode-locked laser maintains definite relative wave phases among many longitudinal resonator modes. Their coherent sum forms a repeating train of short optical pulses, rather than merely producing a broad optical spectrum.
For an ordinary linear resonator, the repetition interval is approximately the round-trip travel time. More phase-aligned spectral bandwidth permits shorter pulses, subject to the time-bandwidth product and spectral shape. A photodetector can measure pulse-train timing, while an intensity autocorrelation estimates much shorter pulse durations under a stated model. Neither a broad spectrum nor a fast periodic intensity signal alone uniquely reconstructs the pulse's complex electric field.
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