= Mode-locked laser
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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