A type of Mode locking that occurs when multiple ultrashort Pulses are circulating in the Laser Resonator with constant spacing. This technique is required for high (multi-GHz) Pulse Repetition Rate fibers since their resonators can not be made short enough to achieve a high repetition rate with a single pulse. Harmonic mode locking requires achieving a stable pulse spacing to avoid pulse dropouts and to avoid so-called "supermode" noise; to achieve this one often requires special measures involving intracavity filters and/or electronic Feedback systems.