Pulse trains with high Pulse Repetition Rate are sometimes obtained with harmonic Mode locking, where multiple ultrashort Pulses are circulating in the LaserResonator with a constant spacing. This technique is required e.g. for high (multi-GHz) pulse repetition rate Fiber lasers, since their resonators can not be made short enough to achieve a high repetition rate with a single pulse.
The difficulty of harmonic mode locking is to achieve a stable pulse spacing, to avoid pulse drop-outs, and to avoid so-called supermode noise. To achieve this, one often requires special measures, involving intracavity filters and/or electronic Feedback systems. On the other hand, harmonically mode-locked lasers have the potential for lower laser Noise (e.g. timing Jitter and phase noise), provided that supermode noise is effectively suppressed.