May 15 2014
Harmonic mode-locking is a method of achieving high pulse repetition rates in an actively mode-locked laser. This method is carried out in the laser resonator, with a constant temporal spacing where multiple ultra-short pulses are circulated.
The harmonic mode-locking method is most commonly applied in high pulse repetition rate fiber lasers. The principle of this method is that multiple pulses circulating at equal spacing are generated by triggering the modulator of an actively mode-locked laser with a harmonic of the resonator's round-trip frequency.
However, the pulses need not necessarily have equal pulse energies. In addition, the mutual phase coherence of the pulses depends on the application. Achieving constant pulse spacing in passively mode-locked lasers is a great challenge, due to the fact that pulses might either occur in bunches or regularly-spaced forms.
Stabilized, harmonically mode-locked lasers usually exhibit lower noise when compared to fundamentally mode-locked lasers, owing to the lesser amount of noise influence per second on a pulse. Rational harmonic mode-locking is a type of harmonic mode-locking that has a higher pulse repetition rate, and varying pulse energy.
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