Fig. 1: Fundamental and harmonic modelocking in the a time and b frequency domains.
Fundamental modelocking: In the time domain, modelocking corresponds to an active modulation that is synchronised to the round-trip time of the pulses. In the frequency domain, this corresponds to a modulation that is equal to the mode spacing, i.e., the round-trip frequency (fRT). Harmonic modelocking: In the time domain, second harmonic modelocking corresponds to an active modulation that is half the round-trip time of the pulses. In the frequency domain, this corresponds to a modulation that is twice the round-trip frequency
