Mode-locking of fibre ring lasers with a coherent-phase relationship among axial mode groups

Abstract A novel method to suppress the noise in harmonically mode-locked lasers, which relies on a coherent-phase relationship among all groups of the axial modes, is reported. An optical amplitude modulator, driven by the harmonic signal (up to ~2 GHz) with its amplitude modulation at a frequency of the axial mode spacing, is used to generate optical pulses of ~40 ps in an erbium-doped fibre ring laser. This method is equally applicable to all multimode lasers whose active gain medium is short as well as long.