Frequency-Shifted-Feedback Mode-Locked Fibre Laser with Tuneable Repetition Rate

Frequency-Shifted-Feedback (FSF) Mode-Locked fibre lasers have been much less studied than other mode-locked sources like SESAM- and NOLM/NALM-fibre lasers. This is probably due to the need for costly active elements in the cavity like Acousto-Optic-Modulators (AOM). In FSF fibre lasers [1], [2], the mode-locking operation relies on the interplay between the AOM which shifts the wavelength of the injected signal to longer wavelength (shift corresponding to acoustic modulation frequency) at each cavity round-trip, the nonlinear effects occurring in the optical fibres generating new spectral components and a filtering element which prevents any possible stable Continuous Wave (CW) operation. In FSF mode-locked lasers, the AOM is driven at a constant power over time leading to an optimal optical transmission and constant wavelength shift. In this work, we propose to take advantage of the fast responsivity of the AOM by modulating its operation with respect to either the fundamental frequency of the laser cavity or its harmonics. Following starting processes like described hereafter, it has been possible to make a laser cavity built at a fundamental frequency of 3.47MHz to operate at a frequency of 6.95MHz or 10.42MHz. We believe that this work will pave the way to more stable and controllable ultrashort pulse fibre laser.