Mode-locked fibre lasers with an adjustable drop-shaped cavity

We report the first implementation of a new cavity topology for mode-locked fibre lasers. This new 'drop-shaped' cavity topology employs a dual-fibre optical collimator and allows for a relatively simple cavity design with continuously adjustable length. It combines the advantages of the conventional ring cavity topology with the adjustment capabilities of hybrid ring-linear cavities (e.g. σ-cavities). Reliable femtosecond pulsed lasing was demonstrated in the experimental drop-shaped Er-doped fibre lasers, which were mode-locked in two different ways - by exploiting nonlinear polarisation evolution (NPE) and by means of a semiconductor saturable absorber. In the case of NPE-based mode locking, the shortest pulse duration was ∼450 fs. The pulse repetition rate could be continuously varied within 3 kHz around ∼25 MHz in the NPE-mode-locked lasers when the distance between the dual-fibre optical collimator and the cavity mirror was adjusted. This feature could, in principle, allow active stabilisation of the pulse repetition rate. Thus, mode-locked fibre lasers with the proposed drop-shaped cavity topology could be applied in metrology and other fields where a high stability of pulse repetition rate or its synchronization with an external clock is required.