5 GHz fundamentally mode-locked Yb-doped silica all-fiber laser using a negative-dispersion SESAM

We demonstrate a 5 GHz fundamentally mode-locked Yb-doped silica all-fiber laser. It exploits a semiconductor saturable absorber mirror (SESAM) with intrinsic negative group-delay dispersion (GDD) for dispersion management, eliminating additional dispersion compensation elements. Using a 2-cm commercially available silica fiber in a Fabry-Perot cavity, self-starting mode-locking is achieved with a pulse width of 1.3 ps at 1030 nm (tunable from 1028 to 1038.5 nm) and a low threshold of 80 mW. Systematic investigations via wavelength tuning and fiber length variation reveal that stable operation requires net cavity GDD below approximately +1000 fs². Experiments with an alternative SESAM confirm that insufficient dispersion compensation, not gain limitation, is the primary obstacle to stable mode-locking. The simplicity of the design, use of standard components, and clear dispersion guidelines establish it as a practical seed source for high-repetition-rate photonic applications.