Threshold reduction of GHz-repetition-rate passive mode-locking by tapering the gain fiber

Passively mode-locked fiber lasers with GHz repetition rates have recently attracted significant attention in frontier research areas, including frequency-comb spectroscopy, coherent optical communication, photonic radar, micromachining, etc. In general, the threshold of passive mode-locking increases with the fundamental repetition rate, which is inversely proportional to the cavity length, and this sets a limit on the scalability of the fundamental repetition rate. To overcome this issue, here we propose to reduce the threshold of continuous-wave mode-locking (CWML) by precisely tapering the gain fiber, which can enhance the power density incident on the semiconductor saturable absorber mirror. Assisted by the analysis of guiding property, an experimental scheme is established for tapering standard Yb-doped fibers (125 µm cladding diameter), and tapered Yb-doped fibers with different waist diameters can be fabricated. Using a tapered Yb-doped gain fiber with waist cladding diameter of 90 µm, we are able to achieve CWML with a fundamental repetition rate of 3.3 GHz, and reduce its mode-locking threshold by 31%. More importantly, the optical spectrum of the CWML is found to be broadened with the waist diameter reduction of the gain fiber, which is beneficial for generating shorter transform-limited pulses. The efforts made in this work can provide a promising route to realize stable high-repetition-rate mode-locked fiber lasers with moderate levels of pump power.