Engineering Carbon Nanotube Saturable Absorber for Boosting Repetition Rate in a Harmonic Mode-Locked GHz Soliton Fiber Laser

GHz femtosecond fiber lasers are being developed to meet the demands of state-of-the-art applications such as high-speed optical communication and ablation-cooled material removal. Harmonic mode locking (HML) opens a portal to generate GHz ultrashort pulses. However, the impact of a saturable absorber (SA) in controlling the laser repetition rate has long been overlooked. Herein, we successfully boost the repetition rate of the HML laser to ∼10 GHz by engineering the nonlinear optical absorption of film-type single-walled carbon nanotube (SWCNT) SA. Our study first reports a new inhibition for HML repetition rate enhancement, the transition from HML to noise-like pulse (NLP) due to reverse saturable absorption (RSA) of SA, and reveals that the optimum SA for HML repetition rate enhancement should be characterized by low effective modulation depth without RSA. Employing SWCNT-SA with ∼0.4% effective modulation depth, we demonstrate the achievement of 9.25 GHz pulses at the 914th harmonic order in a soliton fiber laser. This represents, to the best of our knowledge, the highest HML repetition rate reported for a physical SA mode-locked fiber laser. Our findings may provide general guidelines for the configuration of practical HML fiber laser while opening the possibility for versatile GHz ultrafast laser sources.