Mode-locking threshold decrease in a fiber laser by heterochromic optical pulse injection

A Yb-doped fiber laser mode locked by a nonlinear amplifying loop mirror (NALM) with a figure-9 configuration is experimentally realized. By coupling heterochromic 1580-nm optical pulses into the circular part of the Yb-doped laser oscillator, a new injection-induced Q-switched mode-locking (Q-ML) like transition state is found for the first time to the best of our knowledge. We have investigated the envelope interval and duration of this Q-switched envelope by changing the pump power, injection power, and cavity length detuning. Experimental results have shown the injected pulses serve as a complementary source for establishing mode-locking (ML). As a result, the self-starting ML threshold of the Yb-doped fiber laser is significantly reduced from 355 to 171 mW, which is only half that in the non-injection case. This optical pulse injection assisted ML process is observed with a real-time oscilloscope, revealing stable and consistent buildup dynamics. Our finding may provide a new approach for understanding the buildup dynamics and building mode-locked fiber lasers with low threshold and high consistency.