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

A Yb-doped fiber laser mode locked by nonlinear amplifying loop mirror with 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-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. As a result, self-starting mode-locking threshold of the Yb-doped fiber laser is significantly reduced from 355 mW to 171 mW, which is only a half of that in the non-injection case. This optical pulse injection assisted mode-locking process is observed with a real-time oscilloscope, revealing stable and consistent build-up dynamics. Our finding may provide a new approach for understanding the build-up dynamics and building mode-locked fiber laser with low threshold and high consistency.