“Invisible” pulsation and period-doubling bifurcation of harmonic mode locking in a bidirectional fiber laser

The harmonic mode-locking “invisible” pulsation is reported, here, in a bidirectional passively mode-locked fiber laser. With the help of dispersive Fourier transform technology, it is found that due to the alike nonlinear effects experienced by two pulse-trains in harmonic mode-locking, their evolution is consistent during the “invisible” pulsation. Furtherly, as the increase of pump power, period-doubling bifurcations can be observed based on the “invisible” pulsation phenomenon in harmonic mode-locking scheme, and the period-doubling bifurcation path experienced by the harmonic mode-locking from steady to chaotic is statistically obtained. Finally, the “invisible” pulsation and period-doubling bifurcation in the bidirectional laser are reproduced and studied through numerical simulations. The effect of “invisible” pulsation on the coherence of solitons is further analyzed. We believe our research results will enrich the theoretical system of nonlinear dynamics and can be helpful to the design of pulsed light sources.