Spontaneous pulse temporal position modulation in a bidirectional ultrafast fiber laser

The mode-locked pulses within the laser cavity exhibit temporal characteristics that maintain a constant time interval after each roundtrip. Herein, we report the phenomenon of spontaneous pulse temporal position modulation achieved by introducing two carbon nanotubes (CNTs) saturable absorbers (SAs) into a bidirectional mode-locked fiber laser. By appropriately adjusting the pump power, it is observed that the two CNTs independently trigger counter-propagating pulses in the cavity, resulting in the periodic initiation and annihilation of pulses at different time points. This phenomenon can be attributed to the indirect interaction between the counter-propagating pulses mediated by the CNT colliding mode-locking mechanism, as well as the gain competition between co-propagating pulses. These findings provide an effective approach for controlling the pulse temporal position in dissipative optical systems and also offer new insights into the potential applications of bidirectional ultrafast fiber lasers.