GHz-repetition-rate fundamentally mode-locked, isolator-free ring cavity Yb-doped fiber lasers with SESAM mode-locking.

A novel fundamentally mode-locked, GHz-repetition-rate ring cavity Yb-doped femtosecond fiber laser is demonstrated, which utilizes polarization-maintaining gain fiber and is enable by SESAM mode-locking. Thanks to the isolator-free structure, the ring cavity laser is operated bidirectionally and the two polarization-multiplexed output pulse trains are demonstrated synchronous. As a result, tunable waveforms one of which is with reduced pedestal and shorter pulse width in comparison with each individual, are generated by combination of the two orthogonal-polarized output pulses. Furthermore, a similar ring cavity structure that generates GHz picosecond pulses is demonstrated. We believe such high-repetition-rate polarization-multiplexed mode-locked fiber lasers could find further uses in various applications in need of gigahertz repetition rate and tunable waveforms.