High-energy pulses from all-PM ultra-long Yb-fiber laser mode-locked with quasi-synchronous pumping

We explore, for the first time, mode-locking capabilities and frontiers of a quasi-synchronous pumping technique implemented in an all-polarization-maintaining (all-PM) kilometer-long Yb-fiber laser. This technically simple approach to synchronous pumping relies on sine-wave modulation of pump power at a frequency slightly detuned from the laser cavity inter-mode frequency or its multiple. In combination with the scaled-up all-PMfiber cavity, it has provided stable generation of linearly polarized (scalar) high-energy (up to 130 nJ) pulses with tunable nanosecond duration at the fundamental and harmonic repetition rates ranging from 0.23 to 0.69 MHz. The obtained combination of such a low repetition rate, such a high energy level, and polarization stability is unique for actively mode-locked all-fiber stimulated-emission-based master oscillators. We reveal that detuning of the pump modulation frequency is the key parameter for the pulse shaping control in such mode-locked laser, and further intracavity pulse shortening towards sub-nanosecond duration is theoretically possible with measures preventing pulses from accumulation of excessive nonlinear phase. The demonstrated approach opens up new prospects for pump-modulation-based mode locking in diode-pumped lasers with such inertial active media as rare-earth-doped fibers.