Quasi-coherent noise-like pulses in a mode-locked fiber laser with a 3D rotatable polarization beam splitter

For the first time, to the best of our knowledge, we experimentally observed a novel quasi-coherent noise-like pulse (NLP) in a simplified nonlinear polarization evolution mode-locking fiber laser when appropriate polarization was maintained for the lasing light through a three-dimensional rotatable polarization beam splitter inside the cavity. The degree of first-order coherence was evaluated after an interferogram measurement. The evolution of the measured shot-to-shot spectrum revealed that the NLPs possess quasi-coherence. Self-starting ultrafast soliton pulses switching to quasi-coherent NLPs at higher pump power levels were due to the preservation of the soliton features, mainly the Kelly sidebands in the spectrum. Quasi-coherent NLPs with average power of 56.58 mW and 10.4% slope efficiency were achieved with single pulse energy of 3.22 nJ.