Vector dark-bright pulses from a ytterbium doped fiber laser mode-locked by nonlinear multimode interference

We report dark-bright pulse pair generation from the ytterbium doped fiber laser mode-locked by cascaded nonlinear multimode interference (MMI) effect under an all-normal dispersion configuration. The MMI is implemented in a step index single mode-multimode-single mode fiber structure. The combination of two MMI structures, acting like a saturable absorber, has been used for stable mode-locking operation in the resonator. By adjusting the polarization controller attached to one of the MMI structure, stable dark-bright pulses are observed at fundamental, second and third harmonic repetition rates. The optical spectrum of the dark-bright pulses is doubly peaked at 1036.6 and 1040.6 nm with a spectral width similar to 0.7 nm. The width of the dark pulses varied from 72 to 50 ns and that of the bright pulses from 30 to 14 ns when the pump power is increased from 102 to 170 mW at the fundamental repetition rate mode-locking. The darkness of the dark and intensity of bright pulses increase with the pump power. Dark and bright pulses in the pair are resolved by placing a polarizing component confirming their vector nature.