Evolution of noise-like pulses in mode-locked fiber laser based on straight graded-index multimode fiber structure

Graded-index multimode fiber structure is of interest for implementing all-fiber passively mode-locked laser via nonlinear mode-coupling. We experimentally report the noise-like pulses generation in an all-normal-dispersion Yb-doped fiber laser based on straight graded-index multimode fiber structure. The multimode fiber is precisely cleaved under a microscopic system and fixed in a straight glass tube. Using 108.04 mm-length multimode fiber as saturable absorber, the output noise-like pulses have a central wavelength of 1065.25 nm, fundamental repetition rate of 19.58 MHz, pedestal pulse duration from 2.15 ns to 9.40 ns and peak width from 3.97 ps to 0.43 ps with different pump power. In particular, the optical spectrum evolution of noise-like pulses is associated with the spectral filtering of straight multimode fiber structure. Furthermore, the influence of total cavity dispersion on noise-like pulses are investigated systematically through adding different lengths of HI1060 fiber in the laser cavity. The evolution of noise-like pulses in the easy fabrication and robust all-fiber laser will facilitate potential contribution to the mechanism of noise-like pulses in passively mode-locked fiber laser.