Long Period Pulsation of Mode-locked Soliton Trains in Fiber Laser

Observed is the long period pulsation of the mode-locked solitons and soliton bunches in nonlinear polarization rotation fiber laser. The period of the single soliton and the soliton bunching can reach the order of 103 and 102 cavity round trips,respectively. The pulsation period and the pulsation profile are related with the pumping power and the polarization bias. The long-period pulsation owes to the imprecision of the first order polarization state bias,which leads to the slight polarization modulated filtering of the polarization-dependent isolator on the mode-locked solitons in each round trip. The accumulated modulation loss arouses the long-period pulsation of the mode-locked pulse trains.