Long cavity passive harmonic mode-locked erbium-doped fiber laser

We demonstrate a passive harmonic mode-locked erbium-doped fiber laser with long cavity based on nonlinear polarization rotation (NPR). The total cavity length is 696 m, which can be achieved by incorporating a 650 m long single mode fiber (SMF). The fiber laser produces 6.1 ps, 6.49 nJ mode-locked pulse at a fundamental repetition rate of 306.4 kHz. And from 2^th to 16^th order passive harmonic mode-locked pulse trains have been experimentally observed.