Low-repetition-rate, high-energy, twin-pulse, passively mode locked Yb3+-doped fiber laser.

We report an all-normal-dispersion, low-repetition-rate, high-energy, twin-pulse, passively mode locked ytterbium-doped fiber laser. The mode-locking mechanism of the laser is based on nonlinear polarization evolution and strong pulse shaping with a cascade long-period fiber grating bandpass filtering in a highly chirped pulse. The laser generates a highly stable twin-pulse group with 248 ps and 296 ps duration simultaneously and maximum pulse energy of 26.8 nJ-each pulse at a 2.5445 MHz repetition rate. Energy quantization is observed, which demonstrates the nonparabolic nature of these pulses. The laser can also work in third-harmonic mode locking with 17.8 nJ energy (at a repetition rate of 7.65 MHz and pulse width of 780 ps).