Mode-locked all-fiber laser generating optical vortex pulses with tunable repetition rate

We propose and demonstrate an all-fiber actively mode-locked laser producing optical vortex pulses with high efficiency and a tunable repetition rate. Both vectorial and scalar optical vortices, i.e., cylindrical vector beams and orbital angular momentum beams, have been generated. Highly efficient mode conversion is realized by introducing a two-mode long-period fiber grating into the laser cavity as a mode converter with low insertion loss. A two-mode fiber Bragg grating is employed as a mode selector, a spectrum filter and an output mirror. A LiNbO3 Mach-Zehnder intensity modulator served as a mode-locker to achieve active harmonic mode-locking. The slope efficiency increases from 10.24% to 12.61% with the repetition rate of the pulse train flexibly tuned from 15.65 MHz at the fundamental mode-locking to 626 MHz at the 40th order harmonic mode-locking with superior stability. Switching between vectorial and scalar optical vortices at different repetition rates is realized through the intra-cavity state of polarization control. Published by AIP Publishing.