All-Fiber High Efficiency Wavelength-Tunable Mode-Locked Cylindrical Vector Beam Laser

A wavelength-tunable mode-locked cylindrical vector beam (CVB) fiber laser with a dumbbell-shaped structure was proposed and demonstrated experimentally. A homemade broadband long-period fiber grating (LPFG) and a high-birefringence Sagnac fiber loop work as mode converter and comb filter respectively. The mode-locking mechanism is based on the nonlinear polarization rotation (NPR) effect. The central wavelength can be tuned from 1023.257 nm to 1046.123 nm with mode-locked operating state. The laser has a high slope efficiency of 15.75% with the mode-locking threshold value of 232.5 mW at the central wavelength of 1041.361 nm. The mode-locked CVB pulses have a repetition rate of 7.06 MHz with the duration of 107 picoseconds. Radially and azimuthally polarized beams can be obtained by eliminating the degeneracy of LP11 mode, with the purity of 94.86%. This CVB laser with controllable operating wavelength may have potential applications in mode division multiplexed (MDM) systems, optical manipulation, and electron acceleration.