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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.

In this work, an all-fiber-based mode converter for generating orbital angular momentum (OAM) beams is proposed and numerically investigated. Its structure is constructed by cascading a mode selective coupler (MSC) and an inner elliptical cladding fiber (IECF). OAM modes refer to a combination of two orthogonal LP lm modes with a phase difference of ±π/2. By adjusting the parameters and controlling the splicing angle of MSC and IECF appropriately, higher-order OAM modes with topological charges of l = ±1, ±2, ±3 can be obtained with the injection of the fundamental mode LP 01 , resulting in a mode-conversion efficiency of almost 100%. This achievement may pave the way towards the realization of a compact, all-fiber, and high-efficiency device for increasing the transmission capacity and spectral efficiency in optical communication systems with OAM mode multiplexing.

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