Generation of the Tunable Second-Order Optical Vortex Beams in Narrow Linewidth Fiber Laser

We experimentally demonstrate a narrow linewidth single-longitudinal-mode erbium-doped fiber ring laser with an output of tunable second-order optical vortex beams (OVB). The laser is based on a mode selective all-fiber fused coupler, which is composed of a single-mode fiber (SMF) and a few-mode fiber (FMF). The fused SMF-FMF coupler inserted in the cavity of the laser acts as a mode converter, which converts LP
₀₁
mode to LP
₂₁
mode group. The tunable OVB is produced by combing two even LP
₂₁
modes with orthogonal polarization directions. The tunability of second order OVB in the fiber laser is experimentally realized by adjusting a polarizer at the output end of the fiber. The 20 dB linewidth of the fiber laser is approximate 7.2 kHz.