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 LP01mode to LP21mode group. The tunable OVB is produced by combing two even LP21modes 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.