Generation of Femtosecond Optical Vortex Beams in All-Fiber Mode-Locked Fiber Laser Using Mode Selective Coupler

We experimentally demonstrated a high-order optical vortex pulsed laser based on a mode selective all-fiber fused coupler composed of a single-mode fiber (SMF) and a few-mode fiber (FMF). The fused SMF-FMF coupler inserted in the cavity acts as a mode converter from LP01 mode to LP11 (LP21) mode with a broadband width. Linearly polarized OAM modes can be obtained by combining different vector modes HE2,meven (HE2,modd) and TE0,m (TM0,m) when l = 1 or combining HEl+1,meven (HEl+1,modd) and EHl-1,modd (EHl-1,meven) when l > 1 with a pi/2 phase shift. To the best of our knowledge, this is the first report on the generation of high-order pulse vortex beams in a mode-locked fiber laser. The measured time duration of OVB pulses is 273 fs, 140 fs of OAM(+/- 1) and OAM(+/- 2).