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 LP
₀₁
mode to LP
₁₁
(LP
₂₁
) mode with a broadband width. Linearly polarized OAM modes can be obtained by combining different vector modes HE
_2,m
^even
(HE
_2,m
^odd
) and TE
_0,m
(TM
_0,m
) when l = 1 or combining HE
^even
_{l+ 1,m}
(HE
_{l+ 1,m}
^odd
) and EH
_l-1,m
^odd
(EH
_l-1,m
^even
) when l > 1 with a π/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
.