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 not only acts as mode converter from LP01 mode to LP11 or LP21 modes with a broadband width over 100 nm, but also directly delivers femtosecond vortex pulses out of the mode locked cavity. To the best of our knowledge, this is the first report on the generation of high-order pulse vortex beams in mode-locked fiber laser. The generated 140 femtosecond vortex beam has a spectral width of 67 nm centered at 1544 nm. 
Teng Wang, Feng Wang, Fan Shi, Fufei Pang, Sujuan Huang, Tingyun Wang,   and Xianglong Zeng
physics.optics