Femtosecond Optical Vortices Generation From an All-Fiber Amplifier

High-power femtosecond optical vortex beams have garnered considerable attention in laser-based material processing due to their ability to enhance surface quality and improve processing precision [1]. Recently, we demonstrated the efficient amplification of femtosecond optical vortex beams from a few-mode fiber amplifier, achieving ~245 fs pulse duration, <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\sim 1.4 \mu \mathrm{J}$</tex> pulse energy, and an average output power of 33 W [2]. However, the reliance on free-space component for converting the Gaussian beam into optical vortex beam and coupling it into the final power amplifier introduces the system complexity and renders it highly sensitive to environmental perturbations. To overcome this challenge, we experimentally demonstrate an all-fiber femtosecond amplifier system utilizing an in-house fabricated long-period fiber grating (LPFG) as an inline vortex mode converter.