Generating Femtosecond Vortex Beams Based on Broadband LPFG in Figure-9 Mode-Locked Fiber Laser

We report a scheme to generate broadband first-order linear polarization (LP
₁₁
) modes and optical vortex beams (OVBs) emitted by a figure-9 femtosecond mode-locked (ML) fiber laser connecting a broad long-period fiber grating (BLPFG) inside and outside the cavity, respectively. When the group velocities between the core modes of LP
₀₁
and LP
₁₁
in a two-mode fiber (TMF) are matched, the BLPFG can achieve broadband mode conversion with a 3-dB bandwidth of 160 nm. Moreover, the mode purity of output LP
₁₁
measured through a 4-f system is higher than 98%. The generation of OVBs can be verified by employing a self-interference system to detect the phase singularity and topological charge. The study of broadband OVBs promotes the development in optical trapping, optical communications and so on.