Mode-Locked Vortex Fiber Laser Based on High-Order Mode Conversion

We summarize two kinds of mode-locked fiber lasers (MLFLs) that generate intra-cavity optical vortex beams (OVBs) based on long period fiber gratings (LPFGs) and implement extra-cavity hybrid mode control with another kind of all-fiber mode converter called acoustically-induced fiber grating (AIFG). Firstly, the LPFG we used works at the turn-around point (TAP). Since mode conversion through TAP-LPFG can produce the complete set of eigenmodes of LP
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mode, OVBs are then generated by inducing a phase difference of $\pi/2$ to the eigenmodes. Secondly, the 1.94 W average-power mode-locked pulses come from a MLFL in the dissipative soliton resonance (DSR) regime. The linear cavity is formed by two fiber loop mirrors (FLMs) whose splitting ratios are 50/50 and 5/95. The 5/95 FLM operates as a nonlinear optical loop mirror (NOLM) which aims at initiating stable DSR. The output fundamental mode is then fully or partially converted to high-order mode (HOM) through AIFG. Mode patterns are captured through an optical imaging system. Taking the advantage of all-fiber mode converters, our approach may expand the diversity of mode control under high-power condition and other researches in high-power fiber lasers.