Mode-resolved nonlinear absorption characteristics of few-mode fiber-multimode fiber-few-mode fiber structures

We report a systematic investigation on the mode-resolved nonlinear absorption characteristics of the few-mode fiber-multimode fiber-few-mode fiber (FMF) structure. Firstly, the transmission formulas of the FMF structure are derived, demonstrating that the transmission is mode-dependent. Experiments performed on various FMF structures show that the LP₀₁ and LP₁₁ modes exhibit markedly different nonlinear absorption. By simply adjusting the length of the multimode fiber or the in-line polarization controller, the nonlinear absorption response of each mode can be reconfigured independently, even enabling switching among saturable absorption, reverse saturable absorption, and their hybrid states. Numerical simulations further indicate that different modes exhibit distinct nonlinear absorption behaviors, primarily originating from the mode couplings at the interfaces and the propagation within the multimode fiber. These results provide profound insights into the mode-dependent nonlinear absorption characteristics of the FMF structure and optional explanations for various phenomena reported in spatiotemporal mode-locked fiber lasers.