Harnessing Optical Limiting in Saturable Absorbers for Reconfigurable Spectral Engineering of Ultrafast Fiber Lasers

ABSTRACT Optical limiting is an intrinsic yet often overlooked nonlinearity in nanomaterial‐based composites, traditionally regarded as detrimental in laser cavities and thus deliberately suppressed. Here, we demonstrate that optical limiting can instead be harnessed as a functional degree of freedom for intracavity control in ultrafast fiber lasers. Using a carbon nanotube/polydimethylsiloxane (CNT/PDMS) composite as a representative platform, we show the coexistence of optical limiting and saturable absorption, where the two nonlinearities play distinct and complementary roles. The CNT/PDMS composite exhibits a modulation depth of ∼3% with a low saturation intensity of ∼2.9 MW/cm 2 for stable mode locking, together with a reversible optical limiting induced by photothermal loss under laser radiation. Time‐resolved pump‐probe measurement reveals a thermal recovery time of ∼33 µs, which is on a different temporal scale against ultrafast saturable absorption dynamics. When incorporated into the polarization‐maintaining Er‐doped fiber laser, saturable absorption governs pulse formation, while optical limiting provides intensity‐dependent intracavity loss that reshapes the gain spectrum, enabling pump‐controlled reconfigurable spectral engineering with non‐mechanical and nondestructive nature. This work redefines intracavity optical limiting as a functional degree of freedom for reconfigurable ultrafast lasers and is extendable to various gain media and waveguide platforms.