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/cm2 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.
Loading....