Carbon-Based Composite Reinforced Chitosan Film to Initiate Wavelength -Tunable Q-Switched Erbium-Doped Fiber Laser

Carbon-based materials have gained significant attention as saturable absorbers (SAs) due to their strong nonlinear optical response, ultrafast recovery time, and broadband optical absorption. Among them, reduced graphene oxide (rGO) stands out for its excellent optical modulation properties, making it a promising candidate for Q-switched pulsed laser applications. However, the fragile nature of rGO films limits their practical implementation. To address this issue, we propose a hybrid SA by incorporating rGO with zinc oxide (ZnO) nanoparticles to enhance optical performance and embedding the composite in a chitosan polymer matrix to improve structural stability. The rGO/ZnO composite was synthesized via a hydrothermal method and integrated into chitosan to form a robust thin film. Nonlinear optical characterization revealed a modulation depth of 16 % with a saturation intensity of 0.002 MW/cm². By implementing this composite SA in an Erbium-doped fiber laser (EDFL) system, a tunable Q-switched pulsed laser was successfully demonstrated, achieving a wavelength tuning range of 50 nm from 1523 nm to 1573 nm. The repetition rate increased from 54.0 kHz at 152.3 mW pump power to 64.1 kHz at 254.4 mW, while the pulse width decreased from 2.54 µs to 1.66 µs. These results confirm that the rGO ZnO-chitosan composite film is a highly/ promising SA for tunable Q-switched pulsed laser generation in the C-band wavelength region, offering both enhanced optical properties and mechanical stability.