Optically Nonlinear Graphene-Polymer Composite for Mechanical Reinforcement of Mode-Lockers Incorporating Tapered Fibers

초록

Graphene (Gr)-coated tapered fibers are regarded as highly efficient nonlinear devices for optical signal management. In particular, stable, broadband, and polarization-insensitive laser mode-locking is realized with their nanomaterial-leveraged nonlinear saturable absorption. However, it remains challenging to enhance the mechanical durability of the fiber devices with diameters of only a few micrometers while maintaining the nonlinear functionality of Gr. We encapsulate the waist of a tapered fiber with Gr-dispersed ultraviolet (UV)-curable polymer to demonstrate a mechanically reinforced mode-locker while maintaining the effective nonlinear saturable absorption of Gr. Gr is homogeneously dispersed at an optimized concentration of 0.05 wt % to avoid agglomeration that deforms the sp2 Gr structures, thereby degrading the nonlinearity. The composite polymer is coated around a tapered fiber using a surface morphology-controlled wire-coating method followed by UV curing. Saturable absorption of the prepared device is characterized using a balanced twin-detector setup and fitted to a standard saturable absorption model resulting in a modulation depth of 8.55%. The mechanical robustness is quantified by a single-fiber tensile tester to confirm an improved toughness by a factor of 2.3. Integrated into an erbium-doped fiber ring laser, the device delivers stable femtosecond pulses with a center wavelength of 1563.6 nm, pulse duration of 201 fs, repetition rate of 3.36 MHz, and signal-to-noise ratio of 74 dB. These results establish a robust coating architecture that unifies structural reinforcement with efficient nonlinear optical operation in a single platform. The approach offers a scalable and application-ready solution for next-generation ultrafast fiber lasers requiring both durability and spectral integrity.

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