Femtosecond Soliton Pulse Generation Using Evanescent Field Interaction Through Tungsten Disulfide (WS ₂) Film

We investigated nonlinear optical characteristics of Tungsten disulfide (WS₂) films and experimentally demonstrated their high potential for application as nonlinear saturable absorbers in passively mode-locked fiber lasers. Side polished fiber (SPF) was fabricated and WS₂ film was overlaid to provide an efficient evanescent field interaction. The WS₂ film was prepared using two methods: liquid phase exfoliation to form few-layer nano-sheets, and chemical vapor deposition (CVD) to grow uniform multilayer WS₂ on a SiO₂ substrate. Two SPF saturable absorbers were prepared by either spin coating WS₂ solution or lifting off the multilayer CVD WS₂ on SPF. An all-fiber ring cavity was built and the WS₂ film overlaid on SPF was employed as a mode locker along with Er-doped fiber as a gain medium. Using the spin-coated WS₂ SPF, stable soliton-like pulses were generated with a spectral width of 5.6 nm and 467 fs pulse duration. The fiber laser cavity containing CVD WS2 SPF generated a transform-limited soliton pulse train with a spectral width of 8.23 nm and a pulse duration of 332 fs. Our study confirmed a high potential of WS₂ film as a novel 2-D nonlinear optical material for laser applications.