Direct generation of dip-type sidebands from WS_2 mode-locked fiber laser

In this paper, we demonstrated the direct generation of dip-type sidebands from a passively mode-locked fiber laser based on the evanescent field interaction of two-dimensional material WS2 and microfiber. The WS2 nanosheets in polyvinyl alcohol solution are prepared through the liquid-phase exfoliation method and transferred onto a tapered fiber by the optical deposition method. The WS2-deposited microfiber device could both operate as a mode-locker for soliton generation and induce a strong polarization effect to constitute an artificial polarizer for dip-type sidebands formation. In experiment, six kinds of dip-sidebands, namely, dip-dip type, hybrid peak-dip and dip type (І, ІІ), dual-peak-dip type (І, ІІ), and nearly-flat-top type are achieved by properly adjusting the pump strength and the polarization state in the laser cavity. Study found that the dynamic evolution of the dip-type sidebands along with the pump power is not reversible. In addition, we also obtained the maximum depth of ~16.8 dB and ~22 dB for dual-peak-dip type І and dual-peak-dip type ІІ, respectively, and their second-order forms. Our study shows clearly that WS2-assisted microfiber device can be as a mode-locker for soliton generation and a polarizer for studying nonlinear optics phenomenon in the fiber lasers simultaneously.