Bright-Dark Pulse Pair in a Passively Mode-Locked Fiber Laser Based on Thulium-Doped Fiber

A dual-wavelength bright-dark pulse pair has been experimentally demonstrated in a mode-locked fiber laser based on thulium-doped fiber (TDF). By appropriately adjusting the polarization state, the dual-wavelength bright-dark pulse pairs with fundamental frequency of 20.56 MHz can be successfully obtained under the pump power ranging from 150 to 500 mW. The largest pulse energy is 2.84 nJ, and the conversion efficiency exceeds 10%. The generation mechanism of the bright-dark pulse pair in the cavity is primarily attributed to the cross-phase modulation (XPM) induced by the interaction of the bright soliton and dark soliton locating at different wavebands. Additionally, the dynamics of the bright-dark pulse pair is influenced by the polarization effect. To the best of our knowledge, this is the first demonstration of a bright-dark pulse pair laser employing TDF as the saturable absorber (SA). Its superiority for mode-locking has been fully proved, making it enormous potential in the fields of ultrafast lasers and nonlinear optics. Furthermore, this pulsed laser may offer a novel path for the development of high-power fiber lasers.