Scalar and Vector Solitons in a Bidirectional Mode-Locked Fibre Laser

Recent discovery on scalar-vector features of ultrashort pulses sheds new light on the polarization dynamics of ultrafast fibre lasers. Here, we report on the generation of scalar and vector solitons in a bidirectional fibre laser based on nonlinear polarization rotation (NPR). Through introducing different polarization evolving mechanisms into the counter-propagation branches, the fibre laser respectively approaches the scalar-soliton regime and vector-soliton regime in the clockwise and counterclockwise directions. It is found that the strong local birefringence facilitates the energy to flow between two orthogonal polarization directions, consequently inducing the pulse evolution toward vector solitons even against obvious polarization discrimination effect introduced by the NPR mode-locking mechanism. Furthermore, the polarization resolved measurement is implemented to both analyze the scalar-vector feature of the emitted solitons and realize the manipulation of vector solitons. In addition, numerical simulations are carried out to clarify the manipulation process of the vector solitons. To our knowledge, this is the first observation of scalar-vector operation in a bidirectional fibre laser, which both enriches our understanding toward polarization dynamics of optical solitons and paves potential application scenarios of pulsed fibre lasers.