Nonlinear optical characteristics of germanium and its application for generating bound state solitons within Er-doped fiber laser

Recently, germanium (Ge) was also considered as the two-dimensional (2D) material saturable absorber (SA) because of its excellent nonlinear saturable absorption properties. However, the research of Ge SA Er-doped fiber laser is still rare. In this work, bound state solitons were demonstrated in Ge SA Er-doped fiber laser. To be best of our knowledge, no previous work has ever achieved that. Three kinds of bound state solitons with 9.77 MHz fundamental frequency were investigated, indicating that Ge has excellent nonlinear optical characteristics to promote the development of bound state soliton.