The nonlinear optical property of germanium saturable absorber and its application in normal dispersion Er-doped fiber laser

Recently, germanium (Ge) has been considered as the two-dimensional (2D) material saturable absorber (SA) due to its excellent nonlinear saturable absorption properties. However, previous reports explored the nonlinear saturable absorption properties of Ge SA in anomalous dispersion Er-doped fiber (EDF) laser. To date, the mode-locked operation achieved by normal dispersion EDF laser based on Ge SA has not yet been reported until now. In this work, Ge SA with saturation intensity of 11.69 MW/cm2 and modulation depth of 6.17 % was used to demonstrate a normal dispersion EDF laser. Morphological and nonlinear saturable absorption properties of Ge were also investigated. To the best of our knowledge, the mode-locked pulse with repetition rate of 6.66 MHz were achieved in normal dispersion EDF laser based on Ge SA for the first time. Compared with previous work about Ge SA in anomalous dispersion regime, our result proves that Ge also has excellent optical performance in normal dispersion regime.