Passively Q-switched mode-locked thulium-doped fiber laser using nonlinear polarization rotation technique

A Q-switched mode-locked (QML) thulium-doped fiber laser based on nonlinear polarization rotation (NPR) and negative group velocity dispersion (GVD) is demonstrated. This regime consists in the generation of a mode-locked (ML) pulse train under a Q-switched (QS) envelope. In this regard, by properly adjusting the polarization controllers and increasing the pump power in a range from 1 to 3 W, QML regime is obtained. With a maximum pump power of 3 W, a QS pulse duration of 1.46 μs and a pulse energy of 5.96 μJ are obtained. The maximum frequencies reached are 26 kHz and 2.63 MHz, corresponding to the Q-switched and mode-locked pulses, respectively. This method for generating a QML at 2 μm has the advantages of being eye-safe and robust, providing an ideal dual-frequency comb (DFC) generation method with high potential for various applications.