A 2 μm continuous wave and passively Q-switched fiber laser in thulium-doped germanate glass fibers

In this study, we designed and fabricated a Tm3+-doped germanate glass fiber by using a rod-in-tube technique. The fiber has a core diameter of similar to 13.5 mu m with a cladding of 125 mu m. The Tm3+ doping concentration reached similar to 13 500ppm, and the propagation loss of the fiber was less than 0.03 dB cm(-1) at 1310 nm. Pumping with a 1590 nm fiber laser constructed in-house, a 44.7 mW continuous-wave fiber laser at 1940 nm was achieved in a 22 cm-long germanate fiber, with a slope efficiency of 26%. The passively Q-switched pulse output was first demonstrated by using a carbon nanotube saturable absorber in the germanate fiber. The germanate fiber laser generates approximately 1.5 mu s pulses with a maximum pulse energy of 110 nJ, and the repetition rate varies from 15 to 84 kHz, depending on pump power.