Under the pump of a diode laser at 790 nm, self-pulsing phenomena of ~2 μm Tm3+-doped fiber lasers are investigated.
Laser cavity configuration is changed by using different output-coupling mirrors. When the polished fiber-end (Fresnel
reflection) is used as the output coupler, the cavity is dubbed as "bad" cavity. With this "bad" cavity, regular laser pulse
trains can be obtained near threshold pump level. The pulse width and repetition rate are several micro seconds and several kilohertz, respectively. At higher pump levels, the regular pulse-train shape is broken, and some random pulses occur. At the same time, similar mode-locking phenomenon can be observed. When the output coupler is changed to a T=10% (at 2μm) mirror, regular laser pulse train still can be observed near threshold pump. However, a slight increase of pump leads to randomization of the pulse train. Similar mode-locking phenomenon was not observed with this kind of cavity. When the output coupling is decreased to T=5%, even near the pump threshold, regular pulse train can not be achieved. With increased pump powers, laser output changes from pulse state to continuous wave. Based on the experimental results, a theoretical model is proposed and the origin of self-pulsing is discussed.