Watt-level passively <i>Q</i>-switched mode-locked Tm: LuAG laser with graphene oxide saturable absorber

Abstract

A watt-level passive Q-switched mode-locked operation in Tm: LuAG all-solid-state laser is realized for the first time by using graphene oxide (GO) saturable absorber as a mode-locked starting element. The laser is pumped by a wavelength tunable Ti: sapphire laser operating at 794.2 nm. In this experiment, the maximum continuous-wave (CW) output power of 1440 mW, 2030 mW and 2610 mW are obtained by 1.5%, 3% and 5% output coupled (OC) mirrors respectively, in which the corresponding slope efficiencies are 22.3%, 32.6% and 40.6%, respectively. When the GO is inserted into the cavity, the laser bump threshold is further increased due to more intracavity loss. With a 1.5% OC mirror, the absorbed pump threshold is as low as 325 mW, the maximum output power is 787 mW, and the corresponding slope efficiency is 12.5%. With a 3% OC mirror, the absorbed bump threshold is 351 mW, the maximum output power is 1740 mW, and corresponding slope efficiency is 30.3%. With a 5% OC mirror, the QML operation is not realized due to the increase of intracavity loss. Although the laser pump threshold power of 3% OC mirror differs from that of 1.5% OC mirror by 26 mW, the output power is more than twice higher than that of 1.5% OC mirror. For these reasons, we use a 3% OC mirror in our experiment. In this case, a stable QML operation with a threshold of 3420 mW is obtained. When the pump power reaches 8.1 W, the corresponding maximum output power is 1740 mW, the central wavelength is 2023 nm, the repetition frequency is 104.2 MHz, the maximum single pulse energy is 16.7 nJ, and the modulation depth is close to 100%. According to the symmetrical shape of the mode locked pulse and considering the definition of rise time, we can assume that the duration of the pulse is approximately 1.25 times the pulse rise time. So the width of the mode locked pulse is estimated at about 923.8 ps. The results show that the GO is a promising high power saturable absorber in 2 μm wavelength for the QML solid-state laser. In the next stage, we will increase the pump power, optimize the quality of the GO material, and compensate for the dispersion in the cavity. It is expected to achieve a CW mode-locked operation and femtosecond pulse output.