▎ 摘　　要

A 2.09-mu m in-band pumped passively Q-switched Ho: YAG laser is demonstrated. Single layer graphene deposited on a quartz substrate is used as the saturable absorber for the Q-switched operation. The minimum pulse width of 2.11 mu s is obtained at an average output power of 100mW, corresponding to a pulse repetition frequency of 57.1 kHz and the pulse energy of 1.75 mu J. The beam quality factors M-2 of the Q-switched laser are 1.18 and 1.22 in the horizontal and longitudinal direction, respectively. The optical-to-optical conversion efficiency of the passively Q-switched laser is 4.3%, which is the highest conversion efficiency in the 2 mu m wavelength, to the best of our knowledge. It shows clearly that the Ho: YAG crystal is a potential gain medium in the 2 mu m range for the graphene application.