A 2.09 µm injection-seeded single-frequency Ho:YAG laser resonantly pumped by a 1.91 µm laser diode is demonstrated for the first time. The seed laser is a continuous wave (CW) Ho:YAG non-planar ring oscillator. 15.15 mJ single-frequency output energy is obtained from the injection-seeded Q-switched Ho:YAG laser, with a pulse repetition rate of 200 Hz and a pulse width of 109 ns. The half-width of the pulse spectrum is measured to be 4.19 MHz by using the heterodyne technique. The fluctuation of the center frequency of the singlefrequency pulses is 1.52 MHz (root mean square (RMS)) in 1 h.

The performance of a single-frequency Ho:YAG laser with bow-tie cavity injected by different polarized seed lasers is studied both theoretically and experimentally. With vertically polarized (s-polarized) seeding, a 9.7 mJ single-frequency laser with a pulse width of 183 ns at the repetition rate of 252 Hz is obtained. With horizontally polarized (p-polarized) seeding, an 8.8 mJ single-frequency laser with a pulse width of 220 ns at the repetition rate of 252 Hz is obtained. The laser operating in p-polarized seeding mode performs better than the one operating in s-polarized seeding because of the stable relative center frequency of the pulses (0.9 MHz in 1 h).