Beam quality improvement in a linear ZPG OPO pumped by a Q-switched compact high-power Ho3+:YAG laser

We present a Ho3+:YAG laser pumped by a Tm3+-doped fiber laser that delivers pulses with an energy of 2.2 mJ and a pulse duration of 20 ns at a repetition rate of 25 kHz. The average output power is 58 W, while the pulse peak power is 108 kW. This source is used to pump a linear zinc germanium phosphide optical parametric oscillator (OPO) in different configurations. The resonators are investigated with a focus on maximum output power and beam quality. By introducing a diverging lens in the OPO cavity, the beam quality of signal and idler is improved significantly. For a pump power of 34 W, a combined mid-infrared output power of 14.1 W corresponding to a pulse energy of 0.56 mJ is obtained. For this output power, beam quality factors M2 in the x- and y-axis of 2.1 and 3.3 as well as 2.4 and 3.5 are achieved for the signal and idler beam, respectively.