Growth of crack-free ZnGeP2 large single crystals for high-power mid-infrared OPO applications

Abstract

In this paper, more than 500g of high-purity ZGP polycrystalline material has been synthesized in one run, and high-optical quality ZGP single crystals with dimensions of 50mm in diameter and 140mm in length have been also successfully grown by Vertical Bridgman technique. From data of the near-infrared optical absorption coefficient, we found that the large crystal had lower absorption losses at the near-infrared waveband than the small one. Meanwhile, the absorption coefficient of 24-mm length ZGP–OPO devices for o-polarized 2.05µm light was significantly reduced to about 0.02cm−1 after thermal annealing and high energy electron irradiation. Moreover, improvement in the polishing of ZGP–OPO devices had resulted in increase of laser-induced damage threshold. The average value of laser-induced damage threshold was increased to 2.0J/cm2 under the condition of 2.05µm wavelength, 30ns pulse width and 10kHz pulse rate frequency. As a result, the maximum output of 20.5W was obtained by high pulse repetition frequency Ho:YAG laser emitting at 2.09µm under a pump power of 46W. The corresponding slope efficiency was 68.7% and the conversion efficiency was 44.3%. These results manifested that the large ZGP crystals with high optical quality were acceptable for the fabrication of the infrared OPO devices.