A high energy and high efficiency extracavity KTiOAsO4 (KTA) optical parametric oscillator (OPO) with excellent stability and beam quality, which is pumped by a Q-switched single-longitudinal-mode Nd:YAG laser, has been demonstrated based on a type II non-critical phase matching (NCPM) KTA crystal. The maximum pulse energy of 10 mJ at 3.467 μm is obtained with the repetition rate of 10 Hz and pulse width of 2 ns, corresponding to an optical-optical conversion efficiency of 14% from 1.064 μm to 3.467 μm. After one hour of investigation, the OPO output stability better than 4.5% rms has been obtained. The output stability of 3.467 μm

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