We report on an optical parametric oscillator (OPO) based on periodically poled lithium niobate (PPLN) pumped by a Nd:YAG laser, and on extending its tuning range by difference-frequency mixing of OPO output waves in GaSe crystal. Maximum combined signal and idler pulse energy of 2.7 mJ and slope efficiency of 25% have been achieved. The tuning range is 1.71 to 1.98 μm for the signal wave and 2.81 to 2.30 μm for the idler wave, using a single set of mirrors. As an application of this PPLN OPO, the signal and the idler waves are difference-frequency-mixed in a GaSe crystal to produce tunable mid-IR from 4.35 to 14.25 μm. The efficiency of the mixing has been analyzed, considering the experimental results. © 2002 Society of Photo-Optical Instrumentation Engineers.