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Summary form only given. The development of compact and easy-handling THz-wave sources has been increasingly important from both fundamental and applied points of view. We have recently demonstrated an efficient THz-wave generation by introducing a novel output coupling method into a undoped congruent LiNbO/sub 3/ optical parametric oscillator (OPO), which exhibits multiple advantages such as coherency, wide tunability (frequency: 0.9-2.1 THz, wavelength: 140-310 /spl mu/m), relatively high power (more than 10 mW at the peak), and compactness of its system (table-top size). This scheme is based on an optical parametric oscillation utilizing the lowest A/sub 1/-symmetry polariton mode of LiNbO/sub 3/. Wide continuous tunability is easily obtained by slightly varying the angle between the near-infrared pump and the idler wave vectors due to the noncollinear phase matching condition k/sub p/=k/sub i/+k/sub T/, where indices p, i, T denote pump, idler, and THz wave, respectively. In this report, we describe the performances of the OPO utilizing MgO LiNbO/sub 3/ crystal, which was developed to have remarkably stronger resistance to optical damage than undoped one.