$KNbO_3$ is one of the best materials for efficient nonlinear frequency conversion, due to its large nonlinear optical coefficient ($d_33=20.6pm/V$), wide transparency range (0.4-5㎛) and freedom from photorefractive effects. In this thesis, fabrication of periodically poled $KNbO_3$ crystals and demonstration of a pulsed optical parametric oscillator (QPM OPO) pumped by an Nd:YAG laser are presented. Single domain $KNbO_3$ crystals of 11×14×6 (a×b×c)㎣ size were fabricated by applying an electric field of 500 V/mm along the c-axis at 170℃ for 13 hours. Using the single domain $KNbO_3$ crystal plates, periodically poled structures were fabricated by applying a square pulse with a peak intensity of 280 V/mm and a pulse width of 730ms. A periodically poled $KNbO_3$ of good quality was obtained with a dimension of 7×2.2×0.7(a×b×c)㎣ and a period of 31.5㎛ which is to utilize the $d_33$ component of the second-order nonlinear optical coefficient tensor. Optical parametric oscillation was realized at the signal wavelength of 1.6㎛ using an Nd:YAG laser as a pumping source. The result of this work may contribute to the development of coherent radiation sources in mid-IR region.
