Accurate Measurement of Second-Order Nonlinear-Optical Coefficients of Stoichiometric LiNbO3 and LiTaO3

我们测量了化学计量LiNbO_3的二阶非线性光学系数，这是最近开发的高质量的LiNbO_3晶体。测量波长为1.06 μm和1.31μm。氧化物公司和日立金属公司分别培养了未掺杂和掺氧化镁晶体。将它们切割抛光成楔形样品，楔形角为~ 0.15度，基光正常聚焦在样品上。测量了产生的二次谐波功率与样品厚度或Maker条纹的关系。我们充分考虑了近平面平行板样品的多次反射效应。化学计量LiNbO_3没有精确的折射率，而这是分析数据所必需的。如果楔形角已知，则可以通过测量maker条纹周期来获得折射率。由于很难在10^<-3>精度范围内直接测量楔形角，我们通过测量样品中多次反射引起的干涉周期，以及在基频和次谐波波长下透射率与样品位移的依赖关系来估计楔形角和折射率。这使我们能够以优于5%的精度确定非线性光学系数，并且我们发现在测量精度范围内，化学计量LiNbO_3和MgO:LiNbO_3的非线性光学系数与相同的非线性光学系数相同。

We have measured second-order nonlinear-optical coefficients of stoichiometric LiNbO_3, the high-quality crystals of which have been recently developed.The measurement was performed at the fundamental wavelengths of 1.06 and 1.31μm. Undoped and MgO-doped crystals were grown by Oxide company and Hitachi Metals company. They were cut and polished to the wedge samples with the wedge angle of 〜0.15 degree, and the fundamental light was normally focused onto the sample. The dependence of the generated second-harmonic power on the sample thickness, or Maker fringe, was measured. We fully took account of the multiple-reflection effect in the nearly plane-parallel-plate samples.The accurate refractive indices which are essential for analyzing the data are not available for stoichiometric LiNbO_3. If the wedge angle is known, the refractive indices can be obtained by the measurement of the Maker-fringe period. Because it is difficult to directly measure the wedge angle within 10^<-3> accuracy, we estimated the wedge angle and the refractive indices by measurement of the period of interference caused by the multiple reflection in the sample, and of dependence of the transmittance on the sample displacement in the cross-Nicole configuration, both at the fundamental and the second-harmonic wavelengths. This enabled us to determine the nonlinear-optical coefficients with the accuracy of better than 5%, and we found that the nonlinear-optical coefficients of stoichiometric LiNbO_3 and MgO:LiNbO_3 are same with that of congruent ones within the measurement accuracy.