Birefringent Crystal - An Overview
各向异性透明晶体如方解石、石英等的折射率，是其固有的特性，称为永久双折射。All nonlinear crystals for nonlinear frequency conversion are birefringent. It's because they can have their nonlinearity only by getting non-isotropic, and that also brings about birefringence.Intrinsic birefringence will be the term used