NOT KNOWN FACTS ABOUT BIREFRINGENT CRYSTAL

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. This is due to they're able to have their nonlinearity only by getting non-isotropic, Which also leads to birefringence.直光纤只有很小的随机双折射，�