THE BASIC PRINCIPLES OF 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。For optical fibers as well as other waveguides, it is a lot more appropriate to look at the real difference of efficient refractive indices. That is directly relevant to the real difference in imaginary values of the propagation constants.