在双轴晶体中，除光轴方向外，沿任一方向只存在2个具有不同偏振方向的平面波解，每个解有各自的传播常数。运用拉格朗日乘数法并借助双轴晶体的折射率椭球的概念，在主轴坐标系下求出双轴晶体的平面波解，得到每个解的空间色散关系，并从所得平面波解出发讨论平面波沿光轴方向传输的情况。最后将所得的结果运用到单轴晶体中检验其正确性。

When a plane wave propagates along any direction except the direction of the optical axes in biaxial crystal, there are only two solutions which have different propagation constants and polarizations. The plane wave solutions of a biaxial crystal in principal coordinate system were derived and the spatial dispersion relation of each solution was obtained by using Lagrange multiplier method and the ellipsoid concept of refractive index of the biaxial crystal. Proceeding from the obtained solution of the plane wave, the propagation of the plane wave along the optical axis is discussed. The obtained result was applied in uniaxial crystal for verification.