Kurland I J, Bertoni H L
Appl Opt. 1978 Apr 1;17(7):1030-7. doi: 10.1364/AO.17.001030.
The coupling from a laser to a thin-film optical waveguide by a prism coupler composed of a birefringent material can be strongly dependent on the orientation of the optic axis. It is shown that when the effective index of the wave guided by the film lies between the ordinary and extraordinary refractive indices of the prism, the coupling depends strongly on the orientation of the optic axis. Differences in orientation of only a few degrees separate ranges of orientation in which strong coupling occurs from ranges in which there is no coupling. The orientation dependence of the coupling is considered both for the pure mode case wherein the optic axis lies in the plane of incidence and for the case when the optic axis is rotated out of the plane of incidence, so that mode coupling occurs. When the effective refractive index of the guided wave is less than both the ordinary and extraordinary refractive indices of the prism, the coupling properties are found to be similar to those obtained with an isotropic prism.
由双折射材料构成的棱镜耦合器将激光耦合到薄膜光波导的过程可能强烈依赖于光轴的取向。结果表明，当薄膜所引导的波的有效折射率介于棱镜的寻常折射率和非常折射率之间时，耦合强烈依赖于光轴的取向。仅几度的取向差异就能区分出发生强耦合的取向范围和无耦合的取向范围。对于光轴位于入射面内的纯模式情况以及光轴旋转出入射面从而发生模式耦合的情况，均考虑了耦合的取向依赖性。当导波的有效折射率小于棱镜的寻常折射率和非常折射率两者时，发现其耦合特性类似于用各向同性棱镜所获得的耦合特性。
