We demonstrate that anisotropy of Fresnel diffraction in a birefringent medium can be quantitatively characterized by a planar tensor. Eigenvectors of the tensor correspond to directions of minimum and maximum beam divergence. Zero eigenvalues exist for the slow eigenmode near the optic axis of biaxial crystals and correspond to autocollimated beam propagation along the cone of external conical refraction. Applications of the beam diffraction tensor are demonstrated for analysis of noncritical phase matching in acousto-optics and nonlinear three-wave mixing.

中文翻译：

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双折射晶体中的光束衍射张量

我们证明了双折射介质中菲涅耳衍射的各向异性可以通过平面张量进行定量表征。张量的特征向量对应于最小和最大光束发散的方向。双轴晶体光轴附近的慢本征模式存在零本征值，对应于沿外部锥形折射锥的自准直光束传播。演示了光束衍射张量在声光和非线性三波混频中的非临界相位匹配分析中的应用。