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Paraxial ray methods for anisotropic inhomogeneous media

A new formalism of surface‐to‐surface paraxial matrices allows a very general and flexible formulation of the paraxial ray theory, equally valid in anisotropic and isotropic inhomogeneous layered media. The formalism is based on conventional dynamic ray tracing in Cartesian coordinates along a reference ray. At any user‐selected pair of points of the reference ray, a pair of surfaces may be defined. These surfaces may be arbitrarily curved and oriented, and may represent structural interfaces, data recording surfaces, or merely formal surfaces. A newly obtained factorization of the interface propagator matrix allows to transform the conventional 6 × 6 propagator matrix in Cartesian coordinates into a 6 × 6 surface‐to‐surface paraxial matrix. This matrix defines the transformation of paraxial ray quantities from one surface to another. The redundant non‐eikonal and ray‐tangent solutions of the dynamic ray‐tracing system in Cartesian coordinates can be easily eliminated from the 6 × 6 surface‐to‐surface paraxial matrix, and it can be reduced to 4 × 4 form. Both the 6 × 6 and 4 × 4 surface‐to‐surface paraxial matrices satisfy useful properties, particularly the symplecticity. In their 4 × 4 reduced form, they can be used to solve important boundary‐value problems of a four‐parametric system of paraxial rays, connecting the two surfaces, similarly as the well‐known surface‐to‐surface matrices in isotropic media in ray‐centred coordinates. Applications of such boundary‐value problems include the two‐point eikonal, relative geometrical spreading, Fresnel zones, the design of migration operators, and more.