Diffraction of Gaussian beam in a 3D smoothly inhomogeneous media:   eikonal-based complex geometrical optics approach
  The paper presents an ab initio account of the paraxial complex geometrical optics (CGO) in application to a scalar Gaussian beam propagation and diffraction in a 3D smoothly inhomogeneous medium. The paraxial CGO deals with quadratic expansion of the complex eikonal and reduces the wave problem to the solution of ordinary differential equations of Riccati type. This substantially simplifies description of Gaussian beams diffraction as compared to full wave or parabolic (quasi-optics) equations. For a Gaussian beam propagating in a homogeneous medium or along the symmetry axis in a lens-like medium, the CGO equations possess analytical solutions, otherwise they can be readily solved numerically. As a non-trivial example we consider Gaussian beam propagation and diffraction along a helical ray in an axially symmetric waveguide medium. It is shown that the major axis of the beam's elliptical cross-section grows unboundedly; it is oriented predominantly in azimuthal (binormal) direction and does not obey the parallel transport law. 
P. Berczynski, K.Yu. Bliokh, Yu.A. Kravtsov, and A. Stateczny