Beam dynamics of two modes propagating along the optic axis in a uniaxial crystal.

The Gaussian beam propagation in the direction of the optic axis of a uniaxial crystal is treated by the complex-source-point technique. At the input plane the electric field is linearly polarized. A particular superposition of the ordinary-mode and the extraordinary-mode beams is generated. The electrodynamics of the composite beam has features that are different from those of the two constituent beams. As a result of the anisotropy, on propagation, the cross-polarized component of the electric field is generated except along the beam axis; the cross section of the beam, which is circular at the input plane, becomes elliptical; and the mean squared width of the beam departs from the usual quadratic dependence on the distance from the waist in the direction of propagation.