We show that the propagation equation for the slowly varying envelope of the electric field in a homogeneous dispersive nonisotropic medium contains terms that rotate the 3D wave packet of an optical pulse propagating as an extraordinary wave about an axis perpendicular to the propagation vector. It also possesses Fresnel diffraction coefficients that depend not only on the refractive index but also its derivatives with respect to direction. An analytic expression for the slowly varying envelope is obtained for an initial Gaussian wave packet by keeping terms up to second order in the wave equation for the slowly varying envelope.

• Received 24 July 1995

DOI:https://doi.org/10.1103/PhysRevLett.76.1457