Light distribution close to focus in biaxially birefringent media

The effect of focusing into a biaxially birefringent medium on the light distribution in the focal region of a high-NA optical system is investigated with the Debye approach to vectorial diffraction theory. Attention is limited to media with small birefringence. The electric field in the focal region is the sum of the field of the two polarization eigenmodes of the biaxially birefringent medium. Both modes are generally astigmatically aberrated, are defocused with respect to each other, and have a polarization field that is nonuniform over the pupil. The diffraction integrals are calculated numerically on the basis of an expansion of the field close to focus in terms of partial waves.