Polarization properties of phase gratings recorded in a Bi12SiO20 crystal

Due to the co-existence of optical activity and linear birefringence in a Bi₁₂SiO₂₀ crystal, a phase gratting induced in this medium has many interesting and unusual properties. We study the ellipticity and orientation of the directly transmitted and diffracted beams as a function of the read-beam vector orientation. In image processing experiments the signal to noise ratio can be increased significantly by appropriately choosing the read-beam orientation such that the strong direct beam is linearly polarized and can be eliminated with the help of a polarizer.