Using the method of direct laser writing with femtosecond pulses, a three-dimensional micro-optical element has been formed in bulk of a fused silica plate, functioning as a quarter-wave phase plate at a wavelength of 633 nm. It has been shown that the induced birefringence (Δ n~3·10^-4) is due to the formation of an anisotropic type II modification, accompanied by the generation of oxygen-deficient and nonbridging oxygen hole defect centers. Photoluminescence and Raman scattering spectroscopy confirmed a uniform distribution of defects throughout the modified area and a local reorganization of the amorphous SiO₂ matrix. High transmission (60-80%) in the range of 600-1100 nm and strong UV absorption due to defects were established. It was demonstrated that the position of the area of maximum modification is shifted relative to the geometric focal plane due to the nonlinear self-focusing effect, which was confirmed by a quantitative assessment of the nonlinear focus position. The obtained results demonstrate the potential for creating functional polarization elements in bulk of dielectrics with controlled optical and structural properties. Keywords: Fused silica, 3D micro-optical elements, filamentation, ultrashort laser pulses, wave plate.