The femtosecond laser-modified region in isotropic glass medium shows a big optical birefringence. Transmission of the birefringent regions between two crossed polarizers depends on phase retardation and the orientation angle of the birefringent optical axes. Based on this effect, three-dimensional (3D) multilevel memory was proposed and demonstrated for nonvolatile memory up to eight levels, in contrast to the standard two-level technology. Eight-level writing and reading are distinguishable in fused silica with a near-infrared femtosecond laser. The retention of this memory is characterized for nonvolatile applications.