(physics) The splitting of a ray of light into two parallel rays of perpendicular polarization by passage through an optically anisotropic medium; the property of a material that light passing through it is so split.
2012, Luís A. Fernandes, Birefringence and Bragg grating control in femtosecond laser written optical circuits, Departamento de Física e Astronomia da Universidade do Porto, page 112,
Heat accumulation in borosilicate glass [23] has [been] shown to produce lower birefringence due to radial stress regions as opposed to the more asymmetric stress produced in fused silica [136, 137]; however, these features remain unexplored in most glasses.