A birefringent material shows two different values of refractive index across its section (another term for birefringence is ‘double-refraction’). By definition, the birefringence in a material will be seen along two orthogonal axes or planes known as ‘birefringent axes’. Due to the different values of refractive index, the phase velocity of the light when travelling along the birefringent axes will be different. For this reason, they are commonly known as the slow (higher index) and fast (lower index) axes.
Applied to optical fibers, birefringence is the fundamental principle behind the polarization maintaining fibers.