Principle of PM fibers based on stress-induced linear birefringence: (a) SAPs (panda structure, here) are under quasi-isotropic tensile stress, and they pull on the fiber cladding; (b) in the core region, this yields tensile stress (red arrows) in the x-axis, and a compressive stress (blue arrows) in the perpendicular y-axis; (c) these stresses decrease when one moves away from the core, along the perpendicular y-axis.