It is an optical fiber where birefringence is seen along two orthogonal axes of its section, known as slow and fast axes corresponding to higher and lower refractive indexes respectively. When a light wave travels through a birefringent optical fiber, the birefringence of the fiber will lock the two orthogonal modes of the light into the birefringent axes preserving therefore the polarization state of the light. There are two different families of birefringent fibers depending on the way the birefringence is achieved: shape/form birefringent fibers and stress birefringent fibers.
In the shape/form birefringent fibers family, the birefringence is created as a result of an asymmetric core shape for the two axes. This core shape asymmetry creates both geometrical anisotropy and asymmetrical stress across the core leading onto two different constants of propagation for the birefringent axes. Elliptical core fibers would be a typical example of shape/form birefringent fibers.
In the stress birefringent fibers, the birefringence is created as a consequence of the addition of two stress applying parts (SAPs) at opposite sides of the core in the cladding of the fiber. The stress applying parts are made out of materials with a different thermal expansion coefficient to the fused Silica (which is the typical cladding material in optical fibers). When the fiber cools down at manufacturing the different thermal expansion coefficient of the SAPs creates stress along one axis of the core allowing for a highly birefringent fiber (HiBi fiber). Typical examples of stress birefringent fibers are Elliptical Cladding fibers, Bow-Tie fibers and Panda fibers which show very distinctive SAP shapes.