A Panda fiber is a type of polarization-maintaining fibers. It is a single-mode fiber where the core is sandwiched between two stress rods. These stress-inducing tubes are made of different material, so that when it expands due to changes in temperature, it induces stresses in the core of the fiber and gives rise to a uniform birefringence. This ensures that the polarization is conserved as the light propagates down the fiber.