Wavelength dependence of birefringence in single-mode fiber

Real single-mode fibers generally exhibit elliptical birefringence caused by deviations of the core shape from circularity, by transverse internal stress, and by residual twist. These three contributions can be individually determined by analyzing on a Poincaré sphere the wavelength dependence of the output state of polarization of a short section of fiber with fixed input polarization. Various types of polarization eigenstates of a twisted birefringent fiber are discussed, and the resulting group delay difference is derived.