Birefringence of an irregular segment of a single-mode fiber waveguide with smooth joints

Theoretical and experimental investigations were made of the birefringence in an irregular section of a single-mode fiber waveguide with a partly removed cladding and with smooth joints to a homogeneous fiber. Lifting of the degeneracy in respect of the polarization of orthogonally polarized modes in such irregular sections was described by perturbation theory applicable to the degenerate case. A method was developed for measuring the birefringence in anisotropic and elliptic single-mode fiber waveguides using the prism method for coupling radiation out of a waveguide in the case when the induced birefringence was $\delta\beta\lesssim$10$^{-6}\mu^{-1}$. An analysis was made of the possibility of construction of various fiber-planar devices on the basis of an irregular single-mode fiber waveguide of the type considered here with $\delta\beta\gtrsim$10$^{-3}\mu^{-1}$.