Calculations of the characteristics of a three-layer fiber waveguide with an anisotropic core and an elliptic inner cladding

Approximate analytic expressions are obtained for the calculation of the propagation constants of the fundamental modes and for the determination of the critical wavelength of the first higher mode in a three-layer fiber waveguide with an anisotropic core and an elliptic inner cladding. A comparison is made of the contribution made to the birefringence of such a waveguide by departures of the transverse cross section of the cladding from the circular form and by a transverse anisotropy of the insulating core material induced by thermoelastic stresses. It is shown that the difference between the propagation constants of two orthogonally polarized modes is influenced mainly by the core anisotropy. Therefore, in the case of large dimensions of the inner cladding the departures of its transverse cross section from the circular form can be ignored and the propagation constants as well as the critical wavelengths or frequencies of modes in such a waveguide can be calculated using expressions derived for a two-layer fiber waveguide with a transverse anisotropy of the core insulator.