Mode birefringence of a three-layer elliptic single-mode fiber wavequide

A vector solution is obtained for the fields of the lowest-order modes in a three-layer elliptic fiber waveguide which is valid in the case of a small ellipticity of the boundaries between the layers, but is not restricted in respect of the differences between the refractive indices of the core and claddings. This solution is used to study mode birefringence as a function of the fiber waveguide parameters. It is shown that for a given core and an outer cladding, a waveguide with the W profile ensuring a high concentration of the energy of the lowest-order modes exhibits a stronger mode birefringence than a waveguide with a two-step profile. Near the limit of single-mode behavior an increase in the thickness of the inner cladding enhances the mode birefringence of a W-type waveguide, but reduces the corresponding birefringence of a two-step waveguide. The sensitivity of a three-layer waveguide to elliptic distortions of the boundaries increases on increase in the difference between the refractive indices of the three media forming the waveguide.