Birefringence in microstructure fiber with elliptical GeO2 highly doped inclusion in the core

We studied both numerically and experimentally a birefringence in a microstructure fiber with elliptical inclusion highly doped with GeO(2). We demonstrate that such inclusion increases the phase modal birefringence and modifies its dispersion in the short wavelength range, thus causing that group birefringence crosses zero value at a certain wavelength. Moreover, we numerically analyzed different factors contributing to the overall fiber birefringence, including geometrical birefringence induced by holes distribution and ellipticity of the inclusion as well as stress birefringence associated with thermal shrinkage of the doped glass.