Birefringence measurement in double-clad fiber lasers with large cross section.

We report on birefringence measurements in double-clad fibers with large cross section, doped with neodymium and ytterbium. The experimental results for rectangular double-clad fibers are compared with those for single-clad circular neodymium-doped fibers, taking into account existing models of stress- and geometry-induced birefringence. We demonstrated that the ellipticity of an outer silica cladding has no effect on birefringence in large-area double-clad fibers. The stress-induced birefringence is shown to depend on the ratio between the diameter of an internal silica support and the linear dimensions of the rectangular outer cladding. The stress-distribution pattern is derived to prove the experimental results.