Lifting polarisation degeneracy of modes by fiber design: a platform for polarisation insensitive microbend fiber gratings

Polarisation-dependence in microbend-gratings is an inherent problem, even in perfectly circular fibers, since antisymmetric modes are almost degenerate linear combinations of four distinct, polarisation-sensitive modes. We demonstrate a novel fiber design that lifts polarisation degeneracies of the antisymmetric modes to solve this problem. By intentionally exacerbating the polarisation splittings, we achieve coupling to only the polarisation insensitive doublet, over wavelength ranges exceeding 100nm, thus demonstrating a device with practical useable bandwidths. This enablaes realising all previous applications envisaged with UV-induced long period gratings with the significantly lower cost microbent technology platform.