Numerical verification of degeneracy in hexagonal photonic crystal fibers

Modal birefringences in photonic crystal fibers with six air holes symmetrically arranged around the core region and with multiple air holes in a hexagonal lattice are numerically investigated in detail. It is confirmed from the computed results obtained by a full-vector finite element method that both the hexagonal holey fibers are not birefringent, namely, the two fundamental modes are degenerate.