Higher-order mode couplings in double-helix chiral long-period fiber gratings

All possible resonant couplings of the core modes with the cladding modes are analyzed comprehensively in double-helix chiral long-period fiber gratings (CLPGs). Couplings with higher order cladding modes that were neglected in previous works are emphasized in this paper, and thus interesting properties are revealed. Two kinds of double-helix CLPGs whose cross-section asymmetry originates from small geometric deformation and stress-induced birefringence, respectively, are analyzed in detail. The similarities and differences of mode selection and polarization selectivity of mode couplings are identified for the two structures.