Coupled-mode analysis for double-helix chiral fiber long-period gratings based on elliptical core fiber

Coupled-mode equations (CMEs) for double-helix chiral fiber long-period gratings (CLPGs) based on geometry-induced birefringence fibers are formulated by using the traditional coupled-mode theory. The derivation is based on an expansion in term of local modes, which is better to be understood and easier to be dealt with. The form of the CMEs is the same as that for the other kind of double-helix CLPGs based on stress-induced birefringence fibers, the common properties of double-helix CLPGs are thus proved. Simulated results on a spun elliptical core fiber are also given as an example to demonstrate the properties of the CLFGs.