Origin of apparent resonance mode splitting in bent long-period fiber gratings

The wavelength-selective attenuation notches generated by UV-induced long-period fiber gratings (LPFGs) have been shown to shift in wavelength and change in depth if the LPFG is bent along a curved path. Additionally, the notches frequently appear to "split" into two when the LPFG is curved. These effects have successfully allowed the realization of bend-sensing devices. Publications to date have not yielded a satisfactory physical interpretation for the apparent mode splitting. In this paper, it is shown that the spectral changes witnessed in curved LPFGs are the result of changes in the cladding-mode profiles in a bent fiber. The modes of a curved three-layer optical fiber numerically were determined and these fields are used to show that the uniform UV-induced perturbation allows coupling between the fundamental core mode and both symmetric and antisymmetric cladding modes when the fiber is curved, whereas coupling to only symmetric modes is allowed in the straight fiber. Coupling to these new modes is at the origin of new attenuation notches, which have been misinterpreted so far as mode splitting. Published devices' attenuation-notch wavelength shifts, apparent splitting, and relative depths are all well described using these changes in cladding-mode profiles.