Multicorelike guidance in a triangular-core hollow optical fiber and spectral evolution of its eigenmode degeneracy

A unique multi core-like guidance was achieved in a micro-structured optical fiber composed of a circular air hole at the center surrounded by a high-index triangular core. Unique spectral evolution of the degeneracy was theoretically investigated using a vectorial finite element method to find a 3-fold degeneracy in both the fundamental and the first excited modes in the visible range, which evolved to 2-fold degeneracy as the wavelength increased to IR. Experimental measurements also confirmed evolution of the modal intensity from three spatially isolated patterns confined to individual corners into a super-mode combining the three corners.