Enlarging effective mode area of photonic crystal fibers using defected core structures

In this paper a new strategy to design large mode area photonic crystal fibers with relatively small hole pitches is presented and employed to design fibers with effective mode areas larger than 70µm2 at λ=1.55µm with hole pitch Λ=2.5µm. The idea is based on insertion of circular defect holes in the core area of an HF7 fiber. It is shown that by precisely adjusting the locations and the diameters of these defect holes, field distribution alters in a way that leads to a larger effective mode area than HF7 fiber itself. The validation of the proposed design is carried out by adopting an efficient full-vectorial finite difference frequency domain method with perfectly matched layers for accurate characterization of photonic crystal fibers. Dispersion characteristics of the designed fibers is also investigated and it is shown that the defect insertion has strong dispersion flattening and lowering attitude that can significantly ease the fiber design task for optical communication transmission systems.