Improving triangular-lattice photonic-crystal-fiber couplers by introducing geometric nonuniformities

Abstract

The coupling characteristics of newly proposed photonic-crystal-fiber couplers are rigorously analyzed using a 3-D finite-difference vector beam propagation method. The coupling length and efficiency of the couplers are investigated as a function of their geometrical parameters at different wavelengths. Size variability of the central hole is considered as a means of improving the coupling between the two cores. A determination of the guiding properties of the couplers, such as the propagation constants and the effective indices, is performed using a mode solver based on the plane-wave method. We show that it is possible to design very short photonic-crystal-fiber couplers with submillimeter coupling lengths. Nonuniformities are deliberately introduced in the coupler pattern to decrease the coupling length in comparison with the ideal structures. This study confirms that the designed coupler can act as an efficient ultrasmall-wavelength splitter or polarization-preserving directional coupler for wavelength division multiplexing systems.