Design of graded index type photonic crystal fiber having α-power air hole diameter distribution with α < 1 for fiber collimator application

Abstract

A graded-index type photonic crystal fiber (GI-PCF) that has α-power air hole diameter distribution in radial direction has a characteristic that the beam diameter varies periodically along the propagation direction in the Gaussian beam excited GI-PCF, and it is similar to a GI type multimode fiber (GI-MMF). Since GI-MMFs are used as fiber collimators, it is considered that GI-PCFs are also possible to apply for fiber collimators. However, it is difficult to draw fiber with air hole diameter control in considering higher coupling efficiency at $$1 \le \alpha \le 2$$ . In this paper, we theoretically analyzed light propagation characteristics in Gaussian beam excited GI-PCF having α-power air hole diameter distribution with α < 1 and investigated the coupling efficiency with conventional PCF and its wavelength characteristics. It was found that the coupling efficiency became larger as the air hole diameter d was smaller at α < 1. In particular, a better tolerance for fiber length error and wavelength characteristics were obtained at α = 1/2. From these results and the difficulty of GI-PCF drawing, we designed the suitable structure of GI-PCF for fiber collimator application.