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A graded-index (GI)-type photonic crystal fiber (PCF) that has an α-power effective refractive index profile operates as a multimode optical fiber and can be applied to a collimator for single-mode PCFs. The α-power effective index distribution is realized by an α-power air hole diameter distribution with uniform air hole pitch. However, a pressure controlling in fiber drawing is difficult due to the difference of air hole diameters in fiber cross section. In this paper, we propose a GI-PCF with uniform air hole diameter while the air hole pitch varies in radial direction. Light propagation characteristics in the Gaussian beam excited GI-PCF with uniform air hole diameter were theoretically studied. The period and amplitude of mode field diameter variation along the propagation direction for the innermost air hole pitch, air hole diameter, and radial distribution profile of the pitch were clarified. The collimator application of GI-PCF with uniform air hole diameter for a conventional single-mode PCF was investigated. The suitable GI-PCF structure to obtain high optical coupling efficiency and good tolerance for GI-PCF length error was clarified. Wavelength characteristics of the GI-PCF collimator were also studied. It was clarified that the appropriate GI-PCF length was longer for shorter operating wavelength in collimator application. [ABSTRACT FROM AUTHOR]

A graded-index type photonic crystal fiber (GI-PCF) has a graded effective index distribution that can be realized by an α-power air hole diameter distribution in radial direction. Since a beam diameter varies periodically along the propagation direction in the GI-PCF with Gaussian beam excitation, an appropriate length of GI-PCF should be spliced to a single-mode PCF for collimator application. It is desired that GI-PCF structures to obtain high optical coupling efficiency and good tolerance for fiber length error are designed. In this paper, we theoretically studied coupling characteristics of GI-PCF collimator for conventional single-mode PCF depending on air hole diameter ratio to pitch, where the GI-PCF had parabolic air hole diameter distribution in radial direction, and had the air hole pitch that was equal to that in the conventional single-mode PCF. It was found that the optical coupling efficiency could be higher for larger air hole diameter ratio to pitch, where the coupling efficiency almost equal to that in conventional GI-type multimode fiber (GI-MMF) collimator could be obtained. On the other hand, it was clarified that GI-PCF collimators were more tolerant for fiber length error compared with GI-MMF collimator and that the GI-PCF with smaller air hole diameter ratio to pitch exhibited a better tolerance in fiber length error. We clarified that the GI-PCF collimator had an ability to obtain better wavelength characteristics with fiber length error compared with the GI-MMF collimator. [ABSTRACT FROM AUTHOR]

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 ≤ α ≤ 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. [ABSTRACT FROM AUTHOR]