The Best Paper Award
Bending-Loss Insensitive Fiber with Hole-Assisted Structure
Kazuhide Nakajima, Tomoya Shimizu, Takashi Matsui,Chisato Fukai
Toshio Kurashima
（英文論文誌B　平成23年3月号掲載）
　Bending-loss insensitive fiber (BIF) can reduce the attenuation of a signal light that is induced at a fiber bend. This feature enables us to greatly improve optical wiring efficiency. The core of a hole-assisted fiber (HAF) is the same as that of a conventional single-mode fiber (SMF), but with several air holes arranged around it. These air holes increase the refractive index difference between the core and the cladding by more than 10%. This enables us to realize a BIF with a superior low bending loss characteristic since the high index difference supports the confinement of the signal light in the region surrounded with air holes. Thus, the hole-assisted structure is useful for realizing a BIF. However, the confinement effect obtained with air holes also affects the noise component (higher order mode). It is therefore necessary to design the cross-sectional parameters more precisely to realize a HAF with both a low bending loss and single-mode characteristics.
　In this paper, Authors investigate the optimum design conditions for a single-mode HAF as a BIF both numerically and experimentally. The transmission characteristics of the target HAF are considered to be same as those of conventional SMF, and its allowable bending radius is reduced from 30 to 5 mm. Authors consider three specific parameters, namely the normalized frequency of the core, the distance between the core and the air holes, and the air filling fraction in the fiber cross-section, and they successfully derive a simple relationship with the HAF transmission characteristics. As regards the mode field diameter (MFD), Authors clarify that the MFD of HAF can be the same as that of conventional SMF if we control the normalized frequency of the core and the distance between the core and the air holes. Authors also show that we can design HAF with the desired cutoff wavelength and bending loss characteristics by considering the air filling fraction and the refractive index of the core. The validity of these design conditions is confirmed with the fabricated HAF, and their applicability as an optical cord is also clarified experimentally.
　It should be pointed out that the paper not only clearly describes the design guidelines for a new class of optical fiber with an air hole structure but also clarifies the feasibility of single-mode HAF as a BIF. It should also be noted that the compatibility with conventional SMF plays an important role particularly when we extend the application area of single-mode HAF. We can expect this work to trigger further progress on HAF technologies.

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