Holey Fibers for Low Bending Loss

Recently, broadband-access network has spread out rapidly, and Fiber-To-The-Home (FTTH) has showed a stable growth. Nevertheless, for the prevalence of additional FTTH, the so-called 'last one mile problem', which entails a huge cost for constructing optical fiber networks to end-users, has to be solved. To this end, it is necessary to improve economical efficiency of the products for network access and simplify the installation of the network. For the reasons mentioned above, special performance of optical fibers for dropand indoor-cables in FTTH, such as a low bending loss for small bending diameter and easy storage of surplus fibers, is required 1). Increasing the effective refractive index of a propagating mode or squeezing the electromagnetic field of the mode is effective in reducing bending loss in fibers. For example, FutureGuide® -SR15 which has a core with high refractive index has a permissible bending radius of 15 mm, half of that of a conventional single-mode fiber (C-SMF). Furthermore, an optical fiber using a trench index profile is bendinginsensitive to a bending radius of 7.5 mm and shows low splicing loss to C-SMF 2). However, if the bending insensitivity of a fiber with a conventional index profile is required for an extremely small bending radius, e.g. 5 mm, the mode field diameter (MFD) of the fiber is inevitably reduced and high splicing loss to CSMF is induced. We have applied holey fibers 3) to solve this problem. A hole-assisted holey fiber (HA-HF) has a conventional raised-index core surrounded by air holes and hence possesses the properties of both conventional fibers and holey fibers such as low transmission loss and large anomalous dispersion 4). Since the air holes make a large refractive index difference between core and cladding, an HA-HF can also have very low bending loss without reducing its MFD so much. For the structure of HA-HFs, one layer 5) and two layers 6) of air holes have been already proposed. HAHFs with one layer of air holes have a simple structure, but the problem with them is that suppressing bending loss also strengthens the confinement of higher-order modes and results in long cut-off wavelengths of the higher-order modes. HA-HFs with two layers can solve the problem but they have complicated structures. In this paper, we will propose a novel type of HAHF, which has two layers of air holes with only three holes for each layer. The size of the holes in different layers is allowed to be different so that low bending loss and short cut-off wavelength of a higher-order mode can be obtained simultaneously. A step-index core similar to C-SMFs is used so that the HA-HFs can be spliced to C-SMFs with low loss. One of the fibers we have made in trial has single-mode operation from 1.28 μm and has a bending loss of 0.012 dB/turn for a bending diameter of 5 mm. Furthermore, average splicing losses of 0.05 dB between the HA-HF and C-SMF and 0.08 dB between the HA-HFs by fusion splicing have been obtained.