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60 Feature Articles: Optical Fiber Communications cost directly account for a cost per subscriber, and should be able to be handled easily for quick and economical installation. A surplus optical fiber took out from the cables is wound and stored in a closure or a cabinet. In particular, the fiber in the drop cable is required to be able to be bent small to downsize the closure and the cabinet. Furthermore, a fiber in an indoor cable may be bent with a small curvature for wiring along a wall and for storage. Fig�� Fig�� Fig�� Fig�� Fig�� Configuration Configuration of of cables cables in in in FTTH� FTTH� FTTH� FTTH� FTTH� 3. OPTICAL FIBER USED IN FTTH The structure of a silica optical fiber, which is the most popular optical fiber in telecommunications, is shown in Fig.2. The fiber consists of a cladding which is made from silica and a core whose refractive index is raised slightly higher than that of the Fig�� Fig�� Fig�� Fig�� Fig�� Structure Structure of of silica silica optical optical fiber� fiber� fiber� fiber� fiber� (a) (a) dimensions� dimensions� dimensions� dimensions� dimensions� (b) (b) refractive refractive index index profile� profile� profile� profile� profile� cladding by doping germanium. Light can be confined within the core by the coreto -cladding index difference. The structure is characterized by the core diameter 2a and the relative refractive index difference Light confined in the fiber propagates at a very small attenuation, e.g. 0. 2-0.3 dB/km. The cladding diameter is 125 µm, and the cladding is coated with plastic resin to a diameter of 250 µm. The optical fiber has two main categories; a multimode fiber and a single-mode fiber. The core diameter of the multimode fiber is 50 µm and the fiber is easy to connect with each other and with an inexpensive light source, so that the fiber is applied to short distance communications. The single-mode fiber whose core diameter is about 10 µm is applied to high-speed and/or long-distance communications since more sophisticated techniques are required to connect with each other and with optical devices. The fiber with an index profile as shown in Fig.2 can be operated as a single-mode fiber when the normalized frequency V defined as the following equation is less than 2.405 at a wavelength of . (1) (2) Possible combinations of parameters which result in single-mode operation are infinitely. However, the dimensions and properties of "the single-mode fiber" for telecommunications are defined in ITU-T Recommendation G.652. In Japan, the single-mode fiber complying G. 652 are used even in FTTH whereas FTTH covers short distance and requires economical materials. This is because arc-fusion splice and connector for the fiber have been highly developed and the cost of optical devices for the fiber has become reasonable. 3.1 Solution for small bending radius An optical fiber for a drop cable or an indoor China Communications December 2005
cable should be able to be bent in a smaller curvature as described in Section 2. However, the fiber bent in a small radius causes transmission loss, which is called the bending loss. A single-mode fiber with a smaller mode field diameter (MFD) has a lower bending loss. MFD may be reduced by enhancing the refractive index of the core and reduce the core diameter so as to keep the single-mode conditions shown in Eq. (1). We have developed FutureGuide ® -SR15, which can achieve a negligibly low bending loss at a bending radius of 15 mm by slightly enhancing the refractive index of the core. This allowable bending Feature Articles: Optical Fiber Communications radius is a half of that of the conventional singlemode fiber (C-SMF). The wavelength dependences of the bending losses for FutureGuide ® -SR15 and C- SMF are shown in Fig.3 [3] . The bending loss for FutureGuide ® -SR15 is about one-tenth of that for C-SMF. The bending loss at longer wavelength increases as shown in Fig.4 since the MFD at a longer wavelength becomes larger. The characteristics of FutureGuide ® -SR15 and ITU-T recommendation G.652.B are shown in Table 1. The allowable bending radius of FutureGuide ® - SR15 is a half of that of ITU-T recommendation G. 652.B. The typical MFD of FutureGuide ® - SR15 is smaller than that of ITU-T recommendation G.652.B. However, FutureGuide ® -SR15 is applicable to all regions in FTTH and even in trunk lines since all characteristics of FutureGuide ® -SR15 complies with G.652.B. 3.2 Solution for wideband transmission Wavelengths used for the optical communications are defined as shown in Fig.4. Even Fig�� Fig�� Fig�� Fig�� Fig�� Wavelength Wavelength dependence dependence dependence of of binding binding loss loss on on in FTTH, Wavelength Division Multiplex- FutureGuide FutureGuide FutureGuide ing (WDM) is used to construct passive optical networks (PON) to save the number of optical fibers per subscriber. In WDM for FTTH, Ctransmission band is used for downstreamtransmission ® �SR�� SR�� SR�� SR�� SR�� and and C�SMF C�SMF C�SMF C�SMF C�SMF at at ø�� ø�� ø�� ø�� ø�� mm mm and and �� turn� turn� turn� turn� turn� Table.1 Characteristics of FutureGuide ® -SR15 and ITU-T recommendation G.652.B. China Communications December 2005 61
Page 1: ABSTRACT The configuration of optic
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