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62 Feature Articles: Optical Fiber Communications from a network center to a subscriber, and O-trans- sintering the glass particles. mission band is used for upstream transmission from It has been reported that the fiber without OH the subscriber to the network center. In addition, the absorption peak can be made by optimization of new system that uses C-transmission band for broad- dehydration conditions and fiber structure in the casting and O-transmission band and S-transmission 1980s band for optical communications has started to be deployed. WDM-PON using more wavelengths is in research towards a future FTTH system. In C-SMF, E-transmission band is not used for [5-7] . Recently, this type of fiber, "low water peak fiber," has been commercialized and standardized. We also have developed FutureGuide ® - SR15E, which have both low bending loss and low water peak. The spectral attenuations of FutureGuide ® -SR15E and C-SMF are shown in Fig.5. FutureGuide ® -SR15E has a low OH absorption and can be used for optical communications at Etransmission band [3,8] . Exposing an optical fiber to hydrogen may result in the increase of an attenuation at 1383 nm [9,10] . Therefore, the upper attenuation value of a low- Fig�� Fig�� Fig�� Fig�� Fig�� Wavelengths Wavelengths used used for for for the the optical optical communication� communication� communication� communication� communication� water-peak fiber after hydrogen aging optical communications due to possible larger attenuation at 1383 nm. The attenuation at 1383 nm results from the second overtone of the fundamental Si-OH vibration at 2700 nm. OH groups in a preform must be eliminated for decreasing the attenuation at 1383 nm. An optical fiber preform is made of SiO2 from SiCl through the thermal oxidation 4 and/or the hydrolysis reaction. As for the perform synthesis methods using the thermal oxidation reaction, such as the MCVD and the PCVD methods, the reduction of OH-groups contained in raw material, a substrate and a jacketing tubes, and modification of the apparatus for tighter gas leak have been attempted to reduce the absorption at 1383 nm [4] test (IEC60793-2-50) is defined by ITU-T G.652.C and G.652.D. The attenuations of FutureGuide . As for the perform synthesis methods using the hydrolysis reaction, such as the VAD and the OVD methods, OH-groups in glass particles generate inherently. Therefore, when a preform is made by the hydrolysis, OH-groups in the preform is eliminated by the dehydration process in which a soot perform is heated in the presence of Cl or SOCl before 2 2 ® - SR15E and ITU-T G.652.D are shown in Table 2. The characteristics of FutureGuide ® -SR15E other than those shown in Table 2 are the same as the characteristics of FutureGuide ® -SR15 shown in Table 1. FutureGuide ® -SR15E has an allowable bending radius of 15 mm and is compliant with ITU-T Recommendation G.652. As a result, FutureGuide ® Fig�� Fig�� Fig�� Fig�� Fig�� Spectral Spectral attenuations attenuations of of FutureGuide FutureGuide - SR15E can be used for broadband WDM transmission line in access and core networks. ® �SR��E �SR��E �SR��E �SR��E �SR��E and and and C�SMF� C�SMF� C�SMF� C�SMF� C�SMF� China Communications December 2005
Increasing the relative-index difference between the core and the cladding results in decreasing a bending loss but decreasing a MFD. If two fibers with Gaussian electric fields and with MFDs of 2W and 1 2W , the connection loss in dB, Ls, 2 between the fibers can be calculated by Eq. (3). (3) Obviously, the connection loss is minimized when W =W . When an optical fiber with an enhanced 1 2 Feature Articles: Optical Fiber Communications IV. NEW OPTICAL FIBER IN DEVELOPMENT FOR FURTHER BENDING LOSS IMPROVEMENT We so far have described fibers with a simple refractive-index profile called as the step-index profile as shown in Fig.2 (b). In addition to the requirements for an indoor cable as described in section 2, the cable is desired to be able to be handled without worrying about bending just like a metal cable [11] core refractive index for reduction of bending loss is connected with C- SMF, connection loss increases according as the MFD difference between those two fibers. A fiber with improved bending loss for an indoor cable should be connected with C-SMF applied in a drop cable and an optical network unit (ONU) in a subscriber. Therefore, we should consider not only bending loss but also MFD of the fiber so as to maintain its MFD as large as that of C-SMF, 9.2 µm. We have proposed an improved bending loss fiber with a refractive index profile as shown in Fig.6 . In this section, we introduce the fiber which has a new refractive-index profile so as to achieve advanced bending loss properties. [12] Table 2. Attenuations of FutureGuide . The feature of the profile is that "trench" with a lower refractive-index than that of the fused silica locates between the inner cladding around the core and the outer cladding. The index trench at the edge of the mode field works to confine the mode field stronger ® -SR15E. Fig�� Fig�� Fig�� Fig�� Fig�� Schematic Schematic of of trench trench trench index index profile� profile� profile� profile� profile� than the step index profile even if MFD of the fiber is the same as that for a fiber with the step index profile. As a result, a fiber with optimized parameters of 1 ~ 3 and r2 ~r shown in Fig.6 can achieve improved 3 bending loss and connection loss properties com- China Communications December 2005 63
Page 1 and 2: ABSTRACT The configuration of optic
Page 3: cable should be able to be bent in
Page 7 and 8: decreasing the number of manufactur
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