and improved during about 30 years since practical optical communication systems were first introduced in Japan in 1981. The main issues related to optical fiber connector development changed from performance improve-ment to miniaturization, cost reduction and ease of field assembly when optical communication systems expanded from optical trunk networks to optical access networks. Various different key technologies for optical con-nectors have been developed to meet these requirements, and a large num-ber of optical connectors are currently being used for the flexible and e ffi-cient construction, maintenance and operation of optical access networks. This paper describes the structure, features, and basic technologies of the optical connectors employed in optical access networks in Japan and their standardization and future prospects.
key words: optical connector, optical access network, optical fiber
com-munication, optical connector standardization, connector field assembly
1. Introduction
Progress on communication technologies has led to the ac-tive development of many kinds of broadband service such as voice, data and video communication using access net-works. If we are to provide these services in a timely way, we must construct optical access networks. Optical connec-tors are the key to the flexible and efficient construction, maintenance and operation of these networks. Two cate-gories of optical connector, namely single-fiber connectors represented by the SC (single fiber coupling) type optical connector [1] and multi-fiber connectors represented by the MT (mechanically transferable) type optical connector [2], [3], have been researched, developed and improved for opti-cal networks. The single-fiber connectors, which are mainly used in central offices, buildings and residential premises in access networks, have been developed based on such re-quirements as lower cost, higher packaging density, ease of field assembly and excellent repeatability. In contrast, multi-fiber connectors, which are mainly used in outside plants, have also been developed to meet a number of requirements including lower cost, ease of field assembly and more sta-ble environmental and mechanical performance under harsh outdoor conditions. Various different key technologies have been developed and improved for optical connectors in both
Manuscript received November 5, 2009. Manuscript revised February 19, 2010.
†The authors are with NTT Access Network Service Systems
Laboratories, NTT Corporation, Tsukuba-shi, 305-0805 Japan.
††The author is with Chiba Institute of Technology,
Narashino-shi, 275-0016 Japan.
a) E-mail: [email protected] DOI: 10.1587/transele.E93.C.1172
employed in optical access networks.
This paper describes the structure, features, and basic technologies of the optical connectors used in optical access networks in Japan. Moreover, the paper describes the trend as regards the standardization and future prospects of the optical connector.
2. Access Network Configuration and Optical Fiber Connector
Figure 1 shows the configuration of an optical access net-work. As seen in the figure, the network consists of four areas, namely a central office area, a feeder area, a distribu-tion area and a user area from a central office to residential premises, and six main types of optical fiber connector have been practically employed in these areas. In the central of-fice area, the SC or MU (miniature universal) type optical connector is mainly used to connect optical line terminal (OLT) equipment and optical fiber cables. Moreover, the SC2- or MU optical connector is used in the optical fiber distribution frame (ODF), and an MT connector is used for the connection point between the feeder cable and the ODF. In the feeder and distribution areas, an MT connector is used at the optical cable connection points. In addition, the FAS (field assembly small) type optical connector is used in the drop point, and a FA (field assembly) type, SC-, or MPO (multi-fiber push-on) type optical connector is employed in the user area. The next section describes the structure and features of these connectors, which are used in optical ac-cess networks in Japan.
3. Optical Connector Structure and Features
3.1 SC Optical Connector
The SC optical connector was developed as an econom-ical single-mode opteconom-ical fiber connector with high pack-aging density and superior performance for optical access networks, and it was introduced into optical networks in 1988 [1]. The SC connector structure is shown in Fig. 2. To achieve low cost, high packaging density and superior performance, SC connectors employ zirconia ferrules with small outside diameter deviation, small minute hole ec-centricity and roundness, and a plastic-molded rectangular Copyright c 2010 The Institute of Electronics, Information and Communication Engineers
Fig. 1 Optical access network configuration.
Fig. 2 SC connector structure.
connector housing that uses a push-pull coupling mecha-nism based on four technologies developed for a FC (fiber coupling) optical connector [5]. The aim is to position the two connected optical fibers with an accuracy of 1μm or better, obtain a low loss change for repeated connec-tion/disconnection, a high return loss and maintain stable environmental and mechanical loss characteristics. The four technologies are (1) a precise ferrule with a small eccen-tric minute hole for positioning the optical fiber, (2) a split sleeve arranged for two ferrules, (3) a structure that elimi-nates the need to apply mechanical force at the ferrule con-nection point by floating the ferrule and the sleeve in the plug and the adaptor housing (double engagement structure) and (4) physical contact (PC) connection [6] where direct fiber endface contact is realized by pressing the ferrule ends with a convex spherical shape as shown in Fig. 3. The con-nector with single-mode optical fiber has exhibited average insertion and return losses of less than 0.1 dB and more than 38.6 dB, respectively.
Moreover, a simplified SC receptacle (shown in Fig. 4) was also developed to achieve a further reduction in con-nector cost and a simplified concon-nector structure [7]. For that purpose, the number of parts has been reduced because no mechanical forces are applied to the connector on the
in-Fig. 3 PC contact principle.
side of the ODF. A simplified multi-port receptacle using the above technologies for a SC2 connector, which requires no coupling device as seen in Fig. 4(c), has been used for a 1000- or 2000-fiber ODF.
In addition, the new type of SC connector was recently developed to make it possible to connect or disconnect the fiber without needing expert knowledge. A shutter was pro-vided for this purpose. The structure is shown in Fig. 5. When the connector is not in use, the shutter is closed to prevent the fiber endface from being contaminated and to prevent users from looking into the laser beam. In contrast, when the connector is used, the shutter moves and dust is cleaned automatically from the optical fiber endface of the connector without the need for any special skill. The con-nector is used with a free-bending optical fiber cord [8], which can be bent, folded, and tied as needed in the same way as the metallic cord used for wiring in the home or a local area network (LAN).
3.2 MU Optical Connector
A large number of optical fiber connectors are required that take up little space in an ODF. Therefore, an MU connector [9], [10] with a higher packing density has been developed. The MU connector structure is shown in Fig. 6. A novel self-retentive mechanism that eliminates the pressing force for backplane connector use, a 1.25-mm diameter ferrule, a
Fig. 4 Parts of the simplified SC receptacle compared with those of a conventional SC plug and adaptor.
miniature push-pull type plug and advanced PC technology are used to realize high performance and high packing den-sity with a low insertion loss and a high return loss. The MU connector is 4.4 mm wide and 5.6 mm high, and its cross-sectional area is more than 60% smaller than that of an SC connector. The MU connector is already being used in an ODF, which can terminate and connect 4000-fiber from the feeder cables outside a central office [11] as shown in Fig. 7. 3.3 MT and MPO Optical Connector
Optical fiber ribbons in which several fibers are arranged linearly and accommodated in a common coating are used in feeder and distribution optical fiber cable. Therefore, an MT connector [2], [3] has been developed that can connect a number of fibers in the fiber ribbon simultaneously. The MT connector structure is shown in Fig. 8. The connector consists of a pair of plastic ferrules, fiber ribbons (4- and 8-fiber ribbon), two guide pins and a clamp spring. At the
ferrule end, 4 or 8 fibers are positioned accurately between two guide holes. The fibers to be connected can be easily aligned by two guide holes and two guide pins with index matching material. To obtain high and stable performance, the ferrule was made using a precision plastic molding tech-nique, and an insertion loss measurement technique using a high precision master connector was also developed. This connector has the advantages of ease of handling, small size, low connection loss and stable mechanical characteristics. In addition, to improve MT ferrule productivity and achieve a low cost, the ferrule production method was changed from a transfer molding technique using thermosetting resin to an injection molding technique using thermoplastic resin, which makes the ferrule production process easier [12]. In addition, to reduce the MT connector assembly time at an outside plant and thus make it possible to construct opti-cal networks economiopti-cally and efficiently, a high-speed MT connector assembly method [13] was also developed that uses an adhesive with a short hardening time and high
re-liability and that requires no polishing process.
Moreover, an MPO connector has also been developed [14] for use in a termination cabinet, which is a demarca-tion point between plant for the telecommunicademarca-tion carriers and users in buildings. If we are to use this connector at demarcation points in buildings, we must be able to achieve connection/reconnection easily without the need for index-matching material and to have a low insertion loss and a high return loss. Therefore, the MPO connector employs a push-on pull-off mechanism where the plug and adaptor are engaged by fitting a pair of elastic hooks into corresponding grooves, and the configuration of the ferrule end has been improved as shown in Fig. 9. The ferrule endface is made
Fig. 5 SC connector with shutter.
oblique with an angleθ to the plane perpendicular to ferrule axis so that reflected light is not transmitted in the reverse direction. Furthermore, the fiber ends are designed to pro-trude slightly at the ferrule endface to allow direct fiber end-face contact between multiple fibers. This configuration is obtained using a simple oblique polishing method. The fiber end protrusion is automatically formed during polishing as a result of the difference in the hardness of the plastic ferrule and the fiber. The connector has exhibited average insertion and return losses for single-mode fiber ribbons of less than 0.2 dB and more than 55 dB, respectively, without the use of index-matching material.
3.4 FA and FAS Optical Connector
To realize easy connector assembly in the field and to deal easily with optical fiber drop cable or indoor cable in a cab-inet in a user area, an FA connector was recently developed that includes a ferrule with an optical fiber with a polished endface, and a mechanical splice. This connector holds the optical fiber drop cable or indoor cable sheath directly and
Fig. 6 MU connector structure.
outdoor optical cabinets with long-term reliability. The FA connector consists of a plug and a socket (FA connector plug and FA connector socket), which can be connected directly
Fig. 8 MT connector structure.
optical fiber connection and accommodation time by about 40%.
Fig. 9 MPO connector structure.
4. Optical Connector Standardization and Future Technologies
4.1 Optical Connector Standardization
Various types of optical connector have been researched and developed since the initial development of optical commu-nication networks in Japan. These technologies have been
Fig. 11 FAS connector.
adopted by many telecommunication carriers and equipment vendors worldwide. In addition, positive technology trans-fer and standardization activities with respect to optical con-nectors have progressed thus spreading optical connector use throughout world. As a result, the SC connector has a 50% share, and the zirconia and MT ferrules account for almost 100% of the market. MT connector technology was recently adopted for FTTH systems by Verizon in the United States. Moreover, 19 types of connector have been stan-dardized by the International Electrotechnical Commission (IEC), which is an international organization for the stan-dardization of electric and electronic parts, and of these, seven types of connector (FC, SC, MU, DS, MT, MPO, Mini-MPO) [19]–[25] were developed in Japan. The tech-nology transfer and standardization of the optical connector will continue to be actively pursued.
4.2 Future Technologies
The number of fiber-to-the-home (FTTH) subscribers in Japan had exceeded 15.8 million by the end of June 2009 [26], and will continue to increase steadily. The optical con-nector will be extremely important if we are to construct, maintain and operate optical access networks flexibly and efficiently. In particular, it will become even more important for the connector to be assembled more quickly and easily in the field without special tools or skills and to be produced at lower cost. Therefore, new types of field assembly connec-tors are now being researched and developed. For example, an optical connector that enables us to realize PC connec-tion without having to polish the fiber endface [27] and a method for the easy field assembly of an optical connector [28], [29] have been proposed and demonstrated. Figure 12
works in Japan, their standardization and future connec-tor technologies. In future, optical connecconnec-tors will be ex-tremely important for the more flexible and efficient con-struction, maintenance and operation of optical access net-works. Therefore, we will continue to develop a new optical connector that can be assembled more quickly and easily in the field without special tools and skills and at reduced cost.
Acknowledgements
We thank K. Sakuda, Y. Hibino, M. Tsubokawa, M. Arii and S. Suzuki for their support and encouragement.
References
[1] E. Sugita, R. Nagase, K. Kanayama, and T. Shintaku, “SC-type single-mode optical fiber connectors,” J. Lightwave Technol., vol.7, no.11, pp.1689–1696, 1989.
[2] T. Satake, S. Nagasawa, and R. Arioka, “A new type of demount-able plastic-molding single-mode multifiber connector,” J. Light-wave Technol., vol.LT-4, no.8, pp.1232–1336, 1986.
[3] S. Nagasawa, H. Furukawa, M. Makita, and H. Murata, “Mechani-cally transferable single mode optical fiber connector,” IEICE Natl. Conv. Rec., B-636, March 1989.
[4] “Characteristics of single-mode optical fiber cable,” ITU-T Recom-mendation G.652.
[5] N. Suzuki, Y. Iwahara, M. Saruwatari, and K. Nawata, “Ceramic capillary connector for 1.3μm single-mode fibres,” Electron. Lett., vol.15, no.25, pp.809–811, 1979.
[6] N. Suzuki, M. Saruwatari, and M. Okuyama, “Low insertion- and high return-loss optical connectors with spherically convex-polished end,” Electron. Lett., vol.22, no.2, pp.110–112, 1986.
[7] R. Nagase, K. Kato, S. Iwano, and S. Mitachi, “Simplified SC-type receptacles for optical subscriber systems,” IEICE Trans. Electron. (Japanese Edition), vol.J79-C-II, no.11, pp.641–648, Nov. 1996. [8] M. Kama, H. Tanase, Y. Oda, M. Awamori, T. Nakajima, and
H. Watanabe, “R&D toward cost reduction and quick response to increasing demands for optical services,” NTT Technical Review, vol.5, no.2, pp.25–31, 2007.
[9] S. Iwano, R. Nagase, K. Kanayama, E. Sugita, K. Yasuda, and Y. Ando, “Compact and selfretentive multi-ferrule optical backpanel connector,” J. Lightwave Technol., vol.10, no.10, pp.1356–1362, 1992.
[10] R. Nagase, E. Sugita, S. Iwano, K. Kanayama, and Y. Ando, “De-sign for MU-type single-mode miniature optical connector,” IEICE Trans. Electron., vol.E81-C, no.3, pp.408–415, March 1998. [11] N. Tomita, T. Hirahara, K. Hogari, and K. Niimi, “Optical fiber cable
related technologies focused on cost reduction,” NTT Review, vol.12 no.2, pp.90–99, 2000.
[12] M. Takaya, T. Katagiri, S. Nagasawa, and Y. Murakami, “Develop-ment of MT type connector using an injection molded ferrule and a
“Aerial optical cables and fiber splicing technologies for access net-works,” NTT R&D, vol.47, no.1, pp.91–96, 1998.
[17] “Boxes and box covers of plastic conduits,” JIS C 8435.
[18] T. Nakajima, K. Terakawa, M. Toyonaga, and M. Kama, “Develop-ment of optical connector to achieve large-scale optical network con-struction,” 55th International Wire and Cable Symposium (IWCS), pp.439–443, 2006.
[19] “Fibre optic connector interfaces — Part 4: Type SC connector fam-ily,” IEC 61754-4, 1997.
[20] “Fibre optic connector interfaces — Part 5: Type MT connector fam-ily,” IEC 61754-5, 2005.
[21] “Fibre optic connector interfaces — Part 6: Type MU connector fam-ily,” IEC 61754-6, 1997.
[22] “Fibre optic connector interfaces — Part 7: Type MPO connector family,” IEC 61754-7, 2004.
[23] “Fibre optic connector interfaces — Part 9: Type DS connector fam-ily,” IEC 61754-9, 1996.
[24] “Fibre optic connector interfaces — Part 10: Type Mini-MPO con-nector family,” IEC 61754-10, 2005.
[25] “Fibre optic connector interface — Part 13: Type FC-PC connector family,” IEC 61754-13, 2006.
[26] http://www.soumu.go.jp/menu news/s-news/18957.html
[27] Y. Abe, M. Kihara, M. Kobayashi, S. Matsui, R. Nagase, and S. Tomita, “Novel field installable sagged fiber connector realiz-ing physical contact connection without polishrealiz-ing fiber endface,” OFC/NFOEC, NThC2, 2008.
[28] M. Kihara, Y. Abe, R. Koyama, K, Saito, and S. Tomita, “Study on easily field-assembled optical fiber connector technology,” Proc. IEICE Gen. Conf. 2008, B-13-13, March 2008.
[29] R. Koyama, K. Nakajima, M. Takaya, T. Sekiguchi, and T. Kurashima, “A study of jointing of coated optical fiber,” IEICE Technical Report, OFT2009-62, 2010.
Kazuo Hogari received a B.E. degree in electrical engineering from Ibaraki Univer-sity, Ibaraki, Japan, in 1981, and a Ph.D. de-gree in electrical engineering from Waseda Uni-versity, Tokyo, Japan, in 1994. He joined NTT Electrical Communications Laboratories, Ibaraki, Japan, in 1981, where he has been en-gaged in the research and development of opti-cal fiber cables and optiopti-cal components. He is a Senior Research Engineer, Supervisor, of NTT Access Network Service Systems Laboratories. He is a member of IEEE. He received the most outstanding technical paper award and the best paper award at the 43rd IWCS in 1994, and the 12th OECC/IOOC in 2007, respectively.
Ryo Nagase received B.E., M.E. and Ph.D. degrees in precision engineering from Tohoku University, Miyagi, Japan, in 1983, 1985, and 1998, respectively. In 1985, he joined NTT Laboratories, Nippon Telegraph and Telephone Corp. From 1985 to 2009, He has been en-gaged in research and development of optical fiber connectors. From Oct., 2009, he is a pro-fessor of precision engineering at Chiba Insti-tute of Technology, Chiba, Japan. His current interests are microscopic deformation and opti-cal remote-sensing. He is an Expert of IEC TC86/SC86B and a member of the Japan Society of Mechanical Engineers (JSME) and IEEE.
Kazutoshi Takamizawa joined NTT (Ni-ppon Telegraph and Telephone Corp.) in 1987. He has been mainly engaged in the develop-ment of optical access network systems. He is presently a Senior Research Engineer, supervi-sor, of NTT Access Network Service Systems Laboratories.
