Tu3D.2.pdf
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How to Connect Multicore and Multimode Fibers Ryo Nagase
Chiba Institute of Technology, 2-17-1 Tsudanuma, Narashino, Chiba 275-0016, Japan
[email protected]
Abstract: Multicore and multimode fibers are proposed for use in space-division multiplexing for ultra-wide-band optical transmission systems. This paper introduces recent progress on multicore and multimode fiber connection technologies. OCIS codes: (060.2340) Fiber optics components; (350.3950) Micro-optics
1. Introduction Optical communication traffic continues to increase, however, the transmission capacity of conventional singlemode fiber has now reached around 100 Tbit/s, which is assumed to be the maximum value [1]. Multicore fiber (MCF) and multimode fiber (few-mode fiber: FMF) are promising candidates for achieving ultra-wide-band optical transmission in the near future [2,3]. Optical connectors are essential for constructing optical networks. They are unique in that they require resistance from an external force. They should usually allow some component deformation of around tens of microns and so core alignment accuracy of better than 1 μm is required. Optical connectors used in telecommunication systems have a ‘floating mechanism’ to eliminate the influence of deformation on connection stability. The most difficult issue as regards connecting MCFs is finding a way to align each core under a floating mechanism. For FMF connection, the greatest difficulty relates to accuracy because a core offset causes nonuniform attenuation between each mode. It will require better accuracy than conventional single-mode connectors. This paper describes recent progress on MCF and FMF connection technologies. 2. How to connect multicore fibers (1) Fundamentals of optical connectors Single-mode optical connectors require an attenuation of less than 0.5 dB and a return loss of 40 dB or more. To realize these requirements with the butt joint mechanism, the fiber core offset should be less than 1 μm. There are four conditions for achieving this requirement [4]. (a) Fixing the fiber precisely at the center of the ferrule (b) Precise ferrule alignment with good repeatability (c) Suppression of Fresnel reflection at the connection point (d) Suppression of the influence of housing deformation caused by external force A low-cost technology for manufacturing zirconia ferrules has been established for single-mode simplex connectors. This is a way of achieving condition (a). We can use a zirconia ferrule for the MCF connector if there is precise alignment of the rotation angle around the ferrule axis. A precise ferrule alignment technique with a split sleeve has also been established to satisfy condition (b). We can use the same technique for the MCF connector. For condition (c), physical contact (PC) connection technology has been developed that achieves a return loss of 50 dB or more. We can use PC technology for the MCF connector, however, the specifications of the ferrule endface geometry must be reconsidered because some cores are not located in the center of the fiber. Optical connectors are usually operated by hand, so we sometimes have to consider an external force of tens of newtons interacting with the fiber cables. If such a force interacts with a small sized optical connector, the plug housing will be deformed by more than 10 μm, which is far greater than the alignment tolerance. We can use two techniques to solve this problem (condition (d)), one is butt-joint type coupling with a ferrule ‘floating mechanism’ and the other approach uses a ‘lens coupling’ mechanism. (2) Butt-joint type MCF connector Butt-joint type single-mode optical connectors are widely used, e.g. SC, LC, and MU connectors. It is recommended that these existing mechanical interfaces be used to realize a new MCF connector because the first priority with optical connectors is intermateability. Considerable investment and a long period of development are needed to develop a new mechanical interface that achieves intermateability between the devices supplied by different vendors. The MU-type MCF connector has already been developed based on the following design principles.
Tu3D.2.pdf
OFC 2014 © OSA 2014
φ 1.25 mm
Small size The new MCF connector should have the same or greater packaging density than conventional optical connectors. On the other hand, there is some possibility of using an MCF connector for high-speed optical transceivers. In this regard, small form factor (SFF) connectors are recommended. Symmetrical structure The plug housing will be deformed not only via interaction with an external force but also by interaction with the internal force that is produced by the spring used for PC connection. To realize stable connection for all the cores in an MCF, uniform pressure is recommended for every outer core. There are two types of SFF connectors, one type has only one cantilever for a latch (e.g. the LC connector) and the other type has two cantilevers that are arranged symmetrically (e.g. the MU connector). To realize stable MCF connection, the latter type is recommended because it will deform symmetrically when mated. Floating mechanism An MU connector has a 0.1 mm gap in each direction between the ferrule flange and plug housing. This gap allows a ferrule rotation of ±10°, which does not satisfy the allowable tolerance of ±0.5°. To realize an angle tolerance of ±0.5°, Oldham’s coupling mechanism was used. Figure 1 (a) shows the design principle of the MU-type MCF connector, which incorporates Oldham’s coupling mechanism [4]. Figure 1 (b) shows the attenuation distribution of randomly connected MU-type 7-core MCF connectors [5].
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Figure 1. MU-type multicore fiber connector, (a) design principle and (b) attenuation (3) Lens coupling type MCF connector The other technique for achieving the above-mentioned condition (d) is the lens coupling technique. Expanded beams allow larger misalignment via interaction with an external force at a connection point than butt-joint type connectors. If each core has a pair located in a symmetrical position, e.g. typical 7-core MCF, it can be coupled using only one lens pair [6]. (4) Fan-in and fan-out device At an optical line terminal, an MCF has to connect with single-core fibers. There have been some attempts to achieve optical fan-in and fan-out (FI/FO) devices, e.g. fiber bundle type [7,8], fused taper type [9], waveguide type [10] and lens coupling type [11]. Seven- and 12-port fiber bundle type FI/FOs were realized and used for transmission experiments. A 7-port lens coupling type FI/FO was also realized and it was mentioned that the same structure can use for 19-core MCF [11]. All types of FI/FO devices achieved a typical insertion loss of below 1 dB and crosstalk of less than -50 dB. Figure 2 shows a typical structure of a fiber bundle type optical fan-out [7]. 3. How to connect multimode fibers Mode division multiplexing transmission experiments with FMF have advanced rapidly. When using FMF, we need to consider several additional fiber parameters for inter-mode effects, e.g. differential mode attenuation (DMA),
Tu3D.2.pdf
OFC 2014 © OSA 2014
differential group delay (DGD), linear cross-mode coupling and inter-mode nonlinearity [12]. Any core offset at the connection point of the FMF would influence the DMA or linear cross-mode coupling. Greater accuracy is needed to achieve an FMF connector than a single-mode fiber connector. Finding a way to design an FMF connector with low DMA and multiple connection points in a practical network is the subject for a future study.
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d: Core center-to-center distance d’: Outer diameter of fine fiber
Figure 2. Example of fiber bundle type optical FI/FO device 4. Summary This paper described recent progress on multicore (MCF) and multimode fiber (FMF) connection technologies. MCF connectors and fan-in/fan-out devices are realized that have good potential for practical use. The requirements for an FMF connector are not fixed because there are many types of FMF that can be used for transmission experiments. The design of an FMF connector with low differential mode attenuation will be a future study. 5. References [1] T. Morioka, “New generation optical infrastructure technologies: EXAT initiative towards 2020 and beyond,” in Proc. 14th OECC 2009, 1317 (July, 2009). [2] K. Imamura, K. Mukasa, and R. Sugizaki, “Trench assisted multi-core fiber with large Aeff over 100 μm2 and low attenuation loss,” in Proc. ECOC2011, Mo. 1. LeCervin. 1 (Sep., 2011). [3] R. Ryf et al., “Space-division multiplexing over 10 km of three-mode fiber using coherent 6 x 6 MIMO processing”, in Proc. OFC/NEOFC 2011, PDPB10, (Mar. 2011). [4] R. Nagase, K. Sakaime, K. Watanabe, and T. Saito, “MU-type multicore fiber connector,” IEICE Trans. Electron. Vol. E96-C, No. 9, pp. 1173-1177 (Sep. 2013). [5] R. Nagase, K. Sakaime, K. Watanabe, and T. Saito, “Characteristics of MU-type multicore fiber connector,” submitted to IEICE General Conference 2014 (in Japanese, Mar. 2014). [6] M. Watanabe, Y. Tottori, and T. Kobayashi, “Multi-core fiber free space coupling type connection technology,” in Proc. IEICE Society Conference 2012, C-3-83 (in Japanese, Sep. 2012). [7] K. Watanabe, T. Saito, K. Imamura, and M. Shiino, “Development of fiber bundle type fan-out for multicore fiber,” in Proc. 17th OECC 2012, 5C1-2 (July, 2012). [8] H. Takara, A. Sano, T. Kobayashi, H. Kubota, H. Kawakami, A. Matsuura, Y. Miyamoto, Y. Abe, H. Ono, K. Shikama, Y. Goto, K. Tsujikawa, Y. Sasaki, I. Ishida, K. Takenaga, S. Matsuo, K. Saitoh, M. Koshiba, and T. Morioka, "1.01-Pb/s (12 SDM/222 WDM/456 Gb/s) Crosstalk-managed transmission with 91.4-b/s/Hz aggregate spectral efficiency," in Proc. ECOC2012, Th.3.C.1 (Sep. 2012). [9] H. Uemura, K. Takenaga, T. Ori, S. Matsuo, K. Saitoh, and M. Koshiba, "Fused taper type fan-in/fan-out device for multicore EDF," in Proc. CLEO-PR & OECC/PS 2013, TuS1-4 (Jul. 2013). [10] T. Watanabe and Y. Kokubun, "Laminated polymer waveguide fan-out device for uncoupled multi-core fiber," in Proc. IEEE IPC 2012, ThV3 (Sep. 2012). [11] Y. Tottori, T. Kobayashi and M. Watanabe, " Low Loss Optical Connection Module for Seven-Core Multicore Fiber and Seven Single-Mode Fibers," IEEE Photon. Technol. Lett., 24, 21-24, pp. 1926-1928 (Nov. 2012). [12] Y. Sun, R. Lingle Jr., A. McCurdy, D. Peckham, R. Jensen, and L. Gruner-Nielsen, “Few-mode fibers for mode-division multiplexing,” in Proc. IEEE Summer Topicals 2013, MC3.1 (Jul. 2013).
