Physical contact (PC) optical connectors realize long-term stability by maintaining contact with the optical fiber even during temperature fluctuations caused by the microscopic displacement of the ferrule endface. With multicore fiber (MCF) connectors, stable PC connection conditions need to be newly investigated because MCFs have cores other than at the center. In this work, we investigated the microscopic displacement of connected ferrule endfaces using the finite element method (FEM). As a result, by using MCF connectors with an apex offset, we found that the allowable fiber undercut where all the cores make contact is slightly smaller than that of single-mode fiber (SMF) connectors. Therefore, we propose a new equation for determining the allowable fiber undercut of MCF connectors. We also fabricated MCF connectors with an allowable fiber undercut and confirmed their reliability using the composite temperature/humidity cyclic test.

Various optical fiber connectors have been developed during the 40 years since optical fiber communications systems were first put into practical use. This paper describes the key technologies for optical connectors and recent technical issues.

Multi-core fiber (MCF) is one of the most promising candidates for achieving ultra-wideband optical transmission in the near future. To build a network using MCF, a high-performance and reliable MCF connector is indispensable. We have developed an SC-type optical connector for MCF and confirmed its excellent optical performance, mechanical durability, and environmental reliability. To put the communication system using MCF into practical use, it is necessary to establish a procedure for measuring the initial connection characteristics. Fan-in / fan-out (FIFO) devices are indispensable for measuring the connection characteristics of MCF connectors. To measure the return loss of the MCF connector, it is necessary to remove the influence of reflection at the FIFO itself and at the connection points with the FIFO. In this paper, we compare four types of return loss measurement procedures (three usual method and a new method we proposed) and find that most stable measurement method involves using our new method, the OCWR method without FIFO. The OCWR method without FIFO is considered to be the most advantageous when used for outgoing inspection of connectors. The reason is that it eliminates the measurement uncertainty caused by the FIFO and enables speedy measurement.

Wavelength-division multiplexing multicore fibers can transmit a large amount of information over one fiber, and high-density core allocations enable a large number of fiber lines to be deployed in limited spaces. However, inter-core crosstalk degrades the signal in these systems. This paper describes the design of a frequency interleaving scheme for a 7-core hexagonal multicore fiber. Interleaving schemes shift signal spectra between neighboring cores to reduce the signal degradation caused by inter-core crosstalk. The channel frequency allocation that most efficiently lowers the bit error rate is numerically determined in this study. The results indicate that the optimum frequency interleaving improves the allowable crosstalk ratio by 6.3 dB for QPSK signals, demonstrating its potential for improving wavelength-division multiplexing multicore fiber transmission systems.

We have demonstrated crosstalk mitigation in single-mode MCFs using optical space coding. Four types of single-mode multicore fiber (MCF) models were evaluated by our scheme with the modified prime code and differential detection. Typically, intercore crosstalk was improved by 7-20 dB in 9-core fibers with an original crosstalk of 10-20 dB.

We describe a physical-contact (PC) multicore fiber (MCF) connector with good optical characteristics. To achieve stable physical-contact connection, we clarify the relationship between connector-end deformation and compression force with spherical polished ferrule end structures using finite element analysis and actual measurements. On the basis of the obtained relationship, we demonstrate a design approach that shows the physical-contact condition of all the cores of a multicore fiber with a simplex connector. In addition, we clarify the design criteria for low-loss connection by employing a rotational angle alignment structure, and devise an SC-type rotational MCF connector with high alignment accuracy. Based on our designs for PC and low-loss connection, we demonstrate an MCF connector with PC connection that provides a sufficiently high return loss exceeding 50dB and a sufficiently low connection loss of below 0.2dB for all the cores of a 7-core single-mode MCF.

Prediction of cross-talk is an important facet of multicore fiber (MCF) design. Several approaches for estimating cross-talk in MCF have been proposed but none are faultless, especially when applied to MCF with heterogeneous cores. We propose a new calculation approach that attempts to solve this problem. In our approach, cross-talk in multicore fibers is estimated by coupled power theory. The coefficients in the coupled power equation are theoretically calculated by the central limit theorem and by quantum mechanical time-dependent perturbations. The results from our calculations agree with those of Monte Carlo simulations of heterogeneous MCFs.

We have developed a 7-core-fiber connector. To maintain both the ferrule floating mechanism and precise alignment around the ferrule axis, we employed Oldham's coupling mechanism inside an MU-type connector plug housing and realized an average attenuation of 0.13dB and an average return loss of 48.2dB.