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The transmission capacity of single-mode single-core fibers is approaching the limit of about 100 Tbps owing to nonlinearity and the fiber fuse, which limit the input signal power To avoid the so-called capacity crunch and dramatically increase the transmission capacity, a new approach utilizing space-division multiplexing (SDM) and mode-division multiplexing (MDM) has been proposed and demonstrated In these new multiplexing technologies, a problem that is unprecedented in conventional single-mode fibers, namely, the crosstalk between transmission channels, arises Although the input signal channels are mixed through at the output end, the signal channel can be identified using multiple-input multiple-output (MIMO) signal processing technology at the output end However, even using the MIMO digital signal processing technology, the problem that the computation time increases with the number of channels remains, resulting in increased latency, and so the number of channels is limited To design a transmission system using the SDM and MDM technologies, the crosstalk should be analyzed precisely However, the origin and behavior of crosstalk are different in SDM using multicore fibers (MCFs) and in MDM using few-mode fibers (FMFs) The behavior of crosstalk in single-mode MCFs is predictable to some extent by statistical analysis, and the system can be designed by considering the results of the analysis On the other hand, the behavior of crosstalk of FMFs is less predictable Since the mode launched at the input end is not the eigenmode, mode discrimination or accurate mode demultiplexing is difficult using a conventional mode demultiplexer In addition, the eigenmode itself of FMFs is not always the hybrid mode predicted by the conventional theory but sometimes a linearly polarized (LP) mode, contradicting the conventional theory In other words, the demultiplexed signal always involves crosstalk regardless of the transmission distance, and the quantity of crosstalk cannot be analyzed statistically Therefore, the crosstalk and its behavior are unpredictable In spite of these unpredictable phenomena, the signal channel can be identified using the MIMO signal processing as in single-mode MCFs This means that the MDM technology using FMFs is established in the engineering or inductive logic sense, but still involves unexplained phenomena in the scientific or deductive logic sense In this review, we discuss the predictable behavior of crosstalk in single-mode MCFs and also the unpredictable behavior of crosstalk in FMFs

ieice electronics express

Predictable and unpredictable phenomena in optical fibers for space-division/mode-division multiplexing transmission: statistical analysis of coupling and mysterious behavior of modes