Rigorous mode theory and analysis of few-mode fibers

The research and development of fibers has a long history over half a century. Although the transmission capacity of single-mode fibers has reached 10 Tbps order owing to some breakthroughs, it is approaching the limit of about 100 Tbps owing to nonlinearity and the fiber fuse. To overcome this problem, a new approach utilizing the space-division multiplexing and mode-division multiplexing has been proposed and demonstrated. However, in mode-division multiplexing, even when a mode multi/demultiplexer is used, accurate mode discrimination has been difficult and multiple-input multiple-output (MIMO) signal processing has always been used to identify the input mode channel. This is because the accurate analysis of mode propagation has not been sufficiently investigated. In this review, the history of the mode theory of optical fibers is briefly reviewed and the recent progress of mode theory and related experimental results are introduced. (c) 2018 The Japan Society of Applied Physics