Five-LP-Mode IM/DD MDM Transmission Based on Degenerate-Mode-Selective Couplers With Side-Polishing Processing

Weakly-coupled mode division multiplexing (MDM) techniques supporting intensity modulation and direct detection (IM/DD) have attracted great attention for capacity enhancement of short-reach applications such as optical interconnections, for which mode multiplexers/demultiplexers (MMUX/MDEMUX) with low insertion loss (IL) and modal crosstalk are the key components. In this paper, we firstly propose the design method for degenerate–mode-selective couplers (DMSC) with side-polishing processing. Three kinds of DMSCs for different degenerate linearly-polarized (LP) modes (LP
₁₁
, LP
₂₁
and LP
₃₁
) in a weakly-coupled few-mode fiber (FMF) are designed. Then a pair of MMUX and MDEMUX consisting of cascaded mode-selective couplers (MSC) and DMSCs for all the 5 LP modes of the FMF are fabricated with side-polishing processing. The measurement results show that the MMUX and MDEMUX achieve low back-to-back IL lower than 5.3 dB for all the 5 modes. Finally, stable digital-signal-processing (DSP)-free 5-mode IM/DD MDM transmission utilizing conventional optical modules over 10-km FMF are successfully demonstrated. The proposed low-IL MMUX/MDEMUX is scalable to support more modes and can promote the practical application of MDM techniques.