A Degenerate-Mode-Selective Coupler for Stable DSP-free MDM Transmission

Weakly-coupled mode division multiplexing (MDM) has been intensively investigated in recent years. In order to be compatible with conventional intensity modulation and direct detection systems, effective solutions to handle degenerate non-circularly-symmetric linearly-polarized (LP) modes (
$\text{LP}_{lm{\rm{a}}}$
and
$\text{LP}_{lm{\rm{b}}}$
modes,
l
≥ 1) of weakly-coupled few-mode fibers (FMFs) are highly desired. In this paper, a novel degenerate-mode-selective coupler (DMSC) consisting of an input FMF and an output two-mode fiber (TMF) is proposed based on coupling mechanisms of a FMF directional coupler. Optical powers in both
$\text{LP}_{lm{\rm{a}}}$
and
$\text{LP}_{lm{\rm{b}}}$
modes of the FMF of
$\text{LP}_{lm}$
DMSC can be selectively and simultaneously transferred to the LP
_11a
and LP
_11b
modes of the TMF. Therefore, the DMSC can demultiplex degenerate non-circularly-symmetric LP modes without any digital signal processing (DSP). Necessary conditions for the DMSC are theoretically investigated. LP
₁₁
and LP
₂₁
DMSCs are numerically simulated to verify the principles and fabricated in the form of fused-type fiber couplers. Compared with regular mode-selective couplers (MSCs), the fabricated DMSCs significantly improve the power stability for demultiplexing randomly-rotated non-circularly-symmetric LP modes. Based on a mode demultiplexer composed of cascaded LP
₀₂
MSC, LP
₂₁
DMSC, LP
₁₁
DMSC and LP
₀₁
MSC, 4-LP-mode MDM transmission over 10-km of weakly-coupled FMF is demonstrated with 10-Gb/s on-off-keying and DSP-free direct detection.