Ultra-low-crosstalk mode (de)multiplexers for long-haul weakly coupled few-mode fiber transmission

Suppression of modal crosstalk in a mode (de)multiplexer is a crucial issue for implementing a long-haul weakly coupled mode-division multiplexing system. In this Letter, we present a low-crosstalk mode (de)multiplexer, consisting of cascaded mode-selective couplers with precise manipulation for phase matching or modal area matching between the fundamental mode in a single-mode fiber and the target modes in a few-mode fiber. As a proof of concept, a 2-mode (de)multiplexer is realized with less than -31 dB modal crosstalk and <1.5 dB insertion loss at 1550 nm. Based on this component, 4-λ × 2-mode × 28-GBd quadrature phase-shift keying signal transmission is experimentally demonstrated over a 3733.64-km few-mode fiber only using 2 × 2 multiple-input multiple-output processing. As far as we know, this work achieves the longest reach for a weakly coupled mode-division multiplexing transmission.