Effect of Higher-Order Modal Dispersion in Direct-Detection Mode-Division-Multiplexed Links

Abstract

Short-reach direct-detection links using mode-division multiplexing (MDM) in multimode fiber (MMF) can employ receiver-side optical signal processing (OSP), for example, by Mach-Zehnder meshes, to compensate for crosstalk among modes within mode groups, which have nearly equal propagation constants and couple strongly during propagation. In graded-index (GI) MMF, modes in the lowest-order groups, <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\lbrace LP_{01}\rbrace$</tex-math></inline-formula> and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\lbrace LP_{11}\rbrace$</tex-math></inline-formula> , also have nearly equal group delays. Modes in higher-order groups, such as <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\lbrace LP_{02},LP_{21}\rbrace$</tex-math></inline-formula> , do not, leading to modal dispersion (MD). To enable MDM in these higher-order mode groups, we propose to also employ transmitter-side OSP to launch signals into principal modes (PMs), which are free of MD to first order in frequency. We study the performance of the proposed scheme via simulation of realistic GI-MMFs based on experimental measurements by Carpenter et al. While PM transmission eliminates MD to first order in frequency, higher-order MD limits the ability of frequency-independent OSP to mitigate inter-symbol interference and crosstalk. We quantify the impact of higher-order MD, and show that either the PM coherence bandwidth or the group-delay spread within a mode group can predict the power penalty due to higher-order MD.