A Review of Few-Mode Fibers for Space-Division Multiplexed Transmissions

Abstract

A review of the most recent advances on both weakly-coupled and strongly-coupled fewmode fibers is presented. The challenge of increasing the number of LP modes that can actually be used is also discussed. IntroductionSpace-division-multiplexed transmissions using Few-Mode Fibers (FMFs) have recently received considerable attention because of their potential to multiply the capacity of single-mode transmissions by more than a tenfold.A first weakly-coupled approach 1-5 consists of minimizing mode coupling so that each LP mode can be separately detected using simple 2 2 (non-degenerate LP modes) or 4 4 (two-time degenerate LP modes) MIMO techniques, regardless the number of LP modes. This requires a careful design of the FMF 6,7 in order to reduce mode-coupling phenomena that will ultimately limit the reach of the transmission.A second strongly-coupled approach

Few-Mode Fibers for Mode-Division-Multiplexed Systems

“…This optimization, however, becomes more and more difficult when the number of LP modes increases and when one wants to ensure low bend losses (<10 dB/turn at 10 mm radius). We have found that DMGDs become >100 ps/km as soon as the number of LP modes is ≥4 [18].…”

“…Graded-index-core profiles are nevertheless well adapted to low-DMGD FMFs [7]- [10], [18], [20]. Fig.…”

“…One has thus to find the optimum tradeoff between Max|DMGD| that decreases when the core radius increases and the losses of the least leaky LP 41 mode that increases when the core radius decreases. From 12 to ∼14 μm, Max|DMGD| steeply decreases from 22 ps/km [18] to 8.6 ps/km; then, it decreases at a much more moderate rate and reaches 7.5 ps/km at 15 μm. Above this core radius, the LP 41 mode have losses that become <19.34 dB after 22 m with two 40 mm-radius loops, and can no longer be considered as cut off (see Fig.…”

“…Since the first FMFs demonstrations of early 2011, impressive progresses have been made [18], but increasing the number of LP modes that can actually be used is still a challenging issue. So far, only 4-LP-mode fibers with low DMGDs [5], [10] and with low mode coupling [15] have been reported.…”

Characterization of few mode fibers by OLCI technique

“…Despite these significant advances, the capacity of optical fibers is expected to reach its limit unless other multiplexing strategies, such as mode division multiplexing (MDM) with multiple-input multiple-output (MIMO) digital signal processing, are considered [1]. Performance optimization of MDM transmissions requires management of channel parameters [3] for which precise knowledge of the LP mode characteristics, including their group delay (GD), dispersion (GVD) and mode-dependent loss, is particularly important. Different methods have been proposed to characterize LP modes of few mode fibers (FMF).…”