Mode Coupling in Space-division Multiplexed Systems

Even though fiber-optic communication systems have been engineered to nearly approach the Shannon capacity limit, they still cannot meet the exponentially-growing bandwidth demand of the Internet. Space-division multiplexing (SDM) has attracted considerable attention in recent years due to its potential to address this capacity crunch. In SDM, the transmission channels support more than one spatial mode, each of which can provide the same capacity as a single-mode fiber. To make SDM practical, crosstalk among modes must be effectively managed. This dissertation presents three techniques for crosstalk management for SDM. In some cases such as intra-datacenter interconnects, even though mode crosstalk cannot be completely avoided, crosstalk among mode groups can be suppressed in properly-designed few-mode fibers to support mode group-multiplexed transmission. However, in most cases, mode coupling is unavoidable. In free-space optical (FSO) communication, mode coupling due to turbulence manifests as wavefront distortions. Since there is almost no modal dispersion in FSO, we demonstrate the use of few-mode pre-amplified receivers to mitigate the effect of turbulence without using adaptive optics. In fiber-optic communication, multi-mode fibers or long-haul few-mode fibers not only suffer from mode crosstalk but also large modal dispersion, which can only be compensated electronically using multiple-input-multiple-output (MIMO) digital signal processing (DSP). In this case, we take the counterintuitive approach of introducing strong mode coupling to reduce modal group delay and DSP complexity

Micro-Bend-Resistant Low-Differential-Mode-Group-Delay Few-Mode Fibers

We report the design of micro-bend-resistant low-differential-mode-group-delay (low-DMGD) few-mode fibers that support up to six mode groups (12 LP modes, 21 spatial modes) and that are adapted to multiple-input multiple-output mode-division-multiplexed transmissions. Compared to previous designs, the micro-bending sensitivity is reduced by more than a tenfold factor with acceptable impacts on attenuation, effective area, and DMGD for 2, 3, and 4 mode groups, but with some degradation for five and six mode groups. These fibers also exhibit low coupling between their mode groups, which is beneficial for DMGD compensation and multiple-input multiple-output processing. Based on these designs, we fabricate and characterize a 4-mode-group (6-LP-mode, 10-spatial-mode) fiber with a micro-bending sensitivity reduced by a factor up to 20 compared to previous realizations. This sensitivity is even lower than that of typical standard single-mode fibers, which should allow the deployment in most telecommunication networks without significant induced losses. A DMGD-compensated link with values ≤72 ps/km is also realized. Finally, we show that previous fibers with 5 and 6 mode groups already have low micro-bending sensitivities, which should prevent from using micro-bend-resistant designs which, in these cases, can degrade the attenuation, effective area and DMGD