Analysis of Laser and Detector Placement in Incoherent MIMO Multimode Fiber Systems

Abstract

Abstract-Conventional large-core multimode fibers (MMFs) are preferred for use in short to medium haul optical fiber links, owing to their tolerance to misalignment and low deployment costs; however, data rates through MMFs are limited by modal dispersion. Digital signal processing with multiple-input multiple-output (MIMO) techniques has offered promising solutions to overcome the dispersion limitations of MMFs, but the impact of the geometry of laser and detector arrays on the achievable data rate is not established. To this end, we use a field-propagation-based model to gauge the impact the geometry of lasers and detectors can have on the achievable ergodic and outage rates of incoherent MIMO-MMF links. Laser and detector array geometries were investigated using a grid-based method to optimize the positions of lasers and detectors for a 1 km MIMO-MMF link. Simulations reveal that systems with appropriately designed laser/detector geometries could improve the achievable rate over the fiber by more than 200% over random laser/detector arrays. The grid-based search technique, however, is limited due to high computational requirements for fine grids. As an alternative, we developed a suboptimal "greedy" selection approach to design detector geometries, which produces detector geometries that attain more than 90% of the rate obtained with an exhaustive search, while requiring less than 0.2% of the computation. The low computation requirements and high performance of the greedy selection approach also motivate the use of dynamically reconfigurable detector arrays to achieve high data rates with reduced signal processing complexity. Methods are also presented for clustering detector elements to obtain more consolidated segmented detectors with better fill factors, while still offering significant data rate benefits. The achievable ergodic rate using these systems is verified to be close to the link's ergodic capacity.

Detector and mode subset selection for DSP based multimode fiber links

“…We avoid this cost by using a suboptimal greedy selection to find the subset of modes for decoding for each channel realization. This yields a subset whose data rate is guaranteed to be within (1 − 1/e) of the the capacity obtained with the optimal subset [19]. In practice, the performance is much better, as can be observed in Fig.…”

“…In [19], simulations based on channel models revealed that a significant enhancement in channel capacity could be obtained if laser and detector geometries were designed in accordance with the channel statistics of the fiber. As an example, Fig.…”

Low Complexity Equalization Using Mode-Subset Selection in MIMO Multimode Fiber Links

“…Such an approach, while suboptimal, reduces the number of comparisons required to O(DL), which is much smaller. Moreover, due to the sub-modularity of the capacity of fiber channels with respect to mode subsets [16], the data rate achieved using greedy mode subset selection is guaranteed to be at least (1 − 1/e) times that obtainable with the optimal subset selection approach [17]. Thus, the rate obtained using suboptimal greedy selection is mathematically guaranteed to be close to that obtained using optimal selection.…”

“…from 277 to 665 ps/km for D = 6 to 110. For minimizing the computational complexity, the FFT block length N is optimized based on the criterion described in the previous section, and is found to be to 2 16 . In each of the cases, we generate several channels using the model described in Section II-C, and compare the rate achievable using subset selection for various subset sizes with the capacity obtained using all fiber modes.…”