Offset Coupling, Feedback, and Spatial Multiplexing in 4 $,\times,$ 4 Incoherent-MIMO Multimode Fiber Links

We experimentally evaluate the performance of incoherent multiple-input multiple-output (MIMO) multimode fiber (MMF) links with different fiber media, link lengths, and modulation techniques. The performance of conventional 62.5 μm diameter silica as well as perfluorinated plastic, graded-index MMF sections of various lengths were evaluated using 1 × 1, 2 × 2, 3 × 3 and 4 × 4 MIMO setups over a range of lengths, with both open-loop equalization and feedback-based spatial multiplexing techniques. The data rate was evaluated over silica fiber sections with lengths ranging between 100 m-3 km using distributed feedback lasers and VCSELs at the transmitters. In addition, plastic optical fiber sections with lengths ranging from 1 m - 100 m were evaluated with Fabry Perot lasers. Axial offsets were introduced while launching into, and detecting from the MMF to enhance modal diversity, and the impact of these offsets on the data rates was measured to obtain optimal launch and detection conditions. With optimized launch conditions and modulation parameters, the silica based systems realized data rates of 16 to 26 Gb/s, representing a bandwidth-length product improvement of 28× over MMF standards such as 10 GBASE-SR. The plastic fiber system reached data rates of 24 to 43 Gb/s, with bandwidth-length product increases of up to 12× of rated parameters. The use of off-axis launch and detection with flexible offsets on all four MIMO streams of the 4 × 4 system would enable further performance improvement.