Multi-step-index fiber with a large number of weakly coupled OAM mode groups for IM/DD systems in data centers: design, fabrication, and characterization.

Mode division multiplexing (MDM) technique based on weakly coupled few-mode fibers (FMF) is promising to enhance the capacity of short-reach transmission. We design and fabricate a multi-step-index FMF (MSIF), which supports weakly coupled first-order radial orbital angular momentum mode group (OAMl,1 MG) for MDM transmission. We use three layers of core to regulate the minimum effective refractive index difference (min|Δneff|) between OAMl,1 MG and the adjacent MGs. In experiments, we demonstrate that the fabricated MSIF can support up to OAM6,1 with the interferometric method, and the loss measured by an optical time-domain reflectometer (OTDR) can achieve <0.5 dB/km for the OAMl,1 with an order from |l| = 0 to |l| = 6. The inter-mode-group cross talk (XT) is tested by the power measurement, and the system-level XT after 20 km fiber transmission in the worst case is about -11.1 dB.