209-km Single-Span Mode- and Wavelength-Multiplexed Transmission over Hybrid Few-Mode Fiber

We experimentally demonstrate multiple-input-multiple-output transmission over a 209-km hybrid few-mode fiber span of a combined 3-space-, 2-polarization-, and 5-wavelength-division multiplex, using low-loss 3-spot mode couplers and backward-pumped distributed Raman amplification. Introduction Space-division multiplexing (SDM) over a single fiber is currently under investigation for its potential to overcome the capacity limit of single-mode fibers (SMFs) imposed by the combination of fiber nonlinearities and Shannon's capacity limit. Few-mode fibers [1]­[4] have recently emerged as a viable candidate to gradually extend fiber capacity. In this work we experimentally demonstrate SDM transmission over a single fiber span of 209 km. To our knowledge this is the longest SDM single fiber span reported yet, 55km longer than our previously published result [3]. The record distance is achieved by using three key techniques: First we use low-loss spotbased mode couplers [3], second the loss of the fiber is partially compensated by distributed Raman amplification [5], and third we use a hybrid FMF consisting of a first section with a large effective area fiber, that is more tolerant to nonlinear effects, followed by a small effective area fiber to provide larger Raman gain. In our experiment, 40-Gbit/s QPSK signals are multiplexed over 3 spatial modes, 2 polarizations, and 5 wavelengths, resulting in an aggregate linerate of 1.2 Tbit/s over a bandwidth of 250 GHz. The signals are recovered after transmission by off-line multiple-input multiple-output (MIMO) digital signal processing (DSP).