MIMO equalization with adaptive intermodal interference cancellation for mitigating effect of mode-dependent loss in SDM transmission over coupled multi-core fiber

We propose a multi-input multi-output (MIMO) equalizer architecture with adaptive intermodal interference cancellation without an iterative process for space-division multiplexed (SDM) transmission to mitigate the effect of the mode-dependent loss (MDL). This equalizer uses a MIMO filter to compensate for coupling and spatial mode dispersion and another MIMO filter that generates replicas of intermodal interference from the tentative decision results of the outputs of the first MIMO filter to cancel the interference. The coefficients of the two MIMO filters are adaptively controlled by stochastic gradient descent and gradient calculation with back propagation to minimize the loss function, which consists of the last outputs after interference cancellation. We evaluated the proposed equalizer through numerical simulation of SDM transmission of 32-Gbaud polarization-division multiplexed quadrature phase shift keying (PDM-QPSK) signals over 100-km spans of 4-coupled-core fiber (4-CCF). For 1 dB of the MDL per span, the MIMO equalizer with adaptive intermodal interference cancellation achieved a 17% longer transmission distance without bit errors after forward error correction (FEC) compared with a conventional adaptive MIMO filter. We also evaluated the performance of the MIMO equalizer in wavelength-division multiplexed and SDM transmission of 32-Gbaud PDM-QPSK signals over 52-km 4-CCF spans in a recirculating loop configuration. The MIMO equalizer improved a pre-FEC Q value at 7280 km by 0.8 dB compared with the conventional adaptive MIMO filter.