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We transmit 32 WDM channels over 12 spatial and polarization modes of 177 km few-mode fiber at a record spectral efficiency of 32 bit/s/Hz. The transmitted signals are strongly coupled and recovered using 12×12 multiple-input multiple-output digital signal processing.

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We present the theory for a novel low-loss mode coupler based on multiple Gaussian spots, which can selectively address all spatial and polarization modes of a few-mode fiber. Specifically, we show spot arrangements and how to construct them for few-mode fiber with a large number of modes, and analyze in detail designs for few-mode fibers supporting six and 12 spatial and polarization modes. For six-mode few-mode fibers, couplers with no mode-dependent loss and a coupler insertion loss <2 dB are possible, whereas for few-mode fiber supporting 12 modes, a mode-dependent loss <1 dB and coupler insertion loss <3 dB can be simultaneously achieved.

Low differential group delay (DGD) between the modes of a graded-index few-mode fiber is obtained by combining segments with DGD of opposite sign. Transmission of mode-multiplexed 6×20-GBd QPSK over a record distance of 1200 km is demonstrated.