Spot-Based Mode Couplers for Mode-Multiplexed Transmission in Few-Mode Fiber

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.