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, that can selectively address all spatial and polarization modes of a few-mode fiber. In particular we show spot arrangements and how to construct them for few-mode fiber with large number of modes, and analyze in detail designs for few-mode fibers supporting 6 and 12 spatial and polarization modes. For 6 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 1dB and coupler insertion loss 3dB can be simultaneously achieved.