Wavefront superposition method for accurate and efficient mode conversion

A wavefront superposition (WS) method is proposed for accurate and efficient mode conversion in mode-division multiplexing transmission. The WS method converts an input beam to the WS state, which is composed of the conversion target and radiation modes of a few-mode fiber. The appropriate weighting for the modal component of the WS state enables more efficient conversion than the conventional method in which the output beam consists only of the conversion target. Further, since the components of the radiation modes in the output are eliminated by the mode-filtering property of the few-mode fiber, no modal crosstalk occurs in the WS method. We examine the conversion performance of the WS method by a numerical simulation for the mode-multiplexing numbers 3, 6, 10, and 15. The WS method shows a 2.4 dB higher efficiency than the conventional method, while maintaining an extremely low modal crosstalk (less than -80  dB), even when the number of multiplexed modes is 15.