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Circulator-free on-chip bidirectional four-wave mixing.

On-chip bidirectional four-wave mixing (FWM), which may meet the rapidly increasing demands on integration level and system versatility, has never been implemented for the lack of competent on-chip optical circulators that are indispensable for separating the bidirectional channels. In this scenario, we first propose and demonstrate a circulator-free scheme that fully utilizes the inherent clockwise and counter-clockwise transmission natural of an add-drop microring resonator to implement on-chip bidirectional FWM. It should be noted that the proposed scheme additionally provides a universal solution for multichannel nonlinear operations and the related applications, with the advantages of compact structure and low crosstalk. The implementation for multimode channels is investigated for demonstration, as the mode division multiplexing technique is of particular interest in integrated optoelectronics and the resonance-enhanced nonlinear structure with superior dimension compared to straight waveguides has never been implemented for a multimode case. The experimental results exhibit that the FWM efficiencies of the two-mode channels are -26.78 and -28.69  dB, respectively, and the crosstalk is lower than -35  dB.