Scalable four-mode cyclic converter based on a semi-inverse design approach

The cyclic mode converter offers an appealing solution for eliminating differential mode delay and mode-dependent loss in mode-division multiplexing transmission systems. In this work, we present an innovative four-mode cyclic mode converter. This device boasts high performance, scalability, broad bandwidth, and excellent process tolerance. By employing an efficient semi-inverse design method, we have refined the structural parameters of three directional couplers, leading to enhanced conversion efficiency and a faster optimization process. Simulation results show that the insertion loss is less than 0.4 dB, and the crosstalk is lower than -17 dB over the range of 1500-1600 nm, with a compact footprint of only 15 × 120 µm². Experimental results demonstrate that the present cyclic mode converter exhibits low loss (<1.8 dB) and low crosstalk (<-15 dB) within a bandwidth from 1520 to 1580 nm. The device is insensitive to variations in waveguide parameters. Transmission performance at 1550 nm using non-return-to-zero on-off keying and four-level pulse amplitude modulation shows clear eye diagrams at 64 Gbps and 110 Gbps. Furthermore, this work can be easily extended to more modes and other material platforms. The proposed cyclic mode converter opens up promising avenues for effectively reducing link damage in mode-division multiplexing transmission system.