Broadband MIMO all-optical unitary converter based on compact multimode interference on LNOI

We propose and demonstrate a reconfigurable 4 × 4 multi-input multi-output (MIMO) all-optical unitary converter (OUC) based on non-uniform multimode interference (nMMI-OUC) on a lithium-niobate-on-insulator (LNOI) platform, which has the advantages of broadband operation, compactness, low loss, and fabrication-friendliness. To enhance the bandwidth, compact non-uniform multimode interference is utilized to replace uniform multimode interference, achieving the same cross talk and larger bandwidth. Compared with OUC based on uniform multimode interference (uMMI-OUC), the couplers using nMMI-OUC are only half length. The experimental results show that the reconfigurable four mode demultiplexing have been realized with wavelength-dependent loss of less than 3 dB and cross talk of less than -17.3 dB over 100 nm (from 1520 nm to 1620 nm) wavelength range and ∼0.21 dB/coupler at 1550 nm wavelength for nMMI-OUC.