Broadband three-mode converter and multiplexer based on cascaded symmetric Y-junctions and subwavelength engineered MMI and phase shifters

•A novel architecture to scale the number of modes of mode converters and multiplexers is proposed.•The device is based on cascaded symmetric Y-junctions and subwavelength engineered multimode interference coupler and phase shifters.•This architecture can readily operate up to the third-order mode by including a single switchable phase shifter.•Insertion losses below 1.18 dB and crosstalk lower than −20 dB is achieved over a 161 nm bandwidth (1489–1650 nm).•This approach paves the way for the implementation of next-generation datacom systems with a high channel count. Mode-division multiplexing has emerged as a promising route for increasing transmission capacity while maintaining the same level of on-chip integration. Despite the large number of on-chip mode converters and multiplexers reported for the silicon-on-insulator platform, scaling the number of multiplexed modes is still a critical challenge. In this paper, we present a novel three-mode architecture based on multimode interference couplers, passive phase shifters and cascaded symmetric Y-junctions. This architecture can readily operate up to the third-order mode by including a single switchable phase shifter. Moreover, we exploit subwavelength grating metamaterials to overcome bandwidth limitations of multimode interference couplers and phase shifters, resulting in a simulated bandwidth of 161 nm with insertion loss and crosstalk below 1.18 dB and −20 dB, respectively.