Ultra-broadband and compact mode conversion by controlling the propagation path with a micro-slot embedded in a multimode interferometer

An ultra-broadband and compact mode converter based on a multimode interferometer (MMI) with a micro-slot embedded is demonstrated, where the micro-slot serves as a localized refractive-index perturbation in the MMI that precisely controls the phase evolution of lateral modes. The fabricated device is based on a fiber-compatible polymer platform with a 2700 µm MMI length and a 100 × 14 × 5 µm 3 micro-slot. The fabricated device enables the efficient conversion between the E 11 mode and the E 21 mode with a mode conversion efficiency higher than 92% and modal crosstalk lower than –11 dB across the O band and the C + L band and beyond while maintaining large device fabrication tolerance. Our proposed mode converter offers a robust, CMOS-compatible, and high-performance solution for ultra-broadband mode manipulation and signal processing where high-order modes are required.