Ultra-Compact Multimode Waveguide Routings Based on the Free-Form Curves

With the growing demand for high-capacity optical communication, mode-division multiplexing (MDM) technology has emerged as a promising solution. Multimode waveguide routing (MWR) is critical for the interconnection of various devices in MDM systems. This paper presents a novel design method for small-size, high-performance MWRs based on free-form curves (FFCs) optimized through inverse design. The proposed method facilitates the creation of multimode waveguide bends (MWBs), S-bends (MWSs), and tapers (MWTs) that support three TE modes. These devices are significantly smaller than conventional designs, while maintaining or even enhancing performance. Experimental measurements confirm that all modes of these devices exhibit excellent manufacturing tolerance (±50 nm), low excess loss (<0.17 dB), and low crosstalk (<−28 dB) at the central wavelength of 1.55 μm. Additionally, our approach can be easily extended to support more modes, and the manufacturing process requires only one-step photolithography and plasma etching. This method offers a new direction for efficient, compact, and flexible on-chip optical interconnections, promoting advancements in optical communication systems.