Compact mode-insensitive variable optical attenuation enabled by inverse-designed mode manipulation devices

The growing demand for data capacity in optical communication has heightened interest in mode-division multiplexing (MDM) technology, which provides high parallelism across spatial mode dimensions. Mode-insensitive variable optical attenuation is essential for advanced optical communication systems, ensuring consistent performance across multiple modes. However, current multimode variable attenuation devices are more complex in design and significantly larger than single-mode devices, limiting their integration potential. In this paper, we explore two mode-insensitive phase tuning schemes: waveguide widening and mode transformation. We validate these schemes by developing two compact, mode-insensitive variable optical attenuators (VOAs) on a silicon-on-insulator (SOI) platform. By fully leveraging uniquely inverse-designed mode manipulation devices, the VOAs achieve footprints comparable to their single-mode counterparts, overcoming the size barrier for multimode devices. Experimental results demonstrate robust mode-insensitive attenuation functionality with dynamic ranges of 38.4 dB and 19.6 dB for the two schemes, respectively. The proposed mode manipulation devices and VOAs exemplify the development of compact and efficient multimode components, supporting the ongoing trend toward enhanced integration of on-chip optical systems.