Inverse design and demonstration of on-chip silicon high-order mode pass filter

We propose a concept of a high-order mode (HOM) pass filter based on the inverse-designed mode-routing, which enables an ultra-compact footprint and broad bandwidth. To validate the concept, we experimentally demonstrate two types of HOM pass filters using the direct-binary search topology optimization algorithm. In the first HOM pass filter, the mode-routing region is constructed using an inverse-designed adiabatic coupler, while the second filter utilizes a tapered asymmetric directional coupler. The subwavelength units based on the functional regions of both filters have an ultra-compact footprint of 4 mu m x 800 nm. The experimental results indicate that the insertion losses of two HOM-pass filters are 3.13 and 1.94 dB, respectively, and their mode cross-talks are -15.8 and -27.36 dB at the center wavelength of 1550 nm. Both HOM pass filters exhibit high performance over a broad bandwidth of 130 nm.