Adjoint shape optimization and experimental demonstration of silicon-based 1 × 2 mode splitter

In this paper, we propose and experimentally demonstrate an ultra-compact silicon-based mode splitter via adjoint-based shape optimization. The proposed device can provide efficient TE₀ and TE₁ mode separation while preserving the mode order. By employing an adjoint optimization algorithm, a complex yet fabricable silicon-silica structure was generated within a functional area of 5.5 µm × 4 µm, overcoming limitations of conventional devices such as a large footprint and restricted bandwidth. Experimental results show an operating bandwidth exceeding 130 nm, with insertion loss below 2.5 dB and crosstalk under -10 dB. Leveraging the high integration density and performance advantages afforded by inverse design, the proposed mode splitter offers a flexible and low-loss mode-routing solution for on-chip mode-division multiplexing systems.