A compact broadband four-channel parallel mode converter enabled by adjoint-based inverse design for C-band operation

Aiming to address the capacity bottleneck of traditional single-channel mode converters in mode-division multiplexing (MDM) optical communication systems, this paper proposes a four-channel mode converter based on photonic inverse design. The device adopts a three-section integrated architecture consisting of an input waveguide, a topology-optimized conversion region, and a multi-output waveguide, enabling parallel conversion from the input fundamental mode (TE₀) to TE₀, TE₁, TE₂, and TE₃ modes with four independent outputs. By using a multi-objective optimization algorithm and fully parameterized modeling, a collaborative optimization model of wavelength-channel-mode is constructed to break the coupling constraints in conventional design. The device is designed using Mitsui Chemicals' Diffrar material, with an overall footprint of 18 μm × 20 μm, and operates stably over the 1520-1580 nm communication band. Simulation results demonstrate that the single-channel minimum mode purity exceeds 98.49%, the crosstalk is lower than -18.2 dB, the transmission efficiency is higher than 91.73%, and the total conversion efficiency at 1550 nm reaches 94.50%. The converter features an ultra-compact structure, broadband compatibility, and high performance, providing a promising solution for high-capacity on-chip optical interconnection and photonic AI computing systems.