Compact and scalable mode (de)multiplexer using inverse-designed subwavelength gratings.

A compact and scalable mode (de)multiplexer stands as a pivotal component in on-chip mode division multiplexing systems. Subwavelength gratings can flexibly adjust the waveguide refractive index at the subwavelength scale, while inverse design emerges as a promising methodology for optimizing extensive and intricate parameter sets. Here we propose and experimentally demonstrate a compact mode (de)multiplexer using inverse-designed subwavelength grating structures. The proposed six-channel mode (de)multiplexer is designed with an ultra-short coupling lengths (8.4-15 µm for TE-TE channels) and eliminates the adiabatic transition tapers for cascaded asymmetric directional couplers. This design achieves a total length of only 76 µm. The experimental results reveal that the measured peak insertion losses for all mode channels are less than 0.77 dB, and the crosstalk are below -15 dB in a 100 nm waveband. Furthermore, it is very flexible to extend the dual-polarization mode (de)multiplexer. As a proof-of-concept, a dual-polarization mode (de)multiplexer for supporting eight channels (TE-TE and TM-TM modes) is also experimentally demonstrated. This work provides a promising way for on-chip large-scale and dense-integrated polarization and mode division multiplexing circuits.