Programmable photonic unitary processor enables parameterized differentiable long-haul spatial division multiplexed transmission

The explosive growth of global data traffic demands scalable and energy-efficient optical communication systems. Spatial division multiplexing (SDM) using multicore or multimode fibres is a promising solution to overcome the capacity limit of single-mode fibres. However, long-haul SDM transmission faces substantial challenges due to modal dispersion, which imposes heavy computational loads on digital signal processing for signal equalization. Here we propose parameterized SDM transmission, where programmable photonic unitary processors are installed at intermediate nodes. Instead of relying on conventional digital equalization on only the receiver side, our approach enables direct optimization of the SDM transmission channel itself by the programmable unitary processor, which reduces digital post-processing loads. We introduce a gradient-based optimization algorithm using a differentiable SDM transmission model to determine the optimal unitary transformation. Using this approach, we implement a telecom-grade programmable photonic unitary processor for low-loss, wideband (full C band), polarization-independent and high-fidelity (R2 > 96% across the C band) operation and experimentally report 1,300-km transmission using a three-mode fibre. The optimized photonic processor substantially reduces modal dispersion and post-processing complexity. Our results establish a scalable framework for integrating photonic computation into optical layers, enabling more efficient, high-capacity optical networks. A programmable photonic unitary processor for compensating modal dispersion allows long-haul (1,300 km) high-quality data transmission over a three-mode fibre.