Can programmable optical nodes reduce the digital equalization burden in long-haul spatial division multiplexing? The authors develop a differentiable transmission model that optimizes photonic unitary transformations and implement them on a silica photonic processor. A three-mode recirculating-fibre experiment reaches 1,331.2 km and reduces the reported delay spread from approximately 25 ns to 10 ns compared with an identity transformation; simulations also examine ten modes.

PHOTONICS · NATURE PHOTONICS · 2026

Light can share a fibre and still arrive at different times. A programmable chip, configured using a digital model, brings those journeys closer together.

Original conceptual artwork · not a device photograph

One fibre. Many ways to travel.

Sending information through different spatial modes can increase a fibre’s capacity. But those modes do not all take the same time.

A mode is a pattern in which light travels through a fibre. When different modes accumulate different delays, a short input pulse spreads out. The receiver then needs digital processing to untangle information arriving over a longer time window.

This research puts a programmable optical processor into the link. By deliberately mixing the modes, it can redistribute their journeys and reduce the spread in arrival times.

Improve the optical journey before asking electronics to undo it.

Let the modes take turns.

In this simplified two-mode model, one channel is faster than the other. Switch their paths halfway through and each signal spends one span in each channel.

Two spans. One decision.

Signal A stays fast. Signal B stays slow. Their delay difference grows in both spans.

What this model teaches: swapping does not accelerate light. It balances time spent in fast and slow channels. This exact cancellation is a special two-mode, two-span example, not the paper’s three-mode experimental result.

A digital twin. A physical change.

The real system needs more than a simple swap. A differentiable model helps find useful settings for a programmable optical mesh.

Represent the journey

Combine a model of fibre propagation with a parameterized model of the photonic processor.

Find a better transformation

Use gradients to adjust the processor parameters toward a smaller spread in propagation times.

Send the settings to light

Configure the physical chip to mix the spatial modes. Digital signal processing remains at the receiver.

The fabricated processor contains an 8 × 8 optical mesh . The transmission experiment uses three spatial modes .

A narrower window after a long journey.

Over a 1,331.2 km recirculating link, the optimized processor reduced the reported impulse-response spread from approximately 25 ns to 10 ns.

The receiver’s view

Relative intensity · arbitrary units

Data-inspired illustration of temporal extent; not a raw measured trace. Time axis centered for comparison.

Identity setting: the processor leaves the mode basis unchanged.

Less to untangle. More to investigate.

Optical programmability could help reduce the burden of recovering information from many-mode links.

The contribution is a bridge between a differentiable transmission model and a physical photonic processor. It makes the optical link itself an adjustable part of the system.

Scale. The experiment uses three modes; the paper also studies ten-mode links in simulation.

Coupling. Benefits depend on how modes mix in the fibre. Under strong all-mode coupling, the simulations found no additional improvement over random transformations.

Energy. A shorter digital processing window is promising, but total system energy savings were not measured. Processor loss and heater power also matter.

Mitsumasa Nakajima, Kohki Shibahara, Kohei Ikeda, Akira Kawai, Masaya Notomi, Yutaka Miyamoto & Toshikazu Hashimoto.

Nature Photonics (2026) · DOI: 10.1038/s41566-026-01997-x

An independent interactive explanation based on the supplied research paper. Original AI-generated conceptual artwork; paper figures are not reproduced. Interactive diagrams are explanatory models, with their limits noted above.