[rassibassi]

The upper bound is still the Shannon limit. The experiment does a lot of multiplexing: spatial multi-core fiber, spectral multi channel multiplexing across wavelength, dual polarization.

Each of the multiplexed channels are individually limited by the Shannon limit, and with higher power the fiber's Kerr effect creates interference which creates a sweet spot for the optimal optical launch power.

the novelty here is that the spectral channels are all generated from a single laser source rather than a laser per channel

[igravious]

   ^ Superb answer ^
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Shannon Limit in Information Theory

[1] https://en.wikipedia.org/wiki/Noisy-channel_coding_theorem

[2] https://news.mit.edu/2010/explained-shannon-0115