All work was done at the receive end, without modification to the fibre.
Optical fibre can generally be though of as a medium with infinite capacity where only end-terminal equipment limits bandwidth, but there are non-linear effects within fibres which cause data streams interact, making errors over extreme length at high data rates.
“This is the largest distance increase ever reported for this system architecture,” said Dr Robert Maher of the UCL team. “The challenge was to devise a technique to simultaneously capture a group of optical channels, known as a super-channel, with a single receiver. This allows us to undo the distortion by sending the data channels back on a virtual digital journey at the same time.”
According to the associated paper ‘Spectrally shaped DP-16QAM super-channel transmission with multi-channel digital back-propagation‘ in Nature Scientific Reports, the team received a 16QAM super-channel, consisting of seven spectrally shaped 10Git/s sub-carriers spaced at the Nyquist frequency.
Multi-channel digital back-propagation (MC-DBP) was added to optimised forward error correction achieved 6.6bit/s/Hz data density over the link.
The influence of non-linearity over data bandwidth limit is not fully understood and, for the first time according to the paper, the sensitivity of MC-DBP gain to linear transmission line impairments, and a trade-off between performance and complexity, have been outlined.
UCL researchers will now test their method on denser super-channels commonly used in digital cable TV (64QAM), cable modems (256QAM) and Ethernet connections (1024QAM).
The work was sponsored by the EPSRC (Engineering and Physical Sciences Research Council).
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