Latest Fibre Optic Transmission Record: 400 Petabits A Strand

Long haul fibre optic bandwidth ranges from a few terabits per second into the low thirties with the equipment and operating expense sharply rising as transmission rates go up. Repeatered subsea cables generally lie in the 12 to 25 Tbps window with most spatial division multiplexing deployments pushing 12 to 20 Tbps whereas the traditional 6 to 8 pair coherent optics deployments transmit at least 20 Tbps or higher per strand. 

The key factor determining the optical transmission rate is attenuation, which refers to the fact that a photon or wavelength's intensity or energy diminishes as it travels through fibre optic glass or any other medium. Light is scattered, reflected backwards or absorbed. Other variables that affect transmission rates include the number of distinct wavelength bands (dense wave division multiplexing) that can serve as distinct optical channels in a given spectrum range (usually the C band). The more channels, the higher the transmission rate. Chromatic dispersion (different colors or frequencies travel at different rates except in a vacuum) is DWDM's key foe as it limits the ability to distinguish one color from another. The coherent optics revolution uses digital signal processing to infer the original optical signal from the the scrambled signal that the optical receiver detects. Think of it as nano second speed reconstruction of the original laser output. 

The Japanese National Institute of Information and Communications Technology (NICT) has been exploring ways of boosting optical transmission rates. An international team funded by the agency announced in spring of 2024 that they had achieved 400 petabits per second by combining a variety of new technologies. First step was using virtually all the infrared laser frequencies including the standard O and C bands plus the L, E, S, and U bands. The L band has only slightly higher attenuation than the C band, but the attenuation is much higher for the others. So the researchers developed new optical amplifiers that incorporated bismuth in addition to the standard erbium doping. The O and E bands in standard optical fibre are subject to very high levels of Rayleigh scattering. O and E frequency light is more scattered by minute particles in fibre optic glass than are the workhorse O and C bands. Bismuth more strongly amplifies light in the O and E bands than does erbium. It compensates better for the higher attenuation. 

For more on the promising features of erbium and bismuth doped fibre, see the Nature article.