Pretty wild for a single fiber, but super involved, using 5 different doping type amps. Current systems use just EDFAs (Erbium). These guys used Erbium, Thallium, Bismuth, and others.

The latest one is also just more lambdas. Like that’s impressive but DWDM itself took a decade+ after gear was introduced.

Most of the time it’s easier to just add another few dozen fibers when laying cables.