Wave Division Multiplexing being substantially enabled by all-optical amplifiers in the late 1990's it seems, because otherwise you have to convert optical to electrical signal, amplifiy that, and reconvert that back into an optical signal in order to travel down the next length of fiber-optic.

With optical amplifiers you can amplifiy every color of the rainbow at once, without breaking the chain.

https://en.m.wikipedia.org/wiki/Optical_amplifier