The basic principle has been invented over 30 years ago in 1987: https://ieeexplore.ieee.org/document/4258955 https://ieeexplore.ieee.org/document/4258955 They

The basic principle has been invented over 30 years ago in 1987: https://ieeexplore.ieee.org/document/4258955 https://ieeexplore.ieee.org/document/4258955 They're used in long haul communication for some 15 years or so. They're especially useful in submarine cables, for the sole reason, that new optical transmission schemes(modulation, wavelength multiplexing and so on) can be implemented by just replacing the equipment on either end, without the need to replace components down there on the seabed.