A Technical Update on Submarine Cables [pdf]
3 comments
·September 3, 2025joemaniaci
I was curious to learn more about what the repeater systems look like.
cycomanic
Optical repeaters are 1R repeaters, I.e. they regenerate power. Inside the repeater "boxes" (they are actually cylinders) there is an optical amplifier. For typically these are Erbium doped fiber amplifiers (EDFA). I other words a piece of fibre doped with Erbium (a rare earth). The amplifiers are pumped with laser diodes (typically 1-4 per EDFA) at 980 nm and 1480 nm wavelength. By pumping the doped fibre with these wavelength you provide high gain to the telecom channels which are usually in the optical C-band (~1525-1565nm). This way you can reamplify signals over a large bandwidth (~4 THz) without having to do detection and retransmission (which would be unscalable). Repeaters are typically spaced at 60-80 km in submarine, with a "transparent" design (the gain compensates for the transmission loss of the 60km fibre).
Power delivery to the laser diodes is done through the metal jacket of the cable. The whole submarine cable is essentially a very long DC transmission line. Which is a fascinating topic in itself, E.g. What is ground in such a line, it will differ by 1000s of Volts between continents.
I am surprised there are repeaters involved. Is this because of the imperfections of the surface of the fibreglass tubes that cause decay of precision of the reflection over long distances(a visual noise)?