>"3) Not a dumb question; while loss in fibre optic cable is very low, repeaters are installed at regular intervals along the cable. they are powered by high-voltage DC supplied through the cable armor. The ocean acts as the return path."
That's incorrect, amplification is done via Erbium doping, it is part of the core of the cable. There are no external repeaters.
So to clear this up a bit.
EDFAs (which are the in-line amplifiers which use erbium doping) work by having a section of a fiber doped with erbium ions. Only the part which is going to amplify the signal is doped, the rest of the fiber is supposed to be as clean as it gets.
Those erbium ions get shot with a laser (that's the part that needs power from the shore) and get excited. The source signal then hits the ions, which then emit more photons of the same exact properties, but just way more of them - therefore the amplification part.
Continue 80km down the line, and the same thing has to happen again, as enough light has been attenuated.
I mostly work with land systems, where the EDFAs are pieces of equipment completely separated from the fiber - you get a normal strand of fiber dug into the ground, and once you want to amplify it, you've got to plug it into an EDFA line card with the erbium-doped fiber already contained in it.
In the sea, those systems are way more integrated and the fiber is usually spliced right into the amplifier.
I might be able to dig up a card tomorrow if you're curious as to how the system looks.
That's incorrect, amplification is done via Erbium doping, it is part of the core of the cable. There are no external repeaters.