Erbium-doped fiber amplifiers work by utilizing a nonlinear optical effect where energy is transferred from a pump laser to the signal. This is in principle possible in any optical (glass) fiber, but by doping with exotic elements, the amplification characteristics can be optimized. Erbium is suitable for the conventional communication wavelengths.

For reference I have a PhD in information theory and signal processing for fiber channels.

The crush to the cable could be a number of things but without knowing the terrain, and knowing these cables just lie on the sea floor, it could be caused by the cable sitting on some jagged rock and has been pulled tight elsewhere (perhaps by fisherman dredging the seabed) resulting in the cable being forced onto the jagged rock and it being crushed onto the rock.

Likewise, but unlikely, some heavy object from above has some how landed on the cable, perhaps even a submarine of sorts resting on the seabed.

Again knowledge of the terrain of the sea floor where the cable crush took place is key into gaining some idea of what might have happened, but I think its the first scenario, a fisherman dredging the sea floor elsewhere has caught and pulled the cable tight and the cable crush is the damage from it resting on rocks where its snagged and crushed itself from the tautness.

Rock climbers and abseilers using ropes will see this with their ropes.

However I think there are also fully passive repeaters- https://en.wikipedia.org/wiki/Optical_amplifier