[rdl]

There are repeaters on the cable every ~50-200km (depending on technology), powered by DC power carried on the cable. There are branching units powered the same way -- I believe they handle branching at the fiber level, rather than waves, but not sure of the specifics on modern DWDM cables. (they can run ~80-160 different frequencies of light on the same fiber, and the cables contain multiple fibers...)

[aDfbrtVt]

Quick correction - not repeaters but amplifiers. The term repeater is commonly used to refer to a device that demodulates and remodulates a signal.