[adrian_b]

There are optical amplifiers that use erbium-doped fibers (EDFA = erbium-doped fiber amplifier), which are based on the LASER principle, i.e. amplification by stimulated emission.

Their energy source is not electrical, but also optical, i.e. light produced by a laser with a different frequency from the frequencies used for data transmission and which is also sent through optical fibers.

In current speech, laser is used mostly to mean optical oscillators, which generate light, but actually the laser is only the optical amplifier and it must be inserted in an optical resonator in order to make a generator of light. In the amplifiers for optical fibers, only the laser part is inserted in the fibers, without an optical resonator (i.e. without mirrors), so it does not generate light, but it amplifies the light passing through it.

The erbium-doped lasers function exactly like any other solid-state lasers, which typically use crystals of sapphire or of YAG or glasses doped with ions of chromium (ruby lasers) or of neodymium or of titanium, but erbium is chosen instead of the more common dopants because it amplifies light in the frequency band used for optical fibers.

So the cables with optical fibers do not need any electrical connection for providing amplification in intermediate points when the distance between the ends of the cable is too great.