EDFA

Erbium Doped Fiber Amplifiers are basically optical fibers with a little Erbium doped in it, along with a little external circuitary, used to amplify signals directly in the optical domain without any conversion into the electrical domain. The concept is quite simple, it consists of using a small loop of optical fiber, doped with Erbium, and pumped by Laser Drivers at about 980nm, coupled with the input signal. The result of this is that the Erbium ions get converted into the 4I13/2 state via the 4I11/2. When the signal photons strike these ions, then stimulated emission occurs, sending these back to the ground state that is 4I15/2, and at the same time emitting photons of the same frequency, thus we get additional photons of the same frequency thus causing gain in intensity. You can find more on this at http://www.rp-photonics.com/erbium_doped_fiber_amplifiers.html
This concept revolutionised the world of optical communication, by providing a means to bypass the electronic circuitary to amplify optical singnals, thereby providing a much more efficient system. This was developed in the late 1980s and by early 1990s it was already in use in most optical communication systems. Probably the next big thing in communication may be another step towards the elimination of electronics, i.e. optical processors or the like, leading to faster systems and making quantum computing a possibility.