One technique uses the Raman effect: A "pump" laser at each end of a fiber segment injects light that excites some of the fiber's atoms to higher energy. The atoms use some of that energy for vibration and then emit the rest as photons of a longer wavelength before being excited again. Ordinarily, relatively few atoms participate, but if additional light of the longer wavelength appears at the same time as the pump light--say, from a signal carrying a phone conversation--the Raman process can become much more efficient and produce extra photons to amplify the signal.
But Raman amplification is not uniform throughout the fiber. To prevent the signal from getting too weak in the center of a segment, no segment can be more than some tens of meters long. So a long-distance cable requires a lot of pump lasers.
Juan Diego Ania-Castañón and his colleagues at Aston University in England came up with a simple Raman-based scheme that almost entirely eliminates variations in the signal power along the fiber's length. With such small variations, they could use an extremely long fiber for a single segment--75 kilometers. At each end of their fiber the team placed a mirror that reflected only light of a single wavelength, 1455 nanometers, which was longer than the pump wavelength but shorter than the signal wavelength. The pump laser light passed right through the mirrors, entered the fiber, and generated many Raman photons with a wavelength of 1455 nanometers. These photons bounced back and forth between the mirrors in a giant laser cavity and stimulated more Raman emission at the same wavelength. This 1455-nanometer laser light then acted as a source of pump light spread throughout the fiber, and it Raman-amplified the 1550-nanometer signal uniformly--the second step of a two-stage Raman amplification process.
....which sends messages through noodle soup.
Link found at geekpress.