Cryptonomicon

by Neal Stephenson

So the bad news is that the capacity of modern undersea cables like FLAG isn’t very impressive by Internet standards, but the slightly better news is that such cables are much better than what we have now. Here’s how they work: Signals are transmitted down the fiber as modulated laser light with a wavelength of 1,558 nanometers (nm), which is in the infrared range. These signals begin to fade after they have traveled a certain distance, so it’s necessary to build amplifiers into the cable every so often. In the case of FLAG, the spacing of these amplifiers ranges from 45 to 85 kilometers. They work on a strikingly simple and elegant principle. Each amplifier contains an approximately 10-meter-long piece of special fiber that has been doped with erbium ions, making it capable of functioning as a laser medium. A separate semiconductor laser built into the amplifier generates powerful light at 1,480 nm — close to the same frequency as the signal beam, but not close enough to interfere with it. This light, directed into the doped fiber, pumps the electrons orbiting around those erbium ions up to a higher energy level. The signal coming down the FLAG cable passes through the doped fiber and causes it to lase, i.e., the excited electrons drop back down to a lower energy level, emitting light that is coherent with the incoming signal — which is to say that it is an exact copy of the incoming signal, except more powerful. The amplifiers need power — up to 10,000 volts DC, at 0.9 a...