Subsea Optical Amplifier Fundamentals - Part 1

TAT-8 was the first Atlantic optical cable. It was RFS in 1988 and signified a bandwidth revolution with its capacity 10x that of its coaxial predecessors. By modern day standards it was a pygmy with 280 megabits total throughput. But it heralded the beginning of a long period of rapid throughput growth.

Optical repeaters were spaced every 50 kilometers or 30 US miles. Photo diodes received the weak incoming signals, converted them into a digital representation of zeros and ones, and then laser diodes generated fresh light signals. A copper conductor provided power. Lots of redundancy in terms of components were built into this amplifiers to avoid repairs.

However, this approach had a severe drawback, namely the throughput could not be increased in a time where Moore's law was rapidly increasing transmission rates. The pace of the optical-electrical-optical conversion was set in stone because an upgrade would require replacing all the hardware on all the amplifiers. TAT-8's capacity was maxed out in less than 18 months, yet no upgrade was possible.

However, an American graduate physics student, Robert Mears, had discovered in 1985 an elegant way of boosting laser light that required no conversion of light signals into a digital representation. Shining laser light on a a rare earth element, erbium, raised the energy level of its electrons. A basic physics law is that natural processes will tend towards a ground state of minimal energy. So if an electron's energy is increased to a certain threshold, then the electron will shed that extra energy. Sometimes this happens spontaneously, other times it requires being perturbed by an outside force.

So the basic idea was to create a ten to 20 meter loop of fibre optic cable inside the repeater that has erbium dispersed throughout it. 'Doped' is the terminology. Then a pump laser at 980 nanometers would raise the erbium's energy levels. If the communication light signal hit an erbium atom in the fibre optic strand, it would be absorbed and simultaneously the atom would emit an photon clone identical in wavelength, phase, and direction. This clone would have more energy than the weak incoming signal from the previous repeater. In other words, its amplitude would be jacked up. Otherwise, it is a perfect copy. So no bit errors would result. In fibre optics information is encoded in the dimensions defining the light signal. So information would be preserved as long as the amplifier lasers worked properly.

The beauty of this approach cannot be exaggerated. It means optical amplifiers were essentially agnostic to computer technology upgrades. There would be no computer processing of the light signal and hence no optical-electrical-optical conversions. More's law could be unimpeded so that cable's throughput over time could go ups due to computer technology upgrades of the terminal equipment on land.