More On Subsea Cable Transmission From SubOptic 2025: Optical Amplifiers

The great fibre optic revolution in subsea cables has two components: the fibre optic strands and the optical amplifiers. The first fibre optic subsea cable was the three fibre pair TAT-8, which connected the US to both the UK and France using a simple branching unit. But the amplifiers used computers. Hence there was no way of upgrading them to accommodate faster transmission rates.

Fortunately, quantum mechanics came to the rescue. The most successful scientific theory of all times posits that adding energy to an atom will cause it to emit photons. This was one of Einstein's contributions. An American graduate student in the 80s discovered that the rare earth element erbium had special properties. If it was incorporated into glass, then a pump laser directed at the glass would raise the energy level of the erbium ions and cause them to release photons. This in itself is not that exciting. We know that any object will issue photons depending on its temperature. But if signal photons strike an excited erbium ion in the C band, the ion will absorb the signal photon and emit a clone of it in frequency and phase, but with a higher amplitude or more optical energy. The entire success of fibre optic subsea cables depends on the ability to regenerate signals using purely optical cloning. Optical amplifiers are technologically agnostic about the subsea line termination equipment. Moreover, the amplifiers can transmit per pair up to 40 terabits per second. So they place no practical limit on the transponders at the end points.