Optical Subsea Amplification & The DWDM Revolution

As noted in my previous post, optical amplification allow light signals to be boosted without being first decoded into a digital representation. Optical-electrical-optical conversions go away. Hence the computer necessary for OEO conversions disappears. This in turn sharply improves the amplifier's reliability and life span. It also eliminates the conversion delay so end-to-end latency is improved. Finally, no computer means less cost.

But these benefits are really secondary. More importantly, the advent of optical amplifiers led to a quantum leap in bandwidth that can be attributed to two related developments. The first is that optical amplifiers impose no transmission limits on computer technology. This means that we can lay a cable in the water and then upgrade it at regular intervals as Moore's law improves the ability of computers to process optical signals. Nothing on the wet side changes. Indeed, the introduction of digital processing allowed 10G wave subsea cables like the Hibernia system to be upgraded to 100G backbones with total capacity doubling or tripling. Digital signal processing allows the computer chips in the DWDM hardware to reconstruct the original signal which becomes distorted due to chromatic dispersion and other effects. They also handle the OEO conversions, encryption, and a host of other functions.

The second great impact is that optical amplifiers can boost a range of colors or frequencies. The pump lasers which raise the erbium ions in the transmission glass to higher energy levels themselves generate light at the 980 nanometer wavelength. Their 980 nm photons strike erbium ions, push electrons into higher orbits, and when these electrons are struck by light at longer wavelengths,the ion absorb them and then emit a clone at the same frequency, phase, and in the same direction. So this meant that multiple C band frequencies, which is the preferred spectrum bracket for long haul applications because it has the lowest attenuation, could be used. As soon as engineers realized that optical amplification worked, research turned to developing dense wave division multiplexing.

The diagram below shows the basic idea behind an erbium doped optical amplifier. The pumped laser light is combined with the transmission signal via an optical coupler and then flows through a ten to twenty meter loop of erbium doped fiber. Erbium ions absorb the pumped laser light and when struck by a transmission signal photo, absorb it and emit a clone in terms of wavelength, phase, and direction. The only difference between the original photon and the clone is the latter's higher energy level, which takes the former of greater wavelength amplitude.