The achieved speed has almost reached the Shannon limit, which defines the maximum amount of error-free data that can be sent over a communications channel. Perhaps the most impressive thing about the announcement was that UCL scientists achieved this speed over existing fiber optic cables and didn’t use pristine fiber installed in a laboratory.
The fast signal throughput was achieved by combining several techniques. First, the lasers use raman amplification, which involves injecting photons of lower energy into a high-frequency photon stream. This produces predictable photon scattering which can be tailored to the characteristics needed for optimally traveling through glass fiber.
The researchers also used Erbium-doped fiber amplifiers. To those who have forgotten the periodic table, erbium is a commonly found metal in nature with an atomic weight of 68. Erbium has a key characteristic needed for fiber optic amplifiers in that the metal efficiently amplifies light in the wavelengths used by fiber optic lasers.
Finally, the amplifiers used for the fast speeds used semiconductor optical amplifiers (SOA). These are diodes that have been treated with anti-reflection coatings so that the laser light signal can pass through with the least amount of scattering. The net result of all of these techniques is that the scientists were able to reduce the amount of light that is scattered during the transmission though a glass fiber cable, thus maximizing data throughput.
UCL also used a wider range of wavelengths than are normally used in fiber optics. Most fiber optic transmission technologies create empty buffers around each light bandwidth being used (much like we do with radio transmissions). The UCL scientists used all of the spectrum, without separation bands, and used several techniques to minimize interference between bands of light.
This short description of the technology being used is not meant to intimidate a non-technical reader, but rather show the level of complexity in today’s fiber optic technology. It’s a technology that we all take for granted, but which is far more complex than most people realize. Fiber optic technology might be the most lab-driven technology in daily use since the technology came from research labs and scientists have been steadily improving the technology for decades.
We’re not going to see multi-terabit lasers in regular use in our networks anytime soon, and that’s not the purpose of this kind of research. UCL says that the most immediate benefit of their research is that they can use some of these same techniques to improve the efficiency of existing fiber repeaters.
Depending upon the kind of glass being used and the spectrum utilized, current long-haul fiber technology requires having the signals amplified every 25 to 60 miles. That means a lot of amplifiers are needed for long-haul fiber routes between cities. Without amplification, the laser light signals get scattered to the point where they can’t be interpreted at the receiving end of the light transmission. As implied by their name, amplifiers boost the power of light signals, but their more important function is to reorder the light signals into the right format to keep the signal coherent.
Each amplification site adds to the latency in long-haul fiber routes since fibers must be spliced into amplifiers and passed through the amplifier electronics. The amplification process also introduces errors into the data stream, meaning some data has to be sent a second time. Each amplifier site must also be in powered and housed in a cooled hut or building. Reducing the number of amplifier sites would reduce the cost and the power requirement and increase the efficiency of long-haul fiber.