Recently, a prediction was made that photonics will revolutionize digital computing in 2015 — in particular, technology used in data centers. According to current news, that prediction needs expanding to include photonic technology connecting disparate data centers, even those on different continents.
A research team from University College London (UCL) working under the auspices of the Engineering and Physical Sciences Research Council (EPSRC)-funded program grant UNLOC developed game-changing technology pertaining to long-distance fiber-optic runs. You can read about what the researchers did in their paper Spectrally Shaped DP-16QAM Super-Channel Transmission with Multi-Channel Digital Back-Propagation. If that title means nothing to you, do not fear — Dr. Robert Maher, senior research associate in the Optical Networks Group of UCL’s Electronic and Electrical Engineering Department and one of the paper’s authors, kindly explained the team’s research.
The problem solved
The paper begins by stating over 95% of all digital traffic is transported over optical fiber. And with the insatiable appetite for anything digital, long-distance fiber-optic cables that use a modulation format called Quadrature Phase Shift Keying (QPSK) are becoming a bottleneck.
Interestingly, other modulation formats such as 16 Quadrature Amplitude Modulation (16 QAM) could double the amount of traffic traveling through the same fiber-optic cable. However, there’s a problem with 16 QAM and long fiber-optic runs. “The problem we experience in optically-amplified links such as transoceanic crossings is that the signal power is periodically
increased after every 50 to 100 km,” mentions Maher. “Just after amplification, all the channels interact with each other, causing distortion. This limits the amount of information we can send over an optical fiber. The 16 QAM format is more sensitive to this distortion than QPSK, therefore it is difficult to send data as far using this format.”
According to Maher, the first step was to understand how optical channels interact when traveling over a fiber-optic cable. “Once we figured out the interaction, we could then devise a processing technique that would nullify the distortion,” explains Maher. “That processing technique is 16 QAM super channel.”
16 QAM super channel
Maher then describes how the 16 QAM super channel works. “We first generate a group of individual light channels at different frequencies, which can be coded using amplitude, phase, and frequency: to create a high-capacity optical signal to transport data,” writes Maher.
As expected, the light signals interact with each other and distort, resulting in wrong data being received.
Rather than eliminate the cause of the interference, the research team developed a way to undo the distortion. The first step was to build a high-speed super-receiver that could capture several light frequencies simultaneously. Then the light frequencies were converted into electrical signals.
The next step was getting rid of the distortion. To accomplish that, the researchers got creative. “Once the optical signals are converted into electrical signals, the signals repeat the trip, albeit virtually on a PC,” explains Maher. “The idea is to replicate the inverse of the distortion that the optical signals experienced when they physically travelled over the fibre. This is done using advanced digital signal processing that emulates the fibre cable numerically. The captured channels are then sent on the journey again, but this time in the digital domain.”
This undoes the distortion caused by the optical amplification along the fiber-optic cable, and removing the distortion allows the use of more optical power, which increases the travel distance.
Current QSPK-modulated optical signals can be transmitted over transoceanic distances. However, to keep pace with the current growth in internet traffic, we need to use formats capable of transporting increased amounts of data.
One such format is 16 QAM, and the research team demonstrated that a 16 QAM super-channel could travel over 3,190 kilometers (1,982 miles) without their mitigation scheme — a distance that was almost doubled to 5,890 kilometers (3,660 miles) by undoing the signal distortion. What may be even more impressive is the amount of traffic transmitted through existing QSPK-formatted fiber optic runs could double using the 16 QAM super channel, which is important when one thinks about the cost associated with laying new transoceanic cables.
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