Fiber-optic technology is a fascinating way to move digital bits around. Complicated I dare say as well. Not so, however, for the researchers who continue to make amazing advances.
For example, in a February 2015 TechRepublic article, I wrote about how researchers at University College London (UCL) overcame challenges presented by 16 Quadrature Amplitude Modulation (16 QAM), doubling throughput on long-distance fiber-optic runs, and doubling the distance (5,890 kilometers [3,660 miles]) an optical signal can travel before it needs regenerating.
Just five months later, the distance record of the UCL research team was trounced by a team of electrical engineers from the University of California, San Diego (UC San Diego). In their paper Overcoming Kerr-induced capacity limit in optical fiber transmission, the researchers described how they bested a different set of challenges to once again double the distance (12,000 kilometers [7,456 miles]) traveled before optical-signal regeneration is required.
The optical backstory
As light travels in a fiber-optic cable, two things affect it: attenuation and distortion. The trick is to know when to amplify (reverse attenuation) the signal and when to regenerate (remove distortion) from the signal (Figure A). It is also important to note, as the diagram shows, that amplification happens more frequently than regeneration, which is a good thing because regeneration is expensive. Every wavelength traveling in the cable needs to be processed.
More power, Scottie
In the original Star Trek, Captain Kirk always wanted more power. Scottie would inevitably give reasons why more power was a bad idea — something about distorting the warp field. Still, Scottie managed to give Kirk the power he wanted.
The researchers at UC San Diego were also looking to increase power. The more power you applied to electromagnetic signals (light), the farther they traveled before needing amplification. There is a catch. "Today's fiber-optic systems are a little like quicksand," said team member and research principal Nikola Alic in an interview with Scientific Computing. "With quicksand, the more you struggle, the faster you sink. With fiber optics, after a certain point, the more power you add to the signal, the more distortion you get, in effect preventing a longer reach."
Alic is referring to the Kerr Effect.
The solution: Frequency combs
Team member Stojan Radic, a professor in the Department of Electrical and Computer Engineering at UC San Diego, explains to Scientific Computing how the team made use of the distortion, or what he calls crosstalk.
"Crosstalk between communication channels within a fiber-optic cable obeys fixed physical laws. It's not random. We now have a better understanding of the physics of the crosstalk. In this study, we present a method for leveraging the crosstalk to remove the power barrier for optical fiber. Our approach conditions the information before it is even sent, so the receiver is free of crosstalk caused by the Kerr effect."
The team's research paper mentioned the engineers distorted the information being transmitted in a predictable manner. At the receiving end, using optical tools called frequency combs, the distortion was reversed and the digital information retrieved. Team member, Bill Kuo, responsible for developing the frequency comb, said the process preempted the distortion effect that occurs as optical signals travel along the fiber-optic cable.
Success! Even after upping the power 20 fold, the research team retrieved information from optical signals that traveled 12,000 kilometers. Standard amplification was required, but regeneration was not needed.
What this means
Bridging 12,000 kilometers without regeneration has several advantages: significantly reduced costs, the ability to carry more data, and reduced latency. Less latency results from not having to regenerate the optical signal unless the run is over 12,000 kilometers, an important consideration when routing transoceanic fiber-optic cables.
The research team included Nikola Alic, Vahid Ataie, Bill Kuo, Lan Liu, Evgeny Myslivets, Stojan Radic, and Eduardo Temprana, all members of the UC San Diego Photonics Systems Group. The University of California has filed for a patent on the team's research.
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