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Comment by gethly | Hacker News Reader

I am surprised there are repeaters involved. Is this because of the imperfections of the surface of the fibreglass tubes that cause decay of precision of the reflection over long distances(a visual noise)?

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Even the best optical fiber transceivers and glass are limited (practically) to about 100km; repeaters are typically placed every 60-70km. The technology for delivering power to the repeaters is fascinating. They inject 5,000-10,000VDC at one end and each repeater shunts off a tiny amount of current to power the amplifier. All of this is embedded in the cable itself before being loaded onto the cable ship.

The history behind TAT-1, the first transatlantic telephone cable, and the repeaters used, is fascinating. Bell Labs designed the repeaters. The repeaters used vacuum tubes for amplification and were designed for extreme reliability. The flexible repeaters were integrated into the cable like modern cables.

The tubes were tested to an extremely high standard. Only a small fraction of the manufactured tubes were selected after testing: Bell Labs designed a test regime over 18 years to detect minute flaws in manufactured tubes

The cable and its 306 tubes operated for 22 years with no failures.

Note that this is in stark contrast to the first transatlantic tele_graph_ cable, which did not really have a ground line and consisted of seven copper wires covered with three coats of gutta-percha (natural latex rubber) and then hemp and tar. Many breaks and failures later, the first messages were sent in August 1858. The bandwidth was such that Queen Victoria's message to the US president, James Buchanan, that contained 98 words took 16 hours to send. It ultimately died during a famous dispute between William Thomson – later Lord Kelvin – yes, _that_ Kelvin – and the project's main engineer that ultimately ended in disaster (when the engineer put 2k VDC on the cable, destroying the insulation, against Thomson's advice) and a famous court case that basically saw the role of "the scientist" (the physicist!) as a competition professional for the first time.

It's all fascinating history. By the time of Bell Labs, an awful _lot_ had already been learned from previous failures.

Yeah, trying to build a thousands of km long undersea cable without a good theory of transmission lines is gonna be a painful experience (a lot of this theory was developed to fix these problems!)

To add more details to other replies you received, the primary factors are Rayleigh scattering and impurities absorbing light energy, at 1550nm (where this loss is least pronounced) the number that usually gets thrown around is 0.2dB/km in attenuation. That adds up to needing those repeaters at the intervals we have them.

I think it's mostly that the fiber isn't a perfectly transparent medium, over tens of kilometers attenuation adds up. As said in https://news.ycombinator.com/item?id=45159639 these are just to boost power, they don't reform the signal.

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