The actual fiber joining is done with this device: [1]<p><i>"This fusion splicer automatically aligns a pair of optical fibers in both the X and Y (horizontal and vertical) planes and then fuses them together with heat from an electric arc to form a low-loss splice. Improved image processing software of this fusion splicer enables precise core alignment and accurately estimated splice loss.
A tensile proof test that is carried out by applying tension to the fusion splice verifies the long-term reliability of the fiber splice. Following a successful proof test, a heat-shrinkable splice protection sleeve is installed over the bare glass fiber and cured in the built-in heater."</i><p>Here's a video of a similar fusion splicer for land-based use.[2] The fussy part of aligning the fibers is automated now.<p>The "universal jointing" thing seems to be more about joining all the outer layers of an undersea cable that give it protection and tensile strength. After the fiber has been spliced, all that outer protection has to be joined, too.
Visualize a heavy hose wrapped around a steel strand bridge cable wrapped around a coax cable with a fiber optic strand at the center. Now connect two of those sturdily enough to spend 25 years on the ocean bottom.<p>[1] <a href="https://ujconsortium.com/information/fusion-splicer-ujs-s200" rel="nofollow">https://ujconsortium.com/information/fusion-splicer-ujs-s200</a><p>[2] <a href="https://youtu.be/JP_C0XLLyR0?t=578" rel="nofollow">https://youtu.be/JP_C0XLLyR0?t=578</a>