Well, cost for one. If that fiber isn't being used it's a terrible investment. But more generally it's probably not needed. The bandwidth of optical fibre is theoretically unlimited. A single fibre carries many wavelengths, a wave length is like a channel. This is known as DWDM(dense wave division multiplexing.) A single wavelength commonly caries 100 Gbs. Although recently advances have shown 700 Gbs for a single wavelength[1]. The number of wavelengths and hence bandwidth depends on the transmission gear used on both ends of the cable. You can upgrade the bandwidth by upgrading the transmission gear. The state of the art for years has been 8 pair systems but more recently developments have produced 24 and 36 pair systems.

[1] https://www.lightwaveonline.com/network-design/high-speed-ne...

IIRC, it's mostly to do with power... every Xkm (100?) they need to put a power amp and have to splice 100 pairs at each point...

Presumably because the cable would be (much) heavier and thicker.

There is some movement towards having more pairs in newer cables, e.g. the Grace Hopper will have 16 pairs, cf. https://cloud.google.com/blog/products/infrastructure/announ...

Look up how DWDM works; they don't need more than 8. The optical gear that goes on land, keeps getting more capable of driving ever more bandwidth over the same fiber...

They didn't need 500+ terabits?

The light doesn't really work that way.

u/Denvercoder9 mentioned that there are 8 strands of fiber in this particular cable. Each strand can host multiple wavelengths of light, that do not really interfere with each other. There are different ways of dividing up the usable frequency range; here's a site with some examples: http://www.3coptics.com/News/11.html (Remember, light has a frequency & a wavelength. Human vision covers wavelengths from around 400 nm (nanometers, the UV end) to 700 nm (the IR end)).

The beams of laser light sent down a fiber strand have a wavelength (a central wavelength), and a width. The beam essentially needs exclusive ownership of wavelength ± width. The diagrams on the linked page show how channels are separated to give each channel a set width.

So, when an entity owns a part of a cable, they own a set of wavelengths, likely one or more contiguous blocks of wavelengths, that they can use for whatever. All owners will pay (probably based on the % of wavelengths they control) for a company to operate the cable (running the stations at each end, where the wavelengths are split out on to separate fibers for each owner). The owners will also pay for another company to do repairs as needed.

There's no way to own a physical "slice" of fiber optic cable. The wavelengths all travel together, and there are optical splitters/combiners at each end that separate/combine the wavelengths of light. Think of them like TV and radio broadcasts. Each station broadcasts on it's own frequency (wavelength) over the shared air (fiber).

A metaphor with light would be if you had two lights, one red and one blue. If you shine them both at the same spot, you'll see purple. If you put a prism in that spot, it'll split the red and blue light back out separately. At a very high level, that's what happens with fiber optics.