[cycomanic]

Generally the throughput for a single mode fibre in the C + L Bands (the wavelength regions used for telecom applications), is about 100 Tbit/s for a one span link (50-100km) for a submarine cable across transatlantic distances IIRC the record is around 50-70 Tbit/s. This is research demonstrations, so the 340 Tbit/s would be for a cable with plenty redundancy. Also note that fibres are used in one direction only (one of the main reasons is that one would otherwise create a very long laser), so for duplex operation you need to double the amount of fibres.

[xenadu02]

It seems like the expense would be in the armored outer cable, repeaters, and labor for laying the cable but perhaps at those distances the glass cost matters? Still it seems like you'd want to cram as many fibers into the cable as possible. There must be some limiting factor that prevents you from putting 1000 strands or 10,000 strands in a single cable.

[everfrustrated]

Adding more fibres also means more power needed for more repeaters.