Re: Only part of the problem
Oh, but it is far.
They will remodulate every 40 km, usually a lot less. Even the sea cables remodulate at less than 40 km (I remember each 10 km, but I'm not sure).
Check that table on German wikipedia https://de.wikipedia.org/wiki/Lichtwellenleiter#Kategorien_f%C3%BCr_Multimodefasern
The English wikipedia article is, sadly, not as detailed as the German, compare if you want https://en.wikipedia.org/wiki/Optical_fiber .