I don't believe fibre cables are single length. Between your point of signal origin and the destination are already a number of boxes that act as repeaters. To split the cable, you'd effectively add in another one (or subvert the provider, of course). Or I believe you can tap the cable anyway:

This is a nice primer on fibre:

http://www.redbooks.ibm.com/redbooks/pdfs/sg245230.pdf

And here is one talking about its security, in particular vs tapping (the conclusion: it can be done):

http://www.sans.org/reading_room/whitepapers/physcial/fiber-...

Finally, I always figured that tapping fibre was exactly what this was for:

https://en.wikipedia.org/wiki/USS_Jimmy_Carter_(SSN-23)