Q:

through + relative clause

fibre optics:the use of very thin glass or plastic threads through which light can travel to carry information. 

I am unsure about where through should go to in the relative clause.
Which of the following is correct?  
A) Light can travel through glass or plastic threads to carry information.
B) Light can travel to carry information through glass or plastic threads.

A:

Fibre optics: the use of very thin glass or plastic threads [through
which light can travel to carry information].

First, there's nothing at all wrong with this sentence; it's impeccable.
In the bracketed relative clause, "which" is complement of the preposition "through" and has the nominal "very thin glass or plastic threads" as antecedent. 
Your two alternants are 
A) the use of very thin glass or plastic threads through which light can travel _____ to carry information. 
B) the use of very thin glass or plastic threads through which light can travel to carry information _____
In both cases the gap notation '____' represents the PP "through very thin glass or plastic threads', the only difference being the linear position of gap.
I can't see any motivation for B) which postposes the PP "through very thin glass or plastic beads" over the infinitival clause adjunct "to carry information". Semantically the PP belongs more naturally with "travel".

