What limits the speed at which information is sent/received over a fibre optic line?
The speed at which you turn on and off the light source (laser) and the speed at which you can reliably read it, are typically the major limiters. Fiber optics may allow you to use light as your data medium, and of course at the speed of light, it would seem like an almost limitless way to transmit data. But in reality, the way in which the signal is modulated and encoded are the limiting factors. On each end of a given fiber optic run, equipment has to encode and decode the pulses and frequencies of light. This is also the transition point from optics to electronics.

This is why the fiber optic cable itself is relatively inexpensive, and the communications equipment it connects to can be very costly.