Within this video, the Engineering Guy Bill Hammack, explains how signals are sent over fiber optical cables. Bill uses a bucket of propylene glycol to show how a fiber optic cable works and to show how engineers used them to send signals across oceans. Again a nice tutorial which explains how most of the signals, and therefore most of the internet traffic travels across the ocean.
The essence of how they send information through a fiber optical cable is very simple. I could have a pre-arranged code with someone at the end – perhaps we’ll use Morse code – and I just block the laser so that the person at that end sees flashes that communicate a message. To transmit an analog signal like voice from a phone call along the cable engineers use pulse code modulation.
We take an analog signal and cut it up into sections and then approximate the wave’s loudness or amplitude as best we can. We want to make this a digital signal, which means discrete values of loudness and not just any value. [Bill Hammack]