How does an optocoupler filter noise?
As an illustrative example, below three remote sensors are receiving pulses from a single source:
In the top scheme there is no optocoupler/optoisolator. In the second scheme there is an optocoupler(colored in green) right before each sensor.
And i actually tested this. I first connected the source pulse train to a 100m coaxial cable and at the end of the cable I hooked it up to an oscilloscope. I didn't perform noise analysis but the first thing I noticed was there random was ringing twice the pulse amplitude seen on the scope.
Then I used an optocoupler an the ringing completely disappeared.
This made me believe that the optoisolator really works good for long distances.
One thing I can see that it separates the ground of the source and the receiver. But that would I guess only take care of the 60/50Hz ground loop noise.
How is an optocoupler/isolator filtering the noise? How can we understand the logic behind?