Question:Why do optical fibers not suffer from interference on the received signa?
Interference is random fluctuations added to your signal by external sources or removed from you signal by random losses (attenuations).
With optical fibers it is only possible to add light to the fiber at the ends, so external light or infrared radiation does not alter the signal.
This is because any light entering the fiber from the sides goes straight through.
Because the signals are always digital there is the added benefit that any small amounts of interference are harmless to the quality of the signal.