The cables used in FOTAS are fiber optic cables. The optic fibers used in these cables are made of either transparent plastic or glass, so that the light can travel inside the tube. On top of that; there is another layer which is called “the cladding”.
The cladding is also either glass or plastic; but it is made of a different density than the inside cable which is called the core. The aim here is that, the light should bounce off the cable in a zig zagging way to travel inside the core according to Snell’s Law. It has a lower refractive index than the inner core so light does total internal reflection with minimum signal loss. In the case of an anomaly, the laser light has a different backscatter than normal back scatter. Then the signal processor processes the signal and classifies the anomaly. That’s how the threat is detected.