Most people know that light travels in a straight line unless something acts upon it to change its direction. The three most common methods of changing the direction of light are refraction, diffraction, and reflection. Reflection happens when light bounces off a material, as when light projects onto a movie screen. In diffraction, individual frequencies of light break apart as the light passes the edge of an object. Short wavelengths can’t bend as far around the edge, while longer wavelengths travel further. Refraction refers to light bending when it passes from one medium to another having a different density. This gives rise to the “broken-pencil” effect in a glass of water as light passes from air, to water, and back.
A fiber-optic cable uses refraction to keep light in the center of the central core fiber. A denser cladding glass surrounds the core fiber to refract light back into the core. However, the core fiber also has areas of changing density typically caused by manufacturing defects. When light hits the different density pockets within the core it scatters in multiple directions through a process called Rayleigh backscatter. This process can turn the fiber-optic core into a sensitive vibration monitor.
An acoustic coating placed on the optical fiber concentrates any vibrations into the core. The pressure of the vibration squeezes the fiber-optic core, changing its density at that point. Sensitive detectors monitor the change in the Rayleigh backscatter created by the varying core density and turn it into a representation of the detected vibration. Time-domain reflectometry can be used to determine the exact point along the fiber-optic cable where the vibration took place.
Placing the acoustic coating along different points of the cable creates a number of listening posts on a single fiber strand. It’s then possible to geolocate the sound using triangulation techniques, and send cameras or investigators directly to the source of the vibration. Meanwhile, sophisticated computers can analyze the sound signature for such information as what caused the vibration.
Edited by Robert Repas