Fibre Optic Fence Sensor

Fibre optics is the class of optical technology that uses strands of optically pure glass as thin as a human hair to carry digital information over long distances. A light source, such as a light- emitting diode (LED) or laser diode, is coupled to one end of the fibre, while a receiver (such as a photo transistor or similar device) is coupled to the other end of the fibre. The two major categories of fibre-optic sensors are continuity sensors and micro bending sensors. Major advantages of fibre-optic cable include immunity to radio and electromagnetic interference and durability in conditions of changing temperature and humidity.

Since the fibre does not have to be straight to reflect the light from one end to the other, it can be installed in various configurations on a fence to sense a disturbance. The light diffraction (speckle) pattern and the light intensity at the end of the fibre are a function of the shape of the fibre over its entire length. Motion, vibration, or pressure of the fibre induces modal differences causing a phase shift in the light. Sophisticated processors can detect these phase shifts or minute changes in the light patterns being sent down the cable. These changes can be characterised by the processor as those being produced by someone cutting or climbing the fence, and when the right criteria are met (as set in the parameters of the processor), an alarm will be generated.

Some processors have the capability of allowing long lengths (up to tens of kilometres) of non-sensing cable (single-mode fibre) to be placed between it and a length of sensing cable (multi-mode fibre). This allows the processor(s) to be located in a central location a long distance from the fence being protected.