In this paper we present an in-ground intrusion detection system for security applications. Here, an optical fibre pressure switch is directly connected to a standard digital input of a programmable logic controller (PLC). This is achieved using an intensiometric detection system, where a laser diode and Fibre Bragg Grating (FBG) are optically mismatched, resulting in a static dc offset from the transmitted and reflected optical power signals. Pressure applied to the FBG, as the intruder stepped on it, induced a wavelength shift in the FBG. The wavelength shift was then converted into an intensity change as the wavelength of the FBG matched the wavelength of the laser. The change in intensity, measured by the optical detectors, resulted in a significant change in the DC offset, behaving as on optical switch. When connected to the PLC the optical pressure switch was used to drive a digital output, sounding an alarm in addition to displaying the intrusion event on the human machine interface.
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