Fibre optic acoustic sensing for intrusion detection systems

In this study, acoustic emissions (AEs) were detected through a variety of different flooring materials using a fibre Bragg grating (FBG). The AEs were generated using a low velocity impact test, and by footstep. The acoustic vibrations cause a strain in the optical fibre, and hence alter the wavelength reflected by the FBG. This strain induced wavelength shift can then easily be detected by converting this wavelength shift into an intensity change. This is done using an intensiometric detection system, where a laser is tuned to the 3dB point of the FBG, and the optical power transmitted and reflected is modulated by the spectral shift of the FBG. The intention is to use FBGs as an in-ground intrusion detection system to detect the AEs generated by an intruder walking within range of the sensors. This type of intrusion detection system can be applied to both external (in soil, etc) and internal (within the foundations or flooring of the home) security systems. The results show that the AEs can clearly be detected through wood, ceramic tiles, aluminium and concrete.