LPG-based optical fibre sensor for acoustic wave detection

In this work, a long period grating (LPG)-based optical fibre sensor system has been designed and set up, following which it has been evaluated and optimized for acoustic wave detection. The device relies upon the interaction of the acoustic wave with a LPG placed between two pillars (with one being movable) and exposed to a range of representative acoustic waves generated by a loudspeaker placed at a known distance from the sensor thus created. Through determining the response both to the variation of the magnitude and the frequency of the acoustic signals generated by the loudspeaker, the long period grating-based sensor system created has been successfully characterized, showing clear sensitivities to specific acoustic frequencies, irrespective of the signal intensity variations. In addition, these frequencies are found to be closely related to the configuration of the LPG, i.e. the LPG bending curvature created which can be varied, in this laboratory set-up, by changing the distance between the pillars. The experimental results obtained are in good agreement with those obtained from a modified elastic string theory approach, and thus show potential for the use of LPG-based technique for acoustic wave detection in various media, to create a compact and easy to use sensor system.