Design of optical fibre based highly sensitive acoustic sensor for underwater applications

Fibre optic sensing is a key technology for a variety of underwater sensing and monitoring applications. Fibre optic acoustic sensors are mainly based on interferometric detection approach where the acoustic pressure-induced phase shift of light has been used as sensing principle. Recently, fibre optic acoustic sensors based on speciality fibres like Photonic Crystal Fibre (PCF) were reported. However, interferometry based detection approaches amongst all these fibre optics sensors are intensity based and therefore susceptible to light power fluctuations and require a complex instrumentation related to signal detection. Besides, wavelength based detection approach using FBG (Fibre Bragg Grating) offers significant advantages over the conventional approach. FBG sensors were reported to have higher performance for underwater acoustic sensing applications. This paper reports a novel design of an underwater acoustic pressure sensor using a combination of PCF and FBG to provide high sensitivity. Theoretical investigations were carried out on the PCF-FBG sensor to study the effect of applied pressure and induced strain on the FBG inscribed in the core of PCF. Effect of light confinement in PCF was studied for different geometrical parameters and 4-ring PCF structure was reported. Further, sensitivity enhancement was proposed utilizing air hole structure of the PCF to enhance the impact of acoustic pressure on the induced strain in FBG.