Design Considerations for a Dfb Fibre Laser Based High Sensitivity Broadband Hydrophone

Over the last three decades extensive research has been carried out towards the design and development of fibre optic hydrophones. The inherent advantages of fibre optic sensing technology, like high sensitivity, immunity to electromagnetic interference, intrinsic safety to water leakage, remote measurement capability etc. makes it an ideal choice for underwater acoustic sensing[1, 2]. The initial designs focused on strain sensitivity enhancements through coatings and coiling long fibres on compliant mandrels [3-5]. With the advent of fibre brag gratings and fibre lasers with very high sensitivity to strain, the focus has shifted towards development of fibre laser based hydrophones for underwater water acoustic sensing[6]. In this paper, we present the major design considerations in the development of mechanical encapsulation for distributed feedback fibre laser (DFB-FL) based high sensitivity broadband hydrophone. DFB-FL is a laser cavity created on a rareearth element doped fibre by introducing a π/2 shift in the Bragg grating written on the fibre[7]. When pumped with external laser source, DFB-FL generate very narrow band laser at a wavelength that depends on parameters like the grating pitch, refractive index of the fibre and emission bandwidth of the dopant. DFB-FL hydrophone works on the principle that any changes in the above parameters caused by the pressure changes due to an acoustic wave will result in corresponding change in wavelength of the DFB-FL. Thus the acoustic signals can be measured by monitoring the changes in the wavelength of the laser generated by the fibre laser.