Sensitivity enhancement analysis due to different coating materials of Fibre Bragg Grating-based depth sensor for underwater applications

Ocean depth is a critical parameter for several maritime applications, ranging from surveillance to navigation. One method commonly employed is to measure the pressure at an unknown depth, and compare it with that at the surface. Fibre-optic sensors have made significant advances in this direction, conferring exceptional sensitivity, resolution and compactness. In this work, we present a simulation of an ocean depth sensor based on a uniform Fibre Bragg Grating. We study the effect of introducing a uniform layer of coating on the sensitivity. A range of different materials, including metals and polymers, are studied and compared for their sensitivity performance and general feasibility. Our results show that a coating of Polytetrafluoroethylene (PTFE) provides the highest sensitivity, in terms of wavelength shift per unit change in depth. This sensitivity can be further augmented by introducing modulations in the grating, such as chirp or apodization. We find that apodized PTFE-coated gratings are most suitable for our application, and report a peak sensitivity of 24.54 pm/m. The choice of an optimum coating is critical not only for the sensitivity, but also for parameters such as operating lifetime, robustness and linearity. Our results are anticipated to pave the way to more sensitive, flexible and precise optical sensors for underwater depth gauges.