Fiber Optic Pressure Sensor for Underground Water Level Monitoring

Pressure sensors find many uses in fields ranging from industry to environmental monitoring. In particular, they can play a key role in the monitoring of dikes and embankments, where measurement of the underground water level is a good indicator of the structure stability. This specific application, however, requires high sensitivity and a relatively high longitudinal density of sensing points along the dike. While a distributed pressure sensor would be most befitted for this last requirement, high sensitivity distributed pressure sensors are not yet available nowadays, therefore the viable solution is a dense concatenation of point sensors. Different fiber- based approaches to point measurement of pressure have been presented in the literature. The simplest one is based on transducing pressure into optical losses, but usually this yields low sensitivity. Very high sensitivity can be achieved with interferometric pressure sensors, such as those based on Fabry-Perot cavities; yet both this and the previous approach are difficult to multiplex. From this point of view, fiber Bragg grating (FBG) offer the most effective way to implement a quasi-distributed sensing system; nevertheless, pressure sensitivity of bare FBG is as low as -3 pm/MPa.