Underwater pressure measurement using fibre optic extrinsic Fabry-Perot interferometric (EFPI) sensors

A fibre optic extrinsic Fabry Perot Interferometer (EFPI) sensor is developed for monitoring pressure in the underwater and sub-seabed under simulated conditions. The sensor is robust in design and is fabricated entirely from Silica glass. The EFPI is formed at the tip of the fibre, where the single mode is spliced to a 200ìm capillary, sealed by a 200μ Multimode, which forms the diaphragm. The diaphragm thickness is reduced by polishing and etching with hydrofluoric (HF) acid to about 2-3μ for a high sensitivity. The thickness of the diaphragm is monitored online during polishing and HF etching. The spectrum of the fibre optic sensor (FOS) is interrogated using a broad band optical light source and an optical spectrometer. The sensitivity of the sensor achieved is 0.6cmH₂O, excellent for small depth-changes. Experimental measurements with saturated salt water and chlorophyll pigmentation of different standards were tested, to simulate the sub-sea conditions where a stability of 0.7cmH₂O was reached with a drift of less than 10% under the simulated conditions.