Pressure-independent Brillouin Fiber Optic Sensors for temperature measurements

• Brillouin frequency shift (BFS) optical fiber sensors are dependent both on temperature and strains.
• The negative Brillouin frequency shift under pressure of a silica fiber is a signature of the elastic silica anomaly.
• Elastic anomaly decreases in Ge02 glass and vanishes for sodium alumino-silicate glass compositions.
• A sodium aluminosilicate glass composition with a pressure-independent Brillouin frequency shift is defined.
• In situ high pressure diamond anvil cell experiments allow us to predict the sensitivity of BFS pressure sensors.
• A pressure independent Brillouin temperature sensor for submarine environment can be anticipated.

Fiber Optic Sensors (FOSs) based on Brillouin scattering are widely used in large infrastructures to detect modifications over large distances. In doped silica fibers the Brillouin Frequency Shift (BFS) is proportional both to temperature and strains. In this work we establish that the sensitivity of FOSs to hydrostatic pressure can be forecast from the behavior of the glass under hydrostatic compressions in a diamond anvil cell. It is shown that the BFS under a hydrostatic pressure is a manifestation of the elastic anomaly observed in silica glass. This anomaly vanishes in GeO2 glass and accounts for the decrease of the sensor sensitivity when the GeO2 doping concentration increases in a silica fiber. The progressive vanishing of the anomaly in sodium aluminosilicate glasses which contain the same amount of silicon dioxide (75%) but differ in the Na2O and Al2O3 ratio allows to determine the composition of a glass with a BFS independent of the pressure. Such a glass composition will provide a pressure-independent temperature FOSs.