Investigations on the stability of polarimetric fiber laser sensors in harsh environments

The stability of a polarimetric Fabry-Perot fiber laser sensor for fluid pressure up to 100 MPa is investigated. The fluid acts on one of two elliptical-core fiber sections in the laser cavity producing a shift in the differential phase of the two orthogonal polarization modes and thus a variation in the beat frequencies of the corresponding longitudinal laser modes. The second fiber section, with a 90-degree offset in the core orientation, compensates for temperature-induced phase shifts. Dispersion in the birefringent fiber Bragg grating reflectors is employed to enhance the resolution of the sensor to a few parts in 10(6) of the free spectral range. Investigations on sensor stability address the effect of the fluid on the integrity of the fiber, creep caused by various types fiber coatings, as well as the intrinsic stability of erbium-doped and undoped sensing fibers under pressure and temperature changes. It is found that moisture-free silicone oil does not cause any fiber deterioration even in case of a bare fiber. Silicon nitride and gold coatings (1 mum thick) cause significant signal drift, whereas no drift is observed for carbon coatings. Highly doped elliptical-core sensing fiber exhibited drift in the differential optical phase while un-doped fiber did not.