High resolution fiber laser sensor for hydrostatic pressure

Polarimetric fiber laser sensors are attractive due to their frequency encoded output, which does not require any referencing, and their potential for high resolution. Both Fabry-Perot and distributed feedback fiber (DFB) laser sensors have been investigated for the measurement of strain, uniaxial pressure and magnetic fields (H.K. Kim et al, 1993; G.A. Ball et al, 1993; H.K. Kim et al, 1995; J.T. Kringlebton et al, 1996; S.-Y. Huang et al, 1990). In this paper, a Fabry-Perot type fiber laser sensor for fluid pressure is reported. The pressure acts directly on an elliptical core sensing fiber (S.-Y. Huang et al, 1990; W.J. Bock and W. Urbanczyk, 1996) doped with erbium. As a result, no mechanical transducer is required, which makes possible a sensor with very small diameter. Fiber Bragg gratings (FBG) are used as reflectors for the laser. Configurations with the gratings written into circular core fibers, as well as into the doped elliptical core fibers, are analyzed. It is shown that the dispersion of the highly birefringent gratings can be employed to significantly improve the sensor resolution into the range of a few 10/sup -6/ of the full scale signal. Possible applications of the sensor include pressure monitoring in oil wells and other hostile environments.