Hydrostatic Pressure Measurement With Heterodyning Fiber Grating Lasers: Mechanism and Sensitivity Enhancement

In this paper, dual-polarization fiber grating lasers are exploited as heterodyning sensors for hydrostatic pressure measurement. The laser is formed by photo-inscribing two highly reflective wavelength-matched Bragg gratings over a short section of Er-doped fiber. The measurement is carried out by monitoring the beat frequency between the orthogonally polarized laser output. The pressure sensitivity of a bare sensor is $-0.59 ~{\hbox {MHz}}/{\rm MPa}$ . Theoretical analysis suggests that the pressure response is dominantly determined by the change in intra-cavity birefringence, which is associated with the geometrical imperfection of the fiber core. The effects of differences in elastic properties between fiber core and cladding on the pressure response are examined. We found that the differences in Young\´s modulus and Poisson\´s ratio are mainly responsible for the pressure sensitivity. The pressure sensitivity can be significantly enhanced by packaging the laser into a polymer incorporated with a steel rod. Due to the difference in elastic properties between materials, an additional birefringence is introduced into the laser cavity under hydrostatic pressure. The sensitivity has been enhanced by 228 times, to 170 MHz/MPa. The minimal detectable pressure change of the packaged sensor is about 0.01 MPa. We further demonstrate a miniature packaged sensor with a cross-sectional dimension of only 1.42 mm and a pressure sensitivity of 53.9 MHz/MPa towards practical applications.