Optical fiber sensors for measurement of strain and acoustic waves

A surface acoustic wave (SAW) sensor based on an extrinsic Fabry-Perot interferometer is described. A single-mode fiber, used as the input/output fiber, and a multimode fiber, used solely as a reflector, form an air-gap that acts as a low-finesse Fabry-Perot cavity. The Fresnel reference reflection from the glass/air interface at the front of the air-gap interferes with the sensing reflection from the air/glass interface at the far end of the air-gap. Strains in the silica tube housing the two fibers change the air-gap length, thereby altering the phase difference between the reference and sensing reflections. A theoretical analysis of the interaction between the strain induced by elastic stress wave fields and the fiber sensor is presented. A dual optical wavelength stabilization technique is used to minimize signal drifts. Signal to noise ratios on the order of 39 dB are obtained with a sensitivity of 4 degree(s)/microsecond(s) train cm-1 for strain measurements. The sensor was also attached to a steel cantilever beam and submerged in liquid nitrogen. The sensor was used to measure strain at liquid nitrogen temperatures with typical errors of less than 10%.