A surface acoustic wave 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 mainly 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 in the input/output fiber. 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 and modulating the output intensity. A theoretical analysis of the interaction between the strain induced by the acoustic fields and the fiber sensor is presented. Because signal drifting in interferometric sensors is common, a dual optical wavelength stabilization technique is also incorporated into the sensor to minimize the effect. Signal to noise ratios (SNR’s) on the order of 39 dB are obtained with a strain sensitivity of 4°/ μstrain cm⁻¹.