Stabilized extrinsic fiber-optic Fizeau sensor for surface acousticwave detection

A surface acoustic wave sensor based on an in-line extrinsic Fizeau 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 or Fizeau 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 thin-walled silica tube housing the two fibers change the length of the air-gap, 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 housing is presented