Fiber-Optic Phase-Locked Loop Sensitive to Local Strain Only

The figure schematically depicts an apparatus for measuring strain at only one designated location on a structure. The apparatus is a fiber-optic phase-locked loop, wherein strain in a multimode optical fiber gives rise to a change in the phase of modulation of a laser beam that propagates along the fiber. The optical phase-locked loop makes it possible to perform sensing by use of a multimode optical fiber (in contradistinction to a single- or few-mode fiber as in some other fiber-optic sensor systems). Unlike in other fiber-optic-based sensor systems, the phase change in this system does not occur in response to strain integrated along the entire fiber-optic path. Instead, this system includes lead-in and lead-out strain-insensitive lengths of optical fiber connected to a short strain-sensitive length of optical fiber that is affixed to the structure at the desired measurement location. Strain in the short sensory length produces a phase change, but strain in the lead-in and lead-out portions does not.