A fully distributed optical fiber strain sensor that makes use of the nonlinear Kerr effect and the photoelastic effect. The sensor includes a polarization maintaining optical fiber having a core region and a cladding region where the fiber is embedded within or adhered to a structural element that is to be monitored for strain. Counterpropagating optical pulses interact within the fiber such that the change in refractive index caused by the Kerr effect causes a portion of the pulse intensities to be emitted from the fiber. A strain on the fiber will cause the index of refraction of the core region to change such that a different pulse intensity is emitted from the core region. This change in the loss of pulse intensity can be used to determine the amount of strain on the structure at the interaction region. By taking a strain measurement along two polarization directions and at two different wavelengths, the direction of the strain can also be determined.
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