Static strain measurement with sub-micro-strain resolution and large dynamic range using a twin-Bragg-grating Fabry-Perot sensor

We present experimental results on testing a twin-grating fiber optic sensor for the measurement of static strain and temperature. The sensor is built with two identical Bragg gratings closely spaced in a single-mode fiber. It produces a reflection spectrum modulated by interference. As in traditional fiber Bragg grating sensors, strain of the fiber leads to the wavelength shift of the reflection spectrum. This shift can be determined precisely by measuring phase changes for corresponding components of Fourier transform of the reflection spectrum. Such an approach allows an absolute strain measurement with an interferometric sensitivity. Resolution of 0.2 ?m/m and range to resolution ~ 104: 1 were demonstrated experimentally for static strain measurements using a pair of 0.7-mm-long Bragg gratings with reflectivity of 1% each.