An Ultra-high-resolution FBG Static-strain Sensor for Geophysics Applications

We report an ultra-high-resolution static-strain measurement with a pair of fiber Bragg gratings (FBGs), one for strain sensing and the other for compensating the error due to temperature disturbance and source drift. The difference between the two FBGs' Bragg wavelengths is evaluated by utilizing a cross-correlation algorithm. The mechanism of noise suppression by the cross-correlation algorithm is quantitatively analyzed and the factor that determines the ability of noise suppression of this algorithm is revealed. The temperature dependence is further subtracted by using a vector operation. When no strain is applied, an ultra-high wavelength resolution corresponding to 2.6 n epsilon was obtained, which gives the ultimate performance of the measurement system. With a variable strain applied with a piezo-stage, a resolution of 17.6 n epsilon was demonstrated. This is the first demonstration, to the best of our knowledge, that a real-static strain down to 10 n epsilon is measured, providing a powerful technique for the measurement in geophysics application.