Resonant Fiber Bragg Grating (FBG) Force/Strain Sensor

This paper reports the results of a theoretical and experimental feasibility study of a generic force or strain sensor, incorporating a suspended vibrating optical fiber, incorporating a Fiber Bragg Grating (FBG) as the sensing element. The operational principle is based on monitoring the dependence of the mechanical resonant frequency of a vibrating clamped fiber segment on the tensile force stretching the fiber. It is shown that this frequency change is proportional to the applied force and can be measured using the slope of the reflection spectrum of an inscribed FBG. Furthermore, the fractional frequency change can be made much larger than the change in the strain. Thus, the proposed technique makes possible force measurements in cases, where the strains are too small to be sensed directly by an FBG. Also, the suggested FBG-based approach potentially allows for multiplexed sensing at multiple locations along the host structure. The expected scale factor, ΔFrequency/ΔStrain, is calculated using a simple double-clamped beam model and favorably compared to direct strain measurements. In the framework of the custom-built setup, fiber vibrations were excited by an external shaker, driven by a sweep of frequencies, and a slope-assisted FBG reading method was employed for the measurements of the frequencies in the kHz range. Experimental results, which are consistent with the model predictions, demonstrate the feasibility of using FBG-based measurements of the fiber’s mechanical resonant frequency to the indirect measurement of strains, aiming at eventually achieving sensitivities better than direct strain methods.