Highly Sensitive Strain Sensing Using a Resonant Fiber Bragg Grating

We report on a strain sensing approach that relies on monitoring the mechanical resonant frequency of a suspended, double-clamped, stretched optical fiber with an inscribed Bragg grating (FBG). This resonant frequency, which varies with strain, is measured using the FBG. A reduced-order model of the fiber-beam is used to estimate the expected frequency shift Δf=f-f0, with respect to an unstretched fiber frequency f0, and to determine appropriate design and operating parameters. Using a custom-built setup that provides controlled fiber stretching and excitation by an external shaker, frequency sensing in the kHz range is experimentally demonstrated. The FBG measurements using the slope-assisted interrogation technique are validated by simultaneously detecting the fiber's time-domain response with a laser Doppler vibrometer (LDV). Consistently with the model prediction, an experimental frequency-strain scale factor of Δf/ε≈5.6Hz/με is obtained. The corresponding normalized sensitivity, (Δf/f0)/ε≈682, is more than two orders of magnitude higher than that obtained by direct FBG strain measurement. This resonance-based approach also retains the potential for multiplexed sensing at multiple locations along a single fiber. The applicability of the technique to resonant vibrating-beam accelerometers is demonstrated using the model.