Geometry-tuned PDMS embedded FBG sensors for low- and ultra-low frequency vibration monitoring

This work reports the design and experimental characterization of two femtosecond-laser-inscribed fiber Bragg grating (FBG) vibration sensors based on direct polydimethylsiloxane (PDMS) encapsulation, for low- and ultra-low-frequency monitoring. FBG accelerometers for this regime have so far relied on rigid mass-spring structures with inertial masses; embedding the grating directly in a soft elastomer, trading absolute sensitivity for low mass and conformability, had not been examined for acceleration sensing. Two comparable gratings were embedded in PDMS beams of different geometry, a reference structure ( L = 50 mm, h = 7 mm) and a longer, thinner structure ( L = 60 mm, h = 4 mm). The longer, thinner sensor exhibited a mean sensitivity of 0.0763 ± 0.0056 nm/(m/s 2 ) at 10 Hz, 105.5% higher than the reference (0.0371 ± 0.0010 nm/(m/s 2 )); the measured resonance frequencies (80.6 Hz and 42.0 Hz) matched finite element predictions within 0.31% and 6.03%. Sensitivity was statistically flat across 0.3–20 Hz for both sensors, with a noise-floor-derived dynamic range of 35.0 dB and 42.7 dB. Benchmarked against recent rigid low-frequency FBG accelerometers, the devices did not reach the highest absolute sensitivity, yet both recovered a periodic excitation down to 0.001 Hz, one decade below the lowest frequency reported among them, with signal-to-noise ratios of 6–18 dB. These results demonstrate that a conformable, low-mass PDMS-embedded FBG sensing element recovers a known periodic excitation down to 0.001 Hz, one order of magnitude below the lowest frequency reported for the metallic mass-spring devices it is benchmarked against, complementing rather than replacing rigid mass-spring devices where the highest absolute sensitivity is required.