Ultra-low frequency three-axis FBG accelerometer for seismic monitoring based on multiple flexible beams

Seismic monitoring in the ultra-low frequency range (0.0083-20 Hz) is essential for understanding Earth's structure. Existing electrical instruments are effective but costly and challenging to deploy in complex environments, while FBG accelerometers are adaptable but limited in sensitivity at ultra-low frequencies. This study introduces a high-sensitivity ultra-low frequency three-axis FBG accelerometer with a multi-flexible beam structure. Sensitivity and resonant frequency formulas are derived, and optimization is performed using a differential evolutionary genetic algorithm. A prototype sensor, validated through modal simulations and low- frequency vibration tests, achieves a sensitivity of 1025 pm/g, cross-sensitivity < 4.10 %, and a dynamic range of 97 dB, a flat response in the 0.05-80 Hz frequency range, exhibiting satisfactory sensitivity and precise angular characterization. Additionally, for the first time, a three-channel correlation method was applied to the FBG accelerometer for seismometer self-noise testing. The results enable scalable FBG low-frequency seismic monitoring in harsh environments.