Highly Sensitive Low-Frequency Magnetic Field Sensing System Based on Fiber FPI

A fiber Fabry-Pérot interferometer (FPI)-based sensing system for low-frequency weak magnetic field detection is proposed and experimentally demonstrated. Within the sensor head, a mechanic transform structure is designed and fabricated to amplify the displacement of a magnetostrictive TbDyFe rod under a proper magnetic bias, resulting in significant cavity length change of the FPI with external alternating magnetic field. The dynamic interference spectrum of the FPI is acquired by a spectrometer for high-speed demodulation. A modified Hilbert transform combined with a cumulative average method is performed to recover the signal of magnetic field with suppressed noise. A magnetic field sensitivity of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$2.69\times 10^{-{4}}$ </tex-math></inline-formula> rad/<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu $ </tex-math></inline-formula>T and a minimum detectable magnetic field of up to 12.60 nT/Hz<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$^{\text {1/2}}$ </tex-math></inline-formula> at 60 Hz are achieved. The proposed fiber magnetic field sensing system exhibits good linearity and compactness, high sensitivity and resolution, and flat frequency response under 1 kHz; hence, it has great potential in the field of weak magnetic field sensing.