# Optical Fiber Sensor with Stable Operating Point for AC Magnetic Field Measurement
###### Abstract  
A novel alternating current (AC) magnetic field sensor that has a stable operating point and is insensitive to ambient temperature fluctuations is presented. The sensor is based on a high attenuation fiber Bragg grating (HAFBG) attached to a magnetostrictive rod. A stable operating point is achieved by regulating a heating laser based on a feedback algorithm that compensates the temperature fluctuations of the surrounding environment. Experimental results show that the sensor responds well to dynamic magnetic fields and is able to ensure a stable operating point in the range of at least 15 \({}^{\circ}\)C in an ambient temperature disturbance test. The ease of fabrication and excellent performance suggest that the proposed fiber sensor is suitable for practical AC magnetic field sensing applications, such as health monitoring of transformers and fault diagnosis of induction motors. magnetic field sensor; high attenuation fiber; fiber Bragg grating; stable operating point +
Footnote †: journal: Applied Sciences  
0000-0002-2300-2300-2300-2300-2300-2300-2300-2300-2300-2300-2300-2300-23  
that can, e.g., be utilized to alter the period of an FBG or the length of a Fabry-Perot cavity, resulting in spectral wavelength shifts useable for sensing. This can, for example, be utilized by depositing a magnetostrictive thin film on the surface of FBG [14,17] to encapsulate an FBG directly on magnetostrictive rods [29] or in a mixture of magnetostrictive particles and epoxy resin [30]. Magnetic sensing based on the Faraday rotation effect has high sensitivity but requires a complex and expensive demodulation system. A major challenge facing most optical fiber magnetic field sensors proposed to date is how to effectively reduce temperature crosstalk. Temperature crosstalk elimination can be achieved by different technologies, which the following are examples of. In 2022, [PERSON] et al. proposed a composite magnetic fiber sensor based on an MF-coated nonadiabatic tapered microfiber cascaded with an FBG, and a sensitivity matrix was used to overcome the temperature crosstalk [28]. This kind of sensor requires the use of additional fiber gratings or other structures that can act as a temperature reference. [PERSON] et al. [31] introduced a passive temperature-compensating package to reduce temperature sensitivity by using a passive mount composed of two materials with different thermal expansion coefficients. However, sensors of this kind are either extremely complex in structure or require the use of additional special devices. [PERSON] et al. proposed a quadrature phase-stabilized three-wavelength demodulation technique to avoid imbalances and disturbances between three optical paths [32]. Sensors of this type, however, require either an additional temperature sensor head, temperature-compensating materials, or additional laser sources, greatly increasing cost, size, and complexity. Aiming to resolve the challenge of temperature crosstalk, we propose in this work a high attenuation fiber Bragg grating (HAFBG), which can realize an alternating current (AC) magnetic field measurement with a stable operating point using a magnetic field demodulation algorithm. The sensor consists of a HAFBG and a magnetostrictive rod with a square cross-section. The calibration results show that the sensitivities of temperature, heating laser power, and magnetic field intensity are 7.86 pm/\({}^{\circ}\)C, 0.604 nm/W, and 3.95 pm/mT, respectively. An applied AC magnetic field will dynamically alter the FBG period, resulting in Bragg wavelength shifts of the reflection spectrum. A stable operating point is achieved by adjusting the power of a heating laser that emits 980 nm light. Experimental results show that the sensor can maintain a stable operating point when the ambient temperature is varied in a 15 \({}^{\circ}\)C range.