Highly Sensitive Magnetic Field Sensing Based on Extrinsic Fiber Bragg Grating Fabry-Perot Filter and Optoelectronic Oscillator
Objective Fiber optical magnetic sensors have gained significant interest due to their small size,corrosion resistance,and ability to operate in harsh conditions.Typically,these optical fiber-based sensors are optically demodulated employing an optical spectrum analyzer(OSA)to monitor wavelength shifts or power variations,but this method suffers from slow scanning rate and poor resolution.Therefore,it is essential to suggest a magnetic sensor with a fast interrogation speed and high resolution to meet the needs of certain application fields,such as subsea weak magnetic field detection and exploration of Earth's mineral resources.Recently,optoelectronic oscillator(OEO)-based magnetic field sensing methods have been proposed with different fiber structures,such as fiber Bragg grating(FBG),Mach-Zehnder interferometer(MZI),and FBG Fabry-Perot(FBG-FP)filter.By mapping the sensing information to the frequency of the microwave signal generated from the OEO,the interrogation speed and resolution of the sensor can be strengthened.However,OEO-based magnetic field measurement using FBG and MZI exhibits low sensitivity,and the utilization of FBG-FP or phase-shifted FBG,characterized by narrow notches in their reflection spectrum,proves to be expensive and challenging in manufacturing.In this paper,we put forward an OEO-based highly sensitive magnetic field sensing scheme utilizing an extrinsic fiber Bragg grating Fabry-Perot(EFBG-FP)filter.The proposed scheme not only mitigates the complexity and cost associated with manufacturing the sensing probe but also significantly enhances the sensitivity of magnetic field sensing.Methods We use a pair of FBGs to construct an EFBG-FP filter,with both end faces being carefully milled and axially aligned by the insertion of ceramic ferrules.This is then combined with a grooved magnetostrictive alloy(MA)to create a magnetic field sensing unit.When there is a change in the external magnetic field,the length variation of the MA will effectively induce a change in the air cavity length of the EFBG-FP filter,resulting in a drift in the notch wavelength of the EFBG-FP filter.The EFBG-FP filter exhibits narrowband filtering characteristics.When embedded in the OEO resonant cavity,phase modulation to intensity modulation(PM-IM)can be achieved by filtering one 1st sideband of the phase-modulated signal,and the OEO oscillation frequency will be determined by the difference between the carrier frequency of the light source and the notch center frequency of the EFBG-FP filter.Therefore,the variation in the magnetic field is ultimately mapped to the change in the OEO oscillation frequency.The measurement of the magnetic field can be realized by monitoring the changes in the oscillation frequency with an electrical spectrum analyzer(ESA).In the experiment,the EFBG-FP magnetic field sensing probe is positioned in a solenoid to detect magnetic field changes.To evaluate the sensing performance,the magnetic field is increased in steps of 0.2 mT from 20.2 mT to 21.8 mT,which is within the optimal operating range of the probe,by adjusting the current of the power supply.Results and Discussions The reflection and transmission spectra of the EFBG-FP filter were measured by the OSA with a wavelength resolution of 0.01 nm.The notch's center wavelength is approximately 1550.022 nm,with a free spectral range(FSR)of about 0.098 nm(Fig.4).The frequency response is determined using an ESA.The center frequency of the microwave signal generated by the OEO without a magnetic field applied is 1.2116 GHz,achieving a side mode suppression ratio of 57.31 dB(Fig.5).With the magnetic field increasing from 20.2 mT to 21.8 mT,the OEO oscillation frequency shifts from 1.8540 GHz to 8.6398 GHz(Fig.6).Fitting results indicate that the magnetic field sensitivity can reach as high as 4.258 GHz/mT,the highest compared to other magnetic field sensing schemes based on OEO(Table 1),with a correlation coefficient(R2)of 99.8％(Fig.7).The sensing range of our proposed magnetic field sensing system is limited by the FSR of the EFBG-FP filter and the 3 dB bandwidth of the photodetector(bandwidth is 10 GHz)used in the experiment.The theoretical magnetic field resolution of the proposed sensing system is estimated at 0.2 μT.Furthermore,the magnetic field range of 20.2-21.8 mT falls within the optimal operating range for the proposed sensing system.Conclusions A highly sensitive magnetic field sensing system based on an OEO incorporating an EFBG-FP filter has been proposed and experimentally demonstrated.Two FBGs with reflectivity greater than 95％and well-milled end faces are inserted into ceramic ferrules to form an EFBG-FP cavity,which is bonded to the surface of an MA with two grooves using ultraviolet(UV)glue to constitute a magnetic field sensing probe.With the combination of the OEO,marked enhancements in interrogation speed and resolution are achieved.By simply monitoring the shifts in oscillating frequency,magnetic field measurements can be realized.The proposed sensing system has the advantages of high sensitivity,high resolution,cost-effectiveness,and ease of fabrication.Experimental results reveal that the system can respond to weak changes in the magnetic field.Moreover,by applying a bias magnetic field,highly sensitive magnetic field measurements can be attained over different ranges.
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