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Abstract
A high-sensitivity optical fiber magnetic field sensor based on a multi-Fabry-Perot interference (F-P) cavity in an etched multimode optical fiber (MMF) was proposed. The MMF was etched along the fiber axis and a hole with the length of about 250 µm formed in the MMF. The multi-F-P cavity in the MMF is a sandwich structure, which is composed of UV glue, magnetic fluid and UV glue. The refractive index and effective cavity length of the magnetic fluid cavity change with the changing of the external magnetic field, which will result in changes of the reflection spectra of the multi-F-P. Thus, the external magnetic field could be detected by the changes of spectra. Experimental results showed that the high magnetic field sensitivity of 299.7 pm/mT and 0.164 dB/mT were obtained in the range of 0∼8 mT weak magnetic induction intensity by using the wavelength and intensity demodulations, respectively. The proposed sensor shows the potential applications in the magnetic field measurement in the weak magnetic environment.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
The measurement of magnetic field has been widely used in national defense security, industrial production, and other fields. It is very important for magnetic flux leakage detection in a weak magnetic environment for the defect detection of metal materials for special equipment such as high-temperature boilers and pipes. The magnetic flux leakage testing is that the excitation device emits a weak electromagnetic field to penetrate the metal sample to be tested, and the penetration depth depends on the frequency and intensity of the excitation current [1–4]. When the test piece reaches the magnetization saturation state, the magnetic field lines are uniformly distributed in the test piece and have a large penetration depth. If there are defects such as corrosion and pits in the piece, the magnetic resistance of the surrounding magnetic circuit increases, and the distribution of the internal magnetic field lines changes. And then the magnetic field lines will overflow the surface of the test piece [5–7]. Therefore, the defect information can be obtained by the change of the leakage signal. In the traditional magnetic flux leakage detection method, for example, the method based on the voltage value of the detection signal to analyze defects is vulnerable to external magnetic field interference, low accuracy, and insensitive to a weak magnetic field, which is not suitable for weak magnetic flux leakage signals in internal defects [8–12].
Optical fiber magnetic field sensors based on magnetic fluid (MF) changes for the monitoring of magnetic fields were investigated recently [13–22]. Compared with the traditional magnetic field detection methods, optical fiber magnetic field sensor has the advantages of high sensitivity, anti-electromagnetic interference, safety, stability, anti-oxidation, corrosion resistance, etc. Liu et al. investigated the refractive index of the MF film and found that the refractive index of the MF is linear with its concentration [13]. Zheng et al. inserted a tilted fiber Bragg grating (TFBG) with a grating tilt angle of 2° into a MF-filled capillary for simultaneous measurements of magnetic field and temperature [14]. Deng et al. encapsulated the MF in a capillary to form a core-cladding mode interferometer, which showed a high sensitivity of 162.06 pm/mT in the range of 0∼21.4 mT [15]. Wang [16] and Han [17] proposed a single mode-multimode-single mode (SMS) fiber coated with MF magnetic field sensor, and the sensitivities of 168.6 pm/mT and 905 pm/mT were obtained, respectively. Zheng et al. proposed a fiber Fabry-Perot interference (F-P) sensor with an embedded microfluidic channel, and a magnetic field sensitivity of 418.7 pm/Oe was obtained [18]. Li et al. proposed a vector magnetic field sensor based on U-bent single-mode fiber and MF, and the maximum magnetic field intensity sensitivity of 0.517 nm/mT was obtained [19]. Wei proposed a magnetic field sensor based on a microfiber coupler combined with MF in a Sagnac loop, the maximum magnetic field sensitivity of −488 pm/mT was obtained [20]. Sun investigated a fiber-optic magnetic field sensor based on a tapered two-mode fiber sandwiched between two single-mode fibers, and a maximum sensitivity of 98.2 pm/Oe was obtained [21]. Tian studied an optical magnetic field sensor based on a fiber laser oscillator circuit merged with a Sagnac loop, and sensitivities of 0.07 nm/mT and 0.076 nm/mT were obtained [22]. However, the above existing optical fiber magnetic field sensors use wavelength demodulation to monitor the magnetic field, and most of them are transmission type. These drawbacks seriously limit their applications in practice.
In this paper, a high-sensitivity optical fiber magnetic field sensor based on the MF-filled multimode fiber (MMF) multi-F-P cavities was proposed. The optical fiber was etched by hydrofluoric acid (HF) to form a tapered hole, which was filled with UV glue, MF, and UV glue along the fiber axis, respectively. The effective cavity length and refractive index of the MF cavity changed rapidly when the external magnetic field was applied, resulting in changes in the reflection spectra of the multi-F-P cavities. Thus, the magnetic field can be detected. The magnetic field response characteristics of the sensor were obtained by using wavelength and intensity demodulation methods, respectively. The high magnetic field sensitivities of 299.7 pm/mT and 0.164 dB/mT were obtained.
2. Sensor fabrication and principle
The experimental setup for testing the proposed sensor is schematically shown in Fig. 1(a). The broad-band source with a wavelength range from 1432 to 1632 nm was used as the input light source. The optical spectrum analyzer with a wavelength resolution of 0.02 nm (OSA, AQ6370, Advantest) was used to detect the output spectrum. In the experiment, the variation of different magnetic field intensities (calibrated by a Gauss meter) can be controlled by moving a rectangular magnet close or away from the sensor probe. The reflective optical fiber multi-F-P cavity sensor probe is composed of an etched MMF filled with UV glue, MF and UV glue along the axial direction, respectively. The end of the MMF is coated with a gold film. The thickness of UV glue, MF and UV glue along the optical fiber axis is 45 µm, 80 µm and 130µm, respectively. The core diameter and effective refractive index of MMF are 50 µm and 1.465, respectively. The effective refractive indices of UV glue and MF are 1.496 and 1.506, respectively [23]. The manufacturing process of the sensor probe is as follows: First, vertically immerse a section of MMF with a length of about 15 cm into HF acid solution with a concentration of 49%, and the immersion length of the MMF is 0.3 cm. In general, the optical fiber core is doped with oxide impurities such as germanium dioxide, which dissolves faster in HF acid solution than silicon dioxide. Therefore, the corrosion rate of the MMF core is faster than that of the MMF cladding. After 10 minutes of etching, the MMF end face forms a tapered hole with a depth of several hundred microns as shown in Fig. 1(b). Next, apply UV glue to the innermost part of the tapered hole with a depth of 45 µm; Then fill the hole with a depth of 80 µm of the MF by vertically the end face of MMF to the MF liquid for 5 s; After that, continue to fill the UV glue film with a thickness of 130 µm. Finally, the sensor end is coated with gold reflective film and UV glue protective layer. The UV glue protective layer is solidified by ultraviolet irradiation. The microscopic images of the proposed sensor are shown in Fig. 1(c) and (d).
Fig. 1. Schematic of the MMF-multi-F-P cavities based magnetic field sensor. (a) Experimental setup. (b) The multi-F-P cavities based magnetic field sensing head. (c) Microscopic image of the MMF etched by HF. (d) Microscopic image of MMF after being etched by HF and filled with UV glue and MF.
As shown in Fig. 1(b), the sensing probe contains three main F-P cavities, namely, the MF cavity (C1), the UV glue cavity (C2), and the combination cavity of MF and UV glue (C3). When the incident light reaches C1, part of the light reflects back, and the other part of the light transmits into C2. The light incidents to C2 partially reflects again and enters C1 to form a combination cavity of C3 with the MF cavity, and the length of the C3 cavity is equal to the sum of the cavity lengths of C1 and C2.
Figure 2(a) shows the reflection spectrum of the proposed sensor. It can be seen that the reflection spectrum of the sensor is a modulated multi-cavities F-P interference spectra, which can give the overall information of the spectrum and the details of the resonant peak wavelength. As Fig. 2(b) shows, by using the fast Fourier transform method, we obtained its corresponding spatial spectrum. From Fig. 2(b), one can see that there are three strong peaks located at 0.4412 nm-1, 0.7741 nm-1 and 1.1037 nm-1, which represent the cavities of C1, C3 and C2, respectively. Among them, the UV glue cavity C2 does not change with the change of external magnetic field. On the contrary, the MF cavity C1 is affected by the external magnetic field, and the distribution of nanoparticles in the cavity is orderly arranged and changed with the changing of the external magnetic field, resulting in regular changes in the cavity length and refractive index of the MF cavity. Finally, the reflectivity of MF cavity C1 will change with the change of the external magnetic field. Accordingly, the C3 cavity composed of C1 and C2 will also change with the change of the external magnetic field. Therefore, under the external magnetic field, the two cavities C1 and C3 play the major role in magnetic field measurements.
Fig. 2. (a) Reflection spectrum of the MMF-multi-F-P cavities based magnetic field sensor. (b) Corresponding spatial frequency of the reflection spectrum of the MMF-multi-F-P cavities. Inset shows the enlarged detail spatial frequency curve of the pink rectangular area.
The free spectrum FSR of F-P cavity can be calculated by the following expression,
where λ is the wavelength of the incident light, n is the effective refractive index of the F-P cavity, and Leff is the effective length of the cavity. The cavity lengths of C1, C2 and C3 are 80 µm, 130 µm and 210 µm, respectively. The spatial frequency peak positions of the three F-P cavities C1, C2 and C3 can be calculated as 0.4506, 1.0965 and 0.7791 nm-1 respectively. This shows that C1 and C3 are the main F-P cavities, and the output spectrum of the detection probe is mainly composed of two superimposed F-P interference spectra of C1 and C3 [24].According to the Fresnel reflection principle, the intensity of the sensor output spectrum can be described as,
3. Results and discussions
In the external magnetic field response measurement, we increase the magnetic induction intensity from 0 mT to 8 mT with a step of 2 mT. The changes of the sensor reflection spectrum were recorded by OSA, as shown in Fig. 3. It can be seen that with the increase of magnetic induction intensity, the interference resonance wavelength shifts to the short wavelength direction, which indicates that with the gradual increase of the magnetic field, the high scattering and absorption characteristics of MF make it regularly arranged and clustered, and the refractive index gradually increases. The maximum magnetic field sensitivity of 299.7 pm/mT at the resonance wavelength of 1527 nm is obtained. In addition, as shown in Fig. 4, the longer the resonant wavelength showed relatively higher sensitivity.
Fig. 3. Reflection spectra of magnetic induction intensity in the range of 0∼8 mT. The inset showed the spectra near the resonant wavelength of 1527 nm.
What’s more, the spectrum superimposed by the two F-P cavities can better display more information of the measured magnetic field response. As shown in Fig. 5(a), the reflection spectrum of the proposed structure showed an upper envelope because of the overlap of the two spectra of the two F-P cavities. As shown in Fig. 5(b), one can see that with the increase of external magnetic field intensity, the intensity of the upper envelope's resonant wavelength at 1520 nm increased, correspondingly. Therefore, besides the above wavelength demodulation of the magnetic field measurement, the spectra intensity demodulation of the magnetic field monitoring was also obtained as shown in Fig. 5(c). The maximum magnetic field sensitivity of 0.164 dB/mT with good linearity at the resonance wavelength of 1520 nm was obtained.
Fig. 5. (a) Upper envelope of the reflection spectrum of the MMF-multi-F-P cavities based magnetic field sensor. (b) Upper envelope of the reflection spectrum varied with the magnetic field intensities. (c) Linear fit curve of the magnetic field response for the upper envelope of the reflection spectrum at the resonant wavelength of 1527 nm.
The stability of the reflection interference peak (@1527 nm) and intensity of the upper envelope's resonant wavelength at 1520 nm was tested every 5 min for 1 h at room temperature (about 28 °C). Figure 6 shows that the deviation of the wavelength and the intensity were within ±0.0075 nm and ±0.00045 dB over 1 h, respectively. The proposed sensor has good stability and we think that the deviations are primarily due to micro-fluctuations of the temperature.
As shown in Table 1, we compared the performance of this sensor with those of the similar types of optical fiber magnetic field sensors. Compared with the reported sensors, this interference structure is very suitable for detection in a weak magnetic field environment, and shows a relatively high magnetic induction sensitivity of 299.7 pm/mT. And the intensity demodulation of the magnetic field with a sensitivity of 0.164 dB/mT was obtained, which means that the proposed sensor has the advantages of low cost, convenient manufacture, etc.
Table 1. Performance comparisons for optical fiber magnetic field sensors with different sensing structures
In summary, a highly sensitive optical fiber magnetic field sensor based on an end-etched MMF was proposed. UV glue, MF and UV glue were -filled into the etched hole in the MMF to form a multi-F-P cavity as the magnetic field sensing probe. With the changing of the external magnetic field, the distribution of the MF in the F-P cavity changes, which results in the changing of the reflection spectrum of the sensing probe. Under the magnetic field range of 0∼8 mT, by using the wavelength and intensity demodulations, the highly sensitivity of 299.7 pm/mT and 0.164 dB/mT were obtained. The proposed sensor showed potential applications in weak magnetic field measurements.
Funding
National Natural Science Foundation of China (12274386, 11874332); Natural Science Foundation of Zhejiang Province (LY21F050006); Key R & D plan of Zhejiang Province (2021C01179); National Key Research and Development Program of China (2021YFF0600203).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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