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Abstract
A compact optical magnetic field sensor based on a fiber laser oscillator circuit merged with a Sagnac loop that contains a polarization-maintaining fiber (PMF) and a magnetostrictive rod is proposed. The combination of the PMF-Sagnac loop and fiber laser oscillation circuit significantly increases the signal-to-noise ratio of the reflection spectrum in the system, which is a mixed spectrum that includes interference fringes and lasers. For the proposed system, an erbium-doped fiber amplifier is used to provide gain, a dispersion compensating fiber is used to compensate for dispersion in the system, a PMF is inserted in the Sagnac loop as a microwave filter, while a section of the PMF is bonded to a magnetostrictive rod to achieve magnetic field sensing. The proposed system is shown experimentally to produce a fine reflection interference spectrum. The highest intensity reflection interference peak (from the laser) is over 40 dB with a –3 dB line-width of about 0.03 nm. The system can provide magnetic field sensitivities of 0.07 nm/mT and 0.076 nm/mT for sensor head lengths of 1 m and 2 m, respectively, and exhibits a stability of ±0.029 nm over 1 h. The proposed sensing system has advantages of low cost, high sensitivity, compact structure, and can produce a fine and stable reflection spectrum.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Optical fiber magnetic field sensors have been widely developed in recent years because of features such as high sensitivity, compact size, wide bandwidth, and multiplexing capabilities [1–3]. Magnetic field measurement would be a valuable capability for many sensing applications like current monitoring, bio-medicine, aerospace, manufacturing, and geophysical research [4–7]. Traditional magnetic field sensors are typically Hall sensor and Magneto-resistance (MR)-based systems [6], however they can be vulnerable to electromagnetic interference. Optical fiber sensing technology has come to be a valuable option due to its superior characteristics compared with traditional magnetic field sensors [8]. At present, several frequently-used optical fiber sensing systems utilize interference structures, including a Sagnac loop [9,10], Fabry-Perot [11,12], Mach-Zehnder, and Michelson interferometers [13,14], as well as fiber Bragg grating (FBG)-based structures [4,15]. Shi et al. [9] used two sections of polarization-maintaining fiber (PMF) in a Sagnac interferometer for strain sensing with a sensitivity of 20.67 pm/με, and the highest transmission interference peak intensity was about 30 dB. Yun et al. [10] designed a temperature sensor based on a Sagnac interferometer (SI) combined with a photonic crystal fiber (PCF) in a circular layout. Simulations showed that its temperature sensitivities were 9.47 and 8.57 nm/°C and transmission interference peak was over 35 dB. However, due to a harsh manufacturing process for the sensor head, most results in this study were from simulations. Temperature and strain have been monitored by using a polarization-maintaining few-mode fiber Bragg grating (PM-FMF-FBG)-based structure in which the sensitivities of the wavelength shift to temperature and strain were about 10 pm/°C and 0.73 pm/με, respectively [15]. Zhang et al. [16] have proposed a temperature-insensitive magnetic field sensor based on an optoelectronic oscillator merged into a Mach–Zehnder interferometer with a sensitivity of 1.33 MHz/mT over a magnetic field range of 20.9-58 mT. The fiber laser oscillator and Mach–Zehnder interferometer have a relatively complex frequency-demodulated sensing structure.
Finally, it is well known that optical fiber sensors can be configured in several ways to measure various parameters. However, the sensing system based on a fiber laser oscillation structure has unique advantages such as high signal-to-noise ratio (SNR), narrow line-width, high resolution, and ease of interrogation [17].
In this paper, we first propose a compact and robust optical magnetic field sensing system that can produces a fine and stable mixed spectrum. The system incorporates a fiber laser oscillator circuit coupled to a PMF-Sagnac interference structure. An erbium-doped fiber amplifier (EDFA) is used to provide gain for the sensing system and a dispersion compensating fiber (DCF) compensates for dispersion in the system. The PMF is inserted into the Sagnac loop as a microwave filter, while a section of the PMF is bonded with magnetostrictive rod to achieve magnetic field sensing. As a transducer, the magnetostrictive rod can convert magnetostriction induced by a magnetic field into strain in the PMF. When the magnetic field changes, the length and the birefringence index of the PMF change which causes a shift in the interference peaks. The proposed system has the advantages of low cost, high SNR, and high sensitivity.
2. Principle
A schematic diagram of the proposed magnetic field sensing system is shown in Fig. 1. The system consists of two loops: Loop 1 is the Sagnac loop and is connected to Loop 2 by a 3 dB optical coupler(OC2) with a coupling ratio of 50:50. A PMF is inserted into the Sagnac loop as a microwave filter, and is modulated by magnetostrictive rod. As shown in Fig. 1(b), a section of the PMF is attached to the magnetostrictive rod surface. Loop 2 incorporates an EDFA that integrates the pump source and gain medium to provide gain, a polarization controller (PC) is added to the optical path to control the polarization of the transmitted light, an isolator (ISO) to ensure unidirectional light transmission, and a DCF to compensate for dispersion in the system. The resulting interference spectrum is monitored and recorded by an optical spectrum analyzer (OSA; resolution = 0.02 nm).
Fig. 1. (a) Schematic of the proposed sensing system. (b) Magnetic field sensor head. EDFA: erbium-doped fiber amplifier, PC: polarization controller, OC: optical coupler, ISO: isolator, PMF: polarization-maintaining fiber, DCF: dispersion compensation fiber, OSA: optical spectrum analyzer.
In the proposed system, light of high coherence and narrow bandwidth from Loop 2 is transmitted into the Sagnac ring (Loop 1), wherein it is divided into two identical beams by OC2. The two beams pass through the PMF in opposite directions and finally recombine at the output of the OC2. Due to the birefringence of the PMF, an interference spectrum will be formed in the Sagnac loop. When the laser threshold is satisfied [18], gain in the laser cavity yields stable high coherence light that produces a mixed spectrum of interference fringes [shown in Fig. 2(a)]. Based on the principle of interferometry, the reflection ratio $T$ of the interference spectrum is a periodic function, which can be written as
where $\delta$ is the total phase [19]. Phase variations due to changes in the external environment act on the Sagnac ring. In the interference spectrum, reflection peaks appear when the phase $\delta$ satisfiesFig. 2. Characteristics of the interference spectra for the proposed system: (a) SNR and FSR of the system for different lengths of the PMF; (b) –3 dB line-width and measured wavelength response for different pump currents.
From Eq. (2) we can obtain the interference peak
where ${B_{PMF}}$ represents the birefringence index of the PMF, $L$ is the length of the PMF, $\lambda$ is the working wavelength, and $\textrm{k}$ is a random integer.In Fig. 1(b), the PMF is bonded to a magnetostrictive rod, and the magnetostrictive effect will occurs when magnetostrictive rod is magnetized by the applied magnetic field. The effect is then converted into strain on the PMF which is bonded to the magnetostrictive rod. When the magnetic field changes, the birefringence index ${B_{PMF}}$ and the length $L$ of the PMF will change. By Eq. (3), we can obtain an interference peak shift in the Sagnac loop, and the relationship between the interference peak shift and changes in strain is shown in Eq. (4)
The free spectral range (FSR) used to define the distance between interference fringes is approximately written as
As changes in ${B_{PMF}}$ and the working wavelength $\lambda$ are all minor, the variation in FSR is mainly caused by changes in L, thus, from Eq. (7) it can be seen that there is an inverse relationship between $FSR$ and $L$.
3. Experimental and discussion
The spectra output from the magnetic field sensing system were measured by the OSA. It can be seen in Fig. 2 that the highest reflection interference peak (from the laser) has a high SNR, reaching 40.0 dB, with a narrow (–3 dB) bandwidth of approximately 0.03 nm. From Fig. 2(a), we can see experimentally that there is an inverse relationship between $FSR$ and length $L$ of the PMF. When the length of the PMF was 0.5, 1 m, 2 m, or 4 m, the corresponding FSR was about 12 nm, 6 nm, 3 nm or 1.5 nm, respectively [shown by black square in Fig. 2(a)]. This shows that if a large FSR was needed, a relativity short PMF would be required. The experimental results are also consistent with the simulation results (blue line) in Fig. 2(a) and Eq. (7). It can be seen in Fig. 2(b) that with increasing EDFA current, the wavelength response became narrower and the suppression ratio increased.
Fig. 3. Wavelength shifts for magnetic field changes when the length of the PMF is (a) ${l_1}\textrm{ = }1\textrm{m }$, (b) ${l_2}\textrm{ = }2\textrm{m }$.
To test the performance of the system, sensing fibers with lengths of 1 and 2 m were partly attached to the magnetostrictive rod. A water-cooled magnetic field generator was used to apply different magnetic field strengths and maintain a constant temperature during the experiment. The magnetic field was varied between 0 and 45 mT in increments of 3 mT. The working laser wavelength shifted as the magnetic field increased, the applied magnetic field was then decreased back to 0 mT in 3 mT increments, and the working laser wavelength again recorded. (The linear working region of the magnetostrictive rod used in this experiment was about 10-80 mT).
Figure 3 shows shifts in the wavelength of the working laser for two different sensing fiber lengths over a range of magnetic field strengths. Wavelength was observed to increase with strength of the magnetic field. Furthermore, Figs. 3(a) and 3(b) show that with the length of the sensing fiber further increases, the sensitivity will be enhanced.
Fig. 4. Analysis of measurement results for the proposed system:(a) Analysis results for PMF lengths of 1 m and 2 m. (b) Curves of wavelength variations as the magnetic field increases and decreases for PMF lengths of 1 m and 2 m.
Figure 4(a) shows that the sensitivity of the proposed magnetic fields sensor varied linearly, sensitivities of 0.070 and 0.076 nm/mT for PMFs of 1 and 2 m, respectively. It can be seen from Fig. 4(b) that wavelengths change step were often slightly longer when magnetic field strength was decreasing than when it was increasing. This can be explained by the magnetostrictive rod having magnetic hysteresis, as observed in other magnetic materials [19]. From the results, we can confirm that the sensor was robust and had good reproducibility.
Finally, the stability of the reflection interference peak (working laser wavelength, 1529.19 nm) was tested every 5 min for 1 h at room temperature (approximately 28°C). Figure 5 shows that the deviation of the wavelength was within ±0.029 nm over 1 h. We infer that the wavelength deviation is primarily due to micro-fluctuations in temperature.
In summary, the proposed system has many advantages, including compact size, and low cost, and it yields a fine and stable reflection spectrum. Compared with previously reported fiber magnetic field sensing systems [2,4,19], the proposed system has a high SNR with a narrow (–3 dB) bandwidth. Moreover, compared with fiber magnetic field sensing systems in the literature [20–22], the proposed system is more compact and robust.
4. Conclusion
In conclusion, we have proposed and experimentally demonstrated a high performance magnetic field sensor based on a fiber laser oscillator circuit merged with a PMF-Sagnac interference structure. As a result of combining fiber laser technology, fiber-optic magnetic field sensor technology, and Sagnac interference technology, the proposed magnetic field sensing scheme can generate a stable reflection spectrum, a maximum intensity reflection interference peak of over 40 dB with a –3 dB line-width of about 0.03 nm, and stability of ±0.029 nm over 1 h. The proposed system exhibited sensitivities of 0.07 nm/mT and 0.076 nm/mT for sensor head lengths of 1 m and 2 m, respectively, in response to magnetic field strengths of 10-45 mT (theoretically up to 80 mT). The proposed sensing system is attractive due to its compact size, low cost, fine and stable reflection spectrum.
Funding
National Natural Science Foundation of China (61801134, 61835003); Guizhou Science and Technology Department ((2019)1127).
Disclosures
The authors declare no conflicts of interest.
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