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Abstract
A magnetic field sensor using a dual-frequency optoelectronic oscillator (OEO) incorporating cascaded magnetostrictive alloy-fiber Bragg grating-Fabry Perot (MA-FBG-FP) and FBG-FP filters is proposed and demonstrated. In the OEO resonant cavity, two microwave signals are generated, whose oscillation frequencies are determined by the FBG-FP filter and MA-FBG-FP filter filters with two ultra-narrow notches and two laser sources. Due to the characteristics of MA and FBG, the two generated microwave signals show different magnetic field and temperature sensitivities. By monitoring the variations of two oscillating frequencies and the beat signal using a digital signal processor, the simultaneous measurement for the magnetic field and temperature can be realized. The proposed sensor has the advantages of high-speed and high-resolution measurement, which make it very attractive for practical magnetic field sensing applications. The sensitivities of the proposed OEO sensor for magnetic field and temperature are experimentally measured to be as high as −38.4MHz/Oe and −1.23 or −2.45 GHz/°C corresponding to the MA-FBG-FP filter and FBG-FP filter, respectively.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Magnetic sensors have aroused significant research interest owing to the promising applications in navigation, geophysics, vehicle, medical instrumentation system and current detections [1]. Different kinds of magnetometers have been widely used, such as Hall-effect sensors [2], anisotropic magnetoresistive sensors [3], superconducting quantum interference device (SQUID) magnetometers [4], etc. However, the SQUID magnetometers have the disadvantages of relative bulky structure, complicated operation and high cost, which may restrict the practical applications of the system in some special conditions, such as in small space. Besides, for the electrical transducers, it may be imperfect due to the thermal, radioactive and electromagnetic interference and wiring problems in measurement environments. Compare with the traditional techniques employed in magnetic field detection, the fiber-optic magnetic field sensors are more attractive due to their advantages of compact size, low cost, immunity to electromagnetic interference and strong resistance to chemical erosion. Various approaches have been proposed for optical magnetic field sensing, including the sensors based on magneto-optical material [5–7], magnetic fluid [8,9], fiber laser [10–13], etc. Among them, optical fiber magnetic field sensors combining magnetostrictive material and FBG have aroused significant research interest. The magnetostrictive material is a kind of ferromagnetic materials that change shapes during magnetization. Several schemes have been investigated for magnetic field or electric current sensing by directly bonding an FBG onto a magnetostrictive alloy (MA) [5,14] or coating an fiber Bragg grating (FBG) with magnetostrictive film [15], where strain in a magnetostrictive material resulting from an applied magnetic field is transferred to the FBG. Nevertheless, FBGs and magnetostrictive materials are significantly influenced by temperature which may lead to the difficulty in discrimination between magnetic field and temperature. This temperature cross effect can be eliminated by introducing an additional FBG [16,17]. However, in most of the existing optical fiber-based sensing schemes, optical spectrum analyzer (OSA) is widely used for interrogation in the traditional optical sensor systems. Due to the low resolution and low interrogation speed of OSA, the optical fiber-based magnetic field sensors also suffer from the low resolution and low interrogation speed. Thus, it is highly desirable to propose a magnetic sensor with fast interrogation speed, high resolution and temperature independence for high-performance sensing applications.
Recently, fiber optic sensors using microwave photonics (MWP) technology have attracted significant attention, contributing to new potentials in different sensing applications [18–21]. In particular, optoelectronic oscillator (OEO) is one of the most promising technique for optical fiber sensing [22]. By mapping the sensing information to the microwave domain using OEO resonant cavity, the interrogation speed and resolution are increased effectively since the frequency of a microwave signal can be measured using a digital signal processor (DSP) module with high resolution and high speed. Besides, the microwave signal generated using OEO is featured with low phase noise, high Q factor, and much higher signal-to-noise ratio [23], leading to a high-reliable and high-accuracy measurement. Various schemes based on OEO have been investigated for optical sensing and measurement, including a refractive index sensor [24], a strain sensor [25–27], an acoustic velocity sensor [28], a temperature sensor [28], a transverse load sensor [29,30], an angular velocity sensor [31], etc. Andrey B. Matsko et al. have proposed a magnetometer using an OEO stabilized with an atomic vapor cell [32]. However, this atomic magnetometer is only suitable for weak magnetic field (<1Oe), which may limit the system in practical applications.
In this paper, we propose a novel magnetic field sensing technique with high speed and high resolution using a dual-frequency OEO. The joint use of two laser sources and cascaded MA-FBG-Fabry Perot (MA-FBG-FP) and FBG-FP filters with two ultra-narrow notches operates as a dual-passband microwave photonic filter (MPF) where the passband center frequencies equal to the frequency differences between the optical carriers of laser sources and the two notches. By imbedding the MPF into an OEO loop, two microwave signals at two frequencies corresponding to the center frequencies of the MPF are generated. The two FP filters exhibit different thermal expansion and magnetostriction coefficients, thus the magnetic field and temperature can be simultaneously measured by monitoring the two oscillating frequencies of microwave signals. The proposed approach is theoretically analyzed and experimentally evaluated. The sensitivity of magnetic field is experimentally measured to be −38.4 MHz/Oe and the sensitivities −1.23 GHz/°C and −2.45 GHz/°C of temperature sensing are obtained.
2. Principle of the proposed Sensor
Figure 1 illustrates the schematic diagram of proposed magnetic field and temperature sensing system. Two optical carriers from two tunable laser sources (TLSs) are combined by a 3dB coupler and sent to a phase modulator (PM) via three polarization controllers (PCs). At the output of the PM, two phase-modulated signals are generated and sent to the cascaded MA–FBG-FP and FBG-FP filters through an optical circulator (OC). The −1st sideband of the phase-modulated signals are removed using the cascaded filters. Thus, single sideband (SSB) optical signals are obtained and phase-modulation to intensity-modulation conversion is achieved. The reflected SSB optical signals are converted to an electrical signal using a high-speed PD. Then, the generated electrical signal is amplified by an electrical amplifier (EA) and fed back to the PM to form the OEO. The generated microwave signal from the PD are monitored by an electrical spectrum analyzer (ESA). In the proposed scheme, the cascaded MA-FBG-FP and FBG-FP filters in conjunction with the two laser sources and PM operate as a dual-passband MPF. The MA-FBG-FP filter is fabricated by bonding the FBG-FP filter to a MA from two ends using with UV glue, as shown in Fig. 1. Note that the FBG-FP filter was slightly stretched before being glued onto the MA in order to achieve high mechanical coupling under the stimulus of magnetic fields.
Fig. 1 Schematic of the proposed dual-frequency OEO based on MA-FBG-FP and FBG-FP filters for simultaneous magnetic field and temperature sensing.
Figure 2 depicts the oscillating principle for the proposed OEO. The cascaded filters have two ultra-narrow notches in two reflection bands corresponding to the MA-FBG-FP filter and FBG-FP filter, respectively, as shown in Fig. 2(a). When the phase-modulated signal is fed to the cascaded filters, for the MA-FBG-FP filter, the signal is converted into an intensity-modulated signal since the −1st sideband of the phase-modulated signal is filtered out. After PD, a microwave signal (fOSC1) with the frequency being the wavelength difference between the optical carrier and the notch is obtained. Likewise, for the FBG-FP filter, a second microwave signal (fOSC2) is obtained. Due to the nonlinearity of OEO loop, a beat microwave signal (fBeat) whose frequency equals to the frequency difference between the two microwave signals (fOSC1 and fOSC2) is generated. Thus, as shown in Fig. 2(b), two oscillating signals with the frequencies corresponding to the center frequencies of the passbands of the MPF as well as a beat microwave signal are realized. Assuming that the laser sources have longer wavelengths than the corresponding FP filters. Mathematically, the frequencies of the two oscillating signals can be expressed as [33]
where fs1, s2 or λs1, s2 denote the center frequency or wavelength of the two TLS, fN1, N2 or λN1, N2 denote the center frequency or wavelength of the notch of the sensing filter (1 and 2 represent the MA-FBG-FP filter case and FBG-FP filter case, respectively). neff is the effective refractive index of the fiber core and c is the velocity of light in the vacuum.Fig. 2 The oscillating principle of the proposed OEO. (a) The relationship between the optical carrier and the reflection spectrum of the cascaded MA-FBG-FP and FBG-FP filters. (b) Frequency response of the dual-passband MPF.
For the MA-FBG-FP filter, the applied magnetic field deforms on the MA. Then the strain in the MA resulting from an applied magnetic field is transferred to the FBG-FP filter bonded on it. Thus, the central frequency of the notch of MA-FBG-FP filter will be shifted accordingly due to the variation of the grating pitch with the magnetic field changing. In addition to the magnetic field, the MA-FBG-FP filter is also affected by the temperature. Assuming the external magnetic field and temperature changes are ΔH and ΔT, respectively. The magnetic field induced strain , which is linear to the external magnetic field (in the linear region) and finally transferred to the MA-FBG-FP filter, can be expressed as = k•ΔH. Here, k is a coefficient proportional to the magnetostrictive constant of MA. When the magnetic field changes or environmental temperature varies, the magnetic field- and temperature-induced wavelength shift of the notch of MA-FBG-FP filter can be obtained by [17]
where pe (0.22) is the elastic-optic coefficient of optical fiber, αM is the linear thermal expansion coefficient of MA, ξ ()and αf () are the thermo-optic coefficient and the thermal expansion coefficient of the fiber respectively. The unit of αM, ξ and αf is / °C. The unit of αM, ξ and αf is /°C. The units for k, ,are /Oe, Oe and °C, respectively.For the FBG-FP filter, the wavelength of the notch of FBG-FP filter remains constant as the magnetic field is applied. When the ambient magnetic field and surrounding temperature changes, the wavelength shift of the notch of FBG-FP filter is only determined by temperature change and can be expressed as
From the Eqs. (2) and (3), we can see that the wavelength changes of two notches of the filters are linearly proportional to the magnetic field and temperature variations. When magnetic field and temperature are simultaneously applied to the cascaded filters, the frequencies of the two generated microwave signals can be expressed using a matrix as
where KH1 is the magnetic field sensitivity of the first oscillating signal, KT1 and KT2 are the sensitivities of the two generated microwave frequencies to the temperature. The shift of beat frequency subjected to the applied magnetic field and temperature changes is given byThe elements of the K matrix can be calculated by experimental results. It can be clearly seen that the ambient magnetic field and temperature are mapped to the frequency change in the microwave domain using OEO loop according to Eqs. (4) and (5). Thus, through inversely solving the Eq. (4), simultaneous magnetic field and temperature sensing can be realized by monitoring the frequency changes of oscillating signals.3. Experiment setup and results
The key devices in our experiment are the cascaded FBG-FP and MA-FBG-FP filters. The FBG-FP filter is consisting of two identical FBGs which are directly written in a hydrogen-loaded fiber using a 14 cm phase mask with a period of 1075nm exposed under a 248nm KrF excimer laser. The space of the two FBGs is controlled by switching the excimer laser and changing the writing position in order to introduce an equivalent phase shift at the center of the filter. The detailed fabrication processes of the proposed MA-FBG-FP filter is as follows: 1. Fabricate a second FBG-FP filter based on the same method mentioned above using another phase mask with a different period (1068nm). 2. Fix the second FBG-FP filter using two fiber clamps and make sure that the fiber is slightly stretched. 3. Bond a wire-shaped MA on the FBG-FP filter at two ends of the filter with UV glue. 4. Keep the sensing element at room temperature for 24h to make sure the properties of glue are totally stable. In the experiment, we use Fe83Ga17 wire (Suzhou A-one Alloy, Ltd, China) with diameter of 0.5 mm and length of 2 cm as the MA.
Figure 3 (a) and (b) respectively illustrate the reflection spectra of the MA-FBG-FP filter and FBG-FP filter which are measured by an optical spectrum analyzer (OSA: APEX, AP2051A). The center wavelengths of the MA-FBG-FP and FBG-FP filters are 1546.89nm and 1556.05nm with a 3dB bandwidth of about 100MHz and 80MHz, respectively. Note that the optical power of the narrow notch should be lower than the measured optical power level which is limited by the resolution of the OSA.
For the magnetic field sensing, the sensor head based on the cascaded filters are fixed straight between two poles of permanent magnets, which can generate a uniform magnetic field. The intensity of the magnetic field is adjusted by changing the distance between the two magnets. The magnetic field direction is parallel to the sensing structure. A Gauss meter (HT20, Shanghai Huntoon Magnetic Technology Co., Ltd., China) located next to the sensor head is used to measure the magnetic field strength. In the experiment, the FBG-FP and MA-FBG-FP filters are placed after the OC in close and parallel to experience the same magnetic field. The intensity of magnetic field is increased from 20Oe to 70Oe with a step of 50Oe at a constant room temperature, the frequencies of the generated microwave signals are recorded. Figure 4 shows the superimposed frequency spectra of the generated microwave signals and beat signal measured by an ESA (Agilent N9010A, 9kHz~26.5GHz) at different magnetic field strength. When the strength of magnetic field increases, the first microwave frequency corresponding to MA-FBG-FP filter is shifted linearly to lower frequencies. Meantime, since no magnetic induced strain is applied to the FBG-FP filter, the frequency of the second microwave signal corresponding to FBG-FP filter is almost invariant. Thus, the beat signal is linearly shifted to higher frequencies. Figure 5 shows the relationship between the magnetic field and the frequencies of two microwave signals and the beat signal. Linear fitting is done for the magnetic field sensing in the range from 20 to 70Oe. As shown in Fig. 5(a), the magnetic field sensitivity of the first microwave signal can reach as high as −38.4 MHz/Oe, while the frequency of the second signal remain unchanged. Meantime, a magnetic field sensitivity 38.4 MHz/Oe of the beat signal is obtained as illustrated in Fig. 5(b).
Fig. 4 The superimposed electrical spectra of the generated microwave signals under different magnetic field strength at a constant room temperature.
Fig. 5 (a) The frequency shifts of two generated microwave signals versus the magnetic field strength. (b) The frequency shift of the beat signal versus the magnetic field strength.
Then, the influence of temperature on the proposed sensor is investigated. To do so, the sensing element of OEO is placed in the temperature-controlled container which adjusts the temperature to increase from 20°C to 21°C with a step of 0.2°C. The superimposed frequency spectra of the generated microwave signals and beat signal are shown in Fig. 6 at different temperature without additional magnetic field applied. When the ambient temperature is increased, the frequency spectrum of the OEO sensor shifts to lower frequency, while the frequency of beat signal is increasing. The relationship between the temperature and the oscillating frequencies of two microwave signals and the beat signal is depicted in Fig. 7. Figure 7(a) displays that the temperature sensitivities of the two microwave signals can respectively reach as high as −1.23GHz/°C and −2.45GHz/°C by fitting the experimental data based on linear regression. Meantime, a temperature sensitivity 1.21 GHz/°C of the beat signal is achieved as illustrated in Fig. 7(b).
Fig. 6 The superimposed electrical spectra of the generated microwave signals under different temperature.
Fig. 7 (a) The frequency shifts of two generated microwave signals versus temperature changing. (b) The frequency shift of the beat signal versus temperature changing.
According to the experimental results, the elements of the coefficient matrix K in Eq. (4) can be obtained by analyzing the slopes of the beat signal and two microwave signals. Thus, the magnetic field and temperature can be measured by inversely solving the following relationship
It is clear that the discrimination between magnetic field and temperature can be achieved when the frequency shifts of two microwave signals are known, which shows the proposed sensor has potential applications in the simultaneous measurements of magnetic and temperature. In our experiment, the sensing range is restricted by the small bandwidth of PD (3 dB bandwidth: 10 GHz). In an OEO loop, the phase noise mainly determined by the loop length fluctuation in general. By extending the loop length, the free spectral range (FSR) and the phase noise of the OEO can be greatly reduced. However, when the FSR and phase noise become smaller, the stability of the OEO will decrease. The phase noise performance is a key parameter for the generation of high quality microwave signal using OEO. For the proposed sensing scheme, the phase noise is not critical to the system measurement performance. In our experiment, the fiber loop length of OEO is about 39 m and the FSR is about 5.2 MHz.
Since the electrical signal of OEO is generated by the beating of laser and the −1st sideband of the phase-modulated signal, the linewidth of the lasers will affect the quality of the generated microwave signal. In the experiment, the linewidth of laser is 100kHz (Agilent 81600B). Generally, the linewidth of a commercial laser is much narrower than the FSR. Thus, most of commercial lasers can meet the linewidth requirement for our proposed scheme and low-cost lasers such as the DBF lasers can be applied to perform magnetic sensing. On the other hand, the power of laser should supply the necessary energy for the OEO since the PM and optical circulator may have additional insertion loss. Besides, the relative wavelength drift of the two lasers will affect our sensing performance since the elements of the K matrix (see Eq. (6)) in the sensing system are different for the magnetic field and temperature changes. For our short-term system, the wavelength drifts can be ignored since the commercial laser used in the experiment has a high wavelength stability. For long-term magnetic field sensing, a wavelength calibration system can be adopted to increase the accuracy of the device as the proposed system is applied in practice.
It is worthy to note that although several fiber-laser-based methods using Farady effect have been proposed for magnetic field sensing, which overcome the problems of low interrogation speed and resolution by mapping the sensing information into frequency change of the beat signal. Nevertheless, the sensitivities of these kind of sensors (7.09kHz/Oe in [12] and ~0.4 kHz/ Oe in the range from 0 Oe to 500 Oe in [13]) are relatively low compared with our proposed scheme. In our proposed scheme, the magnetostriction coefficient of MA and the fabrication method of MA-FBG-FP filter will affect the sensitivity and accuracy of the sensor. As a result, in order to further improve the sensitivity of magnetic field measurement, we can choose the MA with high magnetostriction coefficient or adopt microstructure processing [34,35] and surface coating technique [15,36] as the proposed scheme is applied in practice.
4. Conclusion
In summary, we have proposed and experimentally demonstrated a magnetic field fiber-optic sensor based on a dual-frequency OEO incorporating cascaded MA-FBG-FP and FBG-FP filters. The novelty of our work lies on the combination of OEO technology and fiber-optic sensors for magnetic field sensing. Thanks to the unique advantages provided by fiber, the proposed scheme will satisfy the requirement of magnetic field sensors able to work with simple structure also in harsh environments. Besides, the use of OEO technology can greatly improve the interrogation speed and resolution for fiber-optic sensors by translating the magnetic field and temperature information to frequency shifts of the generated microwave signals instead of the wavelength change in optical domain. In addition, the proposed method overcomes the temperature cross effect problem because the central wavelengths of MA-FBG-FP and FBG-FP have different magnetic field and temperature sensitivities. By monitoring the frequencies of the two microwave signals, simultaneous magnetic field and temperature measurements can be implemented. In our scheme, an ESA is used for interrogation and the interrogation speed can reach about several kHz. In the real applications, a DSP unit can be used for measuring the frequency change and the speed interrogation can be greatly improved to MHz range. To realize the large-scale deployment of the sensor, the integration technology of OEO [37,38] can be applied to make the proposed sensor more compact and cheaper. Besides, with the development of heterogeneous integration, the lasers used in our scheme can also be integrated into the chip to achieve monolithic integration. A proof-of-concept experiment is conducted in order to verify the feasibility of the proposed scheme. It our results, the generated signal corresponding to FBG-FP filter can be considered insensitive to magnetic field. The magnetic field sensitivity of the microwave signal corresponding to MA-FBG-FP filter is −38.4 MHz/Oe, and the temperature sensitivities of two microwave signals are −1.23 and −2.34 GHz/°C, respectively.
Funding
National Natural Science Foundation of China (NSFC) (61801017, 61805010, 61475015, 61775015, 61620106014, 61827818); China Postdoctoral Science Foundation (2018M631327); Fundamental Research Funds for the Central Universities (W17RC00020, W17JB00550).
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