Remote Magnetic Field Sensor Based on Intracavity ... - IEEE Xplore

Remote Magnetic Field Sensor Based on Intracavity Absorption of Evanescent Field Volume 8, Number 2, April 2016 Jia Shi Yuye Wang Degang Xu Genghua Su Haiwei Zhang Jiachen Feng Chao Yan Shijie Fu Jianquan Yao

DOI: 10.1109/JPHOT.2016.2538079 1943-0655 Ó 2016 IEEE

IEEE Photonics Journal

Remote Magnetic Field Sensor

Remote Magnetic Field Sensor Based on Intracavity Absorption of Evanescent Field Jia Shi, Yuye Wang, Degang Xu, Genghua Su, Haiwei Zhang, Jiachen Feng, Chao Yan, Shijie Fu, and Jianquan Yao 1

Institute of Laser and Optoelectronics, College of Precision Instrument and Optoelectronics Engineering, Tianjin University, Tianjin 300072, China 2 Key Laboratory of Optoelectronics Information Technology, Ministry of Education, Tianjin University, Tianjin 300072, China DOI: 10.1109/JPHOT.2016.2538079 1943-0655 Ó 2016 IEEE. Translations and content mining are permitted for academic research only. Personal use is also permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.

Manuscript received January 24, 2016; revised February 25, 2016; accepted March 1, 2016. Date of publication March 3, 2016; date of current version March 16, 2016. This work was supported by the National Basic Research Program of China (973) under Grant 2015CB755403 and Grant 2014CB339802; by the National Natural Science Foundation of China under Grant 61107086, Grant 61172010, and Grant 61471257; by the Natural Science Foundation of Tianjin under Grant 14JCQNJC02200; by the Science and Technology Support Program of Tianjin under Grant 14ZCZDGX00030; and by the Specialized Research Fund for the Doctoral Program of High Education under Grant 20120032110053. Corresponding author: D. Xu (e-mail: [email protected]).

Abstract: In this paper, a remote magnetic field sensor based on intracavity absorption of evanescent field is experimentally demonstrated. A single-mode–no-core–singlemode (SNCS) fiber coated with magnetic fluid (MF) is inserted into a fiber ring laser through a circulator and a 3-dB coupler, and it works as the filter and sensing head simultaneously. As the light oscillates in the cavity of the fiber ring laser, intracavity absorption of the evanescent field will be induced in the interface of the no-core fiber and MF. We obtain a magnetic field sensor with a narrow 3-dB bandwidth that is less than 0.05 nm, a high signal-to-noise ratio ∼40 dB, and magnetic sensitivity of 52.1 pm/mT and −0.3679 dBm/mT. Moreover, experiments show that it is convenient to achieve remote sensing by selecting different lengths of the transmission optical fiber connecting the circulator and 3-dB coupler, which has a slight impact on the sensitivity.

Index Terms: Fiber optics sensors, remote sensing, intracavity sensing.

1. Introduction Magnetic field sensors have been investigated with increasing interest due to their significant application in the detection of current and magnetic fields. The conventional methods of measuring magnetic field usually use electromagnetic current transformers. In recent years, some all-fiber magnetic field sensors have been experimentally demonstrated, owing to their advantages such as anti-interference, high sensitivity, compactness, and fast responses [1]. Some special structures of fiber sensors coated with MF have been applied to measure magnetic field. Photonic crystal fiber (PCF) injected with Fe3O4 nanofluid has been designed for magnetic field detection and it has the sensitivity of 242 pm/mT [2]. Tilted fiber grating has been used to measure magnetic field [3], [4]. Chen et al. realize the measurement of magnetic field through the singlemode-multimode-single-mode (SMS) fiber coated with MF and get the sensitivities of 905 pm/mT and 0.748 dB/mT when the particle density of MF is 1.18 g/cm3 [5]. A magnetic field sensor

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Fig. 1. Structure of SNCS fiber coated with MF.

based on a non-adiabatically tapered microfiber is experimentally demonstrated and the sensitivities of 174.4 pm/Oe and −0.02 dB/Oe are achieved [6]. Moreover, various optical fiber devices sensitive to magnetic field are developed, such as microfiber knot resonator [7], fiber-optic Fabry–Pérot interferometer [8], couplers [9], [10], etc. However, the ability of these sensors for remote magnetic field sensing is rarely taken into account. In addition, these passive sensors always use broadband light source which limits the output intensity and the long-distance transmission will contribute to large attenuation of their output intensity. Intracavity sensors based on fiber laser have been investigated extensively and used widely for their excellent performance in high intensity and narrow 3-dB bandwidth [11]. All kinds of fiber laser sensors have been proposed to measure refractive index (RI) [11], temperature [12], vibration [13], and strain [14]. In particular, intracavity sensors based on intracavity gas absorption have a higher sensitivity [15]. However, there have been no active sensors based on intracavity absorption of evanescent field, to the best of the authors’ knowledge. In this paper, we propose a remote magnetic field sensor based on intracavity absorption of evanescent field. The structure of SNCS fiber coated with MF has been inserted into a fiber ring laser through a circulator and a 3-dB coupler. As the intensity of external magnetic field increases, the emission wavelength of the fiber ring laser will have a redshift and the output intensity will decrease, corresponding to the sensitivities of 52.1 pm/mT and −0.3679 dBm/mT, respectively. The proposed sensor has a narrow 3-dB bandwidth less than 0.05 nm and high signal-to-noise ratio (SNR) ∼40 dB. What is more, the ability of the intracavity sensor for remote sensing has been demonstrated by selecting different length of transmission optical fiber (TOF) which connects the circulator and 3-dB coupler. Experiments show that the length of TOF has a little impact on the sensitivity. The proposed intracavity sensor has a narrow 3-dB bandwidth, high SNR, and the ability of remote magnetic field sensing.

2. Theoretical Background The schematic configuration of SNCS fiber coated with MF is shown in Fig. 1. The no-core fiber (NCF) is spliced between two single-mode fibers. The SNCS fiber is sealed in a glass tube which is coated with MF. Both ends of glass tube are wrapped with paraffin. Assuming the single-mode fiber and NCF are ideally aligned, only the symmetric mode LP0m can be excited, and thus only linear polarized radial modes will excited and transmitted in the NCF. Defining the field profile of LP0m mode as m ðr Þ, the input field of NCF can be expressed as E ðr ; 0Þ ¼

M X

cm

m ðr Þ

(1)

m¼1

where r is the radial coordinate in the cross section of the fiber. M is the number of mode order excited in NCF, which is determined by the refractive index of MF, refractive index and diameter of NCF. cm is the excitation coefficient of the LP0m mode, which can be given by R1 E ðr ; 0Þ m ðr Þrdr cm ¼ R01 : (2) m ðr Þ m ðr Þrdr 0


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Fig. 2. Structure of the proposed intracavity sensor.

As the light transmitting in the NCF, the absorption of evanescent field will be induced in the interface of NCF and MF based on evanescent field absorption theory [16]. The field distribution after propagating a distance z can be written as [5] E ðr ; zÞ ¼

M X

cm

m ðr Þexpðm zÞexpðim zÞ

(3)

m¼1

where m is the absorption coefficient of evanescent field, and m is the propagating constant of the LP0m mode. The transmittance of the SNCS can be calculated by " # R 1 2 0 E ðr ; zÞE ðr ; 0Þrdr T ¼ 10 log10 R 1 : (4) R1 2 2 0 jE ðr ; zÞj rdr 0 jE ðr ; 0Þj rdr When external magnetic field of SNCS fiber coated with MF changed, transmission spectrum will shift, and transmission intensity will change, which are both determined by refractive index of the MF and attenuation of the evanescent field.

3. Experimental Setup and Results In this paper, we inserted a SNCS fiber coated with MF into a fiber ring laser to induce intracavity absorption of evanescent field. The schematic configuration of proposed sensor is shown in Fig. 2, which consists of a pump source, a piece of Erbium-doped fiber (EDF), a wavelength division multiplexing (WDM) coupler, an isolator (ISO), a circulator, a 3-dB coupler, a SNCS fiber coated with MF, a piece of transmission optical fiber (TOF), a 10:90 coupler, as well as an optical spectrum analyzer (OSA). The SNCS fiber coated with MF combined with the 3-dB coupler is used as the sensing head. The gain medium is a piece of EDF (Nurfen, EDFC-980-HP) with the length of 3 m, which is pumped by a diode laser centered at 976 nm via a 980/1550 nm WDM coupler (AFR Inc.). The ISO works to operate unidirectionally and prevent spatial hole-burning [17]. After coupled in a circulator, the input light transmits through the TOF and is split into two counter-propagating light beams by a 3-dB coupler. These two light beams pass through the SNCS fiber, recombine at the 3-dB coupler, and transmit back through TOF, which converts the transmission SNCS fiber sensor to a reflected sensing head. In the sensing head, the length of no-core fiber (Tianjin Opticontact Technology Development Co., Ltd) is 10 cm with the diameter of 100 m, and the MF (EMG605, Ferrotec Inc.) has a dilute particle density of 0.59 g/cm3 with the average diameter of


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Fig. 3. Output spectrum of the proposed sensor with 2-m-long TOF and the transmission spectrum of the SNCS fiber coated with MF, which are measured with on magnetic field.

nanoparticles about 10 nm. The SNCS fiber coated with MF works as sensing head and filter simultaneously in the intracavity sensor. TOF is selected as single-mode fiber (Corning, SMF-28) to connect the circulator and the 3-dB coupler. The reflected sensing head make it convenient to realize remote sensing through selecting different length of TOF. The output spectrum of the intracavity sensor is measured by an OSA with the spectral resolution of 0.05 nm (YOKOGAWA, AQ6375) via a 10:90 coupler. The emission wavelength of the fiber ring laser will be at the one that has the minimum spectral transmission loss internal the range of EDF amplifier, which is mainly determined by the filtering characteristics of the SNCS fiber [18], operating wavelength of EDF, ISO, WDM, circulator, and couplers in the system. When there is no external magnetic field, the transmission spectrum of the SNCS fiber coated with MF is shown in Fig. 3, which is measured by a supercontinuum light source (NKT Photonics, SuperK COMPACT) and an OSA (YOKOGAWA, AQ6375). Then, it is inserted into the fiber ring laser using a 2-m-long TOF and the output spectrum of the proposed intracavity sensor with no magnetic field is shown in Fig. 3. The 3-dB bandwidth and SNR are less than 0.05 nm limited by the spectral resolution of OSA and about 40 dB, respectively. As there is no external magnetic field, these nanoparticles of MF are subjected to random thermal Brownian motions, with randomly oriented magnetic momentum [19]. The sensing head is placed in a controlled magnetic field. As the intensity of magnetic field varies, the refractive index of MF and attenuation of evanescent field are both changed, which contributes to the changing of output spectrum wavelength and intensity. By measuring output wavelength or intensity, the intensity of external magnetic field can be measured. The measured corresponding output optical spectra of the fiber ring laser with the external magnetic field of the sensing head increasing from 0 Oe to 120 Oe are shown in Fig. 4(a) with the pump fixed at 150 mW and 2-m-long TOF. Then, the ability of remote sensing is demonstrated by selecting the TOF with the length of 2 km and 5 km, respectively. The pump is increased to 200 mW and the output spectra of the intracavity sensor are shown in Fig. 4(b) and (c). As depicted in Fig. 4, when the intensity of external magnetic field increases, the output spectrum wavelength of the intracavity sensor has a redshift, at the same time, the output intensity has a decrease. The redshift of wavelength and the decrease of intensity are mainly due to the change of the refractive index of the MF and the attenuation of the evanescent field in the SNCS fiber [5]. As the magnetic field controlled from 0 Oe to 120 Oe, the fitted curves of output


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Fig. 4. Output spectra of the intracavity sensor when the external magnetic field of the sensing head increases from 0 Oe to 120 Oe. (a) Length of TOF is 2 m. (b) Length of TOF is 2 km. (c) Length of TOF is 5 km.

Fig. 5. Experimental data and fitted curves of the output of the intracavity sensor when the external magnetic field is controlled from 0 Oe to 120 Oe. (a) Experimental data and fitted curves of output wavelength. (b) Experimental data and fitted curves of output intensity. (c) Wavelength and power stability over 270 seconds measured by fixing the magnetic field at 40 Oe and 100 Oe with TOF∼2 km.


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wavelength are shown in Fig. 5(a), as well as the fitted curves of output intensity are shown in Fig. 5(b). From Fig. 5(a), we can find out that, when using 2-m-long TOF, wavelength sensitivity of magnetic field is 0.00521 nm/Oe, i.e., 52.1 pm/mT, which is pumped by 150 mW. What is more, selecting the length of TOF as 2 km and 5 km has a little impact on the wavelength sensitivity, corresponding to the sensitivities of 56.4 pm/mT and 48.9 pm/mT, which are both pumped by 200 mW. It is depicted in Fig. 5(b) that, the sensitivity of output intensity is −0.03679 dBm/Oe, i.e., −0.3679 dBm/mT with 2-m-long TOF used and pumped by 150 mW. When the longer TOF used and pump power increased to 200 mW, adjacent sensitivities can be achieved as −0.4024 dBm/mT and −0.399 dBm/mT, corresponding to using 2-km-long TOF and 5-km-long TOF, respectively. The external magnetic field can be measured by the output wavelength or intensity of the proposed sensor. Fig. 5(a) and (b) indicate that the length of TOF has a little impact on the sensitivity. In addition, we get the wavelength and power stability over 270 seconds shown in Fig. 5(c) by fixing the magnetic field at 40 Oe and 100 Oe with TOF∼2 km, respectively. The difference between the maximum and minimum values for wavelength is less than 20 pm and that for intensity is less than 0.13 dBm, corresponding to the maximum measuring errors 0.35 mT and 0.32 mT, respectively. It reflects that the sensor possesses a good stability and small measuring errors.

4. Conclusion We demonstrate a remote magnetic field sensor based on intracavity absorption of evanescent field. The structure of SNCS fiber coated with MF is inserted into a fiber ring laser, which works as the filter and sensing head simultaneously in the system. As the external magnetic field increases, the refractive index of the MF and attenuation of the evanescent field are both changed, then the output wavelength has a redshift and output intensity decreases. We obtain an intracavity magnetic field sensor with a narrow 3-dB bandwidth less than 0.05 nm, a high SNR ∼40 dB, and magnetic sensitivity of 52.1 pm/mT and –0.3679 dBm/mT. What is more, the excellent performance of the proposed intracavity sensor for remote sensing has been shown by using different length of TOF of the sensing head. Experiments show that the length of TOF has little impact on the sensitivity.

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