Highly-sensitive magnetic field sensor based on fiber ring laser Ming Deng, Danhui Liu, Wei Huang, and Tao Zhu* Key Laboratory of Optoelectronic Technology & Systems (Ministry of Education), Chongqing University, Chongqing 400044, China; * [email protected]
Abstract: A highly sensitive magnetic field sensor based on a fiber ring laser has been proposed and experimentally demonstrated. The magnetic field sensor was fabricated by introducing a rotary apparatus modulated by an external magnetic field into the fiber cavity to twist one section of the fiber. Due to the remarkable birefringence change induced into the laser cavity, the beat frequency generated between two polarizations of the laser is sensitive to the variation of applied magnetic field intensity. Experimental results show that the polarization mode beat frequency linearly shifts with the increment of the magnetic field intensity and the sensitivity reaches up to 7.09 KHz/Oe in the range of 0 - 437 Oe. Therefore, it will be a promising candidate for the weak magnetic field applications including military, hazard forecast and biomedical fields. ©2016 Optical Society of America OCIS codes: (060.2370) Fiber optics sensors; (280.3420) Laser sensors; (230.3810) Magnetooptic systems.
References and links 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16.
J. E. Lenz, “A review of magnetic sensors,” in Proceedings of IEEE Conference (IEEE, 1990), pp. 973–989. S. C. Rashleigh, “Magnetic-field sensing with a single-mode fiber,” Opt. Lett. 6(1), 19–21 (1981). C. L. Tien, C. C. Hwang, H. Chen, W. Liu, and S. Lin, “Magnetic sensor based on side-polished fiber Bragg grating coated with iron film,” IEEE Trans. Magn. 42(10), 3285–3287 (2006). M. Yang, J. Dai, C. Zhou, and D. Jiang, “Optical fiber magnetic field sensors with TbDyFe magnetostrictive thin films as sensing materials,” Opt. Express 17(23), 20777–20782 (2009). Y. Chen, Q. Han, T. Liu, X. Lan, and H. Xiao, “Optical fiber magnetic field sensor based on single-modemultimode-single-mode structure and magnetic fluid,” Opt. Lett. 38(20), 3999–4001 (2013). M. Deng, C. Huang, D. Liu, W. Jin, and T. Zhu, “All fiber magnetic field sensor with Ferrofluid-filled tapered microstructured optical fiber interferometer,” Opt. Express 23(16), 20668–20674 (2015). A. D. Kersey and D. A. Jackson, “Current sensing utilizing heterodyne detection of the Faraday effect in singlemode optical fiber,” J. Lightwave Technol. 4(6), 640–644 (1986). L. Sun, S. Jiang, and J. R. Marciante, “All-fiber optical magnetic-field sensor based on Faraday rotation in highly terbium-doped fiber,” Opt. Express 18(6), 5407–5412 (2010). G. W. Day, D. N. Payne, A. J. Barlow, and J. J. Ramskov-Hansen, “Faraday rotation in coiled, monomode optical fibers: isolators, filters, and magnetic sensors,” Opt. Lett. 7(5), 238–240 (1982). H. K. Kim, S. K. Kim, H. G. Park, and B. Y. Kim, “Polarimetric fiber laser sensors,” Opt. Lett. 18(4), 317–319 (1993). B. Liu and H. Zhang, “Polarimetric distributed Bragg reflector fiber laser sensor array for simultaneous measurement of transverse load and temperature,” Opt. Fiber Technol. 17(6), 619–625 (2011). A. Frank, K. Bohnert, K. Haroud, H. Brandle, C. V. Poulsen, J. E. Pedersen, and J. Patscheider, “Distributed feedback fiber laser sensor for hydrostatic pressure,” IEEE Photonics Technol. Lett. 15(12), 1758–1760 (2003). J. Gao, H. Wang, X. Guo, and C. G. Lyu, “Directional force measurement technology based on fiber optical laser heterodyning demodulation,” Proc. SPIE 9522, 95220E (2015). C. Lyu, C. Wu, H.-Y. Tam, C. Lu, and J. Ma, “Polarimetric heterodyning fiber laser sensor for directional acoustic signal measurement,” Opt. Express 21(15), 18273–18280 (2013). L. Cheng, J. Han, Z. Guo, L. Jin, and B. O. Guan, “Faraday-rotation-based miniature magnetic field sensor using polarimetric heterodyning fiber grating laser,” Opt. Lett. 38(5), 688–690 (2013). L. Cheng, J. Han, L. Jin, Z. Guo, and B. O. Guan, “Sensitivity enhancement of Faraday effect based heterodyning fiber laser magnetic field sensor by lowering linear birefringence,” Opt. Express 21(25), 30156– 30162 (2013).
#254071 © 2016 OSA
Received 17 Nov 2015; revised 23 Dec 2015; accepted 24 Dec 2015; published 8 Jan 2016 11 Jan 2016 | Vol. 24, No. 1 | DOI:10.1364/OE.24.000645 | OPTICS EXPRESS 645
17. L. Gao, L. Chen, L. Huang, and X. Chen, “Multimode fiber laser for simultaneous measurement of strain and temperature based on beat frequency demodulation,” Opt. Express 20(20), 22517–22522 (2012). 18. J. Wo, M. Jiang, M. Malnou, Q. Sun, J. Zhang, P. P. Shum, and D. Liu, “Twist sensor based on axial strain insensitive distributed Bragg reflector fiber laser,” Opt. Express 20(3), 2844–2850 (2012). 19. R. Ulrich and A. Simon, “Polarization optics of twisted single-mode fibers,” Appl. Opt. 18(13), 2241–2251 (1979). 20. T. Zhu, S. Huang, L. Shi, W. Huang, M. Liu, and K. Chiang, “Rayleigh backscattering: a method to highly compress laser linewidth,” Chin. Sci. Bull. 59(33), 4631–4636 (2014).
1. Introduction Magnetic field measurement is increasing important in many fields such as aeronautics, information storage, environmental monitoring and hazard forecast [1]. Compared to electronic techniques employed in magnetic field detection, fiber-optic magnetic field sensors are more attractive due to their advantages of immunity to electromagnetic interference, high sensitivity, low power consumption, and in situ real-time detection. So, many research efforts have been concentrated on optical fiber magnetic field sensors by employing different mechanisms. A popular scheme is developing fiber-optic magnetic field sensors based on material coatings. For example, magnetostrictive jacket can be deposited on a fiber or an FBG, where strain in a magnetostrictive material resulting from an applied magnetic field is transferred to an optical fiber [2–4]. Moreover, optical fiber magnetic field sensors by combining magnetic fluid with optical fiber interferometers have also been demonstrated [5,6]. These sensors are sensitive to weak magnetic field ranging to several hundred oersted, but their broadband linewidth degrades the measurement accuracy. On the other hand, other research groups have proposed fiber magnetic sensors based on Faraday effect, in which the rotation angle of the polarization plane of a linear polarized light varies with the applied magnetic field strength [7,8]. However, very long sensing fiber has to be employed to improve the sensitivity [9], which results in some disadvantages such as unreliability due to environmental disturbances. Polarimetric fiber laser sensors offer advantages of easy interrogation, high signal-tonoise ratio, absolute encoding, and narrow linewidth which determines that sensors have an inherent high resolution. The excellent properties make them applicable for the measurement of various physical parameters, such as lateral stress [10], temperature [11], hydrostatic pressure [12], directional force [13] and acoustic signal [14]. Recently, Guan et al demonstrated the sensing principle of a polarimetric heterodyning fiber grating laser as a magnetic field sensor [15]. Due to the large intrinsic linear birefringence inside the laser cavity, the nonlinear beat frequency shift is only 520 KHz as the magnetic field intensity is increased to 4500 Oe. Then, they proposed heat treatment of the fiber laser cavity by CO2laser irradiation to permanently modify the linear birefringence and tune the sensitivity. As a result, the beat frequency shift is up to 5 MHz in the range of 4500 Oe, but a still very low sensitivity of ~0.4 KHz/ Oe in the weak magnetic field ranging from 0 Oe to 500 Oe [16]. Moreover, the nonlinear magnetic field response makes it difficult in practical applications. In this paper, we demonstrate a highly sensitive magnetic field sensor based on a fiber ring laser. A home-made rotary apparatus which is modulated by an external magnetic field is introduced into the fiber cavity and utilized to twist one section of the fiber. Due to the remarkable birefringence change induced into the laser cavity, the beat frequency generated between two polarizations of the laser is sensitive to the variation of applied magnetic field intensity. Experimental results show that the linear magnetic field sensitivity is up to 7.09 KHz/Oe, which is about one or two orders of magnitude higher than those of the above mentioned polarimetric fiber laser sensors in the weak magnetic field applications. Therefore, such a device can be operated as a high-sensitivity magnetic field sensor with advantages of easy fabrication, low cost and high accuracy.
#254071 © 2016 OSA
Received 17 Nov 2015; revised 23 Dec 2015; accepted 24 Dec 2015; published 8 Jan 2016 11 Jan 2016 | Vol. 24, No. 1 | DOI:10.1364/OE.24.000645 | OPTICS EXPRESS 646
2. Operating principle There are multi-longitudinal modes in a long fiber ring laser cavity and each longitudinal mode is split into two polarization modes, X-mode and Y-mode, due to their different polarization directions and the effective cavity birefringence. The polarization mode beat frequency (PMBF) is generated by multi-longitudinal modes of different polarization directions, while the linear mode beat frequency (LMBF) is produced by multi-longitudinal modes of the same polarization direction. For lasers with a sufficiently small birefringence, the PMBF can be measured with a commercial RF spectrum analyzer, which is given by [17]
ν PMB = ν LMB + Δν = ν LMB ± McB
2n 2 λ
(1)
B = nx − n y , nx and n y are the effective refractive indices of X-mode and Y-mode,
respectively. c is the speed of light in vacuum, n is the effective refractive index of the fiber, L is the whole laser cavity length and M is the mode number. ν LMB can be expressed by
ν LMB = ν i −ν j =
ic jc c Nc − = (i − j ) = 2nL 2nL 2nL 2nL
(2)
i and j present the ith and jth longitudinal modes in laser cavity, N is the multiple of mode spacing. Due to the effective refractive indices of the two polarization directions are thought to be the same in the calculation of LMBF, so the LMBF is not dependent on the fiber birefringence change caused by any external perturbation. Equation (1) shows that the PMBF is proportional to the fiber birefringence. Therefore, any perturbation resulting in the birefringence change can be detected by discriminating the beat frequency shift through simple electronic signal processing. When a magnetic field parallel to the laser is applied on part of the fiber ring laser cavity, a weak circular birefringence is induced into the laser cavity through Faraday effect, resulting in a low sensitivity to magnetic field [15]. One straightforward method to enhance the circular birefringence may be to apply a twist to the laser cavity with an external transducer. By fixing one end of the fiber and twist the other end, the induced circular birefringence can be described as [18]:
δ B = B0
sin (ω - α ) × Leff ω -α
(3)
B0 is the original birefringence of the fiber cavity, ω is the twist rate and equal to twist angle per the effective twist length, α = gω is the optical rotation, the coefficient of proportionality g is calculated to be ~0.16 for an erbium-doped fiber applied in our system [19]. It can be seen that the birefringence change is dependent on both the fiber twist rate and the effective twist length. According to Eqs. (1)-(3), for a fiber ring laser sensor with given effective twist length, when the laser cavity is subjected to a twist, the fiber circular birefringence will be changed and therefore the polarization mode beat frequency. The relationship between the PMBF and the twist angle is simulated with MATLAB software, which is shown in Fig. 1. Here B0 is set to be 2.3e-7 [19] and Leff is chosen as 80mm. We can see that when the laser cavity is subjected to the twist, the fiber birefringence will be changed periodically as a Sinc function, as well as the beat frequency. Thus, in the twist angle ranging from −90° to 90°, the PMBF is not related to the twisting direction, but determined by the twist angle. In addition, it can be seen that the PMBF monotonously decreases with the increment of the twist angle, and the PMBF shift is about 2.5MHz in the range of 0°-90°, which is far greater than that induced by
#254071 © 2016 OSA
Received 17 Nov 2015; revised 23 Dec 2015; accepted 24 Dec 2015; published 8 Jan 2016 11 Jan 2016 | Vol. 24, No. 1 | DOI:10.1364/OE.24.000645 | OPTICS EXPRESS 647
the Faraday effect. So, when the twist angle is tuned by a magnetic field perpendicular to the laser, such a device offers potentials in a highly sensitive magnetic field sensing applications.
Fig. 1. The relationship between the PMBF and the twist angle
3. Experimental results and discussion
The schematic diagram of the proposed magnetic field sensor based on a fiber laser is shown in Fig. 2(a). A simple and low-cost fiber ring laser is employed in the experiment, in which 10 cm ordinary erbium-doped fiber with peak absorption of 80dB/m near 1530nm provides enough gain for lasing and a phase-shift FBG (central wavelength of 1549.975nm and the 3dB bandwidth of 0.02nm) is utilized to select the beat mode and therefore to enhance the stability of the output laser. The 980 nm laser diode (LD) with a maximum output of 300mW is injected into the laser cavity through a 980/1550 nm wavelength division multiplexer (WDM). Then, the laser is sent into by a 3-dB coupler (OC). One output port of the coupler is connected to an optical spectrum analyzer (OSA, ADVANTEST Q8384), and the other output port of the coupler is injected into a photodetector (PD, THORLABS PDB430C) through a polarizer. The LMBF and PMBF are generated at the PD, and they are measured by an electrical spectrum analyzer (ESA, RIGOL DSA815). The magnetic field is generated by an electromagnet and whose intensity is adjusted by tuning the magnitude of the supply current. The uniform magnetic field zone is Φ50 mm × 25 mm with a uniformity of 20% at one cross section. In order to make the fiber twist under the applied magnetic field, a fiber rotary apparatus is designed to act as a transducer, which is shown in Fig. 2(b). It is mainly composed of a coaxial, non-magnetic fiber clamp and a rotating disk. In addition, a wafer permanent magnet with diameter of 5mm and thickness of 1mm is embedded in the platform to sense the applied magnetic field. It notes that the permanent magnet must be far away from the optical fiber to avoid the circular birefringence which is induced by the intrinsic magnetic field generated in the magnet through Faraday effect. When a magnetic field is applied on the rotary apparatus as shown in Fig. 2(b), the permanent magnet, like a magnetic needle, will continuously rotate with the increment of the magnetic field intensity until its poles are parallel to the magnetic field and therefore leads to a rotation of the whole apparatus. The relationship between the applied magnetic field intensity and the twist angle is calibrated, which is shown in Fig. 2(c). It can be seen that the twist angle monotonously increases with the increment of the magnetic field intensity, and the maximum twist angle change is 80° as the magnetic field intensity is increased to 450 Oe. According to the theory analysis in part 2, we can see that the PMBF decreases with the
#254071 © 2016 OSA
Received 17 Nov 2015; revised 23 Dec 2015; accepted 24 Dec 2015; published 8 Jan 2016 11 Jan 2016 | Vol. 24, No. 1 | DOI:10.1364/OE.24.000645 | OPTICS EXPRESS 648
increment of the applied magnetic field intensity, and the theoretical sensitivity is ~7.78 KHz/Oe. So, this device can be operated as a sensitive magnetic field sensor in weak magnetic field applications.
Fig. 2. (a) Schematic diagram of the magnetic field sensor based on a fiber laser, (b) Schematic diagram of the fiber rotary apparatus, (c) The relationship between magnetic field intensity and the twist angle
To investigate the magnetic field response of the fiber laser sensor, an 8cm length of EDF with the laser cavity was firstly fixed by a fiber holder on one side and the fiber rotator on the other end. Then, the integration was placed in the center of the uniform magnetic field zone and the applied magnetic field was perpendicularly to the laser to eliminate the Faraday effect as well [15]. Because the sensing system is sensitive to the polarization variations inside and outside the ring cavity. So, all parts in our system except for the sensing part are fixed to eliminate the polarization caused by external perturbation. In real applications, an excellent package is necessary to avoid the birefringence being induced into the cavity from external perturbation. In the initial state, namely, the magnetic field intensity is zero, the output spectrum of the fiber laser measured by OSA is shown in the blue curve in Fig. 3(a), whose central wavelength is around 1550.015 nm and the 3-dB bandwidth is about 0.04nm, respectively. The red curves in Fig. 3(a) are the LMBF on the RF spectrum analyzer, detected without the polarizer. It can be seen that there are only three different longitudinal modes in the range of 350MHz, which is consistent with the laser longitudinal mode interval formula Δν LM = c 2nL (L≈1m). When the polarizer is inserted in front of the PD, new beat signals appeared, namely, the PMBF, as seen in Fig. 3(b), which is coincident with the theory mentioned above. Due to the mode selection effect of the PS-FBG filter, the signal noise
#254071 © 2016 OSA
Received 17 Nov 2015; revised 23 Dec 2015; accepted 24 Dec 2015; published 8 Jan 2016 11 Jan 2016 | Vol. 24, No. 1 | DOI:10.1364/OE.24.000645 | OPTICS EXPRESS 649
ratios (SNR) of the beat signals are different. The low-frequency LMB signal has higher SNR, but the high-frequency PMB signal has higher SNR. According to Eq. (1), the highfrequency PMB signal shares the same effective magnetic field sensitivity with the lowfrequency PMB signal. So, the high-frequency PMB signal C of 323.167MHz may be selected for measuring the external magnetic field. It has the SNR and the 3-dB bandwidth of 55dB and ~1kHz, which is much superior to that in [10]. It notes that the linewidth of the PMBF may be further compressed by using the Rayleigh backscattering and therefore to enhance the measurement accuracy [20].
Fig. 3. (a) Optical spectrum of the fiber ring laser sensor observed by OSA and the LMBF without the polarizer observed by RF spectrum analyzer. (b) The original LMBF and the PMBF
The magnetic field intensity applied on the laser cavity for magnetic field sensing is changed from 0 Oe to 437 Oe. Experimental results show that the magnetic field has little effect on the central frequency of the LMBF while a large influence on that of the PMBF, which is mainly because the LMBF does not depend on the birefringence of the cavity caused by the magnetic field. This is in accordance with the theoretical analysis in part 2. The high-frequency PMB signal of 323.167MHz is chosen as the sensing signal that will be detected. As shown in Fig. 4(a), the PMBF shifts to a lower frequency with the increment of the magnetic field intensity with a slight SNR change. The relationship between the PMBF and the applied magnetic field intensity is illustrated in Fig. 4(b) after several repeating test. As the magnetic field intensity increases from 0 Oe to 437 Oe, the PMBF linearly blue shift from 323.167MHz to 320.133MHz. By using a linear fitting of the experimental data as the red line, it is explicit that the magnetic field sensitivity reaches up to 7.09 KHz/Oe with the R2 value of 0.99304, which gets on well with the theoretical analysis mentioned above. The sensitivity of the sensor is one or two orders of magnitude higher than those of the existing polarimetric fiber laser sensors [15, 17], which is mainly because the circular birefringence has been greatly enhanced by applying a twist to the laser cavity with an external transducer. But when we continue to increase the magnetic field intensity, the relationship between the magnetic field and the PMBF will be nonlinear because the fiber birefringence will be changed periodically as a Sinc function with the increment of the magnetic field, as well as the beat frequency, which is in accordance with Eq. (3). In our opinion, the magnetic field sensitivity can be further improved by two methods. One is that changing the thickness and diameter of the permanent magnet so that a greater twist angle will be produced by the same magnetic field intensity, which will introduce a stronger circular birefringence into the laser cavity under the same magnetic field intensity; the other is that increasing the length of the twisted fiber section, which originates from the
#254071 © 2016 OSA
Received 17 Nov 2015; revised 23 Dec 2015; accepted 24 Dec 2015; published 8 Jan 2016 11 Jan 2016 | Vol. 24, No. 1 | DOI:10.1364/OE.24.000645 | OPTICS EXPRESS 650
fact that the birefringence change is proportional to the effective twist fiber length, as well as the beat frequency, according to Eq. (3). Here, we note that the linear response to the magnetic field intensity makes it more competitive in real applications. But, from Fig. 4(b), we can see that such a sensing system cannot be able to implement the measurement of the magnetic field intensity with good repeatability, which is mainly because the rotary apparatus can only clockwise rotate under the magnetic field, a spring can be installed on the rotatable fixture to realize counterclockwise rotation, which will be our next work. Therefore, such a proposed fiber laser sensor offers great potentials in weak magnetic field applications with advantages of high precision, high sensitivity, easy fabrication, and low cost.
Fig. 4. (a) Polarization mode beat frequency of the fiber laser versus different magnetic field intensity, (b) The relationship between the polarization mode beat frequency and the applied magnetic field intensity
4. Conclusions
This paper demonstrates an all-fiber magnetic field sensor which is based on a simple and low-cost fiber ring laser. Due to the remarkable birefringence change caused by a fiber twist which is modulated by a magnetic field perpendicular to the laser, the polarization mode beat frequency is sensitive to the variation of applied magnetic field intensity. Experimental results show that the PMBF linearly shifts to a lower frequency with the increment of the magnetic field intensity, and the sensitivity is up to 7.09 KHz/Oe ranging from 0 Oe to 437Oe. This is one or two orders of magnitude higher than that of the existing fiber laser magnetic field sensors. Therefore, the compact and low-cost fiber laser sensor can be employed in the measurement of weak magnetic field applications including military, hazard forecast and biomedical fields. Acknowledgments
This work is supported by the National Natural Science Foundation of China (NSFC) under Grants No. 61475028, 61475029 and 61575190, the Chongqing Outstanding Youth Project under Grant cstc2014jcyjjq40002.
#254071 © 2016 OSA
Received 17 Nov 2015; revised 23 Dec 2015; accepted 24 Dec 2015; published 8 Jan 2016 11 Jan 2016 | Vol. 24, No. 1 | DOI:10.1364/OE.24.000645 | OPTICS EXPRESS 651
Abstract: A highly sensitive magnetic field sensor based on a fiber ring laser has been proposed and experimentally demonstrated. The magnetic field sensor was fabricated by introducing a rotary apparatus modulated by an external magnetic field into the fiber cavity to twist one section of the fiber. Due to the remarkable birefringence change induced into the laser cavity, the beat frequency generated between two polarizations of the laser is sensitive to the variation of applied magnetic field intensity. Experimental results show that the polarization mode beat frequency linearly shifts with the increment of the magnetic field intensity and the sensitivity reaches up to 7.09 KHz/Oe in the range of 0 - 437 Oe. Therefore, it will be a promising candidate for the weak magnetic field applications including military, hazard forecast and biomedical fields. ©2016 Optical Society of America OCIS codes: (060.2370) Fiber optics sensors; (280.3420) Laser sensors; (230.3810) Magnetooptic systems.
References and links 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16.
J. E. Lenz, “A review of magnetic sensors,” in Proceedings of IEEE Conference (IEEE, 1990), pp. 973–989. S. C. Rashleigh, “Magnetic-field sensing with a single-mode fiber,” Opt. Lett. 6(1), 19–21 (1981). C. L. Tien, C. C. Hwang, H. Chen, W. Liu, and S. Lin, “Magnetic sensor based on side-polished fiber Bragg grating coated with iron film,” IEEE Trans. Magn. 42(10), 3285–3287 (2006). M. Yang, J. Dai, C. Zhou, and D. Jiang, “Optical fiber magnetic field sensors with TbDyFe magnetostrictive thin films as sensing materials,” Opt. Express 17(23), 20777–20782 (2009). Y. Chen, Q. Han, T. Liu, X. Lan, and H. Xiao, “Optical fiber magnetic field sensor based on single-modemultimode-single-mode structure and magnetic fluid,” Opt. Lett. 38(20), 3999–4001 (2013). M. Deng, C. Huang, D. Liu, W. Jin, and T. Zhu, “All fiber magnetic field sensor with Ferrofluid-filled tapered microstructured optical fiber interferometer,” Opt. Express 23(16), 20668–20674 (2015). A. D. Kersey and D. A. Jackson, “Current sensing utilizing heterodyne detection of the Faraday effect in singlemode optical fiber,” J. Lightwave Technol. 4(6), 640–644 (1986). L. Sun, S. Jiang, and J. R. Marciante, “All-fiber optical magnetic-field sensor based on Faraday rotation in highly terbium-doped fiber,” Opt. Express 18(6), 5407–5412 (2010). G. W. Day, D. N. Payne, A. J. Barlow, and J. J. Ramskov-Hansen, “Faraday rotation in coiled, monomode optical fibers: isolators, filters, and magnetic sensors,” Opt. Lett. 7(5), 238–240 (1982). H. K. Kim, S. K. Kim, H. G. Park, and B. Y. Kim, “Polarimetric fiber laser sensors,” Opt. Lett. 18(4), 317–319 (1993). B. Liu and H. Zhang, “Polarimetric distributed Bragg reflector fiber laser sensor array for simultaneous measurement of transverse load and temperature,” Opt. Fiber Technol. 17(6), 619–625 (2011). A. Frank, K. Bohnert, K. Haroud, H. Brandle, C. V. Poulsen, J. E. Pedersen, and J. Patscheider, “Distributed feedback fiber laser sensor for hydrostatic pressure,” IEEE Photonics Technol. Lett. 15(12), 1758–1760 (2003). J. Gao, H. Wang, X. Guo, and C. G. Lyu, “Directional force measurement technology based on fiber optical laser heterodyning demodulation,” Proc. SPIE 9522, 95220E (2015). C. Lyu, C. Wu, H.-Y. Tam, C. Lu, and J. Ma, “Polarimetric heterodyning fiber laser sensor for directional acoustic signal measurement,” Opt. Express 21(15), 18273–18280 (2013). L. Cheng, J. Han, Z. Guo, L. Jin, and B. O. Guan, “Faraday-rotation-based miniature magnetic field sensor using polarimetric heterodyning fiber grating laser,” Opt. Lett. 38(5), 688–690 (2013). L. Cheng, J. Han, L. Jin, Z. Guo, and B. O. Guan, “Sensitivity enhancement of Faraday effect based heterodyning fiber laser magnetic field sensor by lowering linear birefringence,” Opt. Express 21(25), 30156– 30162 (2013).
#254071 © 2016 OSA
Received 17 Nov 2015; revised 23 Dec 2015; accepted 24 Dec 2015; published 8 Jan 2016 11 Jan 2016 | Vol. 24, No. 1 | DOI:10.1364/OE.24.000645 | OPTICS EXPRESS 645
17. L. Gao, L. Chen, L. Huang, and X. Chen, “Multimode fiber laser for simultaneous measurement of strain and temperature based on beat frequency demodulation,” Opt. Express 20(20), 22517–22522 (2012). 18. J. Wo, M. Jiang, M. Malnou, Q. Sun, J. Zhang, P. P. Shum, and D. Liu, “Twist sensor based on axial strain insensitive distributed Bragg reflector fiber laser,” Opt. Express 20(3), 2844–2850 (2012). 19. R. Ulrich and A. Simon, “Polarization optics of twisted single-mode fibers,” Appl. Opt. 18(13), 2241–2251 (1979). 20. T. Zhu, S. Huang, L. Shi, W. Huang, M. Liu, and K. Chiang, “Rayleigh backscattering: a method to highly compress laser linewidth,” Chin. Sci. Bull. 59(33), 4631–4636 (2014).
1. Introduction Magnetic field measurement is increasing important in many fields such as aeronautics, information storage, environmental monitoring and hazard forecast [1]. Compared to electronic techniques employed in magnetic field detection, fiber-optic magnetic field sensors are more attractive due to their advantages of immunity to electromagnetic interference, high sensitivity, low power consumption, and in situ real-time detection. So, many research efforts have been concentrated on optical fiber magnetic field sensors by employing different mechanisms. A popular scheme is developing fiber-optic magnetic field sensors based on material coatings. For example, magnetostrictive jacket can be deposited on a fiber or an FBG, where strain in a magnetostrictive material resulting from an applied magnetic field is transferred to an optical fiber [2–4]. Moreover, optical fiber magnetic field sensors by combining magnetic fluid with optical fiber interferometers have also been demonstrated [5,6]. These sensors are sensitive to weak magnetic field ranging to several hundred oersted, but their broadband linewidth degrades the measurement accuracy. On the other hand, other research groups have proposed fiber magnetic sensors based on Faraday effect, in which the rotation angle of the polarization plane of a linear polarized light varies with the applied magnetic field strength [7,8]. However, very long sensing fiber has to be employed to improve the sensitivity [9], which results in some disadvantages such as unreliability due to environmental disturbances. Polarimetric fiber laser sensors offer advantages of easy interrogation, high signal-tonoise ratio, absolute encoding, and narrow linewidth which determines that sensors have an inherent high resolution. The excellent properties make them applicable for the measurement of various physical parameters, such as lateral stress [10], temperature [11], hydrostatic pressure [12], directional force [13] and acoustic signal [14]. Recently, Guan et al demonstrated the sensing principle of a polarimetric heterodyning fiber grating laser as a magnetic field sensor [15]. Due to the large intrinsic linear birefringence inside the laser cavity, the nonlinear beat frequency shift is only 520 KHz as the magnetic field intensity is increased to 4500 Oe. Then, they proposed heat treatment of the fiber laser cavity by CO2laser irradiation to permanently modify the linear birefringence and tune the sensitivity. As a result, the beat frequency shift is up to 5 MHz in the range of 4500 Oe, but a still very low sensitivity of ~0.4 KHz/ Oe in the weak magnetic field ranging from 0 Oe to 500 Oe [16]. Moreover, the nonlinear magnetic field response makes it difficult in practical applications. In this paper, we demonstrate a highly sensitive magnetic field sensor based on a fiber ring laser. A home-made rotary apparatus which is modulated by an external magnetic field is introduced into the fiber cavity and utilized to twist one section of the fiber. Due to the remarkable birefringence change induced into the laser cavity, the beat frequency generated between two polarizations of the laser is sensitive to the variation of applied magnetic field intensity. Experimental results show that the linear magnetic field sensitivity is up to 7.09 KHz/Oe, which is about one or two orders of magnitude higher than those of the above mentioned polarimetric fiber laser sensors in the weak magnetic field applications. Therefore, such a device can be operated as a high-sensitivity magnetic field sensor with advantages of easy fabrication, low cost and high accuracy.
#254071 © 2016 OSA
Received 17 Nov 2015; revised 23 Dec 2015; accepted 24 Dec 2015; published 8 Jan 2016 11 Jan 2016 | Vol. 24, No. 1 | DOI:10.1364/OE.24.000645 | OPTICS EXPRESS 646
2. Operating principle There are multi-longitudinal modes in a long fiber ring laser cavity and each longitudinal mode is split into two polarization modes, X-mode and Y-mode, due to their different polarization directions and the effective cavity birefringence. The polarization mode beat frequency (PMBF) is generated by multi-longitudinal modes of different polarization directions, while the linear mode beat frequency (LMBF) is produced by multi-longitudinal modes of the same polarization direction. For lasers with a sufficiently small birefringence, the PMBF can be measured with a commercial RF spectrum analyzer, which is given by [17]
ν PMB = ν LMB + Δν = ν LMB ± McB
2n 2 λ
(1)
B = nx − n y , nx and n y are the effective refractive indices of X-mode and Y-mode,
respectively. c is the speed of light in vacuum, n is the effective refractive index of the fiber, L is the whole laser cavity length and M is the mode number. ν LMB can be expressed by
ν LMB = ν i −ν j =
ic jc c Nc − = (i − j ) = 2nL 2nL 2nL 2nL
(2)
i and j present the ith and jth longitudinal modes in laser cavity, N is the multiple of mode spacing. Due to the effective refractive indices of the two polarization directions are thought to be the same in the calculation of LMBF, so the LMBF is not dependent on the fiber birefringence change caused by any external perturbation. Equation (1) shows that the PMBF is proportional to the fiber birefringence. Therefore, any perturbation resulting in the birefringence change can be detected by discriminating the beat frequency shift through simple electronic signal processing. When a magnetic field parallel to the laser is applied on part of the fiber ring laser cavity, a weak circular birefringence is induced into the laser cavity through Faraday effect, resulting in a low sensitivity to magnetic field [15]. One straightforward method to enhance the circular birefringence may be to apply a twist to the laser cavity with an external transducer. By fixing one end of the fiber and twist the other end, the induced circular birefringence can be described as [18]:
δ B = B0
sin (ω - α ) × Leff ω -α
(3)
B0 is the original birefringence of the fiber cavity, ω is the twist rate and equal to twist angle per the effective twist length, α = gω is the optical rotation, the coefficient of proportionality g is calculated to be ~0.16 for an erbium-doped fiber applied in our system [19]. It can be seen that the birefringence change is dependent on both the fiber twist rate and the effective twist length. According to Eqs. (1)-(3), for a fiber ring laser sensor with given effective twist length, when the laser cavity is subjected to a twist, the fiber circular birefringence will be changed and therefore the polarization mode beat frequency. The relationship between the PMBF and the twist angle is simulated with MATLAB software, which is shown in Fig. 1. Here B0 is set to be 2.3e-7 [19] and Leff is chosen as 80mm. We can see that when the laser cavity is subjected to the twist, the fiber birefringence will be changed periodically as a Sinc function, as well as the beat frequency. Thus, in the twist angle ranging from −90° to 90°, the PMBF is not related to the twisting direction, but determined by the twist angle. In addition, it can be seen that the PMBF monotonously decreases with the increment of the twist angle, and the PMBF shift is about 2.5MHz in the range of 0°-90°, which is far greater than that induced by
#254071 © 2016 OSA
Received 17 Nov 2015; revised 23 Dec 2015; accepted 24 Dec 2015; published 8 Jan 2016 11 Jan 2016 | Vol. 24, No. 1 | DOI:10.1364/OE.24.000645 | OPTICS EXPRESS 647
the Faraday effect. So, when the twist angle is tuned by a magnetic field perpendicular to the laser, such a device offers potentials in a highly sensitive magnetic field sensing applications.
Fig. 1. The relationship between the PMBF and the twist angle
3. Experimental results and discussion
The schematic diagram of the proposed magnetic field sensor based on a fiber laser is shown in Fig. 2(a). A simple and low-cost fiber ring laser is employed in the experiment, in which 10 cm ordinary erbium-doped fiber with peak absorption of 80dB/m near 1530nm provides enough gain for lasing and a phase-shift FBG (central wavelength of 1549.975nm and the 3dB bandwidth of 0.02nm) is utilized to select the beat mode and therefore to enhance the stability of the output laser. The 980 nm laser diode (LD) with a maximum output of 300mW is injected into the laser cavity through a 980/1550 nm wavelength division multiplexer (WDM). Then, the laser is sent into by a 3-dB coupler (OC). One output port of the coupler is connected to an optical spectrum analyzer (OSA, ADVANTEST Q8384), and the other output port of the coupler is injected into a photodetector (PD, THORLABS PDB430C) through a polarizer. The LMBF and PMBF are generated at the PD, and they are measured by an electrical spectrum analyzer (ESA, RIGOL DSA815). The magnetic field is generated by an electromagnet and whose intensity is adjusted by tuning the magnitude of the supply current. The uniform magnetic field zone is Φ50 mm × 25 mm with a uniformity of 20% at one cross section. In order to make the fiber twist under the applied magnetic field, a fiber rotary apparatus is designed to act as a transducer, which is shown in Fig. 2(b). It is mainly composed of a coaxial, non-magnetic fiber clamp and a rotating disk. In addition, a wafer permanent magnet with diameter of 5mm and thickness of 1mm is embedded in the platform to sense the applied magnetic field. It notes that the permanent magnet must be far away from the optical fiber to avoid the circular birefringence which is induced by the intrinsic magnetic field generated in the magnet through Faraday effect. When a magnetic field is applied on the rotary apparatus as shown in Fig. 2(b), the permanent magnet, like a magnetic needle, will continuously rotate with the increment of the magnetic field intensity until its poles are parallel to the magnetic field and therefore leads to a rotation of the whole apparatus. The relationship between the applied magnetic field intensity and the twist angle is calibrated, which is shown in Fig. 2(c). It can be seen that the twist angle monotonously increases with the increment of the magnetic field intensity, and the maximum twist angle change is 80° as the magnetic field intensity is increased to 450 Oe. According to the theory analysis in part 2, we can see that the PMBF decreases with the
#254071 © 2016 OSA
Received 17 Nov 2015; revised 23 Dec 2015; accepted 24 Dec 2015; published 8 Jan 2016 11 Jan 2016 | Vol. 24, No. 1 | DOI:10.1364/OE.24.000645 | OPTICS EXPRESS 648
increment of the applied magnetic field intensity, and the theoretical sensitivity is ~7.78 KHz/Oe. So, this device can be operated as a sensitive magnetic field sensor in weak magnetic field applications.
Fig. 2. (a) Schematic diagram of the magnetic field sensor based on a fiber laser, (b) Schematic diagram of the fiber rotary apparatus, (c) The relationship between magnetic field intensity and the twist angle
To investigate the magnetic field response of the fiber laser sensor, an 8cm length of EDF with the laser cavity was firstly fixed by a fiber holder on one side and the fiber rotator on the other end. Then, the integration was placed in the center of the uniform magnetic field zone and the applied magnetic field was perpendicularly to the laser to eliminate the Faraday effect as well [15]. Because the sensing system is sensitive to the polarization variations inside and outside the ring cavity. So, all parts in our system except for the sensing part are fixed to eliminate the polarization caused by external perturbation. In real applications, an excellent package is necessary to avoid the birefringence being induced into the cavity from external perturbation. In the initial state, namely, the magnetic field intensity is zero, the output spectrum of the fiber laser measured by OSA is shown in the blue curve in Fig. 3(a), whose central wavelength is around 1550.015 nm and the 3-dB bandwidth is about 0.04nm, respectively. The red curves in Fig. 3(a) are the LMBF on the RF spectrum analyzer, detected without the polarizer. It can be seen that there are only three different longitudinal modes in the range of 350MHz, which is consistent with the laser longitudinal mode interval formula Δν LM = c 2nL (L≈1m). When the polarizer is inserted in front of the PD, new beat signals appeared, namely, the PMBF, as seen in Fig. 3(b), which is coincident with the theory mentioned above. Due to the mode selection effect of the PS-FBG filter, the signal noise
#254071 © 2016 OSA
Received 17 Nov 2015; revised 23 Dec 2015; accepted 24 Dec 2015; published 8 Jan 2016 11 Jan 2016 | Vol. 24, No. 1 | DOI:10.1364/OE.24.000645 | OPTICS EXPRESS 649
ratios (SNR) of the beat signals are different. The low-frequency LMB signal has higher SNR, but the high-frequency PMB signal has higher SNR. According to Eq. (1), the highfrequency PMB signal shares the same effective magnetic field sensitivity with the lowfrequency PMB signal. So, the high-frequency PMB signal C of 323.167MHz may be selected for measuring the external magnetic field. It has the SNR and the 3-dB bandwidth of 55dB and ~1kHz, which is much superior to that in [10]. It notes that the linewidth of the PMBF may be further compressed by using the Rayleigh backscattering and therefore to enhance the measurement accuracy [20].
Fig. 3. (a) Optical spectrum of the fiber ring laser sensor observed by OSA and the LMBF without the polarizer observed by RF spectrum analyzer. (b) The original LMBF and the PMBF
The magnetic field intensity applied on the laser cavity for magnetic field sensing is changed from 0 Oe to 437 Oe. Experimental results show that the magnetic field has little effect on the central frequency of the LMBF while a large influence on that of the PMBF, which is mainly because the LMBF does not depend on the birefringence of the cavity caused by the magnetic field. This is in accordance with the theoretical analysis in part 2. The high-frequency PMB signal of 323.167MHz is chosen as the sensing signal that will be detected. As shown in Fig. 4(a), the PMBF shifts to a lower frequency with the increment of the magnetic field intensity with a slight SNR change. The relationship between the PMBF and the applied magnetic field intensity is illustrated in Fig. 4(b) after several repeating test. As the magnetic field intensity increases from 0 Oe to 437 Oe, the PMBF linearly blue shift from 323.167MHz to 320.133MHz. By using a linear fitting of the experimental data as the red line, it is explicit that the magnetic field sensitivity reaches up to 7.09 KHz/Oe with the R2 value of 0.99304, which gets on well with the theoretical analysis mentioned above. The sensitivity of the sensor is one or two orders of magnitude higher than those of the existing polarimetric fiber laser sensors [15, 17], which is mainly because the circular birefringence has been greatly enhanced by applying a twist to the laser cavity with an external transducer. But when we continue to increase the magnetic field intensity, the relationship between the magnetic field and the PMBF will be nonlinear because the fiber birefringence will be changed periodically as a Sinc function with the increment of the magnetic field, as well as the beat frequency, which is in accordance with Eq. (3). In our opinion, the magnetic field sensitivity can be further improved by two methods. One is that changing the thickness and diameter of the permanent magnet so that a greater twist angle will be produced by the same magnetic field intensity, which will introduce a stronger circular birefringence into the laser cavity under the same magnetic field intensity; the other is that increasing the length of the twisted fiber section, which originates from the
#254071 © 2016 OSA
Received 17 Nov 2015; revised 23 Dec 2015; accepted 24 Dec 2015; published 8 Jan 2016 11 Jan 2016 | Vol. 24, No. 1 | DOI:10.1364/OE.24.000645 | OPTICS EXPRESS 650
fact that the birefringence change is proportional to the effective twist fiber length, as well as the beat frequency, according to Eq. (3). Here, we note that the linear response to the magnetic field intensity makes it more competitive in real applications. But, from Fig. 4(b), we can see that such a sensing system cannot be able to implement the measurement of the magnetic field intensity with good repeatability, which is mainly because the rotary apparatus can only clockwise rotate under the magnetic field, a spring can be installed on the rotatable fixture to realize counterclockwise rotation, which will be our next work. Therefore, such a proposed fiber laser sensor offers great potentials in weak magnetic field applications with advantages of high precision, high sensitivity, easy fabrication, and low cost.
Fig. 4. (a) Polarization mode beat frequency of the fiber laser versus different magnetic field intensity, (b) The relationship between the polarization mode beat frequency and the applied magnetic field intensity
4. Conclusions
This paper demonstrates an all-fiber magnetic field sensor which is based on a simple and low-cost fiber ring laser. Due to the remarkable birefringence change caused by a fiber twist which is modulated by a magnetic field perpendicular to the laser, the polarization mode beat frequency is sensitive to the variation of applied magnetic field intensity. Experimental results show that the PMBF linearly shifts to a lower frequency with the increment of the magnetic field intensity, and the sensitivity is up to 7.09 KHz/Oe ranging from 0 Oe to 437Oe. This is one or two orders of magnitude higher than that of the existing fiber laser magnetic field sensors. Therefore, the compact and low-cost fiber laser sensor can be employed in the measurement of weak magnetic field applications including military, hazard forecast and biomedical fields. Acknowledgments
This work is supported by the National Natural Science Foundation of China (NSFC) under Grants No. 61475028, 61475029 and 61575190, the Chongqing Outstanding Youth Project under Grant cstc2014jcyjjq40002.
#254071 © 2016 OSA
Received 17 Nov 2015; revised 23 Dec 2015; accepted 24 Dec 2015; published 8 Jan 2016 11 Jan 2016 | Vol. 24, No. 1 | DOI:10.1364/OE.24.000645 | OPTICS EXPRESS 651
