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Vol. 24, No. 16 | 8 Aug 2016 | OPTICS EXPRESS 17980

Highly sensitive and reconfigurable fiber optic current sensor by optical recirculating in a fiber loop JIANGBING DU,1,* YEMENG TAO,1 YINPING LIU,1 LIN MA,1,2 WENJIA ZHANG,1 AND ZUYUAN HE1 1

State Key Laboratory of Advanced Optical Communication Systems and Networks, Shanghai Jiao Tong University, Shanghai, China 2 [email protected] *[email protected]

Abstract: An advanced fiber optic current sensor (FOCS) is proposed based on recirculating fiber loop architecture for significantly enhancing the current sensitivity. The recirculating loop is constructed by a 2X2 optical switch and the standard single mode fiber (SSMF) is used as the sensing head. The proposed FOCS is coupler-free with low insertion loss which results in a significantly improved current sensitivity. We experimentally obtained a sensitivity of 11.5 degrees/A for 1-Km SSMF FOCS and a sensitivity of 21.2 degrees/A for 500-m SSMF FOCS, both of which have been enhanced by more than ten times. The flexible switch control of recirculating can support the FOCS to work for different current scenarios with the same system and thus reconfigurable operation of the FOCS has been achieved. The significantly enhanced high sensitivity with reconfigurable operation capability makes the proposed FOCS a promising method for practical applications. © 2016 Optical Society of America OCIS codes: (060.2370) Fiber optics sensors; (060.2330) Fiber optics communications.

References and links 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14.

Y. N. Ning, Z. P. Wang, A. W. Palmer, K. T. V. Grattan, and D. A. Jackson, “Recent progress in optical current sensing techniques,” Rev. Sci. Instrum. 66(5), 3097–3111 (1995). S. Ziegler, R. C. Woodward, H. H. C. Iu, and L. J. Borle, “Current sensing techniques: A review,” IEEE Sens. J. 9(4), 354–376 (2009). K. Tanaka, K. Fujita, N. Matsuoka, K. Hirao, and N. Soga, “Large Faraday effect and local structure of alkali silicate glasses containing divalent europium ions,” J. Mater. Res. 13(07), 1989–1995 (1998). K. Bohnert, P. Gabus, J. Nehring, and H. Brandle, “Temperature and vibration insensitive fiber-optic current sensor,” J. Lightwave Technol. 20(2), 267–276 (2002). N. Peng, Y. Huang, S. Wang, and L. Wang, “Fiber optic current sensor based on special spun highly birefringent fiber,” IEEE Photonics Technol. Lett. 25(17), 1668–1671 (2013). V. P. Gubin, V. A. Isaev, S. K. Morshnev, A. I. Sazonov, N. I. Starostin, Y. K. Chamorovsky, and A. I. Oussov, “Use of spun optical fibres in current sensors,” Quantum Electron. 36(3), 287–291 (2006). D. Huang, S. Srinivasan, and J. E. Bowers, “Compact Tb doped fiber optic current sensor with high sensitivity,” Opt. Express 23(23), 29993–29999 (2015). W. Lin, H. Zhang, and B. Song, “Magnetic field sensor based on fiber taper coupler coated with magnetic fluid,” in International Conference on Optical Fibre Sensors (OFS24) (2015), paper 96347U. P. R. Watekar, H. Yang, S. Ju, and W.-T. Han, “Enhanced current sensitivity in the optical fiber doped with CdSe quantum dots,” Opt. Express 17(5), 3157–3164 (2009). K. Kurosawa, S. Yoshida, and K. Sakamoto, “Polarization properties of the flint glass fiber,” J. Lightwave Technol. 13(7), 1378–1384 (1995). K. Kurosawa, I. Masuda, and T. Yamashita, “Faraday effect current sensor using flint glass fiber for the sensing element,” in Proceedings of Optical Fiber Sensor Conference (1993), pp. 415–418. H. Zhang, Y. Dong, J. Leeson, L. Chen, and X. Bao, “High sensitivity optical fiber current sensor based on polarization diversity and a Faraday rotation mirror cavity,” Appl. Opt. 50(6), 924–929 (2011). H. Zhang, Y. Qiu, H. Li, A. Huang, H. Chen, and G. Li, “High-current-sensitivity all-fiber current sensor based on fiber loop architecture,” Opt. Express 20(17), 18591–18599 (2012). C. Wang, “Fiber Loop ringdown sensors and sensing,” in Cavity-Enhanced Spectroscopy and Sensing (Springer Berlin Heidelberg, 2014), pp. 411–461.

#268962 Journal © 2016

http://dx.doi.org/10.1364/OE.24.017980 Received 22 Jun 2016; revised 25 Jul 2016; accepted 26 Jul 2016; published 28 Jul 2016

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15. N. S. Bergano and C. R. Davidson, “Circulating loop transmission experiments for the study of long-haul transmission systems using erbium-doped fiber amplifiers,” J. Lightwave Technol. 13(5), 879–888 (1995). 16. J. Li, J. Du, S. Wang, L. Li, L. Sun, X. Fan, Q. Liu, and Z. He, “Improving the spatial resolution of an OFDR based on recirculating frequency shifter,” IEEE Photonics J. 7(5), 6901310 (2015). 17. E. F. Burmeister, J. P. Mack, H. N. Poulsen, J. Klamkin, L. A. Coldren, D. J. Blumenthal, and J. E. Bowers, “SOA Gate Array Recirculating Buffer for Optical Packet Switching,” in Optical Fiber Communication Conference/National Fiber Optic Engineers Conference, OSA Technical Digest (CD) (Optical Society of America, 2008), paper OWE4. 18. R. Zhang, S. Yao, T. Liu, and L. Li, “The effect of linear birefringence on fiber optic current sensor based on Faraday mirror,” in SPIE/COS Photonics Asia. International Society for Optics and Photonics (2014), paper 92741N. 19. P. R. Forman and F. C. Jahoda, “Linear birefringence effects on fiber-optic current sensors,” Appl. Opt. 27(15), 3088–3096 (1988). 20. K. Bohnert, P. Gabus, L. Kostovic, and H. Brändle, “Optical fiber sensors for the electric power industry,” Opt. Lasers Eng. 43(3-5), 511–526 (2005). 21. M. Belal, Z. Song, Y. Jung, G. Brambilla, and T. P. Newson, “Optical fiber microwire current sensor,” Opt. Lett. 35(18), 3045–3047 (2010).

1. Introduction Fiber optic current sensor (FOCS), which employs the Faraday Effect to measure electric current, has been attracting a lot of attentions for the electric power delivery applications [1]. Compared with conventional current sensors, FOCS has advantages including inherently insulation, compact footprint, lowered weight, and immunity to demagnetizing fields [2]. Those advantages make FOCS a safer, more reliable and cost-effective solution for broad application scenarios. Presently, there are two main difficulties which limit FOCS for practical applications: one is its susceptibility to many environment effects such as temperature variation and mechanical vibration [3]; the other is its limited sensitivity to the magnetic field which is determined by Verdet constant of silica material itself [4] and linear birefringence induced by bending. Many studies have been carried out to overcome these problems from aspects of improving sensing media and system design. For example, circularly birefringent spun fibers can be used to reduce its susceptibility to the environment [5, 6]; and other specialty fibers including flint glass fibers, Tb-doped glass fibers, and magnetic fluid fiber devices with larger Verdet constants can be used to improve the current sensitivity [7–11]. However, those specialty fibers are usually expensive and very difficult to fabricate. Therefore, sensing systems utilizing ring-down architecture have also been reported to improve the sensitivity by increasing the effective interactive length between optical fiber and magnetic field [12–14]. In the ring-down architecture, pulsed light recirculate inside the fiber loop with multiple round trips and thus the interactive length can be increased. However, the use of fiber couplers in the ring-down loop inevitably affects the polarization stability of the system and then limits the number of recirculating cycles due to a fixed coupling ratio of the coupler. Meanwhile, the recirculation of the pulse in the ring-down loop will not stop until the pulse power is completely dissipated, which makes the ring-down FOCS difficult for reconfigurable operation [14]. As a matter of fact, reconfigurable operation of an FOCS is a very useful function. In the practical applications, one can find that the FOCS product can only support a certain range of current measurement with certain sensitivity due to the use of a specific sensor head (e.g. a certain length of sensing fiber plus the sinusoidal periodic limitation of polarization angle rotation). Therefore, it is a very desirable function of the FOCS to support different current scenarios with different sensitivities but using a same sensor head (a certain fiber length). To solve the abovementioned problems, we propose a highly sensitive and reconfigurable FOCS by optical recirculating in a fiber loop which is constructed by a 2X2 optical switch and standard single mode fiber (SSMF). The coupler-free and low insertion loss configuration of the system results in a significantly improved current measure sensitivity. The flexible switch control of recirculating can support the FOCS to work for different current scenarios

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with the same system. The use of SSMF rather than specialty fibers for achieving a high sensitivity means that the cost of the FOCS can be kept at a low level. We have experimentally achieved reconfigurable optical fiber current sensing with significantly improved sensitivity. 2. Principle When linearly polarized light passes through a magnetic field parallel to the optical propagation orientation, the light will undertake polarization rotation. This effect is nonreciprocal, depending on whether the light is propagating along or against magnetic field. The rotation angle θ is determined by the length of optical path, magnetic field B and Verdet constant V of the material [13]. Their relationship can be expressed as:

θ = V  Bdl

(1)

when polarization angle of linearly polarized light is changed, light intensity of fast axis and slow axis also changes. By detecting the intensity of polarized light out of PBS, we can calculate the rotation angle. The intensities of two axes can be expressed as:

θ = V  Bdl

(2)

P1 = P0 cos 2 (θ + ϕ )

(3)

P2 = P0 sin 2 (θ + ϕ ) (4) where, P0 is the detected power at polarization beam splitter (PBS). P1 and P2 is the power of fast axis and slow axis, respectively. ϕ is the modulation angel generally set as 45 degrees to make P1 and P2 equal. By calculating the normalized intensity, θ can be obtained as: 1 2

θ = − arc sin PBD

(5)

where, P1 − P2 = cos(2(θ + ϕ )) (6) P0 In our experiment, the system sensitivity is defined by degree per ampere and the resolution is the minimum current variation detected within the measurement uncertainty. From Eq. (1), it can be observed that a larger θ can be obtained with larger l (longer fiber) or larger V. Therefore, by employing a fiber loop architecture, the pulse recirculated inside the fiber loop will undertake multiple times of Faraday effect so that the equivalent sensing fiber length can be increased significantly. PBD =

Fig. 1. Recirculating loop constructed by 2X2 switch and SSMF.

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Rather than a coupler-based ring-down fiber loop, the FOCS proposed in this work is constructed by a recirculating loop based on a 2X2 switch [14–17]. The switch has four ports (port-1 and port-2 are input, port-3 and port-4 are output) and its connection type switches when triggered with a control signal. For example, with a high level control signal, the connection switches from (1-to-3; 2-to-4) to (1-to-4; 2-to-3), as shown in Fig. 1. As a consequence, the pulse can be kept recirculating inside the loop and the recirculating round trips can be flexibly controlled by the control signal, as shown in Figs. 2(a) and 2(b). Since the insertion loss of the switch is comparably small and there is no fixed coupling-loss compared with a coupler-based ring-down loop, one can expect a more significant sensitivity improvement. As presented in Fig. 2(c), the polarization rotation angle of θ increases along with the recirculating round trips. Assume θΝ corresponds to the polarization rotation angle of the N times recirculated pulse and the sensitivity would be magnified by a factor of N under idea circumstance. In practical applications, the PDL introduced by the switch and fiber, together with the linear birefringence, will limit the sensitivity enhancement effect.

Fig. 2. Schematic illustration of the recirculating loop based FOCS. (a): sensing pulse power evolution for different times of recirculating (different round trip pulses); (b): control pulse waveform; (c): polarization rotation evolution for different times of recirculating with enlarged polarization angle rotation.

3. Experimental results and discussions The experimental setup is shown in Fig. 3. We use a linear polarized fiber laser (FL) operating at 1550 nm as the light source and a polarization controller (PC) to control its polarization state. The continuous-wave light from the FL was modulated by an acousto-optic modulator (AOM) to generate pulsed light with a pulse width of 2 μs. The use of AOM can make sure the generated signal pulse has a comparably large extinction ratio (EO), so that the interference noise during recirculating can be kept at a low level. The 2-μs signal pulse is then injected into the recirculating loop constructed by a 2X2 single-mode optic switch and a certain length of SSMF as the sensing head. Under default mode without a bias voltage as the control over the switch, port-1 of the switch is connected to port-2 and port-3 is connected to port-4. When a bias voltage (high level control pulse) is applied, port-1 will be connected to port-4 and port-3 will be connected to port-2.

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Fig. 3. The experimental setup of the proposed FOCS. FL: fiber laser; PC: polarization controller; AOM: acousto-optic modulator; AWG: arbitrary waveform generator; PBS: polarization beam splitter; PD: photodiode; DAQ: data acquisition.

The switch has a response time of 300 ns and the insertion loss is typically 0.6 dB (AGILTRON NS Series 2x2 Switch). The polarization dependent loss (PDL) of the switch is only 0.16 dB. In order to avoid interference, the fiber length has to be long enough to host the whole length of the signal pulse. In the experiment, 1-km SSMF are used as the sensing medium and the delay line as well. In order to minimize the vibrations and noises, the fiber was wound and cured into a fiber coil with a diameter of 11 cm. The delay time of the fiber loop is about 5 μs with a total loss of 4 dB. When the control pulse width from arbitrary waveform generator (AWG) is set as 5(N-1) μs, optical signal pulse circulates for N times inside the fiber loop. After reaching the desired recirculating number, we can stop the recirculating at the end of the control pulse and thus couple out the signal pulse from port-2 to PBS. The signal is then detected by two balanced photodiodes (PD) before acquisition and analysis. The measured rotation angle as a function of current intensity is shown in Fig. 4 with N = 8. The power of the FL was set to 25 mW. The signal pulse width is 2 μs. Figures 4(a) and 4(b) present the detected signal pulse waveforms at the two balanced PDs, corresponds to P1 and P2 in Eq. (5). Along with the linear increase of current, we can observe linear decrease of P1 and linear increase of P2, respectively. In Fig. 4(c), the calculated rotation angle according to Eq. (4) is presented and the curve with a slope of −7.6 indicates a sensitivity of 7.6 degrees per ampere (degree/A).

Fig. 4. The detected signal pulse waveforms and measured sensitivity when N = 8 for 1-Km SSMF. (a) and (b) corresponds to P1 and P2; (c): Relationship between current intensity and rotation angle.

To further improve the sensitivity, we have to increase FL power so that more times of recirculating can be reached with strong enough signal pulse intensity at PBS. However, although the pulse shape at DAQ was distinct and system still had response to the current variation, it wasn’t stable enough to obtain the accurate angle when N exceeded 10 due to

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limited signal-to-noise ratio (the noise is also amplified due to recirculating). The value of angle fluctuated wildly and the linearity was also destroyed when current continued increasing as shown in Fig. 5. One possible explanation for this phenomenon is that the light is actually elliptically polarized light, which makes the linearity deteriorate [18, 19]. We optimized the parameters such as the power of FL and number of recirculation. The highest sensitivity achieved was 11.52 degrees/A as shown in Fig. 5, when the output power of FL was set to 35 mW with N = 10, and the standard deviation of rotation angle was about 0.2 degree. Then we turn the current source on and off to test the variation of P1 and P2 when current intensity was set as 10 mA, angle variation was close to the standard deviation (0.2 degree). Thus, we can define 10 mA as the system resolution under existing experiment condition.

Fig. 5. Slope of 11.5 degrees/A is achieved when N = 10 for 1-Km SSMF.

Fig. 6. The enhancement of sensitivity along with N for 1-Km SSMF.

The recirculating loop in the FOCS also suffers from the linear birefringence due to bending which will retard the system and limit its sensitivity. Longer length of SSMF in the recirculating loop means more turns of the fiber with more bends and therefore introduces larger linear birefringence. The nature that linear birefringence will destroy the sensitivity has been well studied in papers like Ref [18, 19]. Actually, the fact that linear birefringence of the fiber will destroy the sensitivity is exactly one of the most important reason that we like to improve the sensitivity by a recirculating configuration on system level rather than using specialty fibers to reduce the linear birefringence itself. We then utilize 500-m SSMF in the recirculating loop and change the signal pulse to 0.5 μs to reduce the bending-induced linear birefringence. The 500-m fiber is much looser than the wounded 1-Km fiber and the fiber coil diameter is more than 20-cm. By doing so, the

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linear birefringence due to bending is significantly reduced and thus the sensitivity is further enhanced. We obtained 21.2 degrees/A when N = 12 as shown in Fig. 7. The inset picture in Fig. 7 shows the waveforms for P1 and P2 when N = 12, with and without 0.5-A current.

Fig. 7. Slope of 21.2 degrees/A is achieved when N = 12 for 0.5-Km SSMF. The inset picture shows the waveforms for P1 and P2, before and after 0.5-A current.

The further enhanced sensitivity due to recirculating can also be observed from Fig. 8, in which, the increase of sensitivity along recirculating number N is more nonlinear. The reason is mainly caused by the unstable system due to the un-wounded and very loose SSMF. Therefore, the waveforms of the signal pulses also degrade as shown by the inset picture in Fig. 7, compared with that of the FOCS with 1-Km SSMF in Fig. 4.

Fig. 8. The enhancement of sensitivity along with N for 0.5-Km SSMF.

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Fig. 9. Reconfigurable operation of the FOCS for difference current measurement scenarios. The largest current is limited by the 180-degree polarization angle rotation.

In the experiment, small value of current is utilized. Thus, the Faraday effect induced polarization rotation is very small and it can be covered within π range (sinusoidal periodic limitation of polarization angle rotation according to Eqs. (2)-(4)) even for large number of recirculating to enlarge the rotation angle. However, if the application scenario changes, e.g. different current, it is possible that the current may exceed the limited measurement range of a conventional FOCS and the sensor head has to be changed to adapt the requirement. Therefore, the proposed FOCS can provide another important advantage of reconfigurable operation which can flexibly support a broad range of current measurement scenarios as shown in Fig. 9. For example, one can simply use the signal pulse when N = 1 for measurement of large current up to 160.7 A with the FOCS using 1-Km SSMF which has a sensitivity of 1.12 degrees/A as presented in Fig. 6. By adjusting the control signal on the switch to change N to 5, the sensitivity can be enlarged to 4.2 degrees/A and the largest supported current is about 42.9 A which will further be reduced to about 15.7 A if N is turned to 10 (11.5 degrees/A sensitivity). Thus, with the same FOCS, one can use large N for small current measurement with good accuracy (large sensitivity), while large current can also be measured with small N with the sacrifice of accuracy (small sensitivity). Therefore, the FOCS can easily adapt different current measurement scenarios by simply adjusting the control pulse length, which is of great significance for practical applications [20, 21]. The length of SSMF is of great importance to the current sensing performance. On one hand, the fiber has to be short so that the insertion loss and dispersion can be maintained small enough for obtaining a good performance of current sensing. On the other hand, the fiber also needs to be long enough to host the whole length of the sensing pulse to avoid the interference due to recirculating and the distortion due to the limited speed of switching. From the perspective for application, the length of SSMF is related to the sensitivity and measurement range. Thus, one also need to consider the practical application circumstance for choosing a suitable fiber length. Therefore, the minimum length of SSMF is just limited by the AOM speed and the switching speed. In the experiment in this study, the whole range of SSMF in the loop is used for sensing and thus it simultaneously acts as the delay line for hosting the pulse. We choose 500-m and 1-Km for the experiment in this study according to the sub-microsecond modulation speed of AOM and 300-ns switching speed of the 2X2 Switch. One can expect better performance of sensitivity improvement, system configurability and compactness by using higher speed modulator and switch to reduce the fiber length. 4. Conclusion In this study, we propose an FOCS with high sensitivity based on recirculating fiber loop architecture constructed by a 2 × 2 optic switch instead of fiber couplers. Instead of coupling out some portion of the light power after each cycle, the pulsed light can recirculate inside the

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loop without any interruption until the desired recirculating number has been reached. As a result, this architecture can effectively improve both the system stability, sensitivity, and reduce the complexity. Meanwhile, reconfigurable operation can easily be realized by control the switch which enables this FOCS for broad current measurement scenarios. We experimentally achieved significantly enhanced sensitivity of 11.5 and 21.2 degrees/A for FOCSs using 1-Km and 500-m SSMFs, respectively. Funding Natural National Science Foundation of China (NSFC) (61307107, 61405113); Science and Technology Commission of Shanghai Municipality (STCSM) (15511103102, 13ZR1456200).