High sensitivity optical fiber current sensor based on polarization diversity and a Faraday rotation mirror cavity Hongying Zhang, Yongkang Dong, Jesse Leeson, Liang Chen, and Xiaoyi Bao* Fiber Optics Group, Department of Physics, University of Ottawa, Ottawa, K1N 6N5, Canada *Corresponding author:
[email protected] Received 14 October 2010; revised 3 January 2011; accepted 9 January 2011; posted 11 January 2011 (Doc. ID 136599); published 17 February 2011
A novel high sensitivity optical fiber current sensor (OFCS) based on polarization diversity and a Faraday rotation mirror cavity is proposed and demonstrated. Comparing with single-channel detection in a conventional OFCS, a signal power gain of 6 dB and a signal-to-noise ratio improvement of over 30 dB have been achieved in the new scheme. The cavity amplifies magnetic field-induced nonreciprocal phase modulation, while the Faraday rotation mirrors suppress the reciprocal birefringence. A linear response is obtained for current amplitude as low as several mA at an AC frequency of 1 kHz. © 2011 Optical Society of America OCIS codes: 060.2370, 280.4788.
1. Introduction
Optical fiber current sensors (OFCSs) are of great interest because of their immunity to electromagnetic interference, fast response time, compact design, and long-distance signal transmission for remote operation. Typically two categories of OFCS have been demonstrated based on the Faraday effect [1–6] and thermal effect [7,8]. According to the Faraday effect, the rotation of the plane of polarization is proportional to the intensity of the applied magnetic field in the direction of light propagation. This principle can be used for indirect current sensing via interaction between light and a magnetic field induced by the applied current. The expression for the magnitude of Faraday rotation in radians is given by Z ⇀ ⇀ ð1Þ θ ¼v B·dl; where v is the Verdet constant, B is the magnetic flux density induced by the applied current, and l 0003-6935/11/060924-06$15.00/0 © 2011 Optical Society of America 924
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is the length over which the magnetic field and light interact. Because of the nonreciprocal nature of the Faraday effect, many OFCSs use Faraday rotation mirrors (FRMs) [9–11]. The FRM causes a reflection and a nonreciprocal 90° polarization rotation, resulting in the exiting light beam orthogonally polarized to the incoming lightwave. When an orthogonally polarized lightwave travels through a reciprocal birefringence, i.e. temperature gradient, vibration, etc., the polarization modulation can be removed. When the FRM is used in the presence of a nonreciprocal birefringence, i.e. Faraday effect, the measured phase delay will be doubled. The Sagnac loop interferometers are formed [12–14] for high sensitivity current detection with potential of high frequency response [15]. However, an optical fiber delay line must be included in this configuration, and its length needs to be optimized for a given current frequency, which may limit their practical applications. In this paper, a novel sensitive magneto-optical OFCS is designed based on polarization diversity (PD) and a Faraday rotation mirror cavity (FRMC) formed by a coupler and two FRMs. Usually, a
Fig. 1. Experimental setup of the OFCS with PD detection and an FRMC: PC, polarization controller; FUT, fiber under test; BD, balanced detector; OSC, oscilloscope.
polarizer and a single port detector were used in a conventional OFCS, which suffers from the fluctuation noise of the optical source especially for weak current detection. In our scheme, a polarization beam splitter (PBS) in conjunction with a balanced detector is employed for PD to improve the system performance, so that the antiphase signal is doubled while the in-phase noise caused by the fluctuation of the source power is minimized by subtraction of the two input signals of the balanced detector. In addition, FRMC decreases the length of the fiber under test with insensitivity to linear birefringent effects [16]. To the best of our knowledge, this is the first time that an OFCS with PD and FRMC is reported. 2. Experimental Setup
The experimental setup is shown in Fig. 1. The spontaneous emission of an erbium-doped fiber amplifier in the wavelength range of 1530–1560 nm is used as the light source to avoid the interference in the cavity formed by two FRMs. It first passes through a polarizer to form a linearly polarized light beam, with the state of polarization (SOP) changed by using a polarization controller to optimize the system. The light then passes through a circulator and enters a coupler; it forms a cavity with a segment of 100 m standard single mode fiber-28 and two FRMs, which is shown in the dashed frame. This coupler controls the percentage of light inside the cavity. A higher coupling ratio ensures multiple passes within the FRMC. An FRM offers two advantages over a conventional mirror: it maintains the same input SOP in the sensor by compensating the effects of linear birefringence, and it also keeps the recombining light the same SOP as the light being input into the cavity. The returning light coming from the cavity passes through a circulator that allows detection of the returning signal and also provides isolation of the source from returning light. The light then enters another coupler, with the 20% portion being monitored as a power reference, while the 80% portion is sepa-
rated by a PBS into two antiphase signals. These two signals are then detected and subtracted by the two input channels of the balanced detector with PD, and the electrical signal is recorded using an oscilloscope. To generate a magnetic field in the direction of the light propagation, the optical fiber spool is wrapped with copper wire, forming a surrounding toroidal coil, which is visualized in Fig. 2. This method is more convenient than using a single wire because a large magnetomotive force can be generated with low currents inside the toroid, while no magnetic field is generated outside the structure. To generate the magnetomotive forces used in our experiments, the optical fiber spool, whose radius is 4:25 cm, was wrapped by copper wire with 50 turns. 3. Principle of PD Detection
The PD detection in our OFCS is realized by a PBS in conjunction with a balanced detector. Figure 3 shows the beam split in the PBS, where the two optical paths in the fast and slow axes of the PBS are as follows: Ef ¼ E0 cos ω0 t · cos φ;
Es ¼ E0 cos ω0 t · sin φ; ð2Þ
Fig. 2. Toroidal coil formed by wrapping copper wire with 50 turns around a spool of optical fiber. The radius of the spool is 4:25 cm. 20 February 2011 / Vol. 50, No. 6 / APPLIED OPTICS
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π π I ¼ α · P0 cos2 þ θ − sin2 þθ 4 4 ¼ −α · P0 sin 2θ ≈ −2α · P0 θ:
ð5Þ
For the case of a single-channel detector, the output is then given by
Fig. 3. Beam split in a PB: s, slow axis; f , fast axis.
where E0 is the input electric field; Ef and Es are the electric fields in fast and slow axes, respectively; ω0 is the central frequency of the source; and φ is the angle formed by the polarization plane and the fast axis of the PBS. The signal detected by the balanced detector is given by
I ¼ α · P0 cos2
π þθ 4
≈ −α · P0 θ þ
α · P0 : 2
1 ¼ α · P0 ð1 − sin 2θÞ 2 ð6Þ
It should be noted that a sufficiently small θ is assumed to validate the approximations above. Comparing Eqs. (5) and (6), PD detection exhibits two advantages over single-channel detection: (1) doubled sensitivity, and (2) the DC background that corresponds to the source intensity is effectively removed, resulting in a better signal-to-noise ratio (SNR). 4. Experimental Results and Discussion
I ¼ hE2f i − hE2s i ¼ α · P0 ðcos2 φ − sin2 φÞ:
ð3Þ
Then we can obtain dI ¼ −2α · P0 sin 2φ; dφ
ð4Þ
where α is the optical-electrical conversion coefficient, and P0 is the incident light power of the PBS. When φ ¼ π=4 the maximum sensitivity can be obtained as an optimized point. With the current sensor working at the maximum sensitivity point, i.e., φ ¼ π=4, when the input SOP is rotated with a small angle θ by the current-induced magnetic field, the output of the balanced detector is given by
A. Signal Doubling and Noise Suppression of PD Detection
To show the advantages of PD detection over singlechannel detection, measurements were made for an AC current with an amplitude of 3 A and 0:15 A, respectively. All of the experimental results in this paper are obtained for AC frequency of 1 kHz. The results for AC current amplitude of 3 A are shown in Fig. 4, where Fig. 4(a) is the time domain signals with the corresponding current, while Fig. 4(b) is their corresponding power spectra. In both figures the black curve shows PD detection, while the red (gray) curve is for single-channel detection. From Fig. 4(a) one can see that the waveform obtained by single-channel detection shows obvious modulation by the background noise of around 60 Hz, while PD detection shows a good noise cancellation
Fig. 4. (Color online) Measured results for 1 kHz AC current with an amplitude of 3 A: (a) time domain signals; (b) power spectra. 926
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Fig. 5. (Color online) Measured results for 1 kHz AC current with an amplitude of 0:15 A: (a) time domain signals; (b) power spectra.
and represents the sine waveform of the detected AC current. As for the power spectrum, the result with singlechannel detection shows obvious low-frequency noise, leading to a poor SNR, while the result with PD detection shows a 6 dB signal gain with excellent suppression of the background noise, leading to a much better SNR. Figure 5 shows the measured results for a lower current of 0:15 A. In the time domain waveform obtained by single-channel detection [Fig. 5(a)], the weak signal is merged in the background noise, while the sine waveform of the AC current is still obvious in PD detection. The power spectra in Fig. 5(b) shows that compared with the single-channel detection, a 6 dB signal gain with very good noise reduction can still be obtained by the PD detection, indicating that it is especially important for weak current detection. In order to obtain the information of the noise, we made measurements of the background noise using PD detection and the single-channel detection,
respectively, with the result shown in Fig. 6. The typical noise comes from the low-frequency of 60, 128, and 180 Hz, respectively, which mainly correspond to the common electric frequency of 60 Hz and its harmonics. Compared with the result obtained by singlechannel detection, a noise level reduction of around 30 dB is obtained with PD detection. The powers of the three typical noise frequencies are listed in Table 1, where noise reduction of larger than 30 dB is obtained at these three frequencies. B. Improvement of Systematic Sensitivity by the FRMC
Figure 7 shows the signal amplitude as a function of peak current at different coupling ratios, where the incident light power of the PBS was kept the same throughout. It is clear that for all of these coupling ratios the signal amplitude follows a linear relation to the applied peak current. Moreover, the slope of the linear fit increases with coupling ratios, indicating that a larger coupling ratio offers a higher sensitivity. Figure 8 shows the signal gain of the FRMC compared to the case of a single FRM as a function of the power percentage coupled to the second FRM, where the square dots represent the experimental result, while the solid curve is the simulation result with no cavity loss. In both cases the signal gain increases with coupling ratio, which means that a higher coupling ratio ensures a larger signal gain. However, the signal gain is affected to a large extent by the cavity
Table 1.
Fig. 6. (Color online) Comparison of noise levels measured with PD detection and single-channel detection.
Comparison of Typical Noise Measured with PD Detection and Single-Channel Detection
Noise Frequency (Hz)
PD Detection (dBm)
Single-Channel Detection (dBm)
Noise Reduction (dB)
60 128 180
−66:73 −56:16 −66:6
−34 −22:02 −35:58
32.73 34.14 31.02
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Fig. 7. (Color online) Signal amplitude as a function of peak current at different coupling ratios. Fig. 8. Signal gain of the FRMC as a function of the power percentage coupled to the second FRM.
loss, which mainly comes from the reflection losses of the two FRMs. The reflectivity of the FRM is measured to be around 0.75, for which case the simulation result is shown as the dashed line in Fig. 8 for comparison. As one can see from Fig. 8, for the maximum coupling ratio of 99∶1, the signal gain can be as high as 68 dB in the ideal case without any loss, but drops sharply to only 14 dB with an FRM reflectivity of 0.75, while the experimentally obtained signal gain is even smaller, i.e., 8:43 dB, which is due to the insertion losses of the couplers that additionally contribute to the actual cavity loss. The different deviations of the experimental data from the dashed line are caused by the varied insertion losses of different couplers. C.
Weak Current Detection and System Response
In order to explore the capability of weak current detection for this system, measurements were made with a current of several mA. In these measurements a cavity with a coupling ratio of 50∶50 was used. As indicated in Eq. (5), the signal amplitude obtained by the balanced detector depends on the product of light power and the rotation angle of the SOP. As the cou-
pling ratio increases, the rotation angle of the SOP increases through multiple passes of the light within the cavity; however, the detection power also decreases; therefore, a 50∶50 cavity is the optimal trade-off between its moderate power loss and a reasonable multiplication of the rotation angle of the SOP. Figure 9 shows the measured result for a weak current of 6 mA, where (a) is the time domain signal, and (b) is the power spectrum. Although low-frequency noise becomes significant in the case of a weak current, the power spectrum still shows a very good SNR in the vicinity of the signal frequency. Figure 10 shows the measured average signal amplitude as a function of peak current. The lowest current we measured was 1:5 mA. As one can see, even for such a weak current, the detected signal amplitude follows a good linear fit according to the applied peak current, indicating a very good performance of the system for weak current detection.
Fig. 9. Measured results for an amplitude of 6 mA with a 50∶50 cavity. (a) Time domain signal, (b) power spectrum. 928
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Research Chair program for the financial support of this research. References
Fig. 10. (Color online) Signal amplitude as a function of peak current obtained with a 50∶50 cavity.
5. Conclusion
We proposed a sensitive OFCS based on PD detection and an FRMC. The cavity formed by two FRMs effectively increases the sensitivity of the sensor without being affected by the linear birefringent effects. With the method of PD detection, the amplitude of the antiphase signal is doubled while the in-phase noise is suppressed to the largest extent, leading to a much better SNR. Measurements with AC frequency of 1 kHz show that signal gain of 6 dB and noise suppression of larger than 30 dB are obtained using the PD detection compared with the single-channel detection. Using this current sensor with a 50∶50 cavity, AC current as weak as 1:5 mA can be detected with a linear response to current amplitude, which indicates a very good performance of the system for weak current detection. With its high sensitivity, this OFCS is suitable for the measurement of current in most industrial applications, especially in those where weak current is featured. Furthermore, it is an optional flexibility that the coiled copper wire structure can be replaced by the simple scheme of straight wire surrounded by the fiber coil, which is more preferable for large current measurement where high sensitivity is not required. The authors would like to acknowledge Natural Science and Engineering Research Council of Canada (NSERC) Discovery Grants and the Canada
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