In this paper we demonstrate a compact current sensor using the optic fiber micro wire, based on the idea of interferometrically measuring the thermally induced optical phase shifts as a result of heat produced due to the flow of electric current over short transit lengths. A responsivity of 1.28 x 10-4 rad/I2 at 50Hz of current signal has been shown, with capability of measuring alternating current signals up to 500Hz.
© 2010 OSA
In recent years, optical fiber nano wires and micro wires (OFNM) have received considerable attention due to a number of interesting optical properties such as large evanescent field, strong confinement, configurability and robustness [1,2]. One of the attractions of OFNM’s is their flexibility compared to that of normal optical fibers. OFNM offers bend radii of the order of a few microns, allowing for packaging of more complex and compact devices. High sensitivities to environment  due to large evanescent fields, have lead to utilization of OFNM’s as hydrogen, humidity, refractometric and biochemical sensors [4–10] as well. These sensors present excellent properties such as high sensitivity and fast response. In recent years there have been several important advances in OFNM that have contributed towards their increased use for optical sensing, such as recent fabrication techniques of low-loss fiber tapers allow OFNM with lengths from a few millimeters to tens of meters [11–13], thereby increasing their interaction length, embedding/packaging techniques with low refractive index materials have solved problems of device stability [14–16] and the preservation of polarization state with usage of rectangular shaped OFNM with high birefringence [17,18]. All these advances make the OFNM a perfect contender for next generation optical sensing.
Current sensor is a typical application of optical fibers [19,20]. All fiber optical current sensors are typically divided into two categories, where one is based on Faraday effect and the other is based on thermal effect. The former is limited by the extremely small Verdet constant of silica, so in order to increase the device responsivity lots of turns of fibers are needed. A long fiber is necessary to increase the device responsivity because the minimum bend radius of normal optical fiber is a couple of centimeters. The latter needs a short length of fiber but requires complex manufacturing techniques to coat fibers with metals [21–23].
In this paper we demonstrate a compact optical current sensor using an 80mm-long optical fiber micro wire (OFM) with a diameter of 5μm. The OFM was wound for 25 times directly on a 5mm-long, 1mm-diameter copper wire coated with a very thin Teflon film to prevent light coupling. To our knowledge, this is the first time such a miniature optical fiber current sensor is reported.
2. Sensor fabrication and theoretical considerations
Sensor fabrication comprises two-steps. A conventional single mode optical fiber (Corning SMF-28) is tapered into an OFM with a diameter of 5 micrometers using the modified flame-brushing technique , with 80mm length of the central uniform taper region. The taper is then wound on a copper wire of length 5mm and diameter 1mm. Two ends of the copper wire are soldered on two pieces of copper plate to facilitate the assembly in an electric circuit with low contact resistance. Before winding, a very thin Teflon layer is coated on the copper wire to prevent leakage losses due to optical coupling from the OFN into the copper wire. The OFM is carefully wound on the copper wire with a large pitch to eliminate optical coupling between adjacent turns. 25 turns were wound on the copper wire with an 80mm-long OFM. The sensor is then packaged with a UV-curable acrylate polymer (Luvantix PC-373). During the fabrication process, the OFM transmission is measured in situ to ensure low insertion loss. Figure 1(a) shows the transmissions of the OFM as a function of wavelength before and after packaging with a total loss of 1.5dB at 1550nm, while Fig. 1(b) shows the packaged sample.
When an alternating current I 0 cos(ωt) flows through the copper wire, heat is produced in the wire as:Eq. (1), show the effect on OFM due to both D.C and A.C components with thermally induced influence from heated copper wire. The DC term can be used as heat phase modulator, but it cannot be easily distinguished from the low frequency drift present in the interferometer system; for this reason here only the AC term is considered. The expression for the phase temperature responsivity of light propagating in a fiber of length L is given byEquation (4) shows that the AC term oscillates at twice the frequency of input current and is proportional to I02R. This change in phase at twice the input current frequency is used to measure the input current.
3. Experiment Results
The experimental layout as shown in Fig. 2 , comprises a Michelson interferometer, current transformer and a digital signal processing system. Light from a laser diode (LD) at 1550nm with a linewidth of 0.01nm travels through the optical circulator and is split into two arms of the Michelson interferometer by the 3dB coupler. One arm comprises the current sensor, while the other forms a fiber wrapped round a PZT in order to control the phase by locking at the quadrature point. In order to minimise the polarization dependent noise, both the arms of the interferometer are terminated in Faraday rotating mirrors (FRM). The interferometer output is sampled by an A/D convertor and recorded by a computer. Due to electromagnetic induction, passage of alternating current through the coil results in a high current being induced in the frame at the same frequency. .
The current in the frame is measured by a current meter. The interferometer output is sampled by one of A/D convertor channels of National Instrument USB-6221 multifunction data acquisition board. Data is sent to a computer for signal processing. The computer is also used to control the phase bias of the interferometer by adjusting PZT phase modulator through an analog output channel on the USB-6221.
Figure 3 shows the output of the interferometer when a 50Hz, 90A rms alternating current passes through the sensor. Waveform 1 is the detector output, while waveform 2 is the input signal. The frequency of the output signal was measured to be 100Hz, which is in agreement with Eq. (4).
Responsivity was measured in two different ways. For currents in the range 0 to 40A rms, phase bias control was used. From 45 to 120A rms, harmonics were used to calculate the amplitude of phase modulation , and results are shown in Fig. 4 . Linear relationship (R2 = 0.98) between the amplitude of phase modulation and the square of alternating current (1.2A to 120A rms) passing through the copper wire was obtained. Nearly same slopes achieved for the linear fits carried on the data (ref. Fig. 4) corresponding to two different tests suggests repeatability of interferometer phase change measurements with input current. The responsivity of the sensor is at 50Hz. The resistance of the copper wire is 107μΩ. Assuming a minimum-detectable phase change of 10−6 rad , the observed responsivity at 50Hz corresponds to a minimum detectable current variation of 88mA rms.
The sensor response to frequency is presented in Fig. 5 . A digital function generator was used to supply currents with different frequencies. The current flowing through the sensor was kept at 9A rms and the current sensor response was recorded. Figure 5 shows that the sensor reponsivity decays for increasing frequencies and is found to be at 500Hz.
In conclusion, we have demonstrated the sensing capabilities of an interferometric current sensor based on optical fiber micro wires. The current sensor comprised an optical fiber micro wire wrapped round a 1mm-diameter copper wire over a length of 5mm. Success with such compact designs opens possibilities for even smaller copper support rods, carrying current. The sensor was tested in an optical fiber Michelson interferometer configuration with a phase responsivity of at 50Hz with an input impedance of 107μΩ. However, the responsivity was found to vary inversely with increasing frequencies, and was shown to be for a frequency of 500Hz. Responsivity can be increased by reducing the diameter of the copper wire or increasing the material electrical resistivity.
GB gratefully acknowledges the Royal Society (London, U.K.) for his research fellowship. The authors thank the Engineering and Physical Sciences Research Council (EPSRC) for financial support. Z. Song is grateful to CSC for his academic visitor scholarship.
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