An Experimental Study of Acoustic Vibration Effects in Optical Fiber Current Sensors
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This paper presents experimental results comparing the susceptibility of two different optical fiber current sensors to environmental acoustic vibrations. Of the two sensors that are tested, one employs a unidirectional current detection scheme, and the other a bi-directional one. Acoustic vibrations are applied to various optical components of the two different types of current sensors. For each vibration test, several inches of fiber were perturbed at several g's acceleration at a fundamental frequency of 200 Hz. A polarimetric detection sensor was chosen for the unidirectional type sensor. This particular sensor implements two PM fiber communication waveguides which transmit light to a SM fiber Faraday sensing coil. When acoustic waves of 3.0 g peak to peak at 200 Hz were applied to the sensing coil, the sensor detected an apparent current of 141 A rms at the same frequency (Figure 1). When waves of the same magnitude and frequency were applied to the PM fiber leads, there was no false detection. The polarimetric detection sensor is unable to distinguish an acoustic vibration perturbation in the sensing coil from an actual current in the conductor. It does, however, show no sensitivity to mechanical stress perturbation applied to the PM fiber leads. The bi-directional type fiber sensor we tested was an in-line Sagnac interferometer. Acoustic waves of magnitude 3.6 g peak to peak at 200 Hz were applied to the Faraday sensing coil of this sensor (Figure 2), and waves of magnitude 6.0 g peak to peak at 200 Hz were applied immediately after the delay line. For these two tests, there was no false detection of current. The in-line Sagnac interferometer is immune to acoustic vibrations in the Faraday sensing coil. It does, however, suffer from vibration sensitivity far from the sensing coil. When acoustic waves of magnitude 4.0 g peak to peak at 200 Hz were applied before the delay line, i.e. 550 m away, this bi-directional current sensor detected an apparent current of 46 A rms at the same frequency (Figure 3). It should be noted that this sensitivity far from the sensing coil increases linearly with the length of the delay line.
