Fiber-optic acoustic sensors are being developed for the detection and location of partial discharges in oil-filled transformers. The partial discharges can be detected acoustically and located by the triangulation method with the sensors placed inside the transformer tank. In this application, the directionality of the sensor should be as flat as possible within at least 40 from the sensor axis and for a frequency range up to 150 kHz. A calculation of the directionality of a fiber coil acoustic sensor was made using a plane wave approximation. The calculation showed that if the diameter of a fiber coil is less than 20 mm, its directionality is relatively flat within 40 for frequencies lower than 150 kHz. The directionality of an optical fiber acoustic sensor for the frequency range of 50-300 kHz was measured experimentally in an oil tank. The effect of reflected waves was avoided by using a gating technique. The experimental results show somewhat different directionality patterns from the theoretical results but are internally consistent and with maxima and minima at frequencies close to those predicted. The discrepancy is believed to be due to a resonance effect which is not taken into account in the theoretical model. However,the sensor achieves the directionality performance required for the application.
Zhao Zhiqiang, Mark MacAlpine, and leyman Demokan, "The Directionality of an Optical Fiber High-Frequency Acoustic Sensor for Partial Discharge Detection and Location," J. Lightwave Technol. 18, 795- (2000)