Position determination of an acoustic burst along a Sagnac Interferometer

We report on a method to measure the longitudinal position of an acoustic burst, or spark, using an all-fiber Sagnac Interferometer (SI). A focused pulsed similar to10 ns laser is used to ionize air creating the spark. This ionization point is situated near a coil of test fiber connected to the SI but acoustically separated from the bulk of the SI. A fast Fourier transform (FFT) performed on the temporal signal of the SI shows a series of nulls from which the position of the acoustic signal is determined. Over an SI length of similar to35 km, longitudinal positions are determined to within tens of meters, corresponding to a best run percent uncertainty of 0.07%.