Fiber-Optic Sensor for Acoustic Localization

Problems associated with the demodulation and localization of acoustic signal produced by fiber optic interferometer are investigated. It is shown that existing demodulation method utilizing 3 × 3 fiber coupler do not work well for that the acoustic signal cannot reach the full amplitude of the sensor to compensate the dc and normalize the amplitude. Modifications to improve the performance of the previously proposed demodulation method are suggested. After the demodulation, two phase difference signals are received to have two signals with fixed time delay based on a unique data processing method, and hence, the position of the disturbance can be measured by the signal correlation method to extract the time delay. Operation of the modification of the demodulation method is verified experimentally along a 20-km-long sensing fiber with the minimum detectable phase signal closed to the noise equivalent phase (NEP) (0.2 rad). The performance of the sensor is also compared with the conventional sensor, based on the null frequency of the spectrum of the phase difference generated by the acoustic disturbance. Experimental results show that the sensor is advantageous for easy installation, low location error, and flexibility for the application of acoustic localization.