Sound Localization and Restoration using Fiber-based Optical Interferometer

Abstract

Acoustic sensors using electrical signals are well known tools for detecting and extracting acoustic signals. In addition, it has been used not only for simple sound recording, but also for sensor systems that estimate the positions of sound sources using an array of sensors by analyzing frequencies and intensities of collected signals. However, these acoustic sensors are disadvantageous in that electromagnetic noise is exposed, and it is difficult to use them as sensors in a severe environment such as underwater or high-temperature transformers. As an alternative of this sensor, a optical acoustic sensor has been studied. The optical-based acoustic sensor is totally immune to electromagnetic noise, and can be used in severe environments because it uses the propagation medium or traveling path as the sensing unit, also it can be measured in a non-contact state with diaphragm. In addition, based on the wide bandwidth of the characteristic of optics, it is possible to collect signals over the audible range (20 to 20000 Hz) and thus it is attracting attention as a sensor used in acoustic ans vibration sensor network. In this paper we study of sound localization and restoration through optical acoustic sensors, especially based on optical interferometer that obtain information from phase of light. We used an acousto-optic effect to detect acoustic signals using a light interferometer for sound localization. The acousto-optic effect is a phenomenon in which the refractive index of the medium changes due to sound pressure, which ultimately affects the progress of the light. This effect can be inversely analyzed to measure sound sources. An array sensor was constructed using an optical fiber based Mach-Zhender optical interferometer to estimate the location of the sound source. Fiber-based Mach-Zehnder optical interferometers can easily increase the number of sensors by sharing reference part for each sensing part. Also, each sensing unit is made up of optical passive elements, which is easy to install and expand. We propose an array sensor consisting of three sensors using this structure and verify the performance of the proposed position estimation method by measuring the estimation error according to the position of the sound source. Next, this paper deals with a method of restoring acoustic signals based on vibration of reflector exposed to sound using the optical interferometer displacement sensor. In order to measure the micro vibration of the object, the temperature of the light source was controlled to secure precise wavelength stability. This stability is sufficient to measure the sub-nanometer displacement. In order to increase the measurable range and determine direction of the displacement, the quadrature detection method is implemented through the laser modulation with sinusoidal signal. A side effect of this method is a quadrature detection error which distorts the frequency or magnitude of the measured signal. To reduce this error, we propose a signal processing method based on scaling and trigonometric functions and show that this method sufficiently mitigates the error. The results of this dissertation are expected to be an important role in constructing a non-contact acoustic and vibration sensor network in environments where electroacoustic sensors are difficult to use.