Restoration of Voice Signal using a Heterodyne Optical Interferometer

Abstract

Displacement measurement based optical interferometers are used in various elds such as diagnosis and monitoring of facilities in industry, crack detection of structures, scanning of terrain, and acoustic signal detection, etc. In this paper, we propose a method to detect and restore sound in an environment which is dicult to hear the sound using optical interferometer based on displacement measurement. The principle of the optical interferometer is to obtain the path dierence by using the phase information to the interference signal caused by interfering the same optical signals inputted to two dierent paths through each path. However, with only one phase information, it is not possible to know the direction of the change of the displacement position, and the range of the measurable displacement is limited due to the fringe limitation. Therefore, it is possible to overcome this limitation by using the Quadrature detection method which utilizes the in-phase signal and quadrature signal that has a 90 degree phase dierence for an in-phase signal and its signal. The conguratuin of optical interferometer includes a homodyne interferometer using a single frequency and a heterodyne interferometer using two frequencies. In this paper, by constructing a heterodyne interferometer using acousto-optic modulator, it is possible to prevent the problem of wavelength uctuation due to current uctuation applied to laser diodes caused by direct modulation method and to prevent electrical noise in baseband that can occur in the electrical signal processing process. In addition, the interferometer is classied according to the path conguration as Michelson, Fabry- Perot, Mach-Zehnder, and Sagnac Interferometer, and in this paper, the Mach-Zehnder Interferometer was constructed using acousto-optic modulation method, which is easy to congure bulk optical interferometer The voice signal restoration experiment was conducted to obtain the vibration of thin lm through phase information by connecting the optical ber to the sensor head equipped with thin lm by using the direct modulated Fabry-Perot interferometer that was previously studied. As a result, the problem of the Quadrature signal obtained from the interference signal becomes unmeasurable when the re ector's re ectivity is not enough, and this conguration was found to be inappropriate for the purpose of this study to restore the sound source in an environment where the sound source is not well heard. Therefore, the voice signal restoration study was conducted using a Mach-Zehnder interferometer with an acousto-optic external modulation rather than a direct modulation method. In this paper, the aim was to restore the sound source by detecting vibrations of objects in an environment that is dicult to hear, and we expect this paper to be a good guideline for various industrial elds as well as voice restore related industries and research using optical interferometer.