Investigation on Detection of Sound Wave by using an Optical Frequency-modulated Continuous-wave Interferometer and a Heterodyne Interferometer

Abstract

The successful realization of a contactless, non-perturbing, displacement-measuring system to characterize of the material has driven its application field to the recognition of objects including human character with the incessant demand of safety and security. In recognizing the objects, various techniques of electrical as well as optical methods could be tried. However, for more accurate recognition, optical technique has preferred. For example, LiDAR using the laser light is a good example in autonomous vehicle by the good resolution and accuracy in reading of the distance. Especially, the systems based on optical interference between the light from the target object and the reference light has a capability of measuring the minute displacement of the object located far away. Therefore, these systems have often been applied to the bridge safety diagnosis and various military applications.

In this thesis, we discuss the possibility of extracting acoustic wave (sound) with the frequency-modulated continuous-wave optical interferometer as a cheap and simple system. And, with the commercially available components, its limitation in measuring the length difference is investigated. Also, for accurate detection, a fiber-based optical heterodyne interferometer immune to mechanical vibration is constructed and the Doppler frequency due to acoustic wave is displayed using Labview software. These results can be well utilized in detecting the direction of the sound in the air and only with the sound information, without video information, the movement of the object can be traced on the monitor and various applications will be expected in military area.