Comprehensive Noise Analysis for Acousto-Optic Measurement of Airborne Sound

Acousto-optic measurement of sound (AOMoS) has been widely utilized in various acoustic studies owing to its noninvasive nature. However, the presence of significant measurement noise often degrades the acousto-optic signals and thus hinders promising applications. This is partly because the characteristics of the noises in AOMoS have not been fully clarified, which prevents us from adequately predicting and controlling them. To address this challenge, we theoretically and experimentally investigate three specific measurement noises in AOMoS: optical intensity noise, optical phase noise, and environmental noise. Convenient notations for evaluating optical noises in acoustic units are provided, which make it easier to understand the AOMoS noises from an acoustic point of view. Our findings showed that among the six noise sources we investigated, optical shot noise defines the white noise floor at high frequencies while background sound noise determines the low-frequency noise spectrum. The analysis provided in this article will serve as an exhaustive guide for designing and implementing an AOMoS system with a desired noise level.