Airborne Ultrasound Detection via Refractive Index Modulation of Air Using Sagnac Interferometry

Airborne ultrasound is increasingly recognized as a crucial technology for short-range sonar applications in vehicles and drones, as well as for the development of haptic devices, noncontact manipulation of small objects, medical imaging, and material evaluation. This paper presents a novel approach using a fiber-based Sagnac interferometer to detect acoustically induced modulations of the air's refractive index with a minimal setup. This method offers immunity to low-frequency thermal fluctuations and vibratory disturbances while circumventing the complexities and expenses associated with optical components. By eliminating mechanical moving parts, the system benefits from enhanced robustness and operates without the need for recalibration. Initially, a theoretical analysis of the operating principle is provided, followed by experimental validation, which demonstrates the capability to measure ultrasound frequencies ranging from 26.4 to 96.1 kHz using fiber delay coils spanning 100 to 1000 m. Techniques for assessing sound pressure levels are also discussed. Furthermore, the integration of an electro-optic modulator into the system is discussed, highlighting its significant contributions to improving the system's responsivity, linear dynamic range, and stability.