Principle and practical setups for acousto-optic measurements of airborne ultrasound

Ultrasonic proximity sensors are widely used in vehicles, robots, and drones, while non-contact haptic devices and levitation of objects utilizing ultrasonic radiation force have been attracting interest in several novel fields. To develop a new ultrasonic system working in air, quantitative instrumentation and evaluation of ultrasonic field are essential. However, in some cases, especially in standing wave fields, the existence of acoustic sensor may disturb the field distribution even with a 1/8-inch microphone. By using light, we can achieve quantitative measurement of airborne ultrasound without interfering the field. In the first part of this presentation, the principle of acousto-optic measurement of airborne ultrasound is reviewed by introducing the relationship between the sound pressure and modulation in optical refractive index of air. Second, a basic setup for the measurement based on a heterodyne interferometer is explained. Third, a couple of practical optical systems to carry out the acousto-optic measurements of ultrasound are demonstrated. A quasi-heterodyne system using a low-cost diode laser can eliminate the expensive acousto-optic modulator. An application of Sagnac interferometer using a fiber delay coil provides a stable operation with a low coherence light source.