Palm sized airborne ultrasonic sensor with nanofoam acoustic lens and homodyne interferometer

We propose an optoacoustic airborne ultrasonic sensor consisting of a 633-nm He-Ne laser, an Si PIN photodetector, and a sensor head. The device has a sensitivity of 92.7 dB SPL (Sound Pressure Level) for acoustic signal inputs of single 40-kHz pulses, and it exhibits wideband receiving properties because of excluding mechanical moving elements such as diaphragms or voice coils from the sensor. The sensor head, which incorporates an acoustic lens, is made of nanoporous silica in which the sound speed is 50 m/s. Also, it has tapered aperture fields designed to improve its focusing properties and achieve high sound pressure at its focal point, and an integrated Mach-Zehnder homodyne interferometer that translates the pressure into the intensity of optical interference light. The entire device occupies a volume of 65 × 70 × 70 mm³. In addition, we discuss the design methods and derive the equations for the acoustic lens. Finally, a strategy to achieve more stable and highly sensitive optoacoustic sensors is also discussed.