PRISM: PRecise Interferometers as Stratospheric Microphones

The stratosphere, a low acoustic noise environment with enhanced infrasound propagation properties, is an ideal channel for long-range acoustic monitoring. High-altitude balloons (HABs) equipped with low-noise acoustic recorders can effectively utilize this channel to monitor and localize various natural and anthropogenic acoustic phenomena. Mavericks Photonics proposes to develop a lightweight prototype of its record-setting high-resolution optical, acoustic microsensors as precise stratospheric (>20Ākm altitude) microphones. The sensors utilize a microfabricated diaphragm to detect extremely weak acoustic waves. When exposed to an acoustic wave, the diaphragm vibrates parallel to itself relative to a stationary substrate. The diaphragm vibrations are detected interferometrically with a laser beam provided by a single-mode fiber placed ~10ĀĄm from the diaphragm. Our developed acoustic sensors exhibit an exceptionally high resolution of 215 nPa/?Hz from 40 Hz to 4 kHz.ĀIn fact, they are sensitive enough to resolve the small phase noise imparted to the probe light by the thermal noise of the diaphragm structure. The microsensors exhibit anĀoptimal sensitivity independent of the ambient static pressure or the temperature of their environment, areĀlightweight (<5 g), compact (~3Ācm3), low power (~2ĀmW of optical power), and immune to electromagnetic interferences. Simple sensor multiplexing schemes will be investigated for simultaneous measurement of acoustic pressure and acceleration along the 3-axis; enabling acoustic source localization and tracking. In Phase I, Mavericks Photonics will develop a compact microsensor interrogation and recording module to leverage this powerful combination of sensor properties in a robust, lightweight (<2Ākg), and standalone system for field demonstrations onboard HABs in Phase II of the project.