Self-Aligning Optical MEMS Acoustic Sensors With nPa/√Hz Resolution

A critical step in the large-scale assembly of optical interferometric microsensors is the ability to align their tiny interferometers reproducibly. We report here an acoustic MEMS sensor that utilizes a self-aligning two-wave interferometer and exhibits a record pressure resolution. The sensor consists of a circular, spring-loaded diaphragm that vibrates in and out of the plane of a stationary substrate when exposed to an acoustic pressure. This piston-like differential motion between the diaphragm and the adjacent substrate is detected interferometrically with a free space laser beam, delivered by a single-mode fiber, that straddles the diaphragm-substrate boundary. The diaphragm is micro-fabricated in a mechanical chip provided with a reference ledge, and the fiber is bonded to a small hole bored through an optical chip provided with a matched groove. The two chips are aligned with micron-scale precision by mating the ledge and the groove, then gold-bonded. We demonstrate two self-aligning sensor prototypes with similar acoustic sensitivities and a very weak dependence of the sensitivity on the laser wavelength, which is beneficial for multiplexing large arrays of sensors. The sensor exhibits a self-noise limited by residual excess noise below ~300 Hz and by the very small thermo-mechanical noise of the diaphragm above 300 Hz. Its average pressure resolution of 215 nPa/√Hz between 40 Hz and 4 kHz establishes a new record.