An Electronically Tunable Multi-Frequency Air-Coupled CMUT Receiver Array with sub-100µPa Minimum Detectable Pressure Achieving a 28kb/s Wireless Uplink Across a Water-Air Interface

Oceans play a critical role in our ecosystem - they regulate weather and global temperature, serve as the largest carbon sink and the greatest source of oxygen. Maintaining ocean health is of paramount importance and has led to the emergence of the “Internet of Underwater Things (loUT)” with intelligent sensors being deployed for aquaculture, environmental monitoring, surveillance, and exploration. Given that RF and optical signals are heavily attenuated in water, and ultrasound (US) - which has favorable propagation underwater - faces a large water-air interface loss (~ 65dB), deep underwater sensing nodes most often communicate data via ultrasonic links to surface buoys, which then use RF to relay data to a remote station. However, such relay-based water-to-air networking solutions are cost and infrastructure intensive, with the inflexibility of anchored buoys prohibiting operation at scale. Wireless, cross-medium communication approaches that do not require intermediary relays would enable large-scale deployment of next-generation loUT sensors. Previously, laser Doppler vibrometers (LDV) [1] and mm-wave radars [2] have been used to remotely detect displacements on the water surface caused by impinging US waves but suffer from poor sensitivity and low data rates.