Radiation pattern deterioration for phase steered capacitive micromachined ultrasonic transducer arrays due to cross coupling

Cross coupling between neighbor elements in ultrasonic transducer arrays through surrounding fluid is a known effect. Previous work has shown that it can form a mode that is bound to the transducer fluid interface. This paper investigates how cross coupling affects the radiation pattern for finite phase steered arrays of capacitive micromachined ultrasonic transducers (CMUTs). A spatially finite 2D model of a 1D CMUT array with 72 CMUTs was constructed for finite element simulations using COMSOL. The simulated transducer array was loaded with a viscous fluid, and excited by phase modulated pressure to form a steered plane wave. To model typical electrical CMUT interconnects groups of two CMUTs were excited with the same phase. This enables, for distinct frequencies, acoustic power to be coupled into the bound mode induced by cross coupling. Simulations were performed for a frequency range of 1-35MHz for different steering angles and viscosities. To relate the simulations to a physical CMUT array, displacement measurements were performed using an optical heterodyne interferometer. The simulations indicated a narrow band with significant radiation in non steered directions for low viscosity fluids.