Broadband ultrasonic transducer is always preferred for diagnostic ultrasound imaging, because wide frequency bandwidth can reduce duration of ultrasonic pulse and enhance the axial imaging resolution. However, frequency bandwidth of both bulk piezoelectric ceramics based ultrasonic transducer and conventional piezoelectric micromachined ultrasonic transducer (pMUT) is quite limited. To overcome this limitation, a mode-merging pMUT is presented in this paper. By using a rectangular membrane with large length / width aspect ratio, numbers of resonant modes are excited within a narrow frequency range. When this pMUT works in largely damped medium, excited modes are merged together, forming an ultra-wide bandwidth. A -6dB bandwidth of over 94% is measured in water for the proposed pMUT without matching layer, which is significantly broader than that of conventional pMUT. Benefited from such ultra-wide frequency bandwidth, pulse duration of 1 μs is achieved at central frequency of 1.24 MHz. If this broadband pMUT is utilized to replace the conventional ultrasonic transducers for diagnostic ultrasound imaging, the axial resolution can be significantly enhanced without compromising the sensing range.
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