Design and characterization of a sensitive optical micro-machined ultrasound transducer

Novel 3D intravascular or transesophageal ultrasound approaches require transducer arrays containing many small elements. Conventional piezo-electric techniques face fabrication challenges due to narrow kerfs and dense wiring. Micro-machined alternatives like CMUTs and PMUTs lack either sensitivity or bandwidth to fully compete. Therefore we developed an opto-mechanical ultrasound sensor. The absence of wiring makes it MRI compatible. The developed OMUT contains integrated photonics, which is fabricated using standard silicon-on-insulator technology, providing a small footprint and enabling mass production and ease of integration. The sensor consists of a straight waveguide and a micro-ring resonator integrated on a 124 μm wide, 2.7 μm thick acoustical membrane. Light, passing the waveguide, is partly coupled into the ring resonator. A dip appears in the spectrum of the transmitted light at the resonance wavelength of the micro-ring. If the acoustical membrane and hence the micro-ring is deformed due to an incident ultrasound wave, this is observed as a shift in the resonance wavelength of the ring. This paper presents the construction and characterization of our device. Measurement results of the linearity, frequency response, sensitivity and temperature dependence are compared with a model. The results demonstrate that our OMUT is a promising basis for future ultrasound arrays.