Ultrasound Measurement Using On-chip Optical Micro-resonators and Digital Optical Frequency Comb

Optical resonator-based ultrasound detection plays a significant role in the modern ultrasonography. It consists of two steps including the optical readout of the acoustic wave signal and the electrical demodulation of the acoustic response. In this paper, we conduct innovative researches on both of them for improved performance. High quality-factor (Q-factor) chalcogenide glass (ChG) microdisk and microring resonators as optical readout configurations are designed and fabricated. High-precision digital optical frequency comb (DOFC) technique is proposed to monitor the minor resonance frequency shift. The sensitivity up to 2.86 dB/Pa and noise equivalent pressure (NEP) down to 4 Pa are obtained under the 40 KHz ultrasound vibration using a 100-μm radius ${\rm{Ge_{11.5} A_{s24}Se_{64.5}}}$ microdisk resonator with the Q-factor of ${11.5\times10^{6}}$ . As a proof-of-concept, three cascaded ${\rm{SiN_{x}}}$ microring resonators-based ultrasound detector is fabricated and characterized to further validate the versatility of DOFC in the arrayed microcavities. Successful realization of the threedimensional ultrasound detection paves the way to the development of high-performance photoacoustic imaging.