Sensitive broadband ultrasound sensor based on a low-loss high-Q fused-silica plano-concave microresonator

Optical ultrasound sensors can achieve high sensitivity, small element size, and broadband frequency response. Polymer plano-concave microresonators (PCMRs) are among the most sensitive ultrasound sensors, but achieving further increases in sensitivity is limited by the optical absorption of the polymer cavity material. To address this, the use of fused silica as the cavity material is investigated. We present findings supporting the assertion that the low compressibility of fused silica compared to polymeric materials can be overcome by exploiting its extremely low optical absorption to achieve high Q-factors approaching 10⁶. We also show that the high Q-factors can introduce laser phase noise unless very narrow linewidth (<150 Hz) laser sources are employed. In this work, a fused silica PCMR ultrasound sensor is reported that has an NEP of 0.7 mPa/√Hz, a broadband frequency response with a -3 dB bandwidth of 2.5 MHz, and low directional sensitivity.