Amplification of Light‐Ultrasound Interaction in a Pi‐Shifted Fiber Bragg Grating for Enhanced Ultrasonic Sensitivity

ABSTRACT Optical resonators offer a promising alternative to conventional piezoelectric transducers in ultrasound detection because they can be miniaturized without a decrease in sensitivity. However, optical‐resonator‐based detectors still possess a significant sensitivity disadvantage compared to millimeter‐scale piezoelectric transducers, preventing the dissemination of these micro‐detectors. In this manuscript, we present a new detector design that bridges the sensitivity gap between optical‐resonator‐based detectors and piezoelectric transducers by amplifying light‐ultrasound interaction in a pi‐shifted Fiber Bragg grating. The proposed detector has a noise equivalent pressure density <0.5 mPa/√Hz, an ultra‐broad bandwidth of more than 100 MHz. We perform a side‐by‐side comparison between our detector and state‐of‐the‐art piezoelectric bulky transducers employed in pre‐clinical optoacoustic systems, revealing comparable performance of the devices. These demonstrations with the detector length of approximately 500 µm and a diameter of only 125 µm pave the way for miniaturized and high‐performance implementations of our detector in a wide range of minimally invasive applications, especially intravascular and endoscopic.