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.