Ultrasound Detection via Low-Noise Pulse Interferometry Using a Free-Space FabryPérot

In medical application, ultrasound detection is conventionally performed by piezoelectric transducer.
However, with the emergence of hybrid imaging technologies such as optoacoustic tomography, there is
a need for new methods for ultrasound detection that rely on detectors that are transparent and immune
to electromagnetic interference. These properties may be achieved by optical schemes for ultrasound
detection, which often rely on optical resonators to enhance sensitivity. When ultrasound impinges on the
optical resonator, it modulates it central wavelength, where the modulation is often monitored by using a
continuous wavelength (CW) laser. In such schemes, the main noise factor is frequency noise from the laser.
We present a novel scheme for interrogating resonator-based ultrasound detectors, which is based on a
pulse laser. Our scheme, termed coherence-restored pulse interferometry (PI), includes a unique filtering
mechanism that can reduce the optical noise to the shot-noise level - the fundamental quantum limit.
Specifically, our scheme includes a free-space Fabry-Pérot filter whose spectrum coincides with that of
the pulse laser over a bandwidth of 80 nm.
The new CRPI scheme was successfully demonstrated for ultrasound detection in the frequency range of
4-20 MHz, where shot-noise detection was maintained for optical powers up to 5 mW, representing the
lowest optical noise level reported for ultrasound detection, to the best of our knowledge. Accordingly,
CRPI may enable the development of ultra-sensitive optical detector of ultrasound. In addition, the wide
optical band in which CRPI operates enable future wavelength multiplexing of resonators, facilitating the
development of all-optical ultrasound detector arrays