Localized measurement of ultrasonic waves using a Fabry-Perot sensor illuminated by a Bessel beam

Fabry-Perot (FP) ultrasound sensors are a class of optical ultrasound sensors used in photoacoustic tomography (PAT) and other applications. Conventionally, an FP ultrasound sensor comprises an ultrasonically compressible planar microcavity, locally interrogated by a focused Gaussian beam. One way to increase the sensitivity could be to replace this beam with a Bessel beam. The rationale is twofold. First, as a Bessel beam's wavefront better matches the modes of the planar microcavity, this could increase the Q-factor, leading to higher sensitivity. Second, as a Bessel beam provides a focused spatial structure-a central core surrounded by concentric rings-it might retain the ability to locally interrogate the sensor. To explore this idea, we developed an experimental system featuring a custom FP ultrasound sensor interrogated by a Bessel beam and evaluated its on-axis sensitivity, directivity, and image resolution when performing PAT. For comparison, we measured the same characteristics using a conventional focused Gaussian beam with a spot size similar to the core of the Bessel beam. As anticipated, the Bessel beam provided a higher ultrasonic on-axis sensitivity. However, the directivity and spatial resolution were degraded, suggesting that the Bessel beam yielded a larger acoustic element. We conclude that it is feasible to increase the sensitivity of an FP ultrasound sensor using a Bessel beam. Further work is required to establish whether differently designed Bessel beams could concurrently offer a smaller acoustic element.