Coaxial confocal laser-ultrasonic integrated probe for enhanced imaging of seismic physical models

In optical ultrasound systems, the ultrasonic emitter and detector are typically configured in an off-axis arrangement, a geometry that is prone to misalignment issues, thereby increasing system complexity and resulting in a diminished signal-to-noise ratio (SNR). In this study, we present a coaxial and confocal laser-ultrasonic probe, realized by integrating a composite material transducer with a fiber-optic polymer sensor via a custom-designed plano-concave lens. The Ti₃C₂-based laser ultrasound transducer achieves a considerable conversion efficiency of 1.17 × 10^-2 with a -6 dB bandwidth of 14.7 MHz. The polymer waveguide sensor, fabricated using an in-fiber microchannel method, exhibits a low noise equivalent pressure of 15.38 Pa and a wideband response reaching up to 25 MHz. A center-drilled optoacoustic lens is designed to coaxially assemble the ultrasonic transducer and sensor, thereby forming a transmit-receive dual-focusing probe. The collaborative integration largely simplifies the optical ultrasound system and facilitates high-fidelity scan imaging of seismic physical models (SPMs). Compared to the conventional off-axis unfocused configuration, the on-axis focused probe yields an SNR enhancement of 10 dB for imaging a stratified SPM. As an optical ultrasound transceiver, the proposed probe offers advantages including easy fabrication, compact design, efficient transmission, and functional scalability, offering a novel strategy for structural and tissue imaging in optical ultrasonics.