A Miniature Photoacoustic Sensing System with Advanced PMUT and VCSEL Devices.

Abstract

This study presents a high-fill-factor piezoelectric micromachined ultrasonic transducer (PMUT) array fabricated via the cavity silicon-on-insulator (CSOI) process. The center frequency of the PMUT is 4.89 MHz in air and 3.5 MHz in a gel-type couplant, along with a -6 dB photoacoustic (PA) bandwidth of 146%. A miniaturized PA sensing system (4.6 mm × 2.0 mm × 5.2 mm) was developed by integrating the PMUT with a compact vertical-cavity surface-emitting laser (VCSEL). Simulation results reveal critical parameters for optimizing a PA system. The increase in laser excitation energy correspondingly improves the efficiency of PA signal generation, and spatial non-uniformity in optical energy distribution requires algorithmic compensation to prevent signal distortion. The small footprint of the PMUT minimizes phase differences, enabling distortion-free signal detection at close range, while its broad bandwidth ensures high-fidelity signal capture at an optimized center frequency. Guided by these findings, the VCSEL parameters were optimized to a 28 ns pulse width and an average output power of 63.6 μW. Comprehensive characterization, including electrical impedance, acoustic response, and PA bandwidth tests, demonstrated the consistency of the PMUT fabrication process, broadband capability and superiority of the PMUT in close-range sensing. Phantom-based testing showed that the system can acquire multi-depth phantom signals, and the cylindrical wave radiation analysis further highlighted the critical role of laser-PMUT spacing in maintaining signal integrity. This feasibility study validates the compact system as a promising platform for portable PA devices under ideal phantom and ex vivo conditions.