Automated Laser-Fiber Coupling with Three-Axis Fiber-Core Alignment for Optical-Resolution Photoacoustic Microscopy

Optical-resolution photoacoustic microscopy has become an important biomedical imaging modality for visualizing optical absorption contrast with micrometer-scale spatial resolution in superficial tissues. Because this modality relies on tightly focused laser illumination, precise optical alignment is essential, and free-space-to-fiber laser coupling is one of the most critical alignment steps. This issue becomes particularly important in transportable optical-resolution photoacoustic microscopy systems, where repetitive manual readjustment is required. Although our previous automated coupling approach based on motorized mirror control reduced the need for manual beam-path alignment, its optimization capability was limited when the focal position relative to the fiber core is not actively controlled. In this study, we propose an upgraded automated fiber-coupling module that integrates beam steering and fiber-side focusing alignment. In addition to the incident beam path control using motorized mirrors, a piezo-driven three-axis stage carries the coupling objective lens to align the focused beam with the fiber core. The coupled light was divided into imaging and monitoring paths, enabling real-time feedback-based optimization. The proposed module was validated by achieving high-quality photoacoustic images of both phantom and biological tissue through automated coupling. By extending automated coupling from mirror-based beam steering alone to combined beam steering and three-axis fiber-core alignment, the proposed coupling module provides a practical solution for improving the robustness, usability, and transportability of optical-resolution photoacoustic microscopy platforms and other fiber-coupled optical imaging systems.