Development of a laser diode-based self-mixing interferometry scanning imaging system with stray-light-resistance for in-process laser direct energy deposition inspection

This paper presents the first laser diode-based raster scanning imaging system using self-mixing interferometry (SMI) for in-process inspection of directed energy deposition (DED) additive manufacturing. The prototype achieves high-fidelity surface imaging at 20 μm spatial resolution, offering flexible region-of-interest scanning. Validation includes detecting 87 μm defects on DED components. A core innovation is its inherent ambient light immunity, critical in high-stray-light DED environments. This immunity stems from synergistic spectral filtering, temporal photon gating via galvanometric scanning with 5 μs dwell time, and adaptive feedback control (AFC) stabilizing signal-to-noise ratio (SNR) under intense 240 klux illumination. The open-source design with non-customized components, outputs standardized 8-bit grayscale bitmaps up to 3,000 × 3,000 pixels, requiring no preprocessing for AI-driven quality monitoring. Experimental studies show scanning frequency governs stability, with 83.3 kHz being optimal. They also reveal that increased detection distance degrades contrast, and resolution affects image detail quality but is capped by the 200 μm optical spot size. The system's compactness and feasibility for dual-view integration demonstrate transformative potential for real-time, closed-loop DED process control.