Real-time monitoring of fused deposition modeling based on optical coherence tomography

Fused deposition modeling (FDM) has become a widely adopted additive manufacturing technology for producing complex functional structures, owing to its high material utilization, low cost, and design flexibility. However, current quality inspections are largely restricted to post-fabrication evaluation, making it difficult to perform comprehensive quality control and detect defects as they occur. This limitation hinders both process optimization and improvement of print reliability. In this study, we present an integrated real-time in situ monitoring system that combines an optical coherence tomography (OCT) imaging probe with the print head of an FDM printer. The probe consists of a single-mode fiber coupled with a self-focusing lens to achieve efficient collimation and spot focusing. As the print head moves, the co-mounted probe performs volumetric scanning, from which a reconstruction matrix is formulated according to the scanning trajectory. Interpolation-based algorithms are then applied to generate three-dimensional images from the acquired OCT data. We validate the system by printing and scanning a polylactic acid structure and successfully detecting multiple types of defects during fabrication. The results demonstrate that the proposed approach enables real-time defect identification and structural reconstruction, providing a powerful pathway toward enhanced precision, process control, and reliability in additive manufacturing.