Global shutter depth profiling of molten pool and keyhole during laser processing using core-array fiber

Online 3D depth profiling of molten pool/keyhole during laser processing is of great importance to evaluate the metrics of the live process. The indirect methods based on visual image and thermal radiation suffer low correlations with pool/keyhole behavior, thus the accuracy is typically low. Optical Coherence Tomography (OCT) shoots a light probe coaxially with the processing beam into the pool/keyhole, hence being able to provide a direct depth measurement. When it comes to profiling the depth of an area, a galvanometer is typically used to scan the area of interests. However, the pool/keyhole intrinsically flows in a highly dynamic mode, the mechanical scanned image suffers blurs and deformation, as a rolling-shutter camera suffers when imaging object is moving fast. To address this issue, a global shutter imaging method is proposed to image the pool area synchronously. A low coherent light is split into multiple fibers, which are then bundled into a core-array fiber and guided parallelly into the laser head, resulting a multiple of interfering pairs captured and imaged at the same moment. The theoretical model of this global shutter imaging method was created and analyzed in terms of the image performance and limitations. A two-core fiber global shutter imaging system was built to demonstrate the imaging performance on molten pool/keyhole. It shows a great potential to capture high quality 3D points of keyhole/molten pool for further fine closed loop control.