Self-witnessing Coherent Imaging for Automatic Noise Filtering and Artifact Removal in Laser Process Monitoring

Abstract

High-power lasers are rapidly becoming standard tools in advanced manufacturing, mainly in the form of laser welding, laser cutting, and laser additive manufacturing. Of these applications, laser welding in the electric mobility sector—particularly in the manufacturing of battery packs—presents unique challenges. Weld depth needs to be precisely controlled to ensure joint strength and to ensure the weld does not puncture into the lithium ion cell. These processes often involve highly reflective metals (such as copper), which have material properties that lead to unstable welds; this requires an unprecedented level of control to ensure weld quality and depth. To ensure weld quality, we need in-line monitoring during the process. Inline Coherent Imaging (ICI) is a process monitoring technique that has been demonstrated to measure keyhole depth (down to 15 µm axial resolution) at high camera rates ( 200 kHz), with unparalleled sensitivity and dynamic range. Unfortunately, it suffers from sources of noise such as speckle and imaging artifacts that can pose a significant challenge to quality assurance and closed-loop control. To mitigate these problems, I have integrated a second, automatically synchronized, imaging channel into a standard ICI system, by exploiting a previously unused part of the imaging window. This “witness” image makes it possible to identify real signal based on correlation and filter out the uncorrelated noise. This has allowed feature-identification intensity threshold values to be lowered from 12 dB to 8 dB. Using this system, we have demonstrated the complete removal of autocorrelation artifacts with no loss of imaging rate or spatial resolution compared to standard ICI. Between the first and second design for this technique, stability in the system increased by a factor of 12. When applied to imaging laser keyhole welding, I see improvements in the effective imaging rate of up to 79% relative to standard ICI for spot welds performed over 30 ms on 1000 series aluminum at 1100 W.