Inline coherent imaging moved weld depth measurement from inference to observation
Optical coherence tomography aimed down the keyhole measures penetration directly during the weld, replacing indirect proxies that never quite correlated.
For decades, laser weld quality monitoring relied on proxies. Photodiodes watched plasma emission, cameras watched the melt pool, acoustic sensors listened. Each correlated with penetration under controlled conditions and each drifted when material, surface condition or joint geometry changed. The measurement everyone wanted — how deep is the weld, right now — was not available.
What coherent imaging does differently
Inline coherent imaging, drawn from optical coherence tomography in medical imaging, sends a low-power measurement beam down the same optical path as the process beam and into the keyhole. Interferometry between the returned light and a reference arm yields the distance to the bottom of the keyhole, at kilohertz rates.
That is a geometric measurement of keyhole depth rather than an inference from emitted light. It does not care much about surface condition or alloy, and it produces a number in micrometres.
Why the distinction matters industrially
Keyhole depth is not identical to final fusion depth — the melt solidifies behind the keyhole and the relationship depends on speed and material — but it is close enough, and stable enough, to serve as a control variable.
That enables two things proxies could not. The first is closed-loop control: adjusting power in real time to hold penetration constant as conditions vary. The second is per-part documentation, which matters in aerospace, medical and battery work where evidence of process compliance is part of the deliverable.
The practical caveats
Alignment matters, since the measurement beam must find the keyhole. Highly dynamic processes with unstable keyholes produce noisy traces that need filtering. And the technique measures where it is aimed, so a weld whose failure mode is lateral — missing the joint rather than being shallow — still needs seam tracking.
Adoption pattern
Uptake has followed regulatory pressure rather than cost curves. Battery, aerospace and medical device manufacturers adopted it first, because they were already paying for destructive sampling and could substitute measurement for statistics. General fabrication has been slower, which is what usually happens when the benefit is documentation rather than throughput.
This article was produced by the LasersNews AI desk and reviewed by our editors.
Related reading
Shift Toward Data-Driven Beam Modulation and Closed-Loop Monitoring Reshapes Laser Welding
Advanced beam modulation and real-time monitoring are transforming laser welding from a manual craft into a p…
Remote Laser Welding Systems Converge on Dynamic Beam Shaping and Closed-Loop Monitoring
Advanced remote laser welding systems are merging dynamic beam shaping with real-time optical monitoring to m…
Handheld laser welding grows up as fume, safety and training catch up to the hardware
The tool that upended small-shop fabrication is now facing the unglamorous engineering work that decides whet…