Self-witnessing coherent imaging for artifact removal and noise filtering

•Laser welding needs high-speed in situ monitoring for quality assurance and control.•Coherent imaging measures depth but with speckle and autocorrelation artifacts.•We introduce a second imaging channel with no loss of resolution or imaging speed.•Uncorrelated noise and artifacts are now easily filtered out.•Laser keyhole weld tracking rate improves by as much as 67%. In situ coherent imaging provides on-the-fly measurement of morphology changes during high-power laser processing (such as welding and additive manufacturing) at high speeds (exceeding 300 kHz line rates) but suffers from speckle and image artifacts particular to interferometric imaging. We present a self-witnessing inline coherent imaging system that incorporates a second imaging channel to allow complete removal of autocorrelation artifacts, and an increase in signal to noise by a factor of two, with no loss of imaging rate or spatial resolution and no need for balanced detection or specialized light sources. When applied to laser welding of 316L stainless steel, the keyhole tracking rate is increased by as much as 67%.