Auto-focus control with optical monitoring using chromatic aberration of focusing optics in laser welding

The optical monitoring using the chromatic aberration of focusing optics was applied to monitor the focus shift, the power variation and the weld depth in Nd:YAG laser welding. The chromatic aberration was optimized to monitor the temperature drop and the weld pool size reduction rates during the cooling process. The spectral signals were measured at 530 nm and 950 nm at up to 100 kHz sampling rate. A signal processing algorithm was developed to estimate the temperature drop and the weld pool size reduction. The correlation between the profiles of temperature drop and the weld pool size reduction in time, and the focus shift, the power variation and the weld depth were investigated. The results indicate that the focus shift can be monitored with accuracy of better than ±1/4 mm and independent of power variation up to ±10 % at laser spot size of 1 mm with focusing optics of F/3.8. Furthermore, the best focus can be verified by monitoring the weld depth.