Monitoring of Focus Position During Laser Processing based on Plasma Emission

The effects of lens-to-sample distance on plasma emission induced during laser processing were investigated. For tight focusing conditions, a local minimum value of emission intensity was observed for opaque materials. It was attributed to a reduction of ablation efficiency due to saturation. This effect can lead to identify the lens-to-sample distance that produces the highest fluence on the sample. A strategy to detect the working focus position based on monitoring the intensity of plasma emission was proposed. Variations of laser energy, pulse overlap and material type were also studied.