Single-pulse control of ablation by monitoring response of target material

Material exhibits various responses under laser illumination especially when ablation takes place. Some of the material responses are investigated to probe the progression of laser ablation. Our objective is to use the progression signal as a feedback for ablation process control. Often it is desired to be able to actively stop ablation when the target material is totally removed by the laser. This capability is even more beneficial for selective ablation of multi-layer composite materials. We studied the ablation signals as laser ablation progresses on spinning, cylindrically symmetrical, three-layer polymer target. The commencement of each laser pulse is controlled by software which monitors feedback signal in real time. Laser pulsing can be stopped when the feedback signal reaches the pre-determined level. Ablation progression therefore is controlled to single-pulse level. The ablation system uses mask-projection imaging with a 248-nm Excimer as laser source. Ablation emission spectra, surface reflection of the ablating laser and reflection of probing diode laser are used for this study. Currently, the speed is limited by mechanical aspect of system.