Optical diagnostics in laser-induced plasma-assisted ablation of fused quartz

A wideband and ultrafast phototube is applied to diagnose laser-induced plasma-assisted ablation of fused quartz. It is found that signal waveform is closely related to laser fluence and target-to-substrate distance. For the distance less than a threshold, below which quartz ablation takes place even by single pulse irradiation, there are three peaks detected. Signal analyses show that the first peak is attributed to laser scattering, the second and third ones to Ag target and quartz substrate ablation. It confirms that there is a direct influence of target plasma on substrate ablation. The third peak moves forward and overlaps with other peaks as the distance decreases and laser fluence increases. Peak amplitude and its arrival time of the quartz plasma are used to characterize the ablation dynamics. Signal variation with pulse number shows that at a higher distance, there are only two optical peaks attributed to laser scattering and target ablation in the first pulse. While by further pulse irradiation, the peak for quartz ablation is recorded. It is due to Ag thin film deposited on quartz rear side surface after the first pulse irradiation. As the distance increases further, the peak for quartz ablation moves to the right and finally disappears because of no thin film deposited.