Picosecond laser ablation on glass using wavelength of 1064, 532, and 355 nm

The process of removing transparent material such as glass by means of laser ablation is widely performed in the industry. In many cases, the ultrashort pulse laser is used as the laser source, but the laser wavelength is diverse. The ablation mechanism of the transparent material generally is considered nonlinear absorption, but it is known that the absorption coefficient depends on the wavelength. However, there are few examples that discuss the ablation mechanism for multiwavelengths. We performed laser ablation of glass with a 15-ps pulse width and 1064, 532, and 355 nm wavelengths and constructed a unified understanding of the principle by numerical calculation using combined rate equation and beam propagation method. In addition, we measured the three-dimensional profile of the processing traces using a microscope and investigated the dependence of ablation volume to the wavelength. As a result, we acquired the results of ablation efficiency, laser fluence threshold, and inverse absorption coefficient of fused silica glass using various wavelengths in a single set-up. We were able to explain the ablation diameter and depth by numerical calculation considering the multiphoton ionization process and avalanche ionization process. In addition, the calculated laser efficiency indicated that the wavelength suitable for ablation on glass was 355 nm.