Research On Resonant Ablation Rules Of Femtosecond Laser Machining Pr-Nd Glass

Abstract:As one of the novel processing technologies with high precision, femtosecond laser micro/nano-processing refers to many disciplines such as physics, materialogy, fabrication and so on. And because of the complexity of interdiscipline and the extremism of processing requirement, complete acquirement the ablation rules of femtosecond laser is the priority of promoting it to industrial application. This paper is devoted to the study of resonant ablation rules of Pr-Nd Glass processed by femtosecond laser.Firstly, the characteristics of enery level and absorption spectrum are analyzed based on the unique electron shell structure of lanthanide elements Pr and Nd consisted in the Pr-Nd Glass, and it is found that Pr-Nd Glass has strong absorption at the resonant wavelengths585nm and807nm. Furthermore, it is discussed that the ionization mechanism of bound electrons and the physical process of ablation removing when femtosecond laser resonantly ablates Pr-Nd Glass.Secondly, a light path system is built, and a shutter system, used to switch on/off the laser, is designed. In order to automatically control the whole processing system, a piece of software is developed through Visual Basic, which simplifies the operation of processing significantly.Thirdly, based on the theoretical analysis and the experimental platform mentioned above, researches are carried out to investigate the change rules of ablation threshold of Pr-Nd Glass resonantly ablated by femtosecond laser. The results show that initial seed electrons are mainly produced by means of multiphoton ionization near threshold laser intensity. Ablating with resonant wavelengths585nm and807nm, the multiphoton resonance ionization is intense, and the lengths of ablation profiles increase by19.8%and6.8%, respectively, comparing with fused silica, and the ablation threshold, corresponding to807nm, decreases by12.3%. However, these change rules fades away for nonresonant wavelength. Finally, the change rules of resonant ablation effect are studied towards two kinds of femtosecond laser with different repetition rates. With the repetition rate80MHz, the femtosecond laser ablation turns to be hot-working, as there is a coupling effect among impulses. The results show that only resonant wavelengths can damage the surface of Pr-Nd Glass, and the fusing ablation threshold, corresponding to807nm, reduces by18%comparing to another resonant wavelength739nm and. drops two orders of magnitude comparing to cold-working with1kHz. On the other hand, the change rules of resonant ablation efficiency are investigated with the repetition rate1kHz. The results show that the resonant ablation efficiency is influenced by laser intensity greatly. For the807nm laser, when intensity is lower than0.556*1014w/cm2where multiphoton ionization dominates, RAE is increased distinctly. When intensity is higher than0.556×1014w/cm2where tunnel ionization dominates, RAE fades away.42figures,13tables,69references...