Beam shaping for efficient femtosecond laser processing of optical glass

Abstract

Manufacturing optical elements is a process that must efficiently output optics with narrowly defined specifications, like surface quality and shape accuracy. The conventional process consists of different iterative grinding and polishing steps. The aim of conventional grinding is to achieve a polish-able surface quality and to reduce polishing time. Therefore, the shape accuracy and the induced sub surface damage (SSD) of the optical element have to be optimized. Due to the benefits of ultra-short pulse laser radiation like non-thermal ablation and high fluence, the femtosecond laser process is investigated for optics manufacturing. Implementation of such technologies opens up new processing possibilities in the field of optical glass. In this paper, the experiments are focused on the ablation process with an optimized top hat profile to achieve higher productivity in comparison to a classical Gaussian profile. For this reason a femtosecond laser system with a maximum average output power of 20 W is applied. During the laser ablation process the resulting surface depends on various input parameters. In order to establish an optimized ablation procedure, the influence of pulse energy, line overlap, and pulse overlap are analyzed. The experiments are performed for different optical materials such as fused silica and N-BK7. The SSD, surface roughness (RMS), and removal rate are evaluated through various characterization techniques. Based on the achieved results, the dependency on the applied beam profile is demonstrated. Furthermore, the efficiency and the constraints in the optical figuring process are discussed.