Picosecond laser micro-machining of glass for optics manufacture

Abstract

Glass-based aspheric optics are attractive for compact optical setups. In addition, custom optics are useful for the correction of pointing errors of High Power Diode Lasers (HPDL). The machining of fused silica (Corning HPFS® 7980), Schott N-BK7®, Schott N-LaF21 and Ohara S-TiH53 using a Trumpf TruMicro 5X50 laser which provides a 6 picosecond pulse duration with a maximum pulse repetition rate of 400 kHz and maximum average power of 50 W has been explored in this thesis. The machining of these glass materials was carried out using laser wavelengths λ = 1030 nm, 515 nm and 343 nm at different pulse spatial overlap and fluence values. Two scan strategies were used, namely the sequential raster scan strategy and the novel interlaced scan strategy. In this thesis, we show the differences in the machining outcome. The sequential scan strategy is similar to the standard raster scan technique and is limited to lower pulse repetition rates of 20 kHz due to a thermal build-up effect manifesting itself in the formation of fibres and fused debris on the surface. On the other hand, given the same conditions of total pulse energy deposited on the surface, the interlaced scan strategy is found to provide up to 3 times larger ablation depths. Furthermore, as the interlaced scan strategy has been found to suppress the effects of thermal accumulation, higher repetition rates up to 400 kHz could be used providing higher ablation rates up to 60 times more than the sequential scan strategy on the laser used. A high-speed camera was used to observe the mechanism of material removal for the two scan strategies providing insight into the cause of the higher ablation efficiency of the interlaced scan strategy. We demonstrate the capability of this process by machining cylindrical lenses. One batch of the picosecond laser machined lenses was shipped to Fraunhofer ILT while another similar batch of lenses was shipped to PowerPhotonic Ltd. for polishing using their CO2 laser polishing techniques. The polished lenses were tested for their performance, and the accuracy of the process was measured. Finally, the future scope of this technology along with its applications is discussed.