A novel 2 μm Ultrashort Pulsed Laser Source for Selective Laser-Induced Etching of Glass

We present a novel source of ultrashort light pulses in the 2 μm wavelength range and demonstrate its application for selective laser-induced etching of different glasses, such as borosilicate glass and fused silica. A chirped pulse master oscillator power amplifier (MOPA) system is based on a mode-locked seed oscillator followed by a chain of Thulium doped fiber amplifiers. The laser system delivers pulse energies of up to 2 μJ at repetition rates between 30 kHz and 2 MHz and pulse durations as short as 540 fs. The output beam of laser light features an M 2 better than 1.3. Focusing ultra-short laser pulses into a glass work piece generates material modifications at the position where the field intensity exceeds the threshold for modification. This laser-induced structure alteration of the glass material is attributed to a highly nonlinear process involving ultrafast multi-photon excitation of electrons and subsequent energy transfer to the lattice. Here, we demonstrate that modifications inscribed by the 2 μm laser into the different glasses can be selectively removed in a wet chemical etching process. This manufacturing procedure is termed “selective laserinduced etching” or SLE. The key is that the etching rate of modified material is higher than the etching rate of unmodified material. For borosilicate glass we demonstrate etching selectivities of 450 and for fused silica of 500. These high selectivities will allow manufacturing of tunnels, three-dimensional cavities, and even complex networks for microfluidic devices in both materials. The ultrashort 2 μm laser pulse thus makes borosilicate glass – the most important glass for chemical laboratory equipment due to its superior durability, chemical and heat resistance – accessible to manufacturing via selective laser-induced etching.