Effects of thermal annealing on femtosecond laser micromachined glass surfaces

Femtosecond laser micromachining (FLM) of fused silica allows for the realization of three-dimensional embedded optical elements and microchannels with micrometric feature size. The performances of these components are strongly affected by the machined surface quality and residual roughness. The polishing of 3D buried structures in glass was demonstrated using different thermal annealing processes, but precise control of the residual roughness obtained with this technique is still missing. In this work, we investigate how the FLM irradiation parameters affect surface roughness and we characterize the improvement of surface quality after thermal annealing. As a result, we achieved a strong roughness reduction, from an average value of 49 nm down to 19 nm. As a proof of concept, we studied the imaging performances of embedded mirrors before and after thermal polishing, showing the capacity to preserve a minimum feature size of the reflected image lower than μ5μm. These results allow for us to push forward the capabilities of this enabling fabrication technology, and they can be used as a starting point to improve the performances of more complex optical elements, such as hollow waveguides or micro-lenses.