Impact of preheating conditions on form deviation during laser polishing of N-BK7 glass

Laser polishing of glass can achieve a surface roughness comparable to conventional polishing methods in a fraction of the process time. However, the thermal processing with rapid temperature changes of the glass substrate induces thermal stress, resulting in a deviation from the initial shape. The exception being fused silica, were after laser polishing the deviation is often less than a tenth of the value induced in other glasses. A possible cause for this difference is the necessity for preheating of most glasses during laser polishing, whereas fused silica can be polished at room temperature. Therefore, in this study the results of a thorough investigation of the impact of the preheating conditions on the form deviation during laser polishing of N-BK7 are presented. By modifying the preheating conditions to a lower temperature and reducing the heating and cooling rates, we achieved a reduction in form deviation for N-BK7 substrates (diameter 30 mm, thickness 5 mm). Compared to state-of-the-art process parameters resulting in a form deviation of PV ≈ 32 μm, a value of ~6.8 μm could be obtained. The form irregularity RMSi simultaneously has been reduced from RMSi ≈ 4 μm to ~0.3 μm, indicating a significant shift in the form deviation from free-form errors towards spherical errors. Simultaneously, the achieved roughness Sq after laser polishing is maintained at ~1.7 nm.