Efficient laser polishing of silica micro-optic components

We report a study of the basic characteristics of laser polishing of fused silica with a protocol that is particularly suitable for surface smoothing of micro-optic elements fabricated by a laser ablation process. We describe a new, to our knowledge, approach based on scanning a highly controlled small size laser beam and melting areas of tens to hundreds of micrometers of glass using a computer-controlled raster scan process, which does not require beam shaping, substrate preheating, or special atmospheres. Special test samples of silica substrates with prescribed spatial frequency content were polished using a range of irradiation conditions with the beam from a well-controlled ${\text{CO}}_2$ laser operating at a wavelength of $10.59\mu \text{m}$. An analysis is presented of the laser-generated reduction in surface roughness in terms of measurements of the spatial frequency characteristics, and the results are compared with the predictions of a simple model of surface-tension-driven mass flow within the laser-melted layer. This technique is shown to be capable of smoothing silica surfaces with $\sim 1\mu \text{m}$ scale roughness down to levels $<1\text{\hspace{0.17em} nm}$ with no net effect on the as-machined net surface shape, at realistic production rates without a preheating stage, and with noncritical residual stresses.