Highly localised CO₂laser cleaning and damage repair of silica optical surfaces

A technique for localised damage repair of fused silica optical surfaces has been investigated. The study reports the use of a CO₂ laser system at 10.6µm wavelength with 50µm spot diameter (measured at 1/e ²) and pulse duration ranging from 50µs to 200ms. Data of the threshold axial irradiance for the onset of measurable mass loss were produced and compared with heat flow calculations based in "hot" properties of silica, showing a changeover from predominantly 1-d cooling below 300µs to quasi-steady-state 2-d cooling beyond 1ms. Typically, irradiances of about 90% of the threshold for mass loss are then used. Surface melt spots generated with a single laser pulse are found to produce measurable cleaning of the initial polishing swirls and light scratches (-tens of nm deep) at all pulse lengths investigated. A reproducible reference scratch of 1.5µm width and 100-200nm depth made by diamond scribing has been used to simulate smoothing or closing of crack-like features. To fully remove the test scratch requires multiple applications of long pulses. Finally, smoothing of the groups of micron-size surface pits caused by optical damage has been obtained, removing significantly the relative amplitude at high frequencies of the fast Fourier transform with a lower limit of 200 cycles/mm for the 50µm spotsize.