Coherence effects in surface roughness induced by vacuum ultraviolet F2 laser ablation.

We analyze statistical fluctuations in the pulse-to-pulse spatial fluence distribution of a highly multimode F2 laser beam and the influence that coherence effects have on the attainable smoothness of an ablated surface. The magnitude of fluctuations is predicted to be a few percent, with associated roughness increasing as m1/2, where m is the number of ablation pulses. By use of a white-light optical interferometer, measurements have been made of roughness induced on N-BK7 glass surfaces ablated with a 157-nm laser, and reasonable agreement with predictions based on this roughening mechanism has been found.