Continuous-wave laser particle conditioning: Thresholds and time scales

The optical absorption of contaminants on high reflectivity mirrors was measured using photo thermal common-path interferometry before and after exposure to high power continuous-wave laser light. The contaminants were micron-sized graphite flakes on hafnia-silica distributed Bragg reflectors illuminated by a ytterbium-doped fiber laser. After one-second periods of exposure, the mirrors demonstrated reduced absorption for irradiances as low as 11 kW cm−2 and had an obvious threshold near 20 kW cm−2. Final absorption values were reduced by up to 90% of their initial value for irradiances of 92 kW cm−2. For shorter pulses at 34 kW cm−2, a minimum exposure time required to begin absorption reduction was found between 100 μs and 200 μs, with particles reaching their final minimum absorption value within 300 ms. Microscope images of the surface showed agglomerated particles fragmenting with some being removed completely, probably by evaporation for exposures between 200 μs to 10 ms. Exposures of 100 ms and longer left behind a thin semi-transparent residue, covering much of the conditioned area. An order of magnitude estimate of the time necessary to begin altering the surface contaminants (also known as ”conditioning”) indicates about 200 μs seconds at 34 kW cm−2, based on heating an average carbon particle to its sublimation temperature including energy loss to thermal contact and radiation. This estimation is close to the observed exposure time required to begin absorption reduction.