How to Polish Fused Silica to Obtain the Surface Damage Threshold Equals to the Bulk Damage Threshold

We compared the 1064 nm surface damage thresholds of fused silica polished by three different techniques: 1. A conventional polishing technique: that uses loose Alumina abrasives (lapping) followed by a fine Cerium oxide polish. 2. An alumina polishing process producing surfaces very close to super polished. 3. Process 2 followed by a silica polish until the silica surfaces are super polished. We employed the same measurement technique that proved successful for the bulk damage threshold measurement to measure the damage thresholds of bare silica surfaces polished by the above three polishing techniques. We used an 8-nanosecond, single transverse and longitudinal mode pulsed laser, from a Q-switched Nd:YAG laser. We used the surface third harmonic generation technique to precisely place the focus of the laser beam on the surface of the fused silica window, and to measure the laser focus spot size which was found to be 8 mu m in radius. Key findings include: 1. The surface damage threshold can be made equal to the bulk damage threshold. There is a large difference in single-pulse damage thresholds between bare silica surfaces polished using ceria, alumina only, and the surfaces polished using alumina followed by silica. The ceria polished samples have a statistical damage threshold ranging from 50 to 450 GW/cm(2)., where as the alumina polished surfaces show damage at 200-500 GW/cm(2), with half the spots showing damage at the bulk threshold of 500 GW/cm(2). The windows polished by alumina followed by silica show damage almost universally at the bulk damage threshold of 500 GW/cm(2). 2. There are strong conditioning effects for these surfaces. The ceria polished surfaces have reduced thresholds for multiple pulses. The alumina polished surfaces attain the bulk damage threshold at most locations using multiple pulse annealing. 3. We found there was no beam size variation of the damage threshold irradiance for the bare alumina/silica polished samples. 4. By measuring the bulk damage threshold powers at different depth from the front surface, we have found experimental evidence of a self-focusing effect, which gives the correction factor for the measured bulk damage threshold. 5. We showed that surface irradiance is limited by breakdown of the silica and not breakdown of the air. 6. Damage morphologies were recorded for the different surfaces.