Surface Modification of Fused Silica Optics with CO₂ Laser for Improving Optical Performance

Full text: Inertial confinement fusion (ICF) class laser systems require a large area of high quality fused silica optics, which may be exposed to laser irradiation in a certain high fluences. Unavoidably, defects such as scratches and contamination are formed on the optical surface during the mechanical fabrication. Those defects may lead to laser induced damage (LID) by absorbing laser energy and thus affect the overall performance of laser system. Previous research of our group has shown that, since the material was heated to several thousand degrees by CO₂ laser, defects would be removed with surface melting and reconstruction so that the damage resistance could be enhanced. But the surface profile might be changed after laser treatment because of the material flowing driven by thermal expansion effects or surface tension force. Moreover, non-uniform temperature fields can cause changes of local structure which may weaken the surface strengths and degrade the polarization state of laser owing to the stress birefringence. In this work, interaction mechanism between CO₂ laser and fused silica was studied and the response law of the material under different laser parameters such as beam spot size, pulse duration and cumulative treating time was analyzed in detail. Experiment results showed that the damage threshold of CO₂ laser treated area was approximately 40% higher than before without nearly all adverse effects by reasonable control of CO₂ laser energy deposition. And not only that, the surface roughness was also improved. (author)