Surface shape control for large ion assisted deposition mirror

To improve wavefront quality in large scale laser, the residual stress in multi-layer films was controlled precisely by ion-assisted e-beam evaporation. Based on the analysis of the residual stress variation in single HfO2 layer and SiO2 layer, the effects of ion-source beam voltage and beam current on refractive index and residual stress in films were studied. The PV value of 540 mm340 mm60 mm reflective mirror was controlled below 0.5 (=632 nm). The damage threshold of the specimen tested with small-size beam was higher than 30 J/cm2 (N-on-1,1 064 nm, 5 ns). The large-size reflective mirror manufactured with this technology is applied successfully in a large aperture prototype device.