Comparison of production techniques for silicon oxynitride rugates and their effect on laser damage thresholds

A great deal of interest has been shown in the potential of rugate designs for the realization of dielectric mirrors with enhanced resistance to laser induced damage. Some of the benefits are arguably a result of the fact that the mirrors are essentially composed of inhomogeneous materials, with the effective refractive index continuously varying between the two limits defined by the particular design selected. A study has been carried out of the potential of three different processes for the fabrication of mirrors using a 40 period silicon oxynitride mirror design for 532nm, with design goals of R equals 99.9 percent and bandwidth close to 10 percent. The study has compared reactive magnetron sputtering, ion assisted deposition and microwave plasma CVD. Conventional quarter wave stacks were also produced for use at the same wavelength, with the same number of periods to act as a reference. Laser damage thresholds were measured using a Q-switched YAG laser at both the resonance band of the stack and off-resonance at 1064nm. The results highlight the role of mirror design, process characteristics and film morphology.