Deformation-Free Optical Coatings For Extreme Thermal Environments

Wei Liu and Joseph J. Talghader

The mechanical requirements for optical coatings under high power illumination can be quite severe relative to those of their "normal" counterparts, particularly if the structure is thin or lightweight, such as in micromachined or space-based systems. Perhaps the most serious problem for these coatings is deformation due to thermal expansion mismatch. Solving this problem requires both new materials and changes in optical design.

When large temperature gradients exist across optical coatings, optical design must include the mechanical structure of the coating. This can be handled in most systems by analyzing the strains, forces and moments in a free multilayer stack and then empirically modifying the solution for any fixed boundary conditions on the coatings. Recent work in our group demonstrates that optical coating stacks can be designed to minimize thermal deformation. Shape deviations of less than lambda over 60 can be achieved even with micromirrors only 2 microns thick. Among the most important aspects of optomechanical design is having optical coatings with a variety of mechanical properties so that high and low expansion materials can be traded off of one another.

Figure 1 - Concept behind the design of thermally invariant mirrors. The change in radius of curvature of a mirror is related to the mismatch in thermal expansion of its layers weighted by position, thickness, and elastic modulus.

Figure 2 - Interferometric images of stress compensation in micromirrors. By designing coatings with the proper stress and mechanical characteristics one can create essentially deformation free mirrors. The image on the left is a mirror with no deformation compensation (and thus it is highly curved), and the mirror on the right is compensated.

Figure 3 - Surface profiles of mirrors before and after thermal expansion deformation design. Note that the surface profile on the right shows essentially no deformation with increasing temperature even though the mirror structural material is only on the order of 2 microns thick.