March 2020
Spotlight Summary by Carmen S. Menoni
Stress compensation by deposition of a nonuniform corrective coating
Multilayer dielectric interference coatings are ubiquitous in optics and laser systems. These structures consist of alternating layers of typically two transparent materials with contrasting refractive indices at the operation wavelength. Visible and near infrared interference stacks are engineered using amorphous oxides deposited by physical vapor deposition. There have been enormous advances in perfecting the optical response of these devices. It is currently possible to produce high reflectivity quarter-wave stacks with sub-part per million absorption loss near 1 µm. However, all of these coatings are highly stressed. Stress is inherent to the deposition method, whether ion beam or magnetron sputtering or electron beam evaporation. A typical magnitude for the compressive stress of a high reflector deposited by ion beam sputtering and composed of ~30 alternating layers of Ta2O5 and SiO2 is ~300 MPa. For some applications that use thin substrates, as is the case of adaptive optics, this high stress is not tolerable as it leads to significant deformation of the substrate. This work by J. B. Oliver et al. offers an ingenious solution to compensate for the changes in the radius of curvature of the optics due to coating stress. The method consists of depositing a few-micron-thick SiO2 layer with a radially varying thickness profile onto the substrate. This compensating layer enables one to produce a flat optics after a compressively stressed multilayer coating is deposited on top. The team used a shadow mask that conforms the deposition plume to realize the desired thickness profile on the SiO2 intermediate layer. Finite element analysis of the overall structure: substrate+stress-compensating-layer+multilayer-stack were carried out a priori to design the stress-compensating layer. This, in turn, allows for the design of the shadow mask profile. A key advantage of the methodology devised by Oliver et al. is that it can integrate in one process the sequential deposition of the stress-compensating layer and multilayer stack. Owing to the control and repeatability of the deposition process, the methodology is well suited for the realization of wavefront compensated optics for lasers and high-precision optical systems.
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Article Information
Stress compensation by deposition of a nonuniform corrective coating
J. B. Oliver, J. Spaulding, and B. Charles
Appl. Opt. 59(5) A54-A57 (2020) View: Abstract | HTML | PDF