Large-area edge-bonded flat and curved sapphire windows

High strength edge bonds between individual sapphire components have been developed as a means to produce affordable large area windows. Several bonding methods have been demonstrated, with bond fracture strengths ranging from 100-200 MPa. When polished, the bonded windows show excellent transmittance with no degradation in transmitted wavefront quality. The bonding processes have recently been scaled up to 355mm wide, 10mm thick bond lines and multipane windows. Using singly-curved sapphire components for the individual panes, doubly-curved bonded sapphire components have also been produced and polished with excellent results. The edge bonding approach shows promise for fabricating affordable sapphire windows up to 750mm diameter. In addition, recent developments with index-matching glass coatings show the feasibility of substantial cost reductions in optical finishing of sapphire windows, particularly for transparent armor.