Moldable multispectral glasses in GeS2–Ga2S3–CsCl system transparent from the visible up to the thermal infrared regions

• Synthesis of multispectral sulfide glasses transparent from visible to mid-IR
• Molding of the glass on two compositions
• Refractive index measurement and chromatic dispersion analysis
• Protective coating deposition with ZnS

This work is based on the development of glasses with extremely wide optical transmission range for multispectral imaging (visible + thermal imaging for example) with the same optical system. Glass compositions based on the GeS2–Ga2S3–CsCl system have been selected in order to obtain glasses transparent from the visible up to the 8–12 μm region. An increase of CsCl content can greatly improve the visible transmission of the glasses which however will be more sensitive to ambient moisture. Two glasses with different chromatic dispersion, necessary for the design of an optical system, are developed and characterized. Molding tests on aspheric surfaces demonstrate clearly the ability of these two glasses to be molded into complex optics without crystallization and with excellent roughness. In order to test the ability of an anti-reflective coating to protect these glasses against the atmospheric aggression, a ZnS coating has been applied to these glasses. An effective protection can be achieved with a ZnS coating of less than 1 μm of thickness. These results allow considering the use of compositions with higher CsCl content if still more transparency in the visible region is desired.