Herbert Ellsworth Bates

Optical and crystalline characteristics of large EFG sapphire sheet

Edge Defined Film-fed Growth (EFGTM) Saphikon® sapphire crystals have been grown as large, thick sheet. The sheet is then precision-polished and coated into an infrared or laser transmission compatible window. The sapphire windows are subsequently assembled into a multi-panel configuration for advanced targeting, navigation, or reconnaissance applications. As future aerospace programs will require windows with larger apertures, material characteristics and uniformity such as refractive index homogeneity will increase in importance. Optical measurements, x-ray topography data and rocking curve analysis are presented The crystalline properties as they relate to refractive index inhomogeneity and wave front distortion are discussed.

Stress measurement of EFG sapphire using Cr3+ piezospectroscopy

Sapphire’s hardness, strength, and UV-IR transmittance make it an excellent candidate for IR window and transparent armor applications. At Saint-Gobain Crystals, Edge-defined Film-fed Growth (EFG) sapphire crystals are currently being manufactured for IR window and transparent armor applications in sizes up to 305x510x11 mm. However, the demand for even larger sapphire panels continues to increase. In order to aid in the development of larger pieces, a nondestructive measurement has been developed to map planar stress in Clear Large Area Sapphire Sheet (CLASS). The measurement works by utilizing optical excitation of trace amounts of Cr3+ impurities. The resulting luminescence produces a sharp emission doublet whose exact wavelength is dependent on spacing between Cr3+ and O2- ions in sapphire, and therefore the strain in the sample. By recording several data points over an array, it is possible to construct a stress map of large sapphire sheets and gain valuable information on the growth conditions of the sapphire ribbon.

Characteristics of thick (>12 mm) Class225 EFG sapphire sheet for IR window applications

Demand for larger aperture sapphire IR windows is increasing. To withstand the higher dynamic and pressure forces exerted on them these larger windows require thicker material. Edge Defined Film-fed Growth (EFG)TM Sapphire crystals have traditionally been grown with a thickness of ≤ 11 mm, then finished and polished to a nominal thickness of 5.5 mm. We present optical characteristics data here for Class225(R) EFGTM sapphire sheet that is being grown up to 22 mm thick and finished at 16.8 mm.

The production of 225 x 325 mm sapphire windows for IR (1 to 5 μm) applications

Edge Defined film Fed Growth (EFG) SaphikonTM sapphire crystals have been grown and successfully processed into windows measuring 225 x 325 mm with a thickness of 5.6 mm. More than 40 windows have been completed and assembled into customer hardware and delivered. The polished and coated windows have exhibited average transmission >93% from 1 to 5 mm and wavefront measurements of <0.1 waves rms (@ 0.633 μm) over a 125 mm aperture. Optical measurement data are presented and aspects of the crystal growth and polishing processes are discussed.