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 Cr<sup>3+</sup> impurities. The resulting luminescence produces a sharp emission doublet whose exact wavelength is dependent on spacing between Cr<sup>3+</sup> and O<sup>2-</sup> 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.
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)<sup>TM</sup> 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 Class<sup>225(R)</sup> EFG<sup>TM</sup> sapphire sheet that is being grown up to 22 mm thick and finished at 16.8 mm.
EFG sapphire sheet measuring 305 x 510mm and 225 x 660mm have been produced in quantity. The average optical transmission of 6.15 mm thick uncoated polished panels is 84.0% ± 0.5 at 700 nm. This value assures good transmission throughout the 500 to 5000 nm spectral range. Effective absorption coefficients for this spectral range and thickness are calculated and presented. An average index inhomogeneity of 6 ppm ± 2 has been measured and is the requirement for panels polished to 0.1λ at this thickness (@633 nm).
Edge Defined Film-fed Growth (EFG<sup>TM</sup>) Saphikon<sup>®</sup> 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.
EFG sapphire sheet measuring 305 x 510mm and 225 x 660mm have been produced in quantity. The average optical transmission of 6.15 mm thick uncoated polished panels is 84.0% ± 0.5 at 700 nm. This value assures good transmission throughout the 500 to 5000 nm spectral range. Effective absorption coefficients for this spectral range and thickness are calculated and presented. An a verage index inhomogeneity of 6 ppm ± 2 has been measured and is the requirement for panels polished to 0.1λ at this thickness (@633 nm).
Edge Defined film Fed Growth (EFG) Saphikon<sup>TM</sup> 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.
Sapphire's loss of strength between 20 degrees and 1000 degrees Celsius depends on orientation and state of stress. The critical weakness of sapphire occurs in compression along the c-axis of the crystal. In flexure tests of sapphire that is not subject to c-axis compression, the strength actually increases between 20 degrees and 1000 degrees Celsius. Compression on the c-axis causes twinning on rhombohedral crystal planes. When twins on different planes intersect, a crack forms and the specimen is then subject to tensile failure. Doping with Mg<SUP>2+</SUP>, Ti<SUP>4+</SUP>, or introduction of a TiO<SUB>2</SUB> second phase each doubled the c-axis compressive strength of sapphire at 600 degrees Celsius, probably by inhibiting twin propagation. X-ray topography was employed to investigate the relationship between surface and bulk defects and mechanical strength in sapphire. Low angle grain boundaries were not associated with mechanical weakness. Wide, transverse scratches that are evident to x-rays, but not obvious in optical microscopy, can weaken sapphire. Topography demonstrated that annealing reduces long range strain in polished sapphire.
Utilization of the promising Erbium:YAG laser has been hindered by the lack of a truly effective optical fiber delivery system. In a National Eye Institute funded Phase I SBIR, sapphire fibers produced by the Saphikon Edge-defined, Film-fed Growth (EFG<SUB>TM</SUB>) technique were proven effective in delivering 2.94 micron Er:YAG laser energy in pre-clinical in-vitro ophthalmic procedures. A brief overview of the results of both the ab-externo sclerostomy and laser trabecular ablation procedures is given. A Design of Experiments methodology was used to significantly reduce average loss and variability of the EFG fibers, with losses below 1 dB/meter demonstrated in multi-meter lengths of 300 micron diameter fiber. Laser damage threshold levels above 1000 J/cm<SUP>2</SUP>, and power handling capability over 8 watts has been demonstrated. Details of ongoing and planned pre-clinical and clinical studies in ophthalmic, otologic, and dental procedures are discussed, along with other, non- medical applications for the sapphire fibers. Introduction of additional fiber diameters and devices is also reviewed.
Single-crystal sapphire fibers are produced by the Saphikon edge-defined, film-fed growth (EFG<SUB>TM</SUB>) technique. Total losses below 3 dB/meter have been measured at the Erbium:YAG laser wavelength of 2.94 microns. Fibers with lengths greater than a meter have delivered over 400 mJ of Er:YAG laser energy. EFG fibers have been shown to lose less than 10% transmission when bent to a 6 cm radius. The apparent numerical aperture (NA) has been measured as 0.31 for an unclad fiber, and 0.38 for fiber clad with teflon FEP. Scattering losses were shown to dominate loss, with measurements at both visible and 3 micron wavelengths using integrating spheres. Spectroscopy was used to identify absorptive losses in the EFG fibers.
Close attention to crystal growth parameters and characterization of the crystal's thermal environment during growth has led to improvement in the crystal structure of EFG grown dome blanks. These near net shape 80 mm sapphire blanks have been fabricated to produce high quality finished domes. New measurements of the coefficient of thermal expansion (CTE), thermal conductivity, optical scatter, rain erosion and the thermal coefficient of refractive index (dn/dT) as a function of wavelength have been performed and the data are presented.