John W. Locher

Producing large EFG sapphire sheet for VIS-IR (500-5000 nm) window applications

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).

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.

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.

80-mm EFG sapphire dome blanks yield high-quality low-cost single-crystal domes

Close attention to crystal growth parameters and characterization of the crystals 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.

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.

Large EFG sapphire sheet for VIS-IR (500-5000 nm) window applications

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).

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.

Large-area flat and curved sapphire window blanks

With the objective of producing affordable large area windows, high strength edge bonds between individual sapphire components have been developed. Several bonding methods have been demonstrated, with bond fracture strengths ranging from 100 - 200 MPa. The directed energy process, which yields the strongest bonds, has produced bonded sapphire components 600 mm long and 3 mm thick with a 75 mm wide bond line. When polished, the bonded windows show no degradation in transmittance or transmitted wavefront quality. The processes have recently been scaled up to 355 m wide, 10 mm thick bonds lines and multipane window blanks. In addition, doubly-curved bonded sapphire components have been produced and polished with excellent results, using singly curved sapphire components for the individual panes. The edge bonding approaches shows promise for fabricating affordable sapphire windows up to 750 mm diameter.

High-strength edge-bonded sapphire windows

High strength edge bonds have been achieved between individual sapphire components, showing promise for fabricating window blanks up to 600 mm diameter or larger in size. Several bonding methods were investigated, with a directed-energy diffusion-bonding method yielding components with bond fracture strengths of 200 MPa. Bonded sapphire components 600 mm long and 3 mm thick with a 75 mm wide bond line have been produced. When polished, the bonded windows show no degradation in transmittance or transmitted wavefront quality. Process scale up to larger bonds lines is planned. Mechanical and optical characterizations of sub- scale edge-bonded sapphire windows are presented.