Joel Askinazi

Large-area sapphire windows

Large area sapphire windows have been fabricated by edge-bonding multiple panes. A 4-pane edge-bonded 320 x 410 x 7 mm sapphire window with excellent optical characteristics has been successfully finished. Two different bonding methods were used to build up the 4-pane window blank. Pairs of commercially available EFG sapphire panes were first bonded using a 1500°C bonding process. The bonded pairs were then joined using a 1100°C process. Bond strengths for the two methods are approximately 130 MPa (20 kpsi). Optical finishing was completed using standard methods for sapphire with no significant increase in finishing time caused by the bonds. There are no deleterious optical effects or visible optical distortion due to the bond lines. The edge bonding technology can now produce 600 x 600 mm flat window blanks. Conformal windows have also been produced using the edge bonding method. Very high bond strengths of 250 MPa (37 kpsi) have been attained on smaller samples using an optimized solid ceramic fillet.

Current status of very large sapphire crystal growth for optical applications

Sapphire is an ideal visible-MWIR window due to its excellent optical and mechanical properties and its availability in large sizes up to 340-mm diameter boules. Anticipated applications for new, high performance optical systems call for even larger, 450-750 mm diameter, windows. The present effort has focused on producing 500-mm diameter sapphire boules using the Heat Exchanger Method. Three experimental growth runs demonstrated the feasibility of producing 500-mm diameter sapphire boules. Completely crack- free boules have not been grown, but large size sapphire pieces up to 400 mm by 280 mm have yielded from these experimental runs.

Low-cost sapphire windows

Refractive index matching glass coatings have been applied to mechanically-ground sapphire blanks using a modified glazing technique. The as-fired coatings are optically clear and well adhered, producing coated sapphire windows with up to 88 percent in-line transmittance and excellent optical imaging characteristics. Coated sapphire windows up to 150 x 230 mm in size have been produced, with additional scale-up to at least 300 x 350 mm planned for the near future. Glass-coated sapphire (GCS) can be rapidly polished in a small fraction of the time required for sapphire itself, thereby substantially reducing the cost of transparent armor. Glass-coated sapphire windows are also being evaluated for precision airborne reconnaissance and FLIR systems, to determine the limits, if any, to transmitted wavefront quality. The feasibility of applying index matching glass coatings to sapphire dome shapes has also been demonstrated. Index matching glass has also been used as a bonding material to fabricate actively cooled sapphire windows with internal channels for hypersonic missiles.

Development of large-aperture monolithic sapphire optical windows

Large aperture (20-inch diameter) sapphire optical windows have been identified as a key element of new and/or upgraded airborne electro-optical systems. As has been recently reported, Crystal Systems continues in the development of the technology to grow 20-inch diameter, single crystal sapphire boules to meet this need. Owing to the spatial variations in the optical index of refraction potentially anticipated within 20-inch diameter sapphire crystals, computer controlled optical finishing has been identified as a key technology that may be required to enable achievement of transmitted wavefront errors of much less than 0.1 wave rms. BFGoodrich has developed this technology and has previously applied it to finish 8-inch- diameter sapphire optical windows to a transmitted wavefront error of at least four times better than the above requirement. As a key step in the scaling of these critical window technologies to produce 20-inch-diameter sapphire windows, BFGoodrich and Crystal Systems collaborated to apply these technologies to produce a 13-inch-diameter sapphire window having a transmitted wavefront error of 0.059 wave rms. Optical testing of this 13-inch sapphire crystal revealed that it possessed excellent refractive index homogeneity; far better than had previously been encountered in finishing 8-inch sapphire windows. This improvement in material quality implied that conventional optical finishing could potentially have been employed to finish this window to the 0.059 wave rms error. However, due to our desire to demonstrate the process technology for potential future application to 20-inch diameter sapphire windows, it was fabricated using computer controlled optical finishing. This paper addresses the results of this effort, the lessons learned, and the implications associated with the scaling of these technologies to 20-inch-diameter sapphire optical windows.

Demonstration of a conformal window imaging system: design, fabrication, and testing

Design, fabrication, and testing were demonstrated for a conformal window imaging system. The conformal window is sapphire and has a toroidal shape. A pair of axially translating cylindrical lenses were constructed to correct the astigmatism introduced by the window across the full field of regard. A telephoto camera lens was used with the system for imaging objects at infinity. Images of some distant targets were collected, and they compare favorably to those images taken with the camera alone.

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.

Protective broadband window coatings

Optical windows employed in current and future airborne and ground based optical sensor systems are required to provide long service life under extreme environmental conditions including blowing sand and high speed rain. State of the art sensor systems are employing common aperture windows which must provide optical bandpasses from the TV to the LWIR. Operation Desert Storm experience indicates that current optical coatings provide limited environmental protection which adversely affects window life cycle cost. Most of these production coatings also have limited optical bandpasses (LWIR, MWIR, or TV-NIR). A family of optical coatings has been developed which provide a significant increase in rain and sand impact protection to current optical window materials. These coatings can also be tailored to provide either narrow optical bandwidth (e.g., LWIR) or broadband transmittance (TV- LWIR). They have been applied to a number of standard optical window materials. These coating have successfully completed airborne rain and sand abrasion test with minimal impact on optical window performance. Test results are presented. Low cost service life is anticipated as well as the ability to operate windows in even more taxing environments than currently feasible.

Mandrel reusability in precision replication of ZnS conformal domes

Recently precision replication of flat, spherical and aspheric surfaces was demonstrated in ZnS by a chemical vapor deposition (CVD) process. Two-inch size ZnS parts were replicated successfully down to a fraction of a wave in the visible and finish of 20-180 Angstroms RMS. The replication technology was then extended to produce six best-fit-sphere ZnS corrector elements of diameter 2.4-inch by replicating on Al2O3 and SiO2 coated, highly polished and diamond turned ZnS mandrels. These replicated corrector elements measured an inside surface figure of 0.14-0.27 Angstroms RMS and smoothness of about 41 Angstroms RMS. Mandrel reusability was then demonstrated by replicating on several previously used corrector element mandrels which were minimally refurbished (mandrel surface was cleaned with acetone). These replicas and mandrels measured essentially same surface figure and smoothness after second replication as before. After replication, analysis was performed on a few mandrels, which did not release from the replicas. Cause of this adherence was determined to be presence of 10-20 micron size pinholes in the release coating. Very good replication was achieved on those areas where no pinholes were present. The integrity of the release coating determines the durability of the replication mandrel.

Recent advances in the application of computer-controlled optical finishing to produce very high-quality transmissive optical elements and windows

Large aperture (20-inch diameter) sapphire optical windows have been identified as a key element of new and/or upgraded airborne electro-optical systems. These windows typically require a transmitted wave front error of much less than 0.1 waves rms @ 0.63 microns over 7 inch diameter sub-apertures. Large aperture (14-inch diameter by 4-inch thick) sapphire substrates have also been identified as a key optical element of the Laser Interferometer Gravitational Wave Observatory (LIGO). This project is under joint development by the California Institute of Technology (Caltech) and the Massachusetts Institute of Technology under cooperative agreement with the National Science foundation (NSF). These substrates are required to have a transmitted wave front error of 20 nm (0.032 waves) rms @ 0.63 microns over 6-inch sub-apertures with a desired error of 10 nm (0.016 waves) rms. Owing to the spatial variations in the optical index of refraction potentially anticipated within 20-inch diameter sapphire, thin (0.25 - 0.5-inch) window substrates, as well as within the 14-inch diameter by 4-inch thick substrates for the LIGO application, our experience tells us that the required transmitted wave front errors can not be achieved with standard optical finishing techniques as they can not readily compensate for errors introduced by inherent material characteristics. Computer controlled optical finishing has been identified as a key technology likely required to enable achievement of the required transmitted wave front errors. Goodrich has developed this technology and has previously applied it to finish high quality sapphire optical windows with a range of aperture sizes from 4-inch to 13-inch to achieve transmitted wavefront errors comparable to these new requirements. This paper addresses successful recent developments and accomplishments in the application of this optical finishing technology to sequentially larger aperture and thicker sapphire windows to achieve the challenging transmitted wave front error requirements defined above.

High-impact resistance optical sensor windows

Recent field experience with optical sensor windows on both ground and airborne platforms has shown a significant increase in window fracturing from foreign object debris (FOD) impacts and as a by-product of asymmetrical warfare. Common optical sensor window materials such as borosilicate glass do not typically have high impact resistance. Emerging advanced optical window materials such as aluminum oxynitride offer the potential for a significant improvement in FOD impact resistance due to their superior surface hardness, fracture toughness and strength properties. To confirm the potential impact resistance improvement achievable with these emerging materials, Goodrich ISR Systems in collaboration with Surmet Corporation undertook a set of comparative FOD impact tests of optical sensor windows made from borosilicate glass and from aluminum oxynitride. It was demonstrated that the aluminum oxynitride windows could withstand up to three times the FOD impact velocity (as compared with borosilicate glass) before fracture would occur. These highly encouraging test results confirm the utility of this new highly viable window solution for use on new ground and airborne window multispectral applications as well as a retrofit to current production windows. We believe that this solution can go a long way to significantly reducing the frequency and life cycle cost of window replacement.