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High Precision Conformal Sensor Window


OBJECTIVE: Improve the process of manufacturing conformal sensor windows to achieve visible (not just infrared) optical tolerances. DESCRIPTION: Electro-optic sensor windows that conform to the local shape of an airframe are desirable for future aircraft and missiles. Conformal shapes might have no symmetry or just a plane of symmetry, depending on their location. Durable, multispectral materials such as spinel are candidates for conformal windows. The size and curvature of a window will be limited by the availability and cost of a suitable blank. The goal of this project is to improve the precision of the optical manufacturing processes to provide windows that meet visible optical tolerances in addition to infrared optical tolerances. For example, a tenth-wavelength root-mean-square transmitted wavefront error corresponds to a precision of 300 nanometers for infrared operation, but only 60 nanometers for visible operation. Tolerances become even tighter for high off-normal angles of incidence. Proposals might address precision optical fabrication and/or metrology. Metrology solutions capable of measuring objects whose two surfaces deviate more than 5 degrees from being parallel are particularly solicited. A generic conformal window is a toroid with a radius of curvature R in one direction and a radius of curvature 2R in the orthogonal direction. The thickest practical blank material determines the maximum curvature that can be obtained in a demonstration window. Designs with other profiles, such as a parabolic cross section and varying curvature along the perpendicular axis might be provided for demonstrating the capability to make complex shapes. PHASE I: Demonstrate proof of principle of the proposed approach to make a toroidal window with visible optical tolerances. A fused silica toroid with a thickness of 5 mm, footprint of 100 mm x 100 mm and orthogonal radii of curvature of 500 and 250 mm is a candidate demonstration shape. Other shapes and other materials may be proposed. The target root-mean-square wavefront error measured at normal incidence is one tenth wavelength measured at 0.633 microns over an 85% clear aperture area. Proposals may address fabrication (including metrology) or just metrology. A fabrication proposal should state what metrology will be used in Phase I. A metrology proposal should state what artifacts will be used to validate the metrology. Metrology solutions capable of measuring surfaces deviating more than 5 degrees from being parallel are particularly solicited. PHASE II: Demonstrate a conformal window(s) made of spinel with a footprint of at least 150 x 150 mm and thickness of 5 mm. The shape will be chosen by mutual agreement with the Government. The threshold target root-mean-square wavefront error measured at normal incidence is one tenth wavelength measured at 0.633 microns, with one twentieth wavelength being a stretch goal. PHASE III: Implement manufacturing and/or metrology processes for commercial production. PRIVATE SECTOR COMMERCIAL POTENTIAL/DUAL-USE APPLICATIONS: Processes developed for conformal window manufacturing have potential to reduce the cost and widen the scope of manufacturing precision aspheric optics for civilian and space applications. REFERENCES: 1. S. Bambrick, M. Bechtold, S. DeFisher, and D. Mohring,"Ogive and Free-Form Polishing with UltraForm Finishing,"Proc. SPIE 2011, Volume 8016, paper 80160P. 2. J. D. Nelson, A. Gould, D. Dworzanski, C. Klinger,, B. Wiederhold, and M. Mandina,"Rapid Optical Manufacturing of Hard Ceramic Conformal Windows and Domes,"Proc. SPIE 2011, Volume 8016, paper 80160O. 3. S. DeFisher, M. Bechtold, and D. Mohring,"A Non-Contact Surface Measurement System for Freeform and Conformal Optics,"Proc. SPIE 2011, Volume 8016, paper 80160W. 4. M. Gutin, O. Gutin, X.-M. Wang, and D. Ehlinger,"Interferometric Tomography A New Tool for Metrology on Conformal Optics,"Proc. SPIE 2011, Volume 8016, paper 80160X. 5. R. Henselmans, L. Cacace, G. Kramer, N. Rosielle, and M. Steinbuch,"Nanometer Level Freeform Surface Measurements with NANOMEFOS Non-Contact Measurement Machine,"Proc. SPIE 2019, Volume 7426, paper 742606.
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