The continuous drive towards higher performance intelligence, surveillance and reconnaissance (ISR) and high-energy laser (HEL) systems has translated into new requirements for high-performance windows. In these applications a wide range of materials needs to be considered, ranging from amorphous glass (such as fused silica), polycrystalline materials (such as CleartranTM) or hard ceramics (such as AlON, spinel and sapphire). A wide range of sizes (up to and including meter class optics) and geometries are also considered for these applications (high aspect ratio plano surfaces remain prevalent, of course, but “free-form” shapes are also being envisioned and implemented routinely, including conformal windows). As is always the case, increasingly tighter specifications, driven by lower wavelength IR systems as well as visible and/or multispectral systems, require continually more sophisticated metrology techniques to verify and validate. Development of manufacturing processes needed to yield pristine optical surfaces capable of operating at higher laser fluences and/or for highly brittle ceramics capable of withstanding a wide range of temperature, operating pressure and stress are also considered. New high-durability thin film coatings capable of withstanding increasingly harsher environments have been developed for these applications. In a defense environment where cost pressures continue to require less expensive manufacturing processes, several advances are discussed. This paper will present a wide range of examples dealing with these materials, geometries, specifications, metrology and thin film coating developments.
Deterministic grinding of large optical components (2600 - 3700 cm2 area) using Electrolytic In-Process Dressing (ELID) requires strict process controls for several process parameters. In this paper we describe how the voltage and current characteristics of the ELID circuit may be used to establish viable, in-situ feedback monitoring of the grinding process. The specific approach used was to keep the ELID power supply in constant voltage mode and maintain an average power level that was optimized for each material. This was accomplished by monitoring the pulsed waveform and its frequency spectrum. By controlling the down feed rate it is possible to control the electrical characteristics of the wheel. A control loop was developed to over-ride the feed rate based upon the characteristics of the pulsed waveform. A second ELID process monitor was incorporated into the optic support scheme. To insure the part being ground was in a mechanically stable environment the optic was instrumented with eddy current gauges to detect motion during machining. Based on the data obtained from these sensors the support for the optic was optimized to minimize rigid body motion as well as bending. It has been found that creating a stable platform for machining as well as maintaining control of the ELID system is essential for a deterministic process.
Measuring surface figure of large ground surfaces has been done with infrared interferometers or by means of local measurements with a spherometer to obtain a general shape of an optical surface. This paper describes a straightforward technique to obtain surface figure of plano parallel optics with an array of transducers referenced to an optical flat. The instrument utilizes 16 linear variable differential transformers (LVDT) and 16 ultrasonic transducers (UT) to measure surface figure of side 1, side 2 and the wedge in one measurement setup. The transducers are setup in a 4 X 4 array, for a total of 32 in one fixture. The data is acquired via a PC acquiring data through serial ports and an A/D card. The two 4 X 4 data sets are fit to the first ten Zernikes using the method of least squares. The data is displayed with 3D graphics to obtain a view of the optical surfaces. By using 14 bit digital LVDT's and employing the cross correlation technique for acoustic signal processing a system accuracy of plus or minus 1.0 micrometer for the LVDT array and plus or minus 2.75 micrometer for the UT array has been achieved.
High quality optics have been manufactured using both conventional spindle polishing and small tool figuring under computer control. While both techniques have been used to produce precision optics in the past, this paper describes the hybrid process of using both processes to utilize their specific figuring efficiencies. By knowing the frequency content of the surface being corrected, the proper tool size can be selected to address the dominant frequency. By using the most efficient tool for correction fabrication time will be reduced yielding more cost effective processes. Proper process calibrations and controls are required to maximize convergence and material removal.
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