Low-scatter bare aluminum optics via chemical mechanical polishing

Traditionally, aluminum optics have been produced via a combination of machining, lapping, and diamond turning techniques. The surface roughness and diffraction grating effects resultant from diamond turning have largely limited the use of these optics to IR applications. Work arounds for this problem have included nickel coatings which are subsequently polished to a required finish for use in visible and/or ultraviolet spectra. Unfortunately, this introduces additional costs as well as bimetallic effects that can limit the application of such components. We have developed chemical mechanical polishing (CMP) techniques that allow high quality optical surfaces to be produced on bare aluminum alloy such as 6061-T6. Alloy properties such as grain size, inclusions, and voids can impact all types of finishing processes. The CMP method, however, has been very robust in polishing performance over a range of alloy types and properties. Surface roughness <20 A rms is readily attainable with this process, and values below 10 A have been produced with proper process conditions and alloy properties. The monolithic mirrors produced via CMP techniques have been compared against other current alternatives such as diamond turned aluminum, nickel coated aluminum, and aluminized glass. Data indicate the aluminum mirrors produced via CMP can provide performance improvements versus the alternatives based on measurements comparing parameters such as surface roughness, surface quality, reflectivity, and bidirectional reflectance distribution function.