Manufacture of Free-Form Optical Surfaces with Limited Mid-Spatial Frequency Error

Our proposed innovation is a robust manufacturing process for free-form optical surfaces with limited mid-spatial frequency (MSF) irregularity error. NASA and many others have a direct and critical need for high quality free-form optical components. Free-forms can improve the optical performance of many types of optical systems when compared to aspheres. MSF error is a major concern with free-form optics as the standard method for manufacturing free-forms (sub-aperture tool polishing) can lead directly to large MSF error. Simply, MSF error is a height error on the surface in the spatial regime between roughness (micro) and irregularity (macro). MSF errors dramatically degrade performance in optical systems. Our free-form manufacturing process is differentiated by full-aperture polishing step, called VIBE, and by the proposed smoothing step. The VIBE step does not create MSF error as the sub-aperture process does. The smoothing step will reduce any inherent MSF error. In this manner, we will manufacture free-form optical surfaces without MSF errors. Our technical objectives are three fold: 1) Determine most feasible smoothing parameters, 2) Determine feasibility of smoothing for free-forms for reduced mid-spatial frequency error, and 3)Determine the effectiveness of using a computer generated hologram (CGH) for free-form measurements. To accomplish these objectives we have set out the following work plan. First we will design the free-form surface and the associated CGH (with feature for easy alignment). Next, we will perform a study on smoothing to determine the optimized smoothing parameters to remove mid-spatial frequency errors on free-form surfaces. Then, we will manufacture precision free-form surfaces using the optimized parameters. During each step in the manufacturing process (generation, VIBE polishing, smoothing, sub-aperture figure correction, and something) we evaluate both the irregularity and mid-spatial frequency errors.

Free-form optical surfaces with limited mid-spatial frequency error have many applications in NASA's optical systems. Specific examples of NASA optical systems that are improved by free-form surfaces are x-ray and UV imaging instruments on weather satellites, the IRMOS spectrometer at the Kitt Peak National Observatory, and three mirror telescope systems. Three-mirror telescope systems specifically have found significant improvements with free-form surfaces. In addition, many NASA applications require tight mid-spatial frequency specifications to reduce flare in the optical systems. Specific NASA programs that have tight mid-spatial frequency specifications are the International X-ray Observatory (IXO) and the Javian planet finder. The current budgeted error for the IXO mandrels is 1.4 nm rms over the 2 mm

Like with the NASA applications, almost any optical system can have better performance with free-from surfaces with limited mid-spatial frequency errors. High energy laser applications, such as National Ignition Facility (NIF) at Lawrence Livermore National Laboratory are susceptible to mid-spatial frequency errors. The MSF errors are a source of damaging intensity, specifically in the region of interest for high energy laser systems. In EUV lithography, flare is a significant problem, where flare is the optical effect of MSF errors in optical components. The MSF errors cause light to scatter into small angles and reduce image contrast. The MSF errors scales dramatically as lambda is less, which causes a worse problem as the lithography industry heads toward shorter and shorter wavelength systems. Detailed modeling by Youngworth and Stone of MSF errors in diffraction limited imaging systems showed that the variance of the MSF errors strongly affect the imagery of an optical system. Their model was specifically focused on both radial spoke errors induced from ring-tool grinding and rotational polishing marks from CNC deterministic polishing. Similar to X-Ray telescopes, the optics in X-Ray Synchrotron's are grazing incidence also suffer from degraded optical performance from scatter from MSF errors. Several labs around the country have extreme interest in this technology, such as Lawrence Berkeley Laboratory, Brookhaven and Argonne.

This is early/mid-stage (TRL 3) — the most realistic path in is NASA SBIR/STTR, which funds small businesses and research institutions to develop technology aligned with NASA's needs (equity-free, phased funding). Check whether a current SBIR/STTR solicitation topic overlaps with this project's technology area, or contact the project directly (above) to ask.

None of these are guaranteed paths for this specific project — TechPort itself doesn't have an "apply" button. Reaching out to the contact(s) above with a specific question is usually the fastest way to find out what's actually open.