Collaborative Research: Improved Freeform Measurement through Fiber-based Metrology

Metrology is the art and science of measurement, and is central to advanced manufacturing. There is a critical need for improved metrology science in micro- and nano-scale ultra-precision manufacturing of freeform surfaces. The objective of this research is to improve measurement capabilities for such ultra-precision optical surfaces. This surface measurement is important not only after an optic device has been produced, but also during manufacturing to assess the process performance. Existing measurement methods suffer from fundamental errors that limit the achievable accuracy. Additionally, traditional techniques often interrupt the manufacturing process and limit productivity. In this research, on-machine metrology using a novel measurement strategy will be used to improve manufacturing capabilities for challenging optical components. The results of this work will be used to improve freeform measurement capabilities and will therefore lead to scientific advances in the optical, illumination, energy, aerospace, and biomedical fields. Other broader impacts include the two-week 'Precision Engineering-Oriented Multidisciplinary Internship Program' at Tennessee Technological University, a permanent gallery that highlights precision engineering-based historic devices at the Cookeville Children's Museum, Cookeville, Tennessee, and 'Project Engineering for Me (E4Me)' that targets middle school girls in the urban Charlotte-Mecklenburg school system. These educational efforts will expose K-12 students to STEM research and careers with a focus on precision engineering.

The research objective is to eliminate cosine error in freeform surface measurement by integrating a novel fiber-optic sensor in an on-machine measurement configuration. Cosine error in surface profile measurement results from an angular misalignment between the measurement axis and the axis of motion and limits the measurement accuracy. This is especially true in measurements of Fresnel lenses, gratings, spherical/aspherical surfaces, and high-order polynomial surfaces, for example. Cosine error cannot be eliminated in current freeform surface measurement strategies because the measuring tools do not measure along the direction normal to the measurement plane. A novel methodology for freeform surfaces by integrating fiber optic techniques (autofocusing, Fizeau interferometry) in an on-machine configuration is planned. The project's approach also avoids machine tool error motions because the measurement path coincides with the machining path. By answering fundamental questions regarding the performance limits of the approach, it is expected that this research will produce new knowledge in high-precision, on-machine instrumentation.