CAREER: Breaking the Freeform Optics Metrology Barrier with Synthetic Wavelength Interferometry

University Of Rochester, Rochester NY

Investigators

Abstract

Freeform optics use complex surfaces that are not necessarily axisymmetric, potentially very complex, and have many applications. A continuing problem is the measurement (metrology) of such surfaces. Existing metrology solutions use interferometry, but suffer from fringe visibility issues and other errors, while touch systems such as coordinate measuring machines and profilometry have insufficient accuracy (i.e., micrometer scale as opposed to nanometer scale) and may damage the optical surface, especially for softer materials. The PI will execute a multi-faceted research program aimed at improving the metrology of freeform optics. Central to his approach is the combination of two light sources, each with a different wavelength, in order to evaluate the surface profile of the lens. This approach will lead to the measurement of high aspect ratio surfaces, where interference fringes would normally be spaced too close together to discern from each other. This is enhanced by the consideration of trace and retrace (going over the same path in opposite directions with the light source), and the differences between the two measurements. The approach uses mathematical models that will be used to extract the desired geometry. The result is a methodology that can be used in measuring free-form optics to a higher resolution than currently possible. One Broad Impact of this Faculty Early Career Development (CAREER) program research project is to empower optical designers and manufacturers with metrology methodologies that can measure advanced freeform optics. Freeform optics are the foundation for future vision-based technologies and lightweight, portable, unobtrusive mobile devices. Potential devices range from compact satellites for monitoring environmental changes to better optical sensors for autonomous vehicles. This research will have educational impact at the university level through mentoring research assistants, incorporating research in undergraduate student experiential learning, and outreach with high school students. The PI performs outreach via high school outreach, through workshops, and through extensive podcasting and blogging activities. This research will address this fundamental research question: Can synthetic wavelength interferometry (SWI) be used to produce new enabling capabilities in measuring freeform optics? The PI will examine (theoretically and experimentally) the impact of transmission sphere f-number and local slope departure on vignetting, in order to measure optics that normally have fringe ambiguity and retrace errors. This will be also investigated through the application of synthetic wavelength interferometry (SWI), where two light wavelengths are combined so that the phase difference can be examined and the limits to conventional interferometry can be identified and improved upon. The critical aspect of this technique is that the synthetic wavelength can be generated to resolve fringe ambiguities for lengths potentially much larger than the constitutive wavelengths. This is especially important for optics where the local slope departure is high. Fringe ambiguities arise from insufficient sampling of dense fringe patterns in interferometers, which create aliasing in the measurement. These ambiguities manifest from exceeding Nyquist sampling limits from high slope departure optical surfaces like steep aspheric and freeform optics. The investigations in this research will focus on two main aspects: dispersion effects from using synthetic wavelengths and retrace errors from steep slopes causing the returning light to pass through a different spatial location in the optical system. Both Fizeau and Twyman-Green interferometer configurations will be considered, as each system has a different placement of the reference surface and may cause different effects for both dispersion and retrace errors. The wavelength selection for the synthetic wavelength influences in both the non-ambiguity range and extending the Nyquist sampling limit. This, and the coupling with signal processing present potential contrasting criteria for utilizing synthetic wavelength interferometry for measuring freeform optics.