New Design Method for Free-Form Optics

Using a new step-by-step method, these eight different designs for a three-mirror reflective imager were ranked by their potential to be corrected using freeform optics. (Source: J. Rolland, Univ. Rochester)

Lenses and mirrors with free­-form surfaces enable designers to focus light within optical devices that are lighter, more compact, and more effective than ever before. But until now, deter­mining which free-form surfaces will work best in a given confi­guration of mirrors and lenses has been a time-consu­ming and often expen­sive process of trial and error. It doesn’t have to be that way anymore.

Aaron Bauer, a senior research engineer at the Univer­sity of Rochester’s Center for Freeform Optics (CeFO), combines theory and practice in a step-by-step method that eli­minates much of the guess­work. “Aaron has developed a process to design with free-form surfaces that can be applied very gene­rally,” says Jannick Rolland, CeFO director and Brian F. Thompson Professor of Optical Engi­neering. “It’s really beautiful and even at times feels like magic” She believes the findings will help acce­lerate the adoption of free-form optics in industry. “People will no longer say ‘Oh, it’s too expensive to build with freeform optics,’” she says. “Because now you can make some­thing that may cost a tenth of what it would have cost otherwise.”

For as long as mirrors and lenses have been packaged together in tele­scopes, spectro­meters, and a host of other optical devices, perfor­mance has been defined by how well those elements are able to keep a beam of light focused with minimal aber­ration. Tradi­tionally, optical designers have relied on rota­tionally symmetric optical surfaces, because their design and manu­facture was relatively straight­forward. Within the last 20 years, advances in high-speed micro milling, computer-controlled lens polishing, and ion beam etching, among other technologies, have made asymmetric freeform surfaces more feasible. Kyle Fuersch­bach, a former member of the Rolland Lab, laid the theo­retical frame­work for free-form aber­rations theory.

“But we still didn’t have a sys­tematic process to design with that theory,” Rolland says. Bauer, in the meantime, was working along­side Fuersch­bach, designing a head-worn display using freeform surfaces. “I noticed that there were very common patterns of aberrations that were always popping up, and limi­ting my system from going any further,” Bauer says. Moreover, “those patterns of aber­ration matched the ones that Kyle pre­dicted would be corrected by freeform surfaces. So, I put two and two together.”

The method he came up with starts with the initial folding geometry – alignment of mirrors and lenses – contem­plated for a design, and then, based on an analysis of the various aber­rations produced by that alignment, predicts: whether free-form surfaces could minimize those aberrations and, if so, which free-form surfaces should be used for maximum effect. “Free-form surfaces are not a uni­versal solution for correc­ting every aber­ration,” Bauer notes. “So, what our method does is to allow designers to analyze all of these geome­tries ahead of time, in order to predict whether or not there would be a good solu­tion.”

That’s far better than the brute force approach where “people heuris­tically try various freeform surfaces into a design,” Rolland says. “Even if it even­tually works, you could end up with a system where the depar­ture of the surfaces are much larger than they would be otherwise, because all those free­-form surfaces may be fighting each other. And if it does not work, there is nowhere go as a designer.” By using Bauer’s method instead, she says, “you will be able to design some­thing that is a lot simpler, and that will be easier to manu­facture and test. Further­more, the method will quickly and unequi­vocally provide insight into why a given geometry might be intrin­sically limited, which is essential for designers.” (Univ. Rochester)

Reference: A. Bauer et al.: Starting geometry creation and design method for freeform optics, Nat. Commun. 9, 1756 (2018); DOI: 10.1038/s41467-018-04186-9

Link: Optical Diagnostics & Applications, The Institute of Optics, University of Rochester, Rochester, USA

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