Freeform Optics Design Approach Uses Starting-Point Geometries

Researchers have developed a step-by-step design method for determining which freeform surfaces will work best in a given configuration. The new method, which combines theory and practice, could eliminate the time-consuming, expensive process of using trial-and-error to determine which freeform surfaces will best achieve the desired result.

The method developed at the University of Rochester’s Center for Freeform Optics (CeFO) starts with the initial “folding geometry” (alignment of mirrors and lenses) for an optical design and then, based on an analysis of the various aberrations produced by that alignment, predicts whether freeform surfaces could minimize those aberrations and, if so, which freeform surfaces should be used for maximum effect.

Using a 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, with Tier 1 having the greatest potential. Courtesy of the University of Rochester/Jannick Rolland.

The optimization steps of the design process were reported for an F/3 three-mirror imager with a 200-mm aperture and 4° × 4° FOV, resulting in a diffraction-limited solution in the visible band.

Researchers then optimized an alternative starting-point geometry that they quantified as a suboptimal candidate for optimization with freeform surfaces. A comparison of the optimized geometries showed the performance of the optimal geometry was at least 16× better.

Through the development of the design process for optical systems using freeform surfaces, researchers determined that the most critical consideration when searching for a starting point is the geometry, which is determined by the tilt directions of the mirrors, and the method in which the potential of that geometry is evaluated.

Aaron Bauer, a senior research engineer at the University of Rochester’s Center for Freeform Optics, has developed a process that will speed the design and development of optical devices that use freeform surfaces. Courtesy of the University of Rochester /J. Adam Fenster.

“Freeform surfaces are not a universal solution for correcting every aberration," said researcher Aaron Bauer. "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.”

Professor Jannick Rolland said that the new method could help accelerate the adoption of freeform optics in industry. She described it as an improvement over the “brute force” approach, where “people heuristically try various freeform surfaces into a design."

“Even if it eventually works, you could end up with a system where the departure of the surfaces is much larger than they would be otherwise, because all those freeform surfaces may be fighting each other," Rolland said. "And if it does not work, there is nowhere to go as a designer.”

By using Bauer’s method instead, “you will be able to design something that is a lot simpler, and that will be easier to manufacture and test," said Rolland. "Furthermore, the method will quickly and unequivocally provide insight into why a given geometry might be intrinsically limited, which is essential for designers.”

Traditionally, optical designers have relied on rotationally symmetric optical surfaces, whose design and manufacture is relatively straightforward. Within the last 20 years, advances in high-speed micromilling, computer-controlled lens polishing, and ion-beam etching, among other technologies, have made asymmetric freeform surfaces more feasible.

The research was published in Nature Communications (doi:10.1038/s41467-018-04186-9).