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Dissertation/Thesis Abstract

Cascade Optical Coherence Tomography (C-OCT) for Surface Form Metrology of Freeform Optical Components
by Xu, Di, Ph.D., University of Rochester, 2021, 144; 28264934
Abstract (Summary)

Freeform optical components enable dramatic advances in optical system performance and packaging. As the saying goes, “one cannot make what one cannot measure.” Surface form metrology of as-built freeform optics remains a challenge and an active area of research. In this dissertation, a novel cascade optical coherence tomography (C-OCT) technique was conceived towards addressing this metrology challenge. The C-OCT theoretical framework was developed, and a metrology system was built to demonstrate its working principle and metrology capability experimentally.

Three significant areas were investigated to realize a C-OCT metrology system with nanometer-level measurement uncertainty. First, the sample arm requirements concerning telecentricity, distortion mapping, residual wavefront error, and image plane flatness were explicitly addressed. Signal throughput was also discussed, which led to the conception of a pseudo-bistatic configuration. Two custom objective lenses were designed, both highly telecentric and diffraction-limited over a broad spectral bandwidth. One lens was built and tested for wavefront performance and telecentricity. A single-shot telecentricity test method was conceived and experimentally demonstrated, which is versatile for telecentric lens systems in general.

Secondly, a custom afocal pupil relay was developed to enable the necessary telecentric scanning with the custom objective lens. Direct comparison with an integrated galvanometer mirror configuration showed a 65x reduction in measurement artifact, validating the design and its working principle.

Thirdly, a unique secondary interferometer design using a rotating optical cube was developed, so that measurement point-clouds of sufficient sampling density are achieved within reasonable time frames. The acquisition speed requirements were investigated, balancing a range of system-level parameters. A custom actuating assembly was developed with an air-bearing spindle for high-speed continuous rotation while minimizing vibration transfer to the interferometer system. With a 20 mm N-BK7 cube that generates a double-pass optical path difference of up to ±6 mm, the target speed of 1,750 RPM was demonstrated. The assembly was shown to be stable up to 15,000 RPM.

Finally, surface measurements were achieved with the C-OCT metrology system. Results on a flat mirror showed a root-mean-squared residual of 14 nm (~λ/44 at the He-Ne wavelength). Results on a freeform mirror showed a root-mean-squared residual of 69 nm (~λ/9). The surface measurement results validate the C-OCT technique and the working principles of the constituent systems. These investigations and validation provide the groundwork for advancing C-OCT as a viable freeform metrology technique.

Indexing (document details)
Advisor: Rolland, Jannick P.
Commitee: Lambropoulos, John C., Vamivakas, A. Nick
School: University of Rochester
Department: Hajim School of Engineering and Applied Sciences
School Location: United States -- New York
Source: DAI-B 82/8(E), Dissertation Abstracts International
Subjects: Optics
Keywords: Freeform optics, Interferometry, Lens design, Optical coherence tomography, Optical engineering, Testing and metrology
Publication Number: 28264934
ISBN: 9798582542544
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