Abstract
In this paper, we demonstrate the development of a point-cloud metrology method for the noncontact, high resolution, high precision testing of freeform surfaces. The method leverages swept source optical coherence tomography together with a common-path setup in the sample arm configured to mitigate the axial jitter caused by scanning and environmental perturbations. The lateral x-y scanning field was also rigorously evaluated for the sampling step, linearity, straightness, and orthogonality. Based on the finely engineered system hardware, a comprehensive system model was developed capable of characterizing the vertical displacement sensitivity and lateral scanning noise. The model enables predicting the point-cloud surface-metrology uncertainty map of any freeform surface and guiding the selection of optimum experimental conditions. A system was then assembled and experimentally evaluated first with flat and spherical standards to demonstrate the measurement uncertainty. Results of measuring an Alvarez freeform surface with 400-µm peak-to-valley sag show 93 nm (< λ/14) precision and 128 nm (< λ/10) root-mean-square residual from the nominal shape. The high resolution measurements also reveal mid spatial frequency structures on the test part.
Highlights
During the last decade, the optics manufacturing industry has acquired the capability of fabricating freeform optical quality surfaces for imaging applications owing to the machining flexibility offered by an added third independent servo axis in commercial equipment [1]
We investigate a point-cloud method that leverages Fourier-domain sweptsource optical coherence tomography (SS-OCT), a technique based on low-coherence interferometry, for precision freeform metrology
Based on the SS-OCT freeform-metrology system described in Section 2, we developed a comprehensive model of the system that accounts for the various noise sources and predicts the system uncertainty in measuring a freeform surface
Summary
The optics manufacturing industry has acquired the capability of fabricating freeform optical quality surfaces for imaging applications owing to the machining flexibility offered by an added third independent servo axis in commercial equipment [1]. This manufacturing advancement drives the emerging development of freeform surfaces that are characterized by their non-rotationally symmetric departures from base spheres. An impediment to the broad industrial implementation of freeform surfaces in optical imaging systems is the imminent need of a high performance metrology tool capable of measuring significant surface departures and slopes of the parts. The noncontact optical testing techniques being actively pursued are categorized into three main areas: classical interferometry, phase measuring deflectometry, and optical profilometry
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