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Proceedings Paper

Advantages of a low coherence interferometer for optical testing
Author(s): Chris L. Koliopoulos; Klaus R. Freischlad
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Paper Abstract

A new interferometer has been developed using an engineered low coherence LED illumination system, where both the spectral and spatial coherence are tailored to allow single surface interference from plane parallel transparent optical components such as optical windows, glass wafers, glass computer disk substrates, etc. without the need to paint the rear surface to suppress interference from that surface. Only the reflection from a surface at a specific optical path difference can interfere with the reference beam eliminating spurious interference. While the interference region due to spectral coherence is engineered to provide interference within a 280 micron zone, diffraction at surface defects and dust within this zone produces secondary wavefronts which can interfere with the test and reference wavefronts, producing phase artifacts which can limit measurement accuracy in standard coherent laser-based interferometers; these phase errors are eliminated by using a spatially extended LED source. For this low coherence interferometer, the “purity” of interference between the test surface and a high quality calibrated reference surface, along with high spatial resolution phase measurements, offers the ability to review waviness and surface polish variations to sub-nanometer height resolutions over a large 4 inch field-of-view. Examples of polish process variations of optical components, along with power spectral density plots (PSD) are shown where the data were acquired with OptoFlat, a downward-looking LED-based low coherence interferometer newly developed for the rapid measurement of precision polished flat surfaces including thin transparent optical components without special surface preparation.

Paper Details

Date Published: 15 November 2019
PDF: 12 pages
Proc. SPIE 11175, Optifab 2019, 1117516 (15 November 2019); doi: 10.1117/12.2536685
Show Author Affiliations
Chris L. Koliopoulos, InterOptics, LLC (United States)
Klaus R. Freischlad, InterOptics, LLC (United States)

Published in SPIE Proceedings Vol. 11175:
Optifab 2019
Blair L. Unger; Jessica DeGroote Nelson, Editor(s)

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