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Proceedings Paper

'Gedankenspektrum' methods in optical coatings
Author(s): Fred T. Goldstein
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Paper Abstract

Optical coatings are subject to random and systematic errors. Assuming unvarying dispersion, least-squares fitting of measured spectra provides means to solve for non-gross thickness errors. Unlike coating design, in which many acceptable and nearly equivalent solutions are possible, inverse-synthesis requires the unique and correct solution. We introduce a 'Gedankenspektrum' (thought spectrum) method for determining the range and types of spectra required for a correct solution. Starting with an ideal design, we simulate production errors and then calculate the spectrum. Returning to the original design, we solve for the layers corresponding to the modified spectrum. Finally, if each layer is close to its known value, inverse-synthesis is successful; otherwise it fails. The process is repeated until the statistics become clear. Reliability depends on the type of design, number of layers, and measurement specifics. Most importantly, reliability increases markedly when measurements at non-normal incidence are included. This indicates the insufficiency in the (usual?) practice of measuring optical coatings solely according to pass/fail criteria. A second 'Gedankenspektrum' method helps decide which spectral measurements and film thicknesses are required for determining n&k in single films, particularly metals. Starting with given dispersion values, random noise is added to calculated spectra, thereby simulating measurement conditions. We then solve for n&k and compare to given values.

Paper Details

Date Published: 25 September 2008
PDF: 6 pages
Proc. SPIE 7101, Advances in Optical Thin Films III, 710105 (25 September 2008); doi: 10.1117/12.797564
Show Author Affiliations
Fred T. Goldstein, FTG Software Associates (United States)

Published in SPIE Proceedings Vol. 7101:
Advances in Optical Thin Films III
Norbert Kaiser; Michel Lequime; H. Angus Macleod, Editor(s)

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