23 October 2001 Absolute optical path difference measurement with angstrom accuracy over ranges of millimeters
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The stringent figure requirements for optics used in Extreme UV (EUV) projection systems pose a significant challenge to metrologists. The large asphericities combined with low RMS figure error tolerances require novel measurement techniques. Such a method to measure absolute optical path differences (OPD) with 0.1 nm accuracy over ranges of several millimeters is presented. The method combines two different measurement techniques: frequency modulation to determine the absolute OPD with an accuracy of half a wavelength and heterodyning to determine the OPD modulo one wavelength with an accuracy of 0.1 nm. By combining the results of the two methods, the absolute OPD is determined with the large range of the frequency modulation and the high accuracy of the heterodyning. Measurement results for the heterodyning method are presented. In addition, a method to dynamically characterize the frequency behavior of a sweeping tunable external cavity diode laser is shown.
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Lukas Max Krieg, Lukas Max Krieg, Rene G. Klaver, Rene G. Klaver, Joseph J. M. Braat, Joseph J. M. Braat, } "Absolute optical path difference measurement with angstrom accuracy over ranges of millimeters", Proc. SPIE 4398, Optical Measurement Systems for Industrial Inspection II: Application in Industrial Design, (23 October 2001); doi: 10.1117/12.445541; https://doi.org/10.1117/12.445541

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