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2 August 2010 Frequency-stepping interferometry for accurate metrology of rough components and assemblies
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We describe a distance-measuring interferometer based on a novel frequency-stepping laser that is tunable over 30 nm. Conventional tunable lasers provide continuous tuning over a range of wavelengths without any mode transitions. The new frequency-stepping laser was designed to maximize frequency repeatability by exploiting the mode-hopping behavior to achieve equal frequency increments. An interferometric image is collected at consecutive laser mode frequencies making it very easy to perform Fourier transforms. The modulation frequency of the interference on each pixel is directly proportional to the optical path difference between the reference and test arms of the interferometer as well as the laser mode spacing. The inherent stability of the frequency-stepping laser results in a very accurate conversion from the modulation frequency of the pixel to its OPD. A Fourier transform is performed on each pixel to determine the height difference between the reference and measurement arms independent of its neighboring pixels. Our laser mode spacing of 36 GHz results in an unambiguous measurement range of 2.1 mm. Prior knowledge about the features of the part being measured allows us to measure over 300 mm of range with 10 nm resolution. This can be combined with conventional PMI techniques to achieve sub-nanometer resolution. This technique is applicable to both rough and smooth parts making it possible to perform metrology on individual components as well as partial assemblies that require tight tolerances.
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Thomas J. Dunn, Christopher A. Lee, and Mark J. Tronolone "Frequency-stepping interferometry for accurate metrology of rough components and assemblies", Proc. SPIE 7790, Interferometry XV: Techniques and Analysis, 77900I (2 August 2010);


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