A Dynamic Zone Plate Interferometer for measuring Aspherical Surfaces

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Optical interierometers are quite useful for measuring the difference between the shape of an optical surface under test and that of a standard surface. But, conventional methods need considerable modification if the applications are to be extended : since the measurable maximum difference between surfaces is not large, it is quite difficult to obtain directly the error of aspherical surfaces. For this purpose, some special devices such as computer generated holograms or CGH are indispensable for obtaining the deformed or aspherical standard wavefronts.A dynamic zone plate interferometer (DZPI) being studied in this project can measure such surfaces without any need of either CGH or standard surface. The wavelength of the light source is changed in DZPI to generate a series of reference spheres with different radii, Since each interferogram shows the optical path difference between the surface of the optical element under test and one of the many reference shperes, both shape and surfa … More ce error can be calculated from a series of interierograms taken at different wavelengths.Two parabolic mirrors, whose F-numbers are 3 and 4, are experimentally measured As a light source, a ring dye laser is utilized. At each wavelength, an interferogram is recorded and sent to a computer to be processed. Because of the discontinuity of the wavelength change, it is difficult to use a conventional technique for fringe analysis. Then, a Fourier analysis in a continuous space is developed and applied, A theoretical analysis concludes that Fourier method become equivalent to the least square method for a certain set of the parameters used in the analysis.The phase determination error of the proposed method is also theoretically analzed in case of additive noise and nonlinear response through the observation system. It is shown that the measurement error can be minimized with a selection of appropriate system parameters. Experimental results ensure the feasibillity of the DZPI for measuring aspherical surfaces. Less