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Lecture Hall ICM - Internationales Congress Center München SPIE Optical Metrology > Optical Measurement Systems for Industrial Inspection XI > Interferometry II
14:45-15:05 h | ICM - Internationales Congress Center München ICM Room 14c
Subjects: Optical Measurement Systems / Optical Metrology
A tandem white-light interferometer was developed to measure the absolute distance of optical path length L of nanoprofiler, which is being developed to respond to the demand of the freeform measurement within single nanometer-scale uncertainty. Freeform optical elements that have only single nanometer or less figure error and surface roughness are being applied to the numerous fields of science and technology such as hard and soft X-ray science, extreme ultraviolet lithography, satellite optics, large reflecting telescope and so on, however, the uncertainty of common-ways of freeform measurement still remain at dozens of nanometer. Therefore, a new nanoprofiler was developed with a single mode laser beam as the linear probe and four absolute-calibrated goniometers, in order to trace the normal vectors of sample surface without contacting it, in order to achieve the single nanometer uncertainty at freeform measurement. The nanoprofiler finds the figure error of sample surface by fitting the model functions of Legendre polynomial to the data set of normal vector for each coordinate, after removing its numerically simulated assembly errors that are corresponding to the 6 freedom degrees of its 9 components, almost are tilts. Here, it was revealed that the L, the distance between Quadrant Photodiode (QPD) and sample surface of the nanoprofiler, affected second order curvature to the distributions of assembly error sensibility. The tilts could be removed easily, however, the second order curvature ones were not. To reduce the effects of them, the absolute distance of L have to be measured within 0.1 µm uncertainty. Therefore, a tandem white-light interferometer was introduced for the distance measurement. White-light interferometry method is used in many fields of high-precision length and figure measurement. Furthermore, the optical path distance of tandem connection of two Michelson interferometers can be freely designed according to other instrument arrangement. In this study, we developed a tandem interferometer with high-precision linear motion stage and linear encoder, and measured the absolute distance between two optical flats that faced each other for test. As a future prospect, we are going to design the measurement experiment of L by this interferometer, containing several optical elements of different refractive index on the path.