Absolute distance measurement of optical path length of non-contact three-dimensional nanoprofiler based on normal vector tracing method by tandem white-light interferometer

The need for freeform measurement with sub-nanometer precision is increasing with the demand for freeform optics with only single nanometer or less of figure error and surface roughness in numerous fields. Since there are no method to meet the demand, therefore we developed a non-contact three-dimensional nanoprofiler based on normal vector tracing method with the light straightness and absolute-calibrated goniometers as core concepts. The nanoprofiler achieved the sub-nanometer repeatability at the figure measurement of spherical, aspherical, cylindrical, and patterned-flat mirror. However, our numerical analysis about the systematic errors of nanoprofiler revealed that the mismatch between the recognized and the actual optical path length L, the distance between the detector and the sample in nanoprofiler, causes the assembly and motion errors such as the second-order aberration. Therefore, we developed a tandem white light interferometer which references a high-precision linear encoder as a standard of displacement, in order to measure the physical distance of L absolutely. We expect that the second-order terms of systematic errors of nanoprofiler will be decreased below than 0.1 nm or less by measuring the absolute length of L with the uncertainty of 0.1 mu m or less. The first interferometer fringe was obtained for the measuring test.