Non-contact absolute internal distance measurement between two mirrors by using a low-coherence tandem interferometer with optical prism mechanism (1st Report)

Measurement principle and preliminary experiment

Abstract

High-accuracy free form surface and aspherical mirrors are significant in many fields such as lithography applications, digital camera, etc. A novel nanoprofiler using normal vector tracing method can measure the aspherical mirrors without the use of a reference surface. To achieve a high accuracy of the measurement, it is highly demanded to precisely measure the internal distance between a mirror to be measured and a quadrant photodiode (QPD). This paper is the first report of a new optical measurement method for measuring internal distance between two plane mirrors by using a low-coherence tandem interferometer that will be applied for nanoprofiler. In order to observe interference fringes, the optical path difference of the scanning interferometer is adjusted to be the same with the optical path of the second interferometer, which is the internal distance between two mirrors. The internal distance of 100 mm has been measured within a standard deviation of 60 nm.