Short coherence dynamic Fizeau interferometer with internal path matching for radius of curvature measurement

One of the most time-consuming aspects of measuring the radius of curvature of a test article is repeatedly moving between the cateye and confocal positions for stage readings and measurements. Here we introduce a unique short coherence dynamic Fizeau interferometer with internal path matching that facilitates long term stability of the cateye reference position relative to the confocal position, eliminating the need to take repeated cateye measurements. The path matching assembly is equipped with a high-quality encoder and has an athermalized design for long-term stability of the path match locations. In addition, the interferometer uses a polarization based pixelated camera for single frame acquisition of the measurements. Combined with the short coherence source, the single frame measurements facilitate rapid scans of the fringe modulation as a function of stage position for accurately locating the relative position of the test article along the optical axis. Radius of curvature measurement repeatability of less than a micron has been demonstrated. This paper will discuss the theory of operation and present the results of measurement studies.