Interferometric Homogeneity Test using Adaptive Frequency Comb Illumination

Homogeneity tests require the separation of refractive index variations of glass plates from surface deformations or geometrical thickness variations. This separation requires a combination of reflected and transmitted light tests within an interferometer cavity. For the measurement in transmitted light, however, this leads to a nested cavity setup. To separate the interference signals from the different cavities, we illuminate a Fizeau interferometer with an adaptive frequency comb, where the illuminating laser source has been replaced by a broad band light source with a spectral comb structure [1]. In this way, rigid glass plates can be measured, and linear variations in the homogeneity can also be detected. Compared to approaches using a two-beam interferometer as a filter for the broad band light source, the visibility of the fringe system is considerably higher. [1] I. Harder, G. Leuchs, K. Mantel, J. Schwider, "Adaptive frequency comb illumination for interferometry in the case of nested two-beam cavities", Appl. Opt. 50, p.4942-4956 (2011)