Interferometric homogeneity test using adaptive frequency comb illumination

31.10.2012, 16:47

Newsletter 5

Homogeneity tests require the measurement of a glass sample in both reflected and transmitted light so as to separate the homogeneity variations from the geometric thickness variations of the sample. For laser illumination, however, the four boundaries of the glass sample and the two boundaries of the interferometer cavity give rise to six interference terms and a very complicated response. If the cavity lengths are incommensurate, however, it is possible to separate these single interference patterns from each other using specially designed light sources. In this work, we use a superluminescent diode in combination with a tunable Fabry-Perot filter to generate frequency comb illumination for a Fizeau interferometer. By combining measurements of the specimen inside and outside the interferometer, we are able to successfully separate material homogeneities from thickness variations (see figure). Furthermore, measurement of the empty interferometer eliminates phase contributions from the Fizeau plates, allowing absolute measurement of the inhomogeneities. The reproducibility of the procedure is of the order of 0.2 nm/mm peak-to-valley vallue and 0.03 nm/mm rms.

 

Contact: klaus.mantel(at)mpl.mpg(dot)de
Group: Interferometry TDSU
Reference: I. Harder et al., Appl. Opt. 50, 4942-4956 (2011).