Investigation of thermal expansion homogeneity by optical interferometry

Different measurement methods exist for the extraction of the coefficient of thermal expansion (CTE). Among them the observation of the sample length as a function of its temperature is the direct way. In the last decade, the use of phase shifting interferometry in combination with computer-based analysis of interference phase maps drastically improved interferometrical length measurements. In addition to the observation of the length itself, such measurements allow the extraction of a length topography L(x,y) of the sample as shown in this paper. From the behaviour of the length topography at different temperatures an upper limit of CTE-homogeneity can be obtained. It is shown in which way disturbing influences can be removed so that uncertainties of L(x,y) in the sub-nm range can be reached for different shaped samples