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Spherical interferometry in the nm range

The new determination of the Avogradro constant NA requires measuring the volume of an almost perfect silicon single-crystal sphere (1 kg in mass, with a diameter of approximately 93.6 mm) with a relative uncertainty of a few 10–8. A novel interferometer with a purely spherical beam path and a tunable laser system has been developed and set up for this purpose. The new system can determine the diameter as has been common practice to date, yet furthermore, it can display extensive topographies of spherical surfaces.

Eine präzise Temperierung und die Messung der absoluten Temperatur des Prüflings sind Voraussetzung für Messunsicherheiten im nm-Bereich. Um den Einfluss der Luftbrechzahl auszuschalten, werden die Messungen im Vakuum durchgeführt.

The novel interferometer concept relies on spherical waves to probe the sphere. The optical set-up comprises a spherical etalon formed by two reference surfaces integrated into special spherical objectives. These objectives transform plane waves behind the input collimators into spherical waves. The pickup angle of the objectives is 60° so that only a few different orientations of the sphere are needed for a complete topographical image of the sphere surface. To measure a sphere diameter d, first the diameter D of the empty etalon is determined. In a second step, the sphere is introduced into the etalon so that two new interferometers are formed, with spacings d1 and d2 between the reference surfaces and the corresponding sphere surface. The diameter of the sphere results from the difference d = D - d1 - d2.

The interference pattern is photographed with a CCD camera and evaluated using phase shifting interferometry. The phase steps required are produced by a change in the laser wavelength. A novel laser system developed in the laboratory is used for this purpose. This system can be tuned over a range of 12 GHz and, moreover, it can be stabilized to a desired frequency, with an uncertainty of a few 100 kHz.

Initial measurements were carried out on a sphere of black filter glass. The reproducibility of the sphere topography and of selected diameters is at present in the range of a few nanometers. It can therefore be expected that the aspired uncertainty of 1 nm in the measurements of the diameter of the silicon sphere can in fact be achieved.