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Production sequence of Si-spheres and interferometrical determination of the sphere volume

Interferometry on Spheres

Working Group 5.41

Profile

Tasks

The task of WG 5.41 is the precise determination of the volume of silicium spheres (mass 1 kg, diameter about 93.6 mm). The unit of mass is traceably represented at PTB according to the new definition of the kilogram by determining the volume of precision spheres. These have been produced from single crystals of isotope-enriched silicon with roundness deviations <100 nm. Two Fizeau interferometers with spherical reference surfaces were developed in the working group for this purpose, with which the volume is determined from a large number of interferometrically determined diameters. From the positions of the measured subapertures and their overlap, the topography of the spheres can be displayed using a stitching algorithm specially developed for this application.Silicon spheres of highest precision are manufactured in PTB's Scientific Instrument ConstructionOpens external link in new windowhttps://www.ptb.de/cms/en/ptb/fachabteilungen/abt5/fb-55.html ; spheres of natural isotope ratio can be obtained from commercial productionOpens external link in new window https://www.hauser-optik.de/de

 

 

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Research/Development

Research / Development

In the research work on the sphere interferometers, two main objectives are essentially being pursued.
For the realisation of the kilogram, a relative measurement uncertainty of the volume below 1⋅10-8 is aimed for, from which a considerable metrological effort is derived. The dominant contribution in the measurement uncertainty budget is the determination of the temperature of the sphere and reference optics. Absolute measuring Pt25 temperature sensors and relative measuring thermocouples inside the interferometers are used for this purpose. Both systems are monitored by plausibility measurements and traceably calibrated in regular cycles. Another contribution in the uncertainty budget is the intensity and frequency of the laser radiation used, which are therefore stabilised by suitable procedures. In order to estimate the influence of the deviation of the real alignment state from the ideal optical design, simulations of the optical beam guidance are carried out with a beam tracking programme developed in-house. Experimental investigations of wavefront aberration on spheres with significant deviations from the ideal shape (PV>350 nm) are of current interest.
Another focus of the research work is the measurement of spheres with smaller diameters. Here, the determination of radio topographies is in the foreground. Spheres with diameters of about 30 mm (and planned down to 3 mm) require new concepts of mechanical manageability and miniaturisation of drives and adjustment elements. Further development of sphere interferometers can be achieved with adapted sphere supports as well as movement mechanisms. A further development will be achieved by modifying a double-sided probing interferometer of WG 5.43 Opens external link in new windowhttps://www.ptb.de/cms/ptb/fachabteilungen/abt5/fb-54/ag-543.html is being investigated. If spheres of this diameter class are sufficiently characterised, they can be used as reference objects for coordinate metrology Opens external link in new windowhttps://www.ptb.de/cms/ptb/fachabteilungen/abt5/fb-53 and lead to a reduced measurement uncertainty there."



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Services

Services

Measurements with the sphere interferometers to determine the sphere volume and/or the topography can be carried out with spheres with diameters between 89 and 94 mm. The roughness should be below 1 nm and the shape deviation below 1 µm. Uncertainties of 0.6 nm are achieved for the mean diameter. For locally resolved radius determinations - limited by the precision of the positioning - the uncertainty is 1.5 nm. If topographic investigations are to be carried out, (laser) marking of the sphere makes sense. The lowest possible uncertainties are currently achieved for spheres made of silicium and quartz. For other materials, the relative measurement uncertainty can increase significantly, since the optical phase jump is usually not known precisely enough. The measurement of the sphere volume is used, in addition to the contribution to the representation of the unit kilogram, for the traceability of the density measurements of PTB Opens external link in new window Opens external link in new windowhttps://www.ptb.de/cms/en/ptb/fachabteilungen/abt1/fb-11/ag-113.html.


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Information

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