Absolute length measurements at temperatures down to 7 K

PTB's Ultra Precision Interferometer consists of a Twyman-Green interferometer arrangement which is accommodated in an evacuable, temperature-regulated housing. Three lasers whose frequencies are stabilized onto the hyperfine structure transitions in iodine and rubidium, respectively, are used as light sources. As the corresponding wavelengths under vacuum conditions are very precisely known, absolute lengths of gauge-blocktype (prismatic) bodies of up to 400 mm can be measured with subnanometre accuracy by means of the UPI.

In order to characterize the thermal expansion, but also the temporal stability of materials precisely, the interferometer is usually temperature-regulated as a whole, i. e. together with a test piece inside. Hereby, the useful temperature range is, however, limited to a range from 10 °C to 50 °C. In contrast, when designing modern space telescopes, the behaviour of specific materials at cryogenic temperatures is of particular interest.

Within the scope of a project funded by ESA, PTB has, in cooperation with the TransMIT-Center of Adaptive Cryotechnology and Sensors, succeeded in clearly enlarging the temperature range down to cryogenic temperatures. For this purpose, the UPI was  equipped with an extended measurement path which leads the light into an external container where the temperature of a test  piece can be regulated separately. The specially adapted cryostat is based on a two-stage pulse-tube cooler which is connected to the sample container via flexible copper strands. Hereby, a temperature of the samples of down to 7 K is achieved.

One of the greatest challengeswithin the scope of this project consisted in establishing methods to evaluate non-stationary  interferograms from which the absolute length of test pieces of up to 50 mm can be determined over the whole temperature range
from 300 K to 7 K with 1.5 nm accuracy. The first measurements have been performed with samples made of two different materials which come into question as substrates in space telescopes. In a subsequent step, the newly attained possibilities with the ultraprecision interferometer will be exploited to define extremely accurate standards to be used as a reference for indirect measurement procedures.