Structural materials change their geometric dimensions due to a temperature change which can be described via the linear coefficient of thermal expansion (CTE):
where ΔL is the dimensional change depending on a thermal change ΔT over the length L of the sample. High dimensional stability is needed for several terrestrial and space-based applications. In space missions such as NGO/LISA or GRACE-FO up to pm stability of their optical instruments is necessary to meet the mission requirements. Materials with such ultra low CTEs are rare, glass, ceramics such as Zerodur or Clearceram enable high thermal stability but on the other hand they are very heavy. Light weight materials like CFRP can be tuned to have CTE values below 10-7 K-1 in at least on dimension.
To measure CTEs with an accuracy down to 10 ppb/K, not only a high sensitive measurement system is needed, also a thermally and mechanically stable support of the device under test (DUT) and corresponding mirror mounts are necessary. In our setup we use a heterodyne interferometer to measure the displacement of two mirrors as a result of an expansion of the device under test. The sensitivity of our interferometer is demonstrated with a noise level below 2 pm Hz-1/2 at frequencies above 0.1 Hz. The additional dilatometer setup, the support of the device under test decreases the accuracy of the whole setup. The support of the device under test is made of Zerodur to achieve a high mechanical stability and minimize thermal dependency. The measurement mirrors are fixed using clamps made of Invar36 in a tube shaped sample with a maximum length of 120 mm.