Interferometry for picometer-level dimensional stability measurements

The accuracy of high-end industrial equipment and the reliability of produced goods can be limited by dimensional drift phenomena. Examples are the life cycle stability of instrumentation for space missions and the tight requirements for measurement uncertainty in the integrated circuit semiconductor industry. Re-calibration intervals of metrology instrumentation are a relevant cost factor in production. Detailed knowledge of drift stability on various timescales from minutes to weeks is needed for materials and constructional elements, such as glued or bolted joints. We report on the development of an optical heterodyne interferometer concept for the investigation of dimensional stability. The contactless, double-sided optical scheme directly probes dimensional length changes of prismatic samples, whilst the balanced configuration provides common mode rejection of perturbations that act similarly on the measurement and the reference beam. For a detailed description of the optical scheme, see Ref. [1]. In a twin configuration, a second interferometer with similar beam paths is located in close proximity to the sample measurement interferometer. With a vacuum tube included, this second interferometer is considered to act as a refractometer for efficient in-situ correction of air refractive index fluctuations. Alternatively, angular degrees of freedom, such as sample flexure, may be accessed through the twin-configuration. Given a sampling rate larger than approximately 10 Hz, i.e. larger than the thermal expansion rate of typical sample materials (e.g. Zerodur, SiC), sudden intrinsic sample instability events should be observable and discriminated from the slower thermal drift behaviour of both sample and interferometer.