Interferometry with Stabilization of Atmospheric Wavelength

In this contribution we present an approach to incremental interferometric measurements of displacements over a limited range where the atmospheric wavelength of the coherent laser source is either directly stabilized to a mechanical reference or is corrected to fit to the reference. The idea comes from the possibility to use a highly stable material for a reference frame, material which can perform thermal expansion coefficients on the level 10/K within a large temperature range up to 50K. Over a range of several K which may be the practical range for displacement measurements the coefficient is even smaller. This may outperform the best techniques of correction for the variations of the refractive index of air. The mechanics is always a part of the measurement setup and represents one of the sources of uncertainty. A link of the refractive to the mechanical reference can practically eliminate another source of uncertainty. Introduction Interferometric measuring techniques with a highly coherent laser source has become a cornerstone for measurement of geometrical quantities in primary metrology, calibration of mechanical length standards and also in industrial applications where ultimate precision is needed. The overall concept is based on a highly stable laser source with stabilized optical frequency representing a standard of wavelength which is consequently seen as an elementary length counted by an interferometer. Further improvement of resolution of an interferometer below this length element has been achieved by a combination of optical techniques and advanced electronic digital signal processing of the interference signal. Stability of the optical frequency of laser sources which has been achieved recently is very precise. Traditional He-Ne lasers stabilized to the active Doppler-broadened line in Ne can operate with relative frequency stability on the level 10 – 10, He-Ne laser stabilized through subdoppler spectroscopy in iodine on the 10 – 10 level and the potential of iodine stabilized lasers based on frequency doubled Nd:YAG is very close to the 10 level [1]. The reproducibility of their absolute frequencies is another goal in metrology and is limited to 2.1 x 10, resp. 9 x 10 [2]. A conversion of a stable frequency into a precise wavelength relies on the value of 10th IMEKO TC14 Symposium on Laser Metrology for Precision Measurement and Inspection in Industry Braunschweig, GERMANY, 2011, September 12-14