The Turbulent History Of Interferometric Methods Of Precision Positioning

Since the earliest developments 150 years ago, advances in light sources, optics, and data processing have firmly established interferometry as an indispensable tool for measurement of distance, displacement, and angle [1]. While it can be sensibly argued that ever-increasing demands on precision have driven advances in interferometric positioning, here we follow the path to the current state of the art with a different driving force in mind: the presence of air in the beam paths, and the innovations required to deal with refractive index changes and air turbulence.

Free-space, laser-based Interferometers have evolved from a simple Michelson geometry with fringe counting to complex, multi-axis instruments that have sub-nanometer noise levels for monitoring stage motions of several meters per second (Figure 1). These systems readily satisfy the Abbe principle, are directly traceable to the unit of length, and when placed in a vacuum, have unparalleled performance [2]. However, in more common applications where the measurement beams pass through the atmosphere, refractive index variations and turbulence are dominant sources of uncertainty.