The turbulent history of high precision interferometry

Peter de Groot
Zygo Corporation, Middlefield, CT USA

Since the earliest developments 150 years ago, advances in light sources, optics, and data processing have established interferometry as an indispensable tool for measurement of distances and surface shape. While it can sensibly be argued that ever-increasing demands on precision have driven advances in interferometry, here I describe the current state of the art with a different driving force in mind: the presence of air in the beam paths.

The story begins with the invention of linear interferometers, originally developed in the 19^th century as a sensitive measure of the refractive index of gases, when science was searching for answers to the mystery of light propagation. Later interferometry became a tool for measuring distances and displacements, but always keeping in mind that what we are really measuring is an optical path, not a physical one. This creates challenges, including the problem of turbulence in an unsettled atmosphere.

Creative solutions to air turbulence include dispersion interferometry—using the variation of refractive index with wavelength as a signature for air density. I have some interesting stories to tell about attempts to do this for advanced photolithography.

Another approach to displacement measurement relies on optical encoders, which have much shorter beam paths through air than free-space systems, substantially reducing the effects of turbulence and refractive index. Optical encoders have improved significantly in just the past decade, achieving sub nm uncertainties for stages moving at 8 m/s.  

Air turbulence also has a major influence on instruments that measure surface topography, such as laser Fizeau interferometers. For this, the modern solution is using “instantaneous” interferometry—measurements that take only a few thousandths of a second over a million data points. Such systems average air turbulence for high accuracy.

These examples will collectively show how the air that surrounds us, so essential to life, is also a source of innovation in interferometry.