Environmentally friendly interferometry

Abstract

ABSTRACT Combining spatial and time-domain processing in wave-front measuring interferometers is a powerful tool for reducing the influence of environmental disturbances on the measurement. Modern digital imagers and computing platforms eliminate most of the disadvantages typically cited against spatial processing techniques. The ability to perform both spatial and time-domain processing in a single instrument provides the greatest flexibility for precision metrology applications in both static and dynamic environments. Keywords: Interferometry, phase shifting, metrology, carrier fringe, vibration, turbulence. 1. INTRODUCTION Phase Shifting Interferometry 1 (PSI) has been the preferred measurement technique for high-precision wave-front measurements for over 30 years. The technique utilizes an electronic imaging system to store a sequence of interferograms with well-known cavity phase-shifts and a computer recovers the original wave-front phase by an analysis of the variation of intensity as a function of phase-shift. Unfortunately, PSI can perform poorly in the presence of environmental disturbances such as acoustical or mechanical vibrations or air turbulence. The main reason for the environmental sensitivity is that the measurement sequence takes time, so cavity optical path variations due to changing environmental conditions during the measurement alter the expected phase variation and produce errors in the wave-front determination. Different schemes to minimize environmental influences in PSI have been proposed and fit broadly into three categories; least-squares processing, time domain processing and space domain processing. Least-squares processing, described by Greivenkamp