Digitally enhanced heterodyne interferometry

Abstract

Combining conventional interferometry with digital modulation allows interferometric signals to be isolated based on their delay. This isolation capability can be exploited in two ways. First, it can improve measurement sensitivity by reducing contamination by spurious interference. Second, it allows multiple optical components to be measured using a single metrology system. Digitally enhanced interferometry employs a pseudorandom noise (PRN) code phase modulated onto the light source. Individual reflections are isolated based on their respective delays by demodulation with the PRN code with a matching delay. The properties of the PRN code determine the degree of isolation while preserving the full interferometric sensitivity determined by the optical wavelength. Analysis and simulation indicate that errors caused by spurious interference can be reduced by a factor inversely proportional to the PRN code length.