Abstract

The extended range interferometer uses a broadband light source to obtain a monotonically decreasing coherence with increasing optical path difference. The resulting modulation of fringe contrast is then used to resolve the inherent 2π ambiguity of phase shifting interferometers. By circumventing the phase-unwrapping problem, postprocessing is reduced to simple pixel-by-pixel operations. The technique greatly simplifies and speeds up interferogram analysis, making it possible to construct a real-time area microprofilometer. Other advantages of this algorithm over conventional phase shifting interferometers are the capability to accommodate surfaces containing steps more than a quarter of a wavelength high, as well as occlusions and other signal drop-outs. The optical front end of the system is built around a microscope equipped with a Michelson interferometer attachment. For magnifications in excess of 10×, a Mirau interferometer may be used with up to 40×. The coherence length of the light can be tailored to suit various applications by suitably choosing the filters over the tungsten source. The interference pattern is sensed by a high-resolution linear response CCD camera. Image digitization and processing are done on a commercial image processor, and a PC-based system is under development. The system has demonstrated a capability for reliable and repeatable profilometry of poorly reflective, nonmetallic surfaces. Root-mean-square repeatability error was measured as 1 /500th of maximum unambiguous range, typically ~3 nm. Applications to microprofiling of surfaces containing high spatial frequency and high slope detail are presented.

© 1986 Optical Society of America