Edge location by use of optical phase variation

Traditional optical methods for locating an edge are based on light intensi ty variation with respect to a reference triggering level. Since the intens ity Variation is subject to stray Light, the intensity Variation of the lig ht source, and the triggering level variation, the exact position of the ed ge cannot be determined. We describe a method for edge location that uses a phase variation in a modified differential interferometer. The maximal poi nt of the slope of the phase variation across an edge is determined exactly by the relative position between the focused beam spot and the detected ed ge if the initial intensity ratio of the two single-frequency interference beams is kept unchanged. Therefore the phase variation can be used to locat e the edge with high resolution and accuracy. To make practical use of the phase variation, the second derivative of the phase was used as a monotonic zero-crossing signal across the edge. The theoretical and the experimental verification have been conducted in detail. The results of the experiment show the feasibility of edge location when phase variation is used. The sch eme is not affected by stray light and the intensity variation of the Light source. (C) 2000 Optical Society of America OCIS codes: 260.3160, 180.3170 , 260.5430, 120.3180, 120.3940, 120.5060.