Profilometry by Zero-order Interference Fringe Identification

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Abstract
In white light interferometry, the visibility of the fringes is directly related to the optical path difference between the two interfering beams. With a very short coherence length light source, the zero-order interference fringe may be detected by a simple maximum intensity location while the reference mirror is shifted by a piezoelectric transducer. We use this possibility for high-resolution profilometry. We discuss the maximum sampling distance, which guarantees a correct detection, as a function of the self coherence function of the light source. A practical configuration based on Mirau interferometric microscope objective was built. The experimental results show the nanometric resolution of this technique and its ability to profile surfaces with sharp slopes and deep holes. The application field of either heterodyne or confocal profilometers may be extended by associating them with the proposed method.