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Researchers at Ben-Gurion University of the Negev in Israel, led by Joseph Rosen, have developed a method of optical coherence profilometry. The goals of the method are to measure the 3D profile of an arbitrary surface, and to get its topography. In addition, the aim is to measure the optical thickness of a transparent layer along all its area.

Rosen and his colleagues have been able to achieve this by interfering two light beams. One comes from the source to the detector through the measured surface, and the other comes from the source to the detector through a planar reference surface. If the light is incoherent, an interference phenomenon (a pattern of dark and bright stripes called interference fringes) occurs only if the two optical paths are equal. As a result, fringes on the detector plane indicate that the corresponding area on the measured surface has the same altitude as that of the reference surface.

Rosen and his fellow researchers use partial coherence to create interference fringes in two other cases, when the difference between the two paths is a distance called dz or -dz. The event of interference in the path difference of dz is the main useful effect in Rosen's work. As a result, if one sees areas of interference fringes on the detector the only possible conclusion is that these areas on the measured surface are an altitude of dz above the reference plane.

In order to get the topography of the entire measured surface, one must change the coherence of the light so that the value of dz is gradually changed. Knowing the values of dz for all the events of interference enables the mapping of the topography of the measured surface.

But the question is: How do you control the coherence of the light and gradually change the path difference dz? The trick is that behind the laser source, in the input plane of the interferometer, Rosen has put a mask and a rotating diffuser. The diffuser converts the laser beam from coherent to incoherent light. The shape of the mask creates the effect of partial coherence.

Changing the shape of the mask changes the coherence and the value of the path difference dz. The shape of the mask is the well-known Fresnel zone plate - concentric gratings with an increasing cycle toward the circumference.

All that is needed to increase the value of dz is a Fresnel zone plate with a faster grating. After displaying one Fresnel zone plate on the input plane and looking for interference fringes in the output, you change the mask to a faster Fresnel zone plate and look for new fringes, and so on until all the region of measurement is covered.

This technology is in its very initial stage. Many aspects of this system must be improved before it will be ready for commercial use. This is especially true in the case of resolution - the robustness and the efficiency should be improved. However, according to Rosen, this is a new application for a physical effect that has been used very rarely until now.

Basically the method is a different way to do coherence profilometry and tomography. Instead of using the traditional temporal coherence phenomenon this method uses the spatial coherence phenomenon. As a result, all the fields that use coherence profilometry and tomography can benefit from the invention. This will be especially true if the researchers are able to improve the system. The application fields are mostly biomedical imaging and semiconductors inspection, and the like.

The technology has not yet been patented, and Rosen is looking for R&D collaboration of any kind. He would be eager to continue his research under a sponsorship arrangement that will provide the necessary resources.

Details: Joseph Rosen, Associate Professor, Ben-Gurion University of the Negev, Dept. of Electrical and Computer Engineering, PO Box 653, Beer-Sheva 84105, Israel. Phone:+972-7-6477150. Fax: +972-7-6472949. E-mail: rosen@ee.bgu.ac.il.

Copyright 2000, John Wiley & Sons, Inc., New York, NY 10158