Interference of light has numerous metrological applications because the optical path difference (OPD) can be varied at will between the interfering waves in the interferometers. We show how one can desirably change the optical path difference in diffraction. This leads to many novel and interesting metrological applications including high-precision measurements of displacement, phase change, refractive index profile, temperature gradient, diffusion coefficient, and coherence parameters, to name only a few. The subject fundamentally differs from interferometry in the sense that in the latter the measurement criterion is the change in intensity or fringe location, while in the former the criterion is the change in the visibility of fringes with an already known intensity profile. The visibility can vary from zero to one as the OPD changes by a half-wave. Therefore, measurements with the accuracy of a few nanometers are quite feasible. Also, the possibility of changing the OPD in diffraction allows us to use Fresnel diffraction in Fourier spectrometry, to enhance or suppress diffracted fields, and to build phase singularities that have many novel and useful applications.

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When two similar small objects that are installed in the arms of a Mach-Zehnder interferometer are illuminated in such a way that the image of one object in the second beam splitter is located on the other object, the diffracted waves from the objects are exactly superimposed in the interference field. The superimposed diffraction patterns disappear for a phase difference equal to an odd multiple of pi. The phase change induced by imposing changes on the medium surrounding one of the objects can be determined very accurately only by measuring the intensity distribution that appears on the superimposed diffraction patterns. Using this technique we have determined the temperature profile around a wire of diameter 0.26 mm carrying different electric currents by evaluating the phase changes at 700 points on the superimposed diffraction patterns to the accuracy of pi/200. This technique allows us to eliminate the effect of light diffraction from the geometry of an object, to measure the amplitudes of the interfering diffracted waves accurately, and to evaluate the phase difference of the diffracting waves reliably.

Measurement of the source size and specifying its effect on the spatial coherence of propagating light are important for characterizing distant sources such as stars, and imaging with partially coherent light. The common method for measuring spatial coherence is Young’s two-pinhole experiment. For characterizing spatial coherence along a line, one needs to change the location of the pinholes over a large number of pairs of points. But it requires many measurements, which takes significant time. In this paper, we use Fresnel diffraction from a step in reflection to measure the source width and transverse coherence length. It is shown theoretically and experimentally that these quantities are determined by specifying the location of minimum visibility on the diffraction pattern. We utilize a sodium vapor lamp with a variable slit in front of it as an extended one-dimensional incoherent light source. The measurements are made through recording only one diffraction pattern formed by the step. The study is applicable in 2D, and one can characterize weak starlight using highly sensitive equipment.

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