M. P. Kothiyal

Use of spatial phase shifting technique in digital speckle pattern interferometry (DSPI) and digital shearography (DS).

Digital speckle pattern interferometry (DSPI) and digital shearography (DS) are well known optical tools for qualitative as well as quantitative measurements of displacement components and its derivatives of engineering structures subjected either static or dynamic load. Spatial phase shifting (SPS) technique is useful for extracting quantitative displacement data from the system with only two frames. Optical configurations for DSPI and DS with a double aperture mask in front of the imaging lens for spatial phase shifting are proposed in this paper for the measurement of out-of-plane displacement and its first order derivative (slope) respectively. An error compensating four-phase step algorithm is used for quantitative fringe analysis.

Spectrally resolved white-light phase-shifting interference microscopy for thickness-profile measurements of transparent thin film layers on patterned substrates

We describe how spectrally-resolved white-light phase-shifting interference microscopy with a windowed 8-step algorithm can be used for rapid and accurate measurements of the thickness profile of transparent thin film layers with a wide range of thicknesses deposited upon patterned structures exhibiting steps and discontinuities. An advantage of this technique is that it can be implemented with readily available hardware.

Achromatic phase shifting by a rotating polarizer

We describe here an achromatic phase shifting arrangement based on polarization components in which the phase shifting is obtained by rotating a polarizer. These can be used in a polarization interferometer to introduce variable phase shift that is independent of wavelength.

Spectrally resolved phase-shifting interferometry of transparent thin films: sensitivity of thickness measurements

Spectrally resolved white-light phase-shifting interference microscopy can be used for rapid and accurate measurements of the thickness profile of transparent thin-film layers deposited upon patterned structures exhibiting steps and discontinuities. We examine the sensitivity of this technique and show that it depends on the thickness of the thin-film layer as well as its refractive index. The results of this analysis are also valid for any other method based on measurements of the spectral phase such as wavelength scanning or white-light interferometry.

Universal digital speckle shearing interferometer

A speckle shear interferometer is presented which can be used to yield lateral, radial, rotational, and inversion shear fringes in real time in conjunction with a digital image processing system. A modification of the optical arrangement yields reversal or folding shear as well. Experiments are conducted on an edge-clamped diaphragm with concentrated load. Unit contrast fringes have been obtained by resorting to nonlinear processing techniques such as level slicing. The results are presented for various types of shear.

Direct determination of focal length by using Talbot interferometry

A new and simple technique for the determination of the focal length of positive lenses and concave spherical surfaces by Talbot interferometry is described. Results of measurements are presented. We show that high measurement accuracy can be achieved with this technique.

Self-referencing collimation testing techniques

Self-referencing collimation testing techniques are briefly reviewed. New self-referencing configurations using Talbot and doublewedge plate shear interferometric techniques are described. Setting sensitivities of various arrangements are compared.

Talbot interferometry in noncollimated illumination for curvature and focal length measurements

We describe a simple method for measuring the radius of curvature by using Talbot interferometry in a noncollimated light beam. This scheme can also be used to determine the focal length of the collimating lens employed in the setup. Results of the measurements are presented. A discussion of achievable accuracies and the proper choice of parameters is included.

Measurement of discontinuous surfaces using multiple-wavelength interferometry

Interferometric surface profilers using a single wavelength offer excellent vertical resolution, but have an ambiguity-free range of less than half a wavelength. Multiple-wavelength or white light interference techniques are used to overcome the problem. We discuss a three-wavelength interferometric technique used with a phase-shifting phase evaluation procedure. The phase evaluation at the three wavelengths gives wrapped phase at any pixel corresponding to these wavelengths. We use the fact that the variation of phase with wavenumber for a given profile height is linear to determine the absolute value of the profile height. The height is then used to ascertain the fringe order. The fringe order, along with the wrapped phase, gives the profile height with a resolution given by the phase-shifting technique. Experimental results for large step height measurement on etched silicon samples are presented.

Setting sensitivity in Talbot interferometry with modified gratings

Abstract A dual field grating has been introduced for use in Talbot interferometry. We report here results of an experimental investigation on setting sensitivity when using these gratings for testing.