Sonali Chakraborty

Real-time edge detection by cyclic-path polarization interferometer

In a triangular path cyclic interferometer employing a polarizing beam splitter (PBS), the two counterpropagating beams are orthogonally polarized. A sample placed almost equidistant from the PBS is imaged by a lens placed in the path of the emerging beams so that two defocused images of the sample are recorded on a CCD. Using a linear polarizer in the path of the orthogonally polarized imaging beams, it is possible to achieve amplitude subtraction between the two images, resulting in an edge-enhanced image of the sample. The proposed real-time edge-enhancement technique is experimentally demonstrated.

Low-level birefringence measurement by cyclic-path polarization interferometer

A modified cyclic-path interferometer is employed for complete measurement of spatially varying birefringence. An expanded and collimated laser beam intercepted by a birefringent specimen is incident on a polarization-masked cube beam splitter, resulting in two mutually orthogonal polarization components propagating along clockwise and counterclockwise directions in the interferometer. These two wavefronts are made to interfere for four specific orientations of an analyzer. Suitable combinations of the interferograms result in determination of the direction of birefringence and its magnitude. Experimental results are presented.

Edge enhancement of phase objects using a cyclic-path polarization interferometer

The aim of this study is to model edge enhancement effect in purely phase samples. A self compensating interferometer similar to Sagnac’s, employed with a polarizing beam splitter where two counter propagating orthogonally polarized mutually coherent beams are modulated by the phase shifts introduced by a phase sample. An afocal imaging system is used to image the phase sample on the CCD so that one of the images is slightly defocused. Real-time subtraction of two images results in intensity modulation of phase interfaces.

Digital holomicroscopy for nanometer depth resolution

We report a technique which provides the details of three dimensional phase profile of a transparent microscopic sample. The depth resolution achievable is of the order of nanometers. The physical dimensions of the sample are obtained from the numerically reconstructed phase of the object wave. The method involves digital holographic recording of interferograms produced by superposition of the object wave and the coherent reference wave. We sequentially record four different holograms by altering the phase differences between the object wave and the reference wave. To introduce different phase differences between the two interfering beams, we have made use of polarization phase shifting. Therefore, the configuration used is simple and provides accuracy as the phase is introduced by simply rotating a linear polarizer. Experimental results for a microhole etched on glass are presented.