Y. Pavan Kumar

Bio: Y. Pavan Kumar is an academic researcher from Raja Ramanna Centre for Advanced Technology. The author has contributed to research in topics: Interferometry & Wavefront. The author has an hindex of 8, co-authored 38 publications receiving 199 citations.

Step height measurement using lateral shearing cyclic path optical configuration setup and polarization phase-shifting interferometry

Abstract: We present a step height measurement technique using a lateral shearing cyclic path optical configuration (CPOC) setup and polarization phase shifting interferometry (PPSI). In the technique, the CPOC setup generates two orthogonally polarized, laterally sheared, converging beams that get focused at the back focal plane of a positive lens placed at the input beam of the CPOC setup. These focusing beams are reflected back into the CPOC setup by the step optics, positioned in such a way that one of the surfaces of the step optics coincides exactly with the focus point of one of the laterally sheared focusing beams. The CPOC recombines the beams reflected from the step surfaces and the beams re-pass through the lens, which produces collimated and spherical interfering wave fronts for the respective beams. Using PPSI, the radius of the spherical wavefront emerging from the lens, which is related to the focal shift introduced between the reflected beams due to the step height, is determined. The novelty of the technique is the introduction of the CPOC setup and the use of a single wavelength light source to determine the large step height. Results obtained for a step height of 1.340 mm are presented.

Simple technique for the measurement of two-dimensional linear retardation distributions of wave plates with a phase-shifting Nomarski prism.

Abstract: What we believe to be a new technique for the measurement of two-dimensional retardation distributions of a wave plate (WP) is presented Phase-shifting interferometry has been applied for determining the relative retardation distribution using a Nomarski prism (NP) as a phase shifter Absolute retardation distribution is obtained by accurately determining the position of zero retardation in the interference field using white light interference fringes and adjusting the phase distribution with respect to the zero retardation position The measured absolute retardation distribution of the NP is subtracted from that obtained for the combination of the WP and NP, to get the desired retardation distributions for the WP The technique is suitable for the measurement of phase retardation of both single and multiple order WPs, as the actual phase retardation distributions are obtained Results obtained for WPs are presented

Self-referenced technique for the determination of meridional figure errors of a toroidal mirror with a Sagnac interferometer

Abstract: A simple, self-referenced technique for the determination of meridional surface slope errors of toroidal mirrors (TMs) used in synchrotron radiation is presented. An expanded, collimated, linearly polarized laser beam is allowed to fall on a TM, and the reflected toric wavefront from the TM is allowed to enter a Sagnac interferometer based lateral shearing interferometer setup, which duplicates the beam into identical wavefront components having orthogonal linear polarizations, i.e., p and s polarizations, and introduces appropriate lateral shear between the identical pair of wavefronts along the meridional direction. The laterally sheared identical wavefronts interfere when brought to the same state of polarization. The optical path difference (OPD) variation along the direction of the lateral shear in the interference field is a map of the slope variation. Polarization phase shifting interferometery has been applied to find the OPD variation and thus the slope distribution and errors. Results obtained for a TM of moderately long average radius of curvature along the central meridional section and a relatively shorter radius of curvature along the sagittal section are presented and compared with that obtained with a long trace profiler.

Measurement of moderate wedge angles of optical windows using a wedge shear plate

Abstract: We present what we believe to be a new technique for the measurement of moderate wedge angles of optical wedge plates using a wedge shear plate (WSP). Using the technique, a wedge angle of up to 5 deg can be measured. The WSP, oriented at an angle to the incident collimated beam, produces shear fringes. The shear fringes are parallel-shifted by rotating a test wedge plate (TWP), in its plane, placed at the input beam of the WSP. The fringe shift is due to the variation of the optical path difference between the interfering beams reflected from the front and back surfaces of the WSP, brought about by the change in the angle of incidence on the WSP due to the rotation of the TWP. The maximum number of fringes shifted as the wedge direction of the TWP (initially aligned perpendicular to the wedge direction of the WSP) is rotated by 180 deg is used for the measurement of the wedge angle of the TWP. Results obtained for a 59.0-arcmin wedge plate with a measurement uncertainty of ±4.68 arcsec are presented. The technique is very simple, and the results are not affected by external vibrations.

Measurement of displacement and distance with a polarization phase shifting folded Twyman Green interferometer.

Abstract: A Sagnac interferometer (SI), consisting of a polarization beam splitter (PBS), along with two equally spaced plane mirrors that are inclined at 45° to each other, is transformed into a folded Twyman Green interferometer (TGI) by placing a mirrored parallel plate (MPP) into the hypotenuse arm of the SI. The converging input beam produced by a telescope objective (TO) is split into reflected (s-polarized) and transmitted (p-polarized) components by the PBS. The p- and s-polarized focal spots are made to fall on the mirrored end surfaces of the parallel plate (PP). The retroreflected p- and s-polarized beams become collimated after passing through the TO. A linear shift of the PP in either (longitudinal) direction alters the positions of the p- and s-polarized focal spots and results in a set of converging and diverging spherical wavefronts that interfere to form concentric circular fringes. We applied polarization phase-shifting interferometry to obtain the optical path difference (OPD) variation of the interference field. The displacement is calculated from the OPD variation. A validation experiment has been carried out by introducing known shifts to the PP. The setup can be used for displacement as well as distance measurement.

Young`s interference experiment with light scattered from two atoms

Abstract: We report the first observation of interference in the light scattered from two trapped atoms ({sup 198}Hg{sup +} ions localized in a linear Paul trap). The visibility of the interference fringes can be explained in the frame-work of Bragg scattering by a harmonic crystal, but with important differences compared to the case of a large crystal. Comparison of the experimental data with theory shows that the interference pattern offers another method to determine ion temperatures and separations. Furthermore, by exploiting the atom`s internal structure we have found a way to obtain {open_quotes}which path{close_quotes} information without invoking the position-momentum uncertainty relation. If the light scattered by the atoms is detected in a polarization-sensitive way, then it is possible to selectively demonstrate either the particle-nature or the wave-nature of the scattered photons.

Characterization of Bessel beams generated by polymeric microaxicons

Abstract: We present a quick, simple and accurate digital holographic characterization of the Bessel beams produced by polymeric microaxicons. This technique allows the numerical reconstruction of both intensity and phase of the beam at whichever point starting from a single acquired hologram. From these data, it is possible to go back to the axicon structure, and to gather information about their characteristics. In particular, the focal length and the depth of focus of the axicon lens are experimentally measured, and the full width at half maximum of the beam is obtained too. The depth of focus, very large for a Bessel beam with respect to a Gaussian one, is successfully exploited for optical trapping of micrometric objects.

Development of a compact, fiber-coupled, six degree-of-freedom measurement system for precision linear stage metrology

Abstract: A compact, fiber-coupled, six degree-of-freedom measurement system which enables fast, accurate calibration, and error mapping of precision linear stages is presented. The novel design has the advantages of simplicity, compactness, and relatively low cost. This proposed sensor can simultaneously measure displacement, two straightness errors, and changes in pitch, yaw, and roll using a single optical beam traveling between the measurement system and a small target. The optical configuration of the system and the working principle for all degrees-of-freedom are presented along with the influence and compensation of crosstalk motions in roll and straightness measurements. Several comparison experiments are conducted to investigate the feasibility and performance of the proposed system in each degree-of-freedom independently. Comparison experiments to a commercial interferometer demonstrate error standard deviations of 0.33 μm in straightness, 0.14 μrad in pitch, 0.44 μradin yaw, and 45.8 μrad in roll.

A ring-shaped random laser in momentum space.

Abstract: A ring-shaped random laser in momentum space is designed by directly coupling a random laser with a commercial optical fiber. By using a simple approach of selectively coating the random gain layer on the surface of the fiber, red and yellow random lasers are respectively achieved with low threshold values and a good emission direction due to the guiding role of optical fibers. The unique coupling mechanism leads to a random laser with a ring shape in momentum space, which is an excellent illuminating source for high-quality imaging with an extremely low speckle noise. More importantly, a triple-state color-switchable random laser with yellow, red and yellow-red dual-colors can be flexible, and is obtained by simply moving the pump position. The results may promote the practical applications of random lasers in the fields of sensing, in vivo biological imaging, and high brightness full-field illumination.

A simple technique for measuring thickness distribution of transparent plates from a single image by using the sampling moiré method

Abstract: A simple measurement technique for the thickness distribution of transparent plates is proposed by using the sampling moire method. The thickness distribution is automatically analyzed by measuring the phase difference of the moire fringe obtained from a single image, consistent with light refraction. The relationship between the actual thickness and phase difference of the moire fringe is calibrated by using glass plates of known thicknesses. The thickness measured by the present method was found to be in agreement with measurements using a digital micrometer. In our experiment, the average error in glass plate thickness was 1.9% over a 3.5 mm measurement range. Experimental results indicate that the present method can be useful for nondestructive measurements of the thickness distribution of various transparent plates.