P. Senthilkumaran

Interferometric optical vortex array generator

Two new interferometric configurations for optical vortex array generation are presented. These interferometers are different from the conventional interferometers in that they are capable of producing a large number of isolated zeros of intensity, and all of them contain optical vortices. Simulation and theory for optical vortex array generation using three-plane-wave interference is presented. The vortex dipole array produced this way is noninteracting, as there are no attraction or repulsion forces between them, leading to annihilation or creation of vortex pairs.

Optical phase singularities in detection of laser beam collimation

Optical phase singularities, also called optical vortices, are known to have applications in various branches of optics. Here the role played by optical vortices in collimation testing is explained. Interference and a diffractive experimental setup in which the presence of an optical vortex permits collimation testing are presented. It is shown that the moire fringes that aid in collimation detection are due solely to the presence of vortices and not to the accompanying phase factors that are involved in producing a grating structure.

Tight focusing of vortex beams in presence of primary astigmatism

The tight focusing of vortex carrying beams is studied using the Debye-Wolf diffraction integral in the presence of primary astigmatism. The roles of topological charge, polarization distribution of the input beam, and handedness of the beam polarization are investigated in the intensity distribution of the focal plane of a high-numerical-aperture lens. The effect of tight focusing in the presence of astigmatism on the dark core of the azimuthally polarized beam is also investigated and compared with the dark core of a circularly polarized vortex beam. The effect of an aberration has been discussed in the context of the fluorescent spot size in the focal plane of a stimulated emission depletion microscope for two different polarization setups.

The effect of astigmatism on the diffraction of a vortex carrying beam with a Gaussian background

The effects of astigmatism and defocusing on the intensity distribution and encircled energy for a beam with an on-axis vortex embedded in a Gaussian background have been investigated using the Fresnel–Kirchhoff diffraction integral. A study has been carried out for two values of the topological charge for a truncated beam, and the stability of the optical vortices under the influence of aberration has been analysed. Some results are also presented for the influence of the truncation parameter of the beam. The aberration results in flattening of the dark core, and elongation of the core along a particular direction, thus resulting in a tail-shaped point spread function. Twofold symmetry is observed for selected defocused planes, and the diffraction pattern becomes more complex with an increase in the value of the aberration. The combined effect of astigmatism and coma on the PSF is also investigated.

Vortex array generation by interference of spherical waves

Interferometric methods of vortex generation involve the interference of three or more plane waves. We show that spherical wave interference can produce vortex lattices similar to the one produced in the three-beam interference of plane waves. Three spherical waves of the same curvature are made to interfere in a shear interferometer introduced in a Mach-Zehnder configuration.

Effect of primary spherical aberration on high-numerical-aperture focusing of a Laguerre-Gaussian beam.

Effect of primary spherical aberration on the tight focusing of linearly and circularly polarized Laguerre-Gaussian (LG) beams is studied by using the vectorial Debye integral. Results are presented for the intensity distribution and square of the polarization components. In the case of the linearly polarized LG beam with unit and double topological charge, the presence of aberration reduces the residual intensity at the focal point and spreads the sidelobes. If the beam is circularly polarized, the aberration results in an increase in the size of the dark core along with a reduction in the intensity at the periphery of the bright ring. The effect of aberration is also discussed in the context of the fluorescent spot size in the focal plane of a stimulated-emission-depletion microscope.

Double-wedge-plate interferometer for collimation testing : new configurations

Some new configurations of a wedge-plate interferometer are described that use two wedge plates for collimation testing with a built-in reference unlike a single wedge plate for which an external reference is required. Theory and measurement results are presented.

Selective edge enhancement using anisotropic vortex filter

In optical image processing, selective edge enhancement is important when it is preferable to emphasize some edges of an object more than others. We propose a new method for selective edge enhancement of amplitude objects using the anisotropic vortex phase mask by introducing anisotropy in a conventional vortex mask with the help of the sine function. The anisotropy is capable of edge enhancement in the selective region and in the required direction by changing the power and offset angle, respectively, of the sine function.

Fiber-optic Sagnac interferometer for the observation of Berry’s topological phase

Few experiments have been reported for the observation of Berry’s phase for the photon. This may be due to the difficulty of configuring suitable experiments. We start by repeating previous work on the observation of the optical activity of a helically wound fiber coil in the transmissive mode, but then we go on to demonstrate that when the light is reflected back through the same coil from a mirrored surface, it cancels this optical activity. We also report on a new experimental configuration in which a helical fiber coil is built into the loop of a Sagnac interferometer. The changes in Berry’s phase acquired by the two counterpropagating beams can be varied by stretching the coil with the result that fringes are produced at the output. One of the major benefits of this experimental setup is that it enables observations of Berry’s phase to be made for light in any state of polarization.

Adaptive helical mirror for generation of optical phase singularity

We report a specially designed adaptive mirror that can be bent into a helical shape for generation of an optical phase singularity. The adaptive helical mirror (AHM) reported here is a reflective device that can provide a continuous phase variation of the optical field in the azimuthal direction. The construction details and evaluation of the AHM are presented. A Michelson interferometer is used for the detection of the phase singularity. The AHM can be used for generation of a singular beam having multiple topological charges, positive or negative, just by controlling the excitation voltage of the AHM.