Showing papers by "Nirmal K. Viswanathan published in 2010"

Generation of tunable chain of three-dimensional optical bottle beams via focused multi-ring hollow Gaussian beam.

Abstract: We report here the generation of a chain of three-dimensional (3-D) optical bottle beams by focusing a π-phase shifted multi-ring hollow Gaussian beam (HGB) using a lens with spherical aberration The rings of the HGB of suitable radial (kr) and axial (kz) wave vectors are generated using a double-negative axicon chemically etched in the optical fiber tips Moving the lens position with respect to the fiber tip results in variation of the semi-angle of the cones of wave vectors of the HGBs and their diameter, using which we demonstrate tunability in the size and the periodicity of the 3-D optical bottle beams over a wide range, from micrometers to millimeters The propagation characteristics of the beams resulting from focusing of single- and multi-ring HGBs and resulting in a quasi-non-diffracting beam and a chain of 3-D optical bottle beams, respectively, are simulated using only the input beam parameters and are found to agree well with experimental results

System and Method for Generating an Optical Vector Vortex Beam

Abstract: Optical vector vortex beams may be generated from few-mode or multi-mode optical fiber by introducing linearly or circularly polarized light into the fiber. By adjusting at least one of the polarization of the light, the incident angle of the light coupled into the fiber, and the length of the fiber, one or more vector modes supported by the fiber may be selected. The resulting output from the fiber may be a vortex beam that can be collimated and used for precise manipulation of objects on order of a micrometer or a nanometer in diameter.

Generic dark hollow beams using negative cones chemically etched in fiber tips

Abstract: We report here the characteristics of dark hollow beams (DHBs) generated using negative micro-axicons fabricated via selective chemical etching process in the tip of optical fibers. The DHB output from the etched fibers were further manipulated to generate single and multiple period optical bottle beams, 0 th - and 1 st - order Bessel beams and astigmatic Bessel beams and beams with helical wavefront. Selective excitation of the guided modes in single and multimode optical fibers with V-numbers ranging from 2.405 – 5.69 are etched to different negative cone dimensions and are used to generate in a controllable way DHBs and related optical beams. Negative cones are etched in the fiber tips via selective chemical etching process wherein the different etch rates of the fiber core and cladding dopants and dopant concentration results in different cone angles and cone depths. Input laser beam guided through the optical fiber diffracts and refracts at the tip generating beams with one or many bright rings surrounding the dark central spot. By positioning the DHB in the front focal plane of a bi-convex lens we generate Bessel beams whose characteristics are compared with that generated using positive cone etched in the fiber tip and bulk axicon. Under certain excitation conditions we also observe helical wavefront DHBs with phase dislocation embedded in the beam.

System and method for generating an optical vector vortex beam having two lobes

Abstract: Optical vector vortex beams may be generated from few-mode or multi-mode optical fiber by introducing linearly or circularly polarized light into the fiber. By adjusting at least one of the polarization of the light, the incident angle of the light coupled into the fiber, and the length of the fiber, one or more vector modes supported by the fiber may be selected. The resulting output from the fiber may be a vortex beam that can be collimated and used for precise manipulation of objects on order of a micrometer or a nanometer in diameter.

Rotational frequency shift in cylindrical vector beam due to skew rays in few-mode optical fibers

Abstract: We report here the demonstration of rotational Doppler Effect (RDE) and measurement of rotational frequency shifts (RFS) in single-charge helical-phased cylindrical vector beams directly generated using a two-mode optical fiber. The vector-vortex beam with a shifted vortex core, generated by propagating the Gaussian laser beam as an offset-skew ray selectively excites both the fundamental and first low-order waveguide modes simultaneously in the two-mode optical fiber. Rotation frequency of the output beam around a shifted axis of the beam is measured as a function the analyzer rotation for changing handedness of the input circular polarization to demonstrate RDE in the directly excited cylindrical vector-vortex beams. Even small variations in the input launch conditions were found to dramatically alter the stability of the vortex beams and hence the demonstration of RDE.

Predicting the Decay of Fiber Bragg Gratings from Their Growth

Abstract: We report the development of a model that predicts the decay of fiber Bragg gratings based on growth data and validate the same using experimental data obtained from accelerated aging studies.