Measurement and analysis of reversible transformations of vortex-free and singular light fields produced by PDLC cells

In this work, we have firstly realized and investigated reversible transformations of vortex-free and singular optical complex landscapes appearing in a He-Ne laser beam transmitted through a PDLC cell, which is controlled by an applied DC voltage. Biaxes of all micro size LC domains are oriented approximately along the applied electric field, and vortex-free complex landscape exists at 200 V. At 160 V, biaxes orientation spreads, and the first ‘embryo’ of vortices in the form of a smooth phase step appears. The first vortex pair nucleates at 140 V. Multitude of vortex pairs is realized when electric field is switched off. Reversed evolution is realized when the applied DC voltage grows from zero value to 200 V. It was shown that the observed topological evolution is fully defined by changes in structure and localization of real and imaginary zero-intensity lines for output complex light field. The developed technique of parametric dynamics can be applied successfully to realize and investigate various types of controlled optical elements including media with nanoadditions

Optical vortices and the flow of their angular momentum in a multimode fiber

The problem of propagation of optical vortices in multimode fibers is considered. The structural changes experienced by the wave and ray surfaces in their transformation from the free space to a fiber medium are determined. The continuity equation is obtained for the flow of the vortex angular momentum in an unhomogeneous medium.Розглядається проблема розповсюдження оптичних вихорів в багатомодових волокнах. Показано які структирнихзмін дізнають хвильова і променева поверхні при переході із вільного простору в середовище волокна. Записане рівняння неперервності для потоку кутового момента вихоря в неоднорідному середовищі.Рассматривается проблема распространения оптических вихрей в многомодовых волокнах. Показано какие структурные изменения испытывают волновая и лучевая поверхности при переходе из свободного пространства в среду волокна. Записано уравнение непрерывности для потока углового момента вихря в неоднородной среде

Detection and metrology of optical vortex helical wave fronts

Spatial structure of optical vortex helical wave fronts is for the first time directly tested using various interference arrangements and precise measuring techniques. Experimental data are compared with simulation results. On this base, advantages of interference technique of any kind for testing of optical vortices in solving the specific problems are revealed

Spin-orbit interaction in a generic vortex field transmitted through an elliptic fiber

Any small external perturbation on an ideal round optical fiber induces cross - section deformation and transforms eigen guided vortices into generic vortex fields, which can change their structure and angular momentum. It is the properties of these vortical fields in an elliptic weakly guiding optical fiber that we consider in this paper. The eigenfunctions and the spectrum of polarization corrections to the scalar propagation constant in the case of relatively large and small values of a fiber ellipticity are obtained by means of the spin-orbit interaction operator method. Discussed is the conversion process of a spin and orbit angular momenta on a vortex propagation along a deformed fiber

Synthesis, recording and metrology of laser beams with phase singularities

The main task of the work was obtaining the high efficient diffraction elements to generate high quality laser beams with phase singularities. The method of recording such diffractive structures on the organic photoresist and further replication to the transparent and reflective materials is described. Off-axis and on-axis holograms of optical vortices with different topological charges were computer generated and recorded. The exposure characteristics of the photoresist Shipley S1800 was investigated. The maximum achieved diffraction efficiency was 30 % (for the normal incidence case). The quality of the intensity and phase distribution was investigated. As criteria of the beam quality we accept the relative coincidence between experimental and theoretical data. We assume that ratio more than 0.8 indicates the good fitness. For this assumption over 90 % of the beam energy produced by our computer generated hologram (CGH) concur with theoretical distribution. Our diffraction elements recorded on photoresist are much better that ones recorded on silver emulsion. They were successfully used in the experimental investigations in our laboratory

Dispersions of carbon nanotubes in cholesteric liquid crystals: features of aggregate formation

For liquid crystal dispersions of carbon nanotubes (CNT), experimental data on time stability of their characteristics were obtained by three different methods (optical transmission vs. temperature, electric conductivity vs. time, and electric conductivity vs. voltage in conditions of Freedericksz transition). The results clearly suggest that induction of helical twisting in the nematic matrix substantially slows down the process of CNT aggregation, with cholesterol derivatives being more efficient as compared with non-steroid optically active dopants. This allows considering such dispersions as promising functional materials with enhanced time stability