Lidiya Mikhaylovskaya

Edge diffraction of optical-vortex beams formed by means of the fork hologram

We present the experimental and numerical study of the transverse profile for a beam obtained by the screen-edge diffraction of optical-vortex (OV) Kummer beams with topological charges 1, 2 and 3, generated with the help of a “fork” hologram. The main results concern the behavior of the secondary OVs formed in the diffracted beam due to splitting of the incident multicharged OV into a set of single-charged ones. When the screen edge moves across the incident beam, OVs in every cross section of the diffracted beam describe complicated spiral-like trajectories, which distinctly manifests the screw-like nature and the energy circulation in the OV beam. At certain conditions, positions of the separate OVs as well as their mutual configuration (singular skeleton of the diffracted beam) shows high sensitivity to the screen edge dislocation with respect to the incident beam axis. This can be used for remote measurements of small displacements and deformations.

Singular skeleton evolution and topological reactions in edge-diffracted circular optical-vortex beams

Abstract Edge diffraction of a circular optical vortex (OV) beam transforms its singular structure: a multicharged axial OV splits into a set of single-charged ones that form the ‘singular skeleton’ of the diffracted beam. The OV positions in the beam cross section depend on the propagation distance as well as on the edge position with respect to the incident beam axis, and the OV cores describe regular trajectories when one or both change. The trajectories are not always continuous and may be accompanied with topological reactions, including emergence of new singularities, their interaction and annihilation. Based on the Kirchhoff-Fresnel integral, we consider the singular skeleton behavior in diffracted Kummer beams and Laguerre-Gaussian beams with topological charges 2 and 3. We reveal the nature of the trajectories’ discontinuities and other topological events in the singular skeleton evolution that appear to be highly sensitive to the incident beam properties and diffraction geometry. Conditions for the OV trajectory discontinuities and mechanisms of their realization are discussed. Conclusions based on the numerical calculations are supported by the asymptotic analytical model of the OV beam diffraction. The results can be useful in the OV metrology and for the OV beam’s diagnostics.

Displacements and evolution of optical vortices in edge-diffracted Laguerre–Gaussian beams

Based on the Kirchhoff-Fresnel approximation, we consider behavior of the optical vortices (OV) upon propagation of the diffracted Laguerre-Gaussian (LG) beams with topological charge |m| = 1, 2. Under conditions of weak diffraction perturbation (i.e. the diffraction obstacle covers only the far transverse periphery of the incident LG beam), the OVs describe almost perfect 3D spirals within the diffracted beam body, which is an impressive demonstration of the helical nature of an OV beam. The far-field OV positions within the diffracted beam cross section depend on the wavefront curvature of the incident OV beam so that the input wavefront curvature is transformed into the output azimuthal OV rotation. The results can be useful in the OV metrology and for the OV beams diagnostics.

Influence of discharge parameters on space charge in narrow gas discharge tubes

The expression for magnitude of a volume charge on an axis of a positive column of a glow discharge is obtained depending on parameters of discharge. The extreme dependence of a uncompensated positive charge at centre of a gas discharge tube from a density of electrons on an axis is established. The electronic temperature and gas pressure determine location of extremely (maxima) point.

Influence of thermal processes on the electron temperature in the active media of gas-discharge lasers

It was shown that in estimation of the electron temperature value in the gas-discharge positive column on the basis of the Schottkys relation, it is necessary to include not only pressure reduced value p0R0, but to account for the discharge system structure and the thermal regime of the whole discharge system during the discharge.

Influence of the electron-ion recombination on the positive column characteristics in narrow discharge tubes

The theoretical analysis of the influence of a volume electron-ionic recombination on energy and spatial characteristics of discharge plasma of positive column in narrow gas-discharge tube is developed. It is shown that under some conditions even in a diffusion mode of glow discharge this influence reduces to essential modifications of interior gas discharge parameters. The basic equations are: the equation of energy balance (in local approximation) of electrons in positive column for determination the intensity of the longitudinal electric field, the balance equation of the number of charged particles and the equation for the discharge current. The interior parameters of discharge plasma (electronic temperature and electron concentration on an axis) experience variations caused by transition of glow discharge from diffusion mode to volume at increase of gas pressure and discharge current. The numerical calculations for the discharge in helium are carried out.

Characteristic properties of the light-sound interaction in a “thick ” acoustic layer at orthogonal orientation of the interaction fields

The theoretical analysis of the case of normal incidence of light onto a traveling supersonic wave in the isotropic medium is developed. The investigation is based on solution of the general wave differential equations obtained from Maxwell equations. The case of moderate length of the acoustic-optics interaction area was considered. For this length, the conditions necessary for observation of the Raman-Nath diffraction as well as Bragg diffraction are not satisfied. The features of this diffraction regime are analyzed.

Influence of Discharge Parameters on Radial Distributions of Charge Particles in Positive Column in Narrow Gas Discharge Tubes

The calculations of the radial distributions of charge particles in positive column in narrow gas discharge tubes are presented. At that the balance equation and Poissons equation have been solved without condition of the quasineutrality of plasma. The calculations show that in narrow gas discharge tubes the concentration of the not compensated space positive charge at the center may be much greater than concentration of the electrons at the center

Characteristic properties of thermal regime in small diameter gas discharge tube

Machine solutions of the closed equations system for determination the intensity of the longitudinal electric field, electronic temperature and the temperature of the working gas that are stated in plasma of gas discharge of the one-component gas mixture in the case of diffusion regime of the glowing discharge are presented. Taking into consideration the wall sheath of space charge the thermal regime in a small gas discharge tube is examined.

Express-control method of frequency nonreproducibility sources in frequency-stabilized He-Ne lasers

The discharge current influence on the homogeneous broadening of the spectral lines in the gas-discharge media by direct collisions of the excited atoms with free electrons and also through collision broadening changing because of the gas heating by discharge current is considered. It was established that homogeneous broadening and hence the frequency shift of the spectral line depend essentially on the thermal condition of the laser work. The theoretical calculations are confirmed by the experimental measurements of the dependence of the homogeneous broadening of spectral line 3s2 - 2p4 neona in the He-Ne laser on the discharge current magnitude. For the homogeneous broadening determination by the experimentally measured Lamb dip characteristics a new computation method was developed, which can be used both in single-mode and two axial modes conditions of laser generation. It has been shown that on its basis and using known dependencies of the collision broadening of the spectral lines on the pressure, there can be realized an express-control of the main sources of the generation frequency unreproducibility in the sealed off discharge tubes, such as gas pressure and relative excitation of the laser system.