We report on an experimental study aimed to suppress the speckle structure produced by a coherent optical field. The technique proposed is based on the reduction of a laser output coherence utilizing enriching the emission spectrum lines. We achieved a temperature-controlled simultaneous emission of two components at 1.064 m and 1.066 m with nearly equal intensities in IR from diode-pumped Nd:YVO4 laser with intracavity second-harmonic generation The emission lines 532 nm, 532.6 nm and 533.1 nm were recorded in the second-harmonic output. The influence of the spectrum variation on the formation of a speckle field was checked. We succeeded to remove intensity zeroes and reduce the contrast (visibility) from 0.92 to 0.65 in a light scattered by a ground glass diffuser at the angle 35°. A simple consideration of the speckle field dumping mechanism is presented.
We demonstrated in theory and experiment superluminal properties of optical field propagation in the shadow area
behind an opaque disk (Poisson's spot). The wave front of the field in the shadow zone is initially delayed with respect
to unperturbed field. This phase shift gradually diminishes along the optical path, and therefore insures variation of the
phase velocity of the field at the axis. This fact was verified experimentally by the method of interference with a tilted
reference plane wave. A resulting excess of about c×10-5 was detected. Moreover, the result does not depend on the
frequency of light oscillations. It means that a non-monochromatic field (e.g. short light pulse) will propagate without
dispersion, what insures the equality of the group velocity to the phase velocity, both exceeding c.
Laguerre-Gaussian (LG) light beams possess discrete values of orbital angular momentum (OAM) of l&barh; per photon, where l is the azimuthal index of the mode. In principle l can take on any integer number, resulting in an unlimited amount of information that can be carried by any part of the beam - even a single photon. We have developed a technology demonstrator that uses OAM to encode information onto a light beam for free-space optical communications. In our demonstrator units both the encoding and decoding of the orbital angular momentum states is achieved using diffractive optical components (holograms). We use 9 different OAM values; one value is used for alignment purposes, the others carry data.
This work presents an analysis of optical vortex beams with integer and fractional topological charges, produced by binary computer-generated holograms. In the case of integer topological charge, diffraction of a Gaussian beam by the hologram creates an optical vortex beam with amplitude distribution, which can be described in the far field by the Kummer function. We introduced for such beams the name “Kummer beams”. An analysis of the optical vortex beam with 1/2 fractional topological charge shows that it can be represented in the far field as a superposition of a vortex-free wave and single-charged anisotropic optical vortex.
We present experimental results for passing of optical vortex (OV) through cylindrical lens. Process OV transformation by cylindrical lens doesn't considered in detail before. OV change the sign of topological charge after focus. At this situation emerged the question: 'What does happen with orbital angular momentum?' because the conservation laws of topological charge and orbital angular momentum must be safe. Answer on this question can give more information about propagation of OV and about of transformation orbital angular momentum within combined beams.
It is presented numerical simulations for Optical Vortex (OV) diffraction on the basis of Rayleigh-Sommerfeld solution of an edge diffraction problem and experiments that establish the effect of surviving and restoring of OV in a diffracted beam. Dependence of singular point position on screening investigated.
Currently, optical vortices (OVs) which are areas of circular motion of light flux in an electromagnetic wave, are being actively studied. Optical vortices can be divided into longitudinal OVs, where the axis of circular flow coincides with the direction wave propagation, and transversal OVs, where the axis is perpendicular to the wave propagation direction. Airy rings in a focal plane of a lens may serve as an example of transversal OVs. For both longitudinal and transversal OVs the OV axis is the line where the field amplitude is equal to zero. The phase is assumed to be uncertain, or singular, along the OV axis. We report a detailed study of a structure of a transversal OV created in an interference field of two two-dimensional (2D) Gaussian waves and the topological reactions which can happen in the interference field. A physical reason, which determines the direction of light flow circulation is discussed.
An optical device is proposed and realized for spatial separation into different channels of the Laguerre-Gaussian modes with even and odd mode indices. It was shown that a Gaussian beam diffracted by special computer-synthesized hologram attains in the first order of diffraction a single off-axis optical vortex, which may be represented as superposition of such modes. The proposed separator is able to create 'pure' axial vortex in one output channel and vortices- free beam in another channel. The obtained result is interpreted as a possibility to separate photons with zero and nonzero orbital angular momentum.
We analyze the properties of light beams carrying phase singularities, or optical vortices. The transformations of topological charge during free-space propagation of a light wave were presented in a theory and experiment. We report theoretical calculations of orbital angular momentum density (AMD) in a of combined beam, which is a superposition of LG01, LG10 and LG00 modes with different relative phase shifts and intensity ratio. We show that the total orbital angular momentum in a combined beam is constant during the propagation, in spite of any variation in the absolute value of AMD.
We report the nucleation of wave front phase dislocations in a Gaussian beam with initially smooth wave front experienced the self-action in a nematic liquid crystal cell. It is shown experimentally that the structure of the aberrational rings appeared due to inducing of a nonlinear lens in liquid crystal bulk has more complicated character than mentioned earlier.
It was shown that the well-known Sommerfeld's solution of the problem of a plane wave diffraction on a perfectly conductive half-plane screen can be expressed as a superposition of two waves, one being a plane wave with the amplitude twice smaller than the incident wave amplitude, and the other infinite-aperture wave with infinitely- extended edge dislocation. On the contrary with traditional components of the diffraction field, these wave can exist and propagate in free space separately with self-similar features.
Computer simulation of optical vortex diffraction on edge of nontransparent screen demonstrates the possibility for optical vortex to restore from the part of beam passing trough aperture. Regeneration of vortex is shown to take place for any part of the beam passing trough aperture, even if it does not contain vortex core.
Novel approach for the analysis of singularities in vector fields has been proposed. The essence of this approach is scalar consideration of the phase vortices at the orthogonal field components. The new type of vortices are introduced, namely the phase-difference vortices. The sign principle for the phase-difference vortices is formulated. An interconnection between the characteristics of the complete system of phase vortices associated with orthogonal field components, the behavior of an azimuth of a linearly polarized electrical field at an s-contour, and the polarization singularities, dislocations is established. The feasibility for comprehensive estimation of the characteristics of temporal singularities based on the measurement of the stationary field parameters is shown both theoretically and experimentally. Some elementary polarization situations are analyzed on the basis of the developed approach. The obtained results are extended to the case of quasi-uniform in polarization fields.
Polarization structure of vectoral optical fields is investigated. Interconnection among polarization structure of the field and the vortex networks at its orthogonal components is established.
We study the cross-talk between the competing holographic gratings in photorefractive cubic crystal Bi12SiO20. The cross-talk technique applies to measure a reduction factor which distinctly appears at low spatial frequency free electron distribution in diffusion regime under conditions so that direct observation of photorefractive gratings can not be performed. We suggest a hypothesis for the reduction factor of the carriers distribution with respect to light modulation is a long-distance photoelectron migration related to a high initial kinetic energy of the free carriers.
Recent measurements revealing new aspects of the nature and physics of space-charge waves in photorefractive Bi12SiO20 crystals are presented. A Bessel-function family of space-charge wave resonances are shown to exist for excitation by a frequency modulated running interference pattern. It is also demonstrated that space-charge waves may continue into regions of the crystal not subjected to the driving light field. Subharmonic space-charge gratings are shown to have an internal structure that for running waves moves with a group velocity opposite the driving optical interference pattern, which is moving with the phase velocity of the space-charge field and the associated refractive-index grating. We also describe how the subharmonics evolve dynamically from a speckle-like pattern towards a regular wave structure. Finally, self-excitation of spontaneous scattered beams ins described, and it is demonstrated that space-charge waves with a long decay time and a high Q-factor are associated with this phenomenon.
We show directly that a light beam carrying optical vortex rotates in space around the axis of propagation. The spatial rotation and related angular momentum result from the helical wavefront with axial phase singularity. Using Gaussian envelope of a 'singular' beam, we calculate the angular velocity of rotation. Experimental observation of light beam rotation is performed at the first time to our knowledge and confirms the theoretical predictions.
We show how to synthesize a wave with monochromatic mixed screw-edge dislocation. We obtained theoretically and experimentally that a mixed screw-edge dislocation embedded in a monochromatic Gaussian beam does not propagate as a self- similar stable object, but transforms into several pairs of opposite-sign optical vortices in near field, and produces single-charge optical vortices in far field.
Optical Vortex (OV), or wavefront screw dislocation represents a wavefront phase defect, where the wavefront attains a helicoidal shape. The wave possessing axial OV is one of the possible solutions of scalar wave equation. It means the expression for OV has in the simplest case a form E(r,(phi) ) varies direct as rexp(i(phi) ), where r, (phi) are cylindrical coordinates. Therefore OV is a discontinuity of the phase such that the phase circulation around its axes is an integral multiple of 2(pi) . The phase change for circulation around the axis may have different signs. It determines the topological sign of screw dislocation and correspondingly the helicoidal form of dislocation: with right twirl (positive) or left twirl (negative). Moreover, in a general case a wavefront around vortex axis may have a form of multistart helicoid, with a pitch m(lambda) , when (lambda) is the wavelength, and m is integer. The objective of this paper is to present experimental reversal of screw dislocation by four-wave mixing and to investigate the sign of dislocation within phase- conjugated beam with respect to the incident signal beam.
We provide a direct experimental observation of space charge waves in photorefractive crystals with point group 23 based on their penetration into an area with uniform light illumination. Those are the results of excitation of eigen modes of space charge filed excited by moving light fringes. We have found experimental conditions when the quality factor of the space charge waves attempts dramatical increase with respect to what current theory predicts. This leads to self- excitation of the waves and results in self-oscillation of light. The competition between space charge waves and fundamental grating results in appearance of K/2, K/4 components in the fundamental grating and to spontaneously occurring beams called subharmonics. The subharmonic beams are generated in a pattern of domains that evolve dramatically as they move through the crystal. The domains are result of non- homogeneity of the space charge waves and separated by narrow boundary regions, where the phase of subharmonics changes by pi. We experimentally prove the domains move with the group velocity of running space charge waves.
We report experimental and numerical investigation of optical vortices nucleation in a wavefront of a laser beam passed through LiNbO3 crystal with light-induced self- defocusing nonlinear lens and through SBN crystal with self-focusing nonlinear lens.
We describe main properties of optical wavefront dislocations, or optical vortices. The problem of vortices birth and annihilation is analyzed. Holographic method for vortices creation is discussed. We report at the first time four-wave mixing experiments with screw dislocation wavefronts.
We report experimental and numerical investigation of optical vortices nucleation in a wavefront of a laser beam passed through LiNbO3 crystal with light-induced self- defocusing nonlinear lens and through SBN crystal with self-focusing nonlinear lens.
We report experimental and numerical investigation of a process of ingenious optical vortices nucleation in a wavefront of a laser beam passing through photorefractive LiNbO3 crystal with self-induced nonlinear lens.
We report the result of investigation of the screw, edge and mixed edge-screw dislocations of wavefront in monochromatic light waves. Methods for the experimental determination of the topological charge value and helicity of screw dislocation are elaborated. By use of computer- synthesized binary gratings waves with edge, mixed screw-edge and anisotropic screw dislocation were obtained and studied experimentally.
The structure of laser beam with screw dislocations is discussed. The separation of m-order screw dislocation into m first-order dislocations is observed. Rotation of a dislocation array nested into Gaussian beam caused by the beam propagation is measured.
Winner-takes-all dynamical behavior of transverse structure in a large aperture dye laser with intracavity saturable absorber on bacteriorhodopsin controlled by bleaching and coloring external beams is demonstrated both theoretically and experimentally. As a result of mode competitive interactions in the multistable dye laser the strongest component of multimode structure incoherently initiated by uniform external bleaching beam suppresses the rest modes and the final stage of transverse structure evolution is the single-mode regime of operation.
The transverse nonlinear dynamics of an active system (laser with nonlinear saturable absorber in self-imaging cavity) are considered theoretically. The soliton-like light field structure formation as well as new effects of optical multistability originating from the transverse oscillation phase distribution are shown to be possible.
The transverse nonlinear dynamics of switchings in an active system (laser with nonlinear saturable absorber on bacteriorhodopsin in a self-imaging cavity) is studied both experimentally and theoretically. The soliton-like light field structure formation and continuously cycled self-switching process are investigated.
Spatio-temporal effects of thresholding and switching are investigated theoretically and experimentally in bistable single- and multimode cavities containing two nonlinear elements: photochromic absorber on bacteriorhodospin controlled by bleaching and coloring external beams and amplifier on organic dye pumped by periodic laser pulses.
Optically controlled bistability and signal competition are investigated theoretically and experimentally at an incoherent beam interaction in Fabry-Perot cavity containing two nonlinear elements: photochromic absorber on bacteriorhodopsin controlled by bleaching and coloring external beams and amplifier on organic dye pumped by periodic laser pulses.
This paper reviews some results of the modern investigations on holographic spectral elements based on volume phase gratings recorded in sensitized gelatin films. Physical principles, fabrication techniques, and main characteristics of the holographic spectral elements are discussed. Some applications of the holographic spectral elements for tunable laser sources are demonstrated.
We discuss the competition between oscillating waves in two ring cavities coupled by a common amplifier on a photorefractive crystal. We determine the threshold
conditions and steady-state intensity for each wave. Different steady-state regimes
of oscillations were revealed by analytical treatment. Conditions for single wave
oscillation and two waves simultaneous oscillation were found. Oscillation with a
bistable switching is also shown to be possible. Numerical simulation of transient
effects in the system has shown the possibility of controlling the oscillation
regimes by external optical signals
The results obtained may be of interest in the systems with optical information
processing.
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