E. A. Melnikova

Controlled diffraction liquid-crystal structures with a photoalignment polymer

The characteristics of electric-field-controlled diffraction elements based on a nematic liquid crystal and a new photoalignment polymer have been studied. The photocross-linking of this polymer allows periodic oriented structures and liquid crystal phase gratings with a 30% diffraction efficiency in the first order to be formed.

Optical switch based on the electrically controlled liquid crystal interface.

The peculiarities of the linearly polarized light beam reflection at the interface within the bulk of a nematic liquid crystal (NLC) cell with different orientations of the director are analyzed. Two methods to create the interface are considered. Combination of the planar and homeotropic orientations of the NLC director is realized by means of a spatially structured electrode under the applied voltage. In-plane patterned azimuthal alignment of the NLC director is created by the patterned rubbing alignment technique. All possible orthogonal orientations of the LC director are considered; the configurations for realization of total internal reflection are determined. The revealed relationship between the propagation of optical beams in a liquid crystal material and polarization of laser radiation has enabled realization of the spatial separation for the orthogonally polarized light beams at the interface between two regions of NLC with different director orientations (domains). Owing to variations in the applied voltage and, hence, in the refractive index gradient, the light beam propagation directions may be controlled electrically.

Electrically controlled waveguide liquid-crystal elements

Liquid-crystal (LC) elements with an electrically controlled spatial topology of director orientation have been developed and manufactured. Waveguide light-beam propagation mode in a spatially structured LC cell has been realized for the first time; the possibility of creating electrically controlled waveguide dividers and adders has been demonstrated experimentally.

Controlling light fields by means of liquid-crystal diffraction elements

Methods of correcting light fields by means of optoelectronic diffraction elements based on an oriented layer of nematic liquid crystal and photopolymeric material, making it possible to form anisotropic phase diffraction structures, are experimentally investigated.

Electrically controlled spatial-polarization switch based on patterned photoalignment of nematic liquid crystals.

A switching scheme for two orthogonal modes of laser radiation that is based on the total internal reflection effect realized at the interface of two liquid crystal regions with orthogonal director orientations is proposed. To create the photorefractive interface within the bulk of a liquid crystal, an original technique based on self-alignment of azo dye photoalignment and absorbing electrode patterns has been developed. Spatial separation of the orthogonally polarized light beams and their switching (when the positions of reflected and transmitted light beams are switched) due to the voltage applied has been experimentally realized.

Methods of creation and optimization of anisotropic liquid-crystal photonic structures

In this paper the techniques to create the liquid-crystal (LC) photonic structures with the controllable configuration of anisotropy are proposed. The regularities in propagation and transformation of the light beams in such structures are analyzed. Experimental implementation of the waveguide propagation and signal multiplexing regimes based on the total internal reflection of radiation from the electrically controlled interface of two LC regions with planar orthogonal orientation of the director, and also the formation of waveguide channels due to the graded index distribution on application of the voltage to the LC cell electrodes is described.

Optical limiters for laser radiation based on reverse saturable polymethine dyes

The paper presents an optical device that permits dynamic limiting of the power of nanosecond pulsed laser radiation. Different possibilities to control the characteristics of optical limiters based on reverse saturable polymethine dyes with the use of schemes including lenses and diaphragms have been analyzed. It is proposed to change the geometric position of a dye cell relative to the focal region to control an operation threshold of the limiter. It is established that the efficiency of optical limiting may be improved using additionally the effect of thermal defocusing.

Laboratory works to study fiberoptic communication and processing systems

The laboratory works are meant to train the university students mastering modem information technologies and techniques based on optical data transmission and processing. These works involve the use of fiberoptic systems and transmission methods for optical signals. Students study the operation principles of fiberoptic communication and processing systems, optoelectronic components, one- and multimode fibers, optical input/output couplers, multiplexers, optical dividers and connectors.

Multiwave mixing and soliton-like spatial structures in photorefractive crystals

A theoretical model has been developed for multiwave mixing in media with photorefractive nonlinearity, making it possible to describe the process of interaction between the waves in the conditions exhibiting nonlinearities of different orders with due regard for the mixing geometry. Theoretically, the process of N-wave mixing at the N - 1th-order nonlinearity has been described with the help of a band model for the photorefractive nonlinearity mechanism that includes the transitions from impurity levels within the band gap and enables description of the diffusion or drift processes in the external electric field. The energy efficiency of multiwave mixing and geometric parameters of spatial solitons in photorefractive Bi12TiO20 crystals in the conditions of pulsed and continuous laser excitation have been studied experimentally. It has been found that switching-on of the photorefractive nonlinearity mechanism with 532 nmwavelength laser pulses requires a time interval in excess of 20 - 50 ns, with saturation beyond 80 ns. The formation dynamics of spatial solitons in photorefractive Bi12TiO20 crystals has been analyzed with the use of continuous-wave radiation of a He-Ne laser. It has been determined that the formation conditions and dynamics are influenced by a number of factors including the geometry of radiation input into a crystal, power of the light beam, orientation of its polarization in the directions of the crystal axes, applied electric field and its direction.

Laboratory practical work on optical data processing

A laboratory bench has been developed and a laboratory manual has been prepared for the practical course in optical data processing including the following laboratory exercises: “Spatial Filtering of Images”, “Optical Bistability”, “Intensity Self-Oscillations and Optical Chaos”, “Logic Optical Elements”. These laboratory exercises enable the students to study the analog and digital information conversion methods and the effects underlying them. The laboratory bench was created on the basis of the electrically controlled liquid-crystal elements and involved a computerized recording system for temporal and spatial characteristics of light beams.