M. A. Bolshtyansky

Stabilization of the speckle pattern of a multimode fiber undergoing bending

The propagation of light in a multimode optical fiber waveguide results in speckle, a complicated mapping of the input field onto the output field. The speckle pattern changes as the fiber undergoes twists and bends. We show theoretically and demonstrate experimentally that changes in the mapping caused by bends in one region of the fiber can be compensated with bends applied along a fixed region of the fiber. In this way the output speckle pattern of the fiber can be stabilized. In this method it is supposed that the modes evolve adiabatically without intermixing and that the analytical dependence of perturbed propagation eigenvalues of the modes follows R(-2) behavior, where R is the local value of bend radius.

Polarization effects on induced chi (2) tensor properties in bulk glass

The authors explored tensor properties of chi (2) gratings induced in bulk glass under different polarization conditions. Three cases were studied: recording of gratings by waves with parallel, orthogonal and 45 degrees -tilted polarizations. The results obtained point out the presence of a static electric field recorded in proportion to E(EE).

Use of skew rays in multimode fibers to generate speckle field with nonzero vorticity

Beams at the output of an axially symmetric multimode fiber were produced with mostly positive or negative values of angular index m through skew illumination of the input end of the fiber. In this way rays with predominantly positive or negative helicity were excited inside the fiber. The output cw speckle field at the wavelength lambda = 632.8 nm contained large densities N(+) and N(-) of wave-front dislocations, or optical vortices, of both positive and negative sign. Vorticity was defined as V = N(+) - N(-) , was expressed analytically through the Wigner function of the field, and was found experimentally to be in good agreement with theory.

BRIEFING: beam reconstruction by iteration of an electromagnetic field with an induced nonlinearity gauge

Measurement of the local intensity and phase distributions in the small focal waist of a laser beam with the use of a thin nonlinear-optical layer is discussed. We suggested and tested in a computer experiment the beam reconstruction by iteration of an electromagnetic field with an induced nonlinear gauge (BRIEFING) algorithm, which allows one to extract the absolute intensity (in watts per square centimeter) and phase distributions from two intensity patterns in the far-field zone: with and without nonlinearity.

Transmission of the image signal with the use of a multimode fiber

Two schemes of the image acquisition through a multimode fiber are suggested. Both are based on the assumption that the input-to-output propagation Green function is stabilized by some or other technique. In both schemes the object is characterized by the transverse distribution of reflection pattern, and the reflected signal is to be collected by a photodetector for further computer processing. The applications for endoscopy are discussed.

Phase properties of χ(2) gratings in glass

A new method for the measurement of the phase shift between second-harmonic light generated by a bulk glass and the injected second-harmonic seed is presented. A linearly polarized pump and a second-harmonic seed with noncollinear polarizations were used for preparation of cerium-doped lead-silica glass. Independence of the phase shift from the angle between the polarizations of the recording waves is demonstrated, and a phase shift value close to 90 degrees is observed, i.e., the X((2)) grating was in phase with the real ?E(EE)?.

BRIEFING: Beam reconstruction by iteration of electromagnetic field with induced nonlinearity gauge

Measurement of the local intensity and phase distributions in the small focal waist of a laser beam with the use of a thin nonlinear-optical layer is discussed. We suggested and tested in a computer experiment the beam reconstruction by iteration of an electromagnetic field with an induced nonlinear gauge (BRIEFING) algorithm, which allows one to extract the absolute intensity (in watts per square centimeter) and phase distributions from two intensity patterns in the far-field zone: with and without nonlinearity.

Correlation dimension measurement of a process trajectory by photorefractive crystal

A scheme for correlation dimension measurements is suggested. It is based on the holographic grating recording in a photorefractive crystal and can operate in a real-time scale without restrictions on the values of the dimension measured.

Second-order polarizability gratings in glass

Optically induced second harmonic generation in glasses with large PbO content was studied. Tensor and phase properties of encoded (chi) (2) susceptibility indicate the presence of electrostatic field, which is proportional to E(EE) where E is a sum of real recording waves E(omega ) and E2(omega ). The model describing charge separation as a result of (chi) (3) optical rectification is discussed.