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.

Stabilization of transmission function: theory for an ultrathin endoscope of one multimode fiber.

Propagation of a monochromatic light through a multimode fiber is considered in connection with the problem of image transmission. We discuss the variations of throughput scrambling, i.e., of the pixel-to-pixel Greens' function for transmission under the bends of the fiber axis. Two main statements are elaborated theoretically, which in principle allow the stabilization of the throughput scrambling. First, if the deformations are smooth enough, the modes are adiabatically modified without intermixing. Second, the variation deltaphi(m,n) of the phase of a mode M (m,n) can be expressed as a power series in a small parameter (rho/R)(2), where rho is the core radius and R(-1) is the curvature of bending. A scheme for the compensation of those phase shifts is suggested based on prebending of the fiber with subsequent controllable adjustment of that prebending.

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.

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.

Adjustable half-wave plate.

An adjustable construction of a half-wave plate is described. The device consists of three birefringent plates with arbitrary thicknesses. Owing to the variable angles between the optical axes of these plates, it could be easily adjusted to a different wavelength of laser radiation.

Bound-bound transition in a field with a nonzero average cube in a molecule in a Langmuir-Blodgett film.

Interference of single- and two-photon transitions that is due to an E(2omega) + E(2omega) field may result in asymmetry of the excitation diagram and hence in reorientation of molecules in a Langmiur-Blodgett film.

Intensification of a photorefractive response by an external ac field in the ferroelectric crystal Ba(2)NaNb(5)O(15).

Static photorefractive gratings are recorded by a running interference pattern in the ferroelectric crystal Ba(2)NaNb(5)O(15) with an externally applied ac field. Both two- and four-wave mixing based on this recording mechanism are investigated.

Phase properties of X((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)?.

Nonlinear theory of nondegenerate beam coupling through phase-locked detection in an ac field.

Different frequency beams interact through the static grating recorded by their running interference pattern in an external ac field. The influence of the self-diffraction through the regular degenerate and nondegenerate interaction is studied theoretically. The depletion of all the waves is taken into account.

Nondegenerate two-wave interaction in a photorefractive crystal in an external detecting field.

A hologram recording mechanism in photorefractive Bi(12)TiO(20) crystal by different-frequency laser beams in an ac field is studied. The frequency difference between the beams is essentially more than the reverse of the response time of the hologram.

Polarization dependence of the phase of interference between single- and two-photon ionization.

The interference between single-photon and two-photon ionization has been observed for electrons ejected in the direction n from the photomultiplier cathode exposed simultaneously to the picosecond pulse of a Nd laser E(omega) and its second harmonic E(2omega). The change of the phase difference of such interference was measured for the switch of the polarizations from E(omega) ||n, E(2omega) ||n to E(omega) ? n, E(2omega) || n.

Second-order polarizability holograms in few-mode fibers: phase conjugation and angular selectivity.

Second-order polarizability holograms in optical fibers were recorded by picosecond radiation of a neodymium laser together with its second harmonic. High angular selectivity of holograms was observed for various input angles of the reading IR beam. We observed phase conjugation of the green signal by using an oppositely directly IR beam to read the holograms. Coherent erasure of a hologram was demonstrated by means of a second exposure by an interference pattern of opposite sign.

Resonant optical activity induced in gases by collisions with right-left nonsymmetrical molecules.

Frequency-dependent collision-induced circular dichroism and optical rotation are predicted for gases in the visible and infrared regions near resonance. For the appearance of those effects, the gas should contain right-left non-symmetrical molecules, and the pressure should correspond to the condition (l/lambda) ~ 1, where l is the free-path length. Possible right-left nonsymmetrical effects in nonlinear spectroscopy of gases at l >> lambda are mentioned.