Tunable middle infrared radiation with HgGa2S4 crystal

Optical properties of orange phased HgGa₂S₄ crystal is investigated. Generation of tunable middle infrared radiation by second harmonic of tunable CO₂ laser radiation has been demonstrated. Second harmonic conversion efficiency with respect to other infrared crystal has also been discussed.     

Third-order nonlinear susceptibilities of dye solutions determined by third-harmonic generation

The third-order nonlinear susceptibilities _xxx ⁽³⁾ (–₃; ₁, ₁, ₁) versus concentration are determined for rhodamine 6G, fuchsin, and methylene blue in methanol. Third-harmonic generation of a modelocked Nd-glass laser is applied. The real and imaginary part of the nonlinear susceptibility are measured. A resonance enhancement due to absorption bands is observed.     

On control of chaos in the Kerr lens mode locked Ti:Sapphire laser

In this paper, I present the numerical demonstration of a simple way to reduce the pulse duration of a Kerr lens mode locked Ti:Sapphire laser. The core of the method lies in the low dimensional deterministic chaos that displays the laser. By adding an acousto optic modulator and applying an adequate control algorithm, it is possible to reduce the pulse duration by a factor of two with a laser that otherwise operates in the range of 40 fs.     

Determination of nonlinear refractive indices by external self-focusing

The nonlinear refractive index of benzene is determined by measuring the reduction of the beam divergence of picosecond ruby laser pulses when passing through a benzene sample. Time-integrated spatial beam profiles give an effective refractive index while time-resolved beam profiles measured with a streak camera allow the determination of the temporal evolution of the nonlinear refractive index.     

Efficient multi-frequency generation of ultrashort light pulses using stimulated Raman scattering and optical parametric amplification

Optical parametric amplification of multi-frequency seed pulses generated in a mixture of compressed hydrogen and methane by stimulated Raman scattering of 1 ps, 1 kHz laser pulses at 395.8 nm has been studied. Efficient generation of spectrally narrow ultrashort pulses with a spatial distribution close to the Gaussian profile of the pump beam was obtained in the visible and near infrared ranges.     

Three-photon absorption and its limitation of third-order nonlinear optical effects in rutile

Three-photon absorption and three-photon-induced excited-state absorption of rutile are studied by transmission measurements using picosecond pulses of a mode-locked Nd: glass laser. The nonlinear absorption limits efficient stimulated Raman scattering. It reduces the efficiency of two-photon absorption of a picosecond probe continuum. Three-photon absorption coefficients, excited-state absorption cross sections, a Raman gain factor, and two-photon absorption cross-section spectra are determined. The arrangementsEc andE c are considered.     

Multiline coherent oscillation in photorefractive crystals with two species of movable carriers

The coherent oscillation, because of nearly degenerate four-wave mixing in photorefractive crystals with two types of movable charge carriers, occurs at two spectral lines symmetrically shifted with respect to the pump frequency. Consequently the output oscillation exhibits the high contrast intensity modulation. The frequency separation of two oscillation modes (and modulation frequency of the output intensity) depend on the incident light intensity and spatial frequency of the developing grating. A model is presented explaining this type of oscillation by the two-maxima shape of the gain spectrum in crystals with sufficiently different relaxation times of two space-charge gratings, one formed by movable electrons and the other one by movable holes. The experimental data for coherent oscillator with tin hypothiodiphosphate (Sn₂P₂S₆) are in reasonable quantitative agreement with the calculations. Received: 12 November 1998 / Revised version: 11 January 1999 / Published online: 7 April 1999     

Influence of gas circulation on stimulated Raman scattering and amplification of ultrashort laser pulses in methane

Applying gas recirculation in a high pressure cell, laser pulses of 1 ps at 400 nm and with a repetition rate of 1 kHz were frequency shifted by stimulated Raman scattering and amplification in methane gas at high pressure. We studied the influence of gas recirculation on the conversion efficiencies into the Stokes and anti-Stokes components as well as on their spatial distributions and spectral shapes using generator and generator-amplifier arrangements. For high pump energies, recirculation in the generator cell decreases conversion efficiency into the first Stokes component whereas it increases conversion into higher Stokes and anti-Stokes components. It results in a significantly improved spatial characteristics of the frequency-shifted radiation, however, is accompanied by a substantial spectral broadening. Using gas recirculation in the generator-amplifier arrangement we achieved a conversion efficiency into the first Stokes component of about 50% with highly improved spatial and spectral characteristics.     

Temporal behavior of dark spatial solitons in closed-circuit photovoltaic media

We show that the time-dependent nonlinear wave equation in closed-circuit photovoltaic media can exhibit quasi-steady-state and steady-state spatial solitons. We demonstrate that the formation time of open-circuit quasi-steady-state and open-circuit steady-state dark solitons decreases with an increase in the intensity ratio of the soliton, which is the ratio between the soliton peak intensity and the dark irradiance. We find that for the time-dependent nonlinear wave equation that exhibits only an open-circuit steady-state dark soliton, changing the electric current density J₀ does not generate quasi-steady-state dark solitons and affects the formation time of steady-state dark solitons and that for the time-dependent nonlinear wave equation that exhibits an open-circuit quasi-steady-state dark soliton, changing J₀ gives rise to three different time evolution regimes of the full width half maximum of the soliton’s intensity. The first regime shows that the formation time of steady-state dark solitons increases with J₀ whereas the formation time of quasi-steady-state dark solitons is independent of J₀. The second regime shows that the formation time of steady-state dark solitons decreases with an increases in J₀ and the formation time of quasi-steady-state dark solitons increases with J₀. The third regime shows that changing J₀ enables only steady-state dark solitons in the time-dependent nonlinear wave equation, of which the formation time increases with J₀.     

Optical modes in a nonlinear double-channel waveguide

We analytically address different types of optical modes in a coupler composed by two nonlinear optical waveguides. It is shown that the coupler not only supports symmetry-preserving modes but also symmetry-breaking modes. In addition, the properties on the existence and bifurcation of those modes are analyzed in detail.     

Effects of dark soliton crystal temperature on the self-deflection of bright soliton in a biased serial photovoltaic photorefractive crystal circuit

Based on the theory of one-dimensional separate soliton pairs formed in a serial photovoltaic photorefractive crystal circuit, the effects of the dark soliton crystal temperature on the self-deflection of the bright one in a bright-dark soliton pair are investigated. The numerical results indicate that the spatial shift of the bright soliton can experience obvious increase in their self-deflection with the increase of the temperature of the dark soliton. The self-bending process is further studied using perturbation techniques and the results are found to be good agreement with that obtained by numerical method.     

Defect vector gap solitons in photonic lattices

The existence and general properties of different kinds of defect vector gap solitons in one dimensional optically induced photonic defect lattice with focusing saturable nonlinearity in photorefractive crystal are analyzed. The defect is well localized in a single site with two existence forms, namely repulsive and attractive defect. Propagation constants of two beams that compose defect vector gap solitons could be from same gap or from different gaps. We show that some kinds of unstable scalar defect gap solitons could be stabilized by their corresponding vector cases.     

Self-compression and controllable guidance of multi-millijoule femtosecond laser pulses

Self-compression of multi-millijoule femtosecond laser pulses and dramatic increase of the peak intensity are found in pressurized helium and neon within a range of intensity in which the ionization modification of the material parameters by the pulse is negligible. The pulse propagation is studied by the (3 + 1)-dimensional nonlinear Schrödinger equation including basic lowest order optical processes - diffraction, second order of dispersion, and third order of nonlinearity. Smooth and well controllable pulse propagation dynamics is found. Construction of compressed pulses of controllable parameters at given space target point by a proper chose of the pulse energy and/or gas pressure is predicted.     

Gray spatial solitons in biased two-photon photovoltaic photorefractive crystals

This paper predicts that gray spatial solitons can exist in biased two-photon photovoltaic photorefractive crystals. Under appropriate conditions and in the steady state, the gray spatial solitons solution of the optical evolution equation is obtained. The properties associated with these solitons, such as their intensity profile, intensity full width at half-maximum, width, transverse velocity and phase distribution, are discussed as functions of their normalized intensity and degree of “grayness”. Relevant examples are provided.     

Self-steepening is an abrupt process

We discover that self-steepening occurs so fast with slight change in the input energy that it could be considered as an on-off process whose potential application could be far reaching. This observation was carried out by generating a stable uniform pattern of femtosecond filaments inside a methanol cell and measuring the evolution of the white light spectra. We found that the white light supercontinuum, a consequence of self-steepening, turned on almost instantaneously with respect to a very slight change in the energy.     

Quasi-two-dimensional dark spatial solitons and generation of mixed phase dislocations

This paper presents experimental evidence that orthogonally crossed dark soliton stripes form quasi-two-dimensional spatial solitons with a soliton constant equal to that of singly charged optical vortices. Besides the pairs of oppositely charged optical vortex solitons, the snake instability of the dark formation at moderate saturation is found to lead to generation of steering mixed edge–screw phase dislocations with zero total topological charges. Received: 26 October 1998 / Revised version: 19 January 1999 / Published online: 12 May 1999     

Rotating surface soliton complexes in annular waveguides

We address surface soliton complexes formed at the edge of annular guiding structures containing several concentric rings. Such soliton complexes feature a π-phase difference between neighboring spots. It is shown that the multipole-mode solitons can rotate steadily upon propagation, and the existence domain is strongly affected by the rotation frequency. The rotation may enhance the stabilization of surface multipole-mode solitons.     

Self-Deflection of Dark Screening Spatial Solitons Based on Higher-Order Space Charge Field

The effects of higher-order space charge field on the self-deflection of dark screening spatial solitons in biased photorefractive crystals are numerically investigated under steady-state conditions. The expression for an induced space-charge electric field including higher-order space-charge field terms is obtained. Numerical results indicate that dark solitons possess a self-deflection process during propagation, and the solitons always bend in the direction of the c axis of the crystal. The self-deflection of dark solitons can experience considerable increase especially in the regime of high bias field strengths.     

Steering one-dimensional odd dark beams of finite length

This paper presents arguments for the existence and dynamics of one-dimensional odd dark beams of finite length in bulk Kerr nonlinear media. Their characteristic mixed edge–screw phase dislocations force them to steer in space. The modulational stability of these dark beams and one possible interaction scheme are discussed. Received: 26 October 1998 / Revised version: 19 January 1999 / Published online: 12 May 1999