Spectral and temporal study of picosecond-pulse propagation in a single-mode optical fibre

The power-dependent pulsewidth variations for input 85 ps pulses from a cw mode-locked Nd: YAG laser propagating through 125 m of single-mode optical fibre have been examined using a Synchroscan streak camera. Simultaneous spectral and temporal measurements provide information as to the optimum parameters for pulse compression in fibres.     

Time-dependent propagation of high energy laser beams through the atmosphere

The computation of time-dependent three-space-dimensional laser beam propagation is described. The methods are applicable to the propagation of high energy laser beams through the atmosphere in the presence of a horizontal wind and turbulence for most situations of interest. Possible cases are propagation of cw beams through stagnation zones, multi-pulse propagation, including the self-consistent treatment of pulse self-blooming, and propagation involving transonic slewing. The solution of the Maxwell wave equation in Fresnel approximation is obtained by means of a discrete Fourier transform method, which, surprisingly, gives excellent results for diffraction problems. The latter provide a stringent test for the accuracy of any solution method. Considerable use is also made of discrete Fourier transform methods in solving the hydrodynamic equations. The treatment of turbulence is based on the generation of random phase screens at each calculation step along the propagation path. In a time-dependent calculation the random phase screens can be either made to move with the wind at a given propagation position or generated anew for each successive time. This work was done under contract to the Advanced Research Projects Agency of the Department of Defense, the Army Missile Command, Huntsville, Alabama, and the U.S. Energy Research and Development Administration.     

Light propagation in twisted dielectric media

A periodically stratified medium is investigated. Especially, we use a dielectric tensor, which describes an optically uniaxial crystal, whose axis lies obliquely in space and rotates about thez-axis as function ofz. The interpretation of the results differs considerably from that for another often treated kind of anisotropy.     

Reverse saturable absorption and optical limiting in indanthrone dyes

Fluence dependence of transmission in three dyes, viz. indanthrone, dichloroindanthrone and violanthrone, is reported at 532 nm as well as at 1.06 µm. These dyes show optical limiting at both these wavelengths for nanosecond pulses. The limiting is due to the combined effect of reverse saturable absorption from the triplet state and thermal defocussing in the dye solutions.     

Time-dependent propagation of high-energy laser beams through the atmosphere: II

Various factors that can effect thermal blooming in stagnation zone situations are examined, including stagnation-zone motion, longitudinal air motion in the neighborhood of the stagnation zone, and the effects of scenario noncoplanarity. Of these effects, only the last offers reasonable hope of reducing the strong thermal blooming that normally accompanies stagnation zones; in particular, noncoplanarity should benefit multipulse more than cw beams. The methods of treating nonhorizontal winds hydrodynamically for cw and multipulse steady-state sources are discussed. Aspects of pulse “self-blooming” are also considered. Work was performed under the auspices of the U. S. Energy Research and Development Administration under contract W-7405-Eng-48, U. S. Navy Contract N00014-76-F-0017 and U. S. Army Contract W31U31-73-8543.     

Spectral and temporal investigations of nonlinearities in a non-polarization preserving single-mode optical fibre

Intense green and red components generated in a non-polarization preserving optical fibre using a Q-switched and mode-locked cw Nd: YAG laser have been examined spectrally and temporally with picosecond resolution. In addition to the red (four-photon interaction) and green (second-harmonic) components, many other frequency mixing processes were detected. From theoretical considerations, the red component (anti-Stokes) which is generated through parametric four photon mixing should be in the same propagation mode as the Stokes component (ir) and this was confirmed experimentally.     

Utilization of photoreversible optical nonlinearities intrans-cis photochromic molecules for spatial light modulation

In this paper we examine the nonlinear optical properties of molecular systems undergoing acis-trans photoisomerization process. Such a process gives rise to dynamic photochromism on a picosecond time scale significant for designing a molecular Spatial Light Modulator (SLM).A kinetic analysis of a generalcis-trans nonlinear organic absorber is performed. The analysis is used to derive an expression for the imaginary part of the complex nonlinear molecular index of refraction. The properties of a molecular SLM based on such an absorber are discussed by examining the azo-type molecular system. These calculation predict an optimum contrast ratio and fast on-off switching for the SLM performance, achieved at low read-write laser intensities.     

Transient optical properties of excimer-laser-irradiated polyimide

Reflectivity, transmission and scattering measurments in ArF (193 nm) excimer-laser irradiated polyimide are reported. The time dependence of transient changes in all three quantities were measured together for the first time, confirming a transient refractive-index change in the first 150 ns after irradiation. The relaxation time of the effect was measured and found to consist of both a fast and slow component similar to that previously reported in acoustic wave measurements. The fast decay in about 60 ns and the slow decay is 115 ns. Several potential mechanisms are examined which may explain the result.     

Transient optical properties of excimer-laser-irradiated polyimide

Experiments involving time-resolved scattering and transmission in ArF (193 nm) excimer-laser-irradiated polyimide are reported. Scattering experiments determined that constituents of the carbon layer responsible for laser-induced electrical conductivity form between 150 and 400 ns after the excimer pulse. Transmission experiments indicate that absorbing carbon is present before the formation of organized crystallites responsible for electrical conduction and scattering.     

Solutions for the propagation of light in nonlinear optical media with spatially inhomogeneous nonlinearities

In this paper, we present solutions for the nonlinear Schrödinger (NLS) equation with spatially inhomogeneous nonlinearities describing propagation of light in nonlinear media, under two sets of transverse modulation forms of inhomogeneous nonlinearity. The bright soliton solution and Gaussian solution have been obtained for one set of inhomogeneous nonlinearity modulation. For the other, bright soliton solution, black soliton solution and the train solution have been presented. Stability of the solutions has been determined by exact soliton solutions under certain conditions.     

On a fourth-order wave equation for EM propagation in chiral media

A fourth-order wave equation is derived to study the propagation of an electromagnetic (EM) wave in a medium composed of chiral objects. This approximate wave equation does not reflect the handedness of the medium directly, which is more readily apparent, however, in a pair of two coupled second-order wave equations obtaineden route.     

Beam propagation through parabolic-index waveguides with distorted optical axis

An integral expression is derived for the fields in parabolic-index waveguides with distorted optical axis. It is based on well-known ray matrix techniques and generalized Huygens integrals. The distortions give rise to beam oscillations which in turn are identified as the excitation mechanism for radiation modes. The model gives some better understanding of the transition regions between waveguides. On leave from the Department of Electronics onthe Weizmann Institute of Science, Rehovot, Israel     

Some characteristics of resonant electromagnetic modes in a dielectric sphere

A set of formulae have been developed for calculations of the optical characteristics of droplets under conditions of whispering gallery mode (WGM) excitation. The WGM excitation is shown to cause the ripple structure in the Mie extinction cross section. The Chýlek equation for the distance between two neighboring morphology-dependent resonances (MDRs) in a spectrum has been revised. The equation is presented for the spherical-resonator finesse allowing for light scattering on inhomogeneities of the resonator surface, etc. A comparison of the theory and data in the literature is made and some peculiarities of spectra are explained.     

Pump-pulse induced chirp production in semiconductors

Nonlinear absorption and refractive index changes in wide-gap semiconductors (CdS, CdSSe, and GaSe) after ps light irradiation are determined with an excite-and-probebeam spectrometer. Based on a simple model pump-pulse induced chirp parameters on a signal pulse of different wavelength are calculated. Possible applications are discussed.     

Optical beams in lossy non-local Kerr media

It is discussed that optical beams propagate in non-local Kerr medium waveguides with losses. A variational principle is carried out for the 1 + 1-D non-local non-linear Schrödinger equation in the presence of the losses. In the strongly non-local case, the approximate analytical solutions are obtained. The lossy soliton solution shows that, Unlike its local counterpart, such lossy strongly non-local soliton does not possess the adiabatic property anymore. In addition, the general approximate results for non-soliton cases are gained. The comparisons between our approximate analytic solutions and numerical simulations confirm our variational approximate solutions.     

Modulational instability of optical beams in photovoltaic and photorefractive media due to two-photon photorefractive effect under open circuit condition

The modulational instability of broad optical beams in two-photon photorefractive (PR) photovoltaic materials under open circuit conditions has been investigated. Under linear stability framework, the one dimensional modulational instability growth rate has been estimated by considering the space charge field. Gain of the instability is shown to exist only when the photovoltaic fields orients in the same direction with respect to the optical c-axis of the medium. It is found that the behavior of the gain spectrum is different in low and high power regions. We have found by numerical simulations that the evolution of the soliton induced by the modulational instability at low photovoltaic field show the dynamical behaviors similar to those of the localized beam as the initial profile. However, it has been shown that increasing photovoltaic fields produce traveling, breathing, and mutually interacting solitons.     

Thermally-induced optical bistability with two holding beams

Thermally-induced optical bistability with two holding beams has been experimentally investigated in a metal-dielectric-metal interference mirror. Steady-state responses different from the usual S-shaped curve and with potential interest for composite logic operations are reported.     

Off-resonance dispersion in molecular double resonance experiments

This paper analyses effects related to the real part of the gaseous electric susceptibility in microwave-microwave and infrared-microwave double resonance experiments on two transitions with a common level. The most interesting features are obtained when the gaseous refractive index is probed far off resonance, i.e. in the tail of the dispersion characteristic and when the pump frequency is scanned around the resonance: a resonant change of the refractive index for the probe field is observed, the shape of which is that of the saturated pump line. A calculation using a model of strong collisions gives results in good agreement with the experimental ones. Applications of these phenomena to the determination of frequencies and transition matrix elements of infrared lines are discussed in the general case of lack of overlapping of the laser gain profile and of the infrared line. It is proposed to use these dispersion effects to detect Doppler-free infrared two-photon molecular absorption.     

Dynamical tunneling of counterpropagating beams mediated by an optical photonic lattice

In a numerical study, we demonstrate the dynamical tunneling (DT) of two counterpropagating (CP) mutually incoherent beams in a two-dimensional photonic lattice, recorded in a photorefractive (PR) crystal. The beams are launched head-on from the opposite faces of a PR crystal in which an optically induced two-dimensional photonic lattice is established. The DT is caused by the spontaneous symmetry breaking of CP beams, which is induced by the nonlinear interaction between the beams and is mediated by the lattice. To observe DT we found no need to introduce a specific external tilt potential, as is done in the conventional Zener tunneling; the tilting is provided by the repulsive interaction between the beams, which causes ejection of one beam from the launching region of the other. As the beams propagate, they move laterally in real time, causing the leakage of radiation from the first Brillouin zone to the second and higher zones. In the process the beams also tunnel from the first photonic band zone to the higher zones, which by definition is the DT.     

Reverse-saturable absorption in aluminophthalocyanine-doped xerogels

Aluminophthalocyanines were encaged in different silica xerogel matrices. Nonlinear reverse-saturable absorption behavior was observed for both nanosecond and picosecond pulse excitation at 532 nm. In order to determine the relaxation paths of the molecules after optical excitation, we measured the evolution of the transmission of a continuous probe beam through the samples following absorption induced by a pump pulse. The transmission recovery was a few tens of microseconds. The fluorescence lifetime was about 10 ns.