Pulse Propagation In Waveguides Research Articles

Dispersive waves and radiation trapping in optical fibers with a zero-nonlinearity wavelength

The formation of dispersive waves and radiation trapping are two most relevant phenomena in supercontinuum generation. In spite the fact that the nonlinear coefficient of many waveguides depends strongly on wavelength and even changes sign, the influence of a frequency-dependent nonlinearity on these phenomena have seldom been studied. The goal of this work is to fill this gap in the literature by thoroughly studying the behavior of dispersive waves and radiation trapping in the presence of a zero-nonlinearity wavelength (ZNW). Although the generalized nonlinear Schrödinger equation (GNLSE) is widely used to model optical pulse propagation in waveguides, it has been shown that the GNLSE may lead to unphysical results in the case of frequency-dependent nonlinearities. For this reason, we perform our analysis by resorting to the newly introduced photon-conserving generalized nonlinear Schrödinger equation (pcGNLSE). Further, we discuss results obtained with both equations and put in evidence the inadequacy of the GNLSE to appropriately account for radiation trapping.

Ultrabroadband supercontinuum generation through filamentation in a lead fluoride crystal

We report the filamentation and supercontinuum generation of a femtosecond pulse in a piece of bulk lead fluoride (PbF2) crystal with a high bandgap and an ultrabroadband frequency window covering 5.6 octaves. A broadband supercontinuum spanning 4.7 octaves from 350 to 9000 nm is demonstrated. The filament traces and bright conical visible emission patterns of the supercontinuums are observed. Additionally, simulations are performed to investigate the supercontinuum generation in the PbF2 crystal by using a waveguide model that considers the supercontinuum generated in the filamentation as a process of pulse propagation in waveguides written by filamentation. The findings of this study demonstrated that a PbF2 crystal is a very suitable nonlinear medium for use in supercontinuum generation by filamentation. This is of significance for the development of ultrabroadband supercontinuum sources based on bulk media and high-peak-power lasers.

Solitary Waves in Chains of High-Index Dielectric Nanoparticles

We study both linear and nonlinear propagation of pulses in waveguides composed of high-index dielectric nanoparticles supporting both electric and magnetic resonances. We reveal that short pulses (∼100 fs) broaden significantly in the linear regime after propagating only several tens of micrometers, due to a strong waveguide dispersion. In the nonlinear regime, the pulses propagating in the chains of spherical nanoparticles broaden even more strongly than in the linear regime due to defocusing nonlinearity. However, for the chains of nanodisks the pulse broadening can be compensated by the nonlinear effect, due to the interplay of the electric and magnetic resonances that can change the sign of the group-velocity dispersion for some frequencies, making possible the formation and propagation of solitary waves and effective generation of the new frequencies. Our results demonstrate that considered systems can serve as a promising platform for nonlinear and ultrafast nanophotonics, allowing the observation ...

Simulation of ultrashort pulse transmission in a chain of fused silica microspheres

The paper is devoted to the simulation of ultrashort pulse propagation in waveguides of two types. The first type (type 1) represents an ordinary waveguide made of cylindrically shaped fused silica without coating. The second type (type 2) consists of microspheres made of the same fused silica following one another. A three-parameter Sellmeyer model is used to take into account the dependence of electrical permittivity on the frequency of radiation. The coefficients of pulse broadening and amplification as well as the coefficient of the pulse spectrum narrowing have been calculated. The numerical simulation using the FDTD method which takes into account the frequency dependence of the permittivity and implemented in the FullWAVE software package showed that there is no temporal broadening in the case of propagation of linearly polarized ultrashort pulse 3.55 fs long with a central wavelength of 532 nm in a waveguide consisting of a sequence of fused silica microspheres with the radius of 1 mkm, while there is a two-fold temporal broadening of the pulse in the case of transmission of this pulse in a conventional silica cylindrical waveguide.

Spatial effects in supercontinuum generation in waveguides.

An economic, space- and time-resolved method to model ultra-short, intense pulse propagation in waveguides is described. Simulations of supercontinuum generation on a chip demonstrate the utility of the approach. Comparisons with the generalized nonlinear Schrödinger equation elucidate spatial effects, which influence pulse dynamics and the generation of new spectral components.

Dispersion-to-spectrum mapping in nonlinear fibers based on optical wave-breaking

In this work we recognize new strategies involving optical wave-breaking for controlling the output pulse spectrum in nonlinear fibers. To this end, first we obtain a constant of motion for nonlinear pulse propagation in waveguides derived from the generalized nonlinear Schrödinger equation. In a second phase, using the above conservation law we theoretically analyze how to transfer in a simple manner the group-velocity-dispersion curve of the waveguide to the output spectral profile of pulsed light. Finally, the computation of several output spectra corroborates our proposition.

Dispersion of nonlinearity in subwavelength waveguides: derivation of pulse propagation equation and frequency conversion effects

Description of pulse propagation in waveguides with subwavelength features and high refractive index contrasts requires an accurate account of the dispersion of nonlinearity due to the considerable mode profile variation with the wavelength. The corresponding model derived from asymptotic expansion of Maxwell equations contains a complicated network of interactions between different harmonics of the pulse, and therefore is not convenient for analysis. We demonstrate that this model can be reduced to the generalized nonlinear Schrodinger-type pulse propagation equation under the assumption of factorization of the four-frequency dependence of nonlinear coefficients. We analyze two different semiconductor waveguide geometries and find that the factorization works reasonably well within large wavelength windows. This allows us to utilize the pulse propagation equation for the description of a broadband signal evolution. We study the mechanism of modulational instability induced by the dispersion of nonlinearity and find that the power threshold predicted by the simple model with three interacting harmonics is effectively removed when using pulses, while the efficiency of this process grows for shorter pulse durations. Also, we identify the effects of geometrical and material dispersion of nonlinearity on spectral broadening of short pulses in semiconductor waveguides.

Inversion of dispersive GPR pulse propagation in waveguides with heterogeneities and rough and dipping interfaces

Abstract We investigate the influence of interface roughness, heterogeneous media, and dipping layers on the inversion of dispersive GPR pulse propagation in a surface waveguide, using 3D FDTD modelling. For both broadside and endfire source–receiver configurations, we calculated responses for different interface roughnesses, heterogeneities in dielectric properties, and dipping interfaces. For increasing roughness and heterogeneity, increased backscatter energy is visible in the data. The use of multiple source–receiver offsets to calculate the phase-velocity spectrum produced a relatively good signal-to-noise ratio. For low interface roughness the medium properties could be reasonably well reconstructed. For the largest interface roughness, significant diffracted energy was present and the medium properties could not be reliably reconstructed. For models having stochastic relative permittivity variations with a Std up to 15% and correlation lengths between 0.1 and 0.5 m, the model parameters could still be relatively well reconstructed. For models with a dipping interface, the shot and countershot configuration clearly indicate lateral changes. Single-mode inversions return a wide range of medium property values, whereas combined TE-TM inversion return permittivity values that are remarkable close to the e 1 and e 2 values for both the shot and countershot configuration. In general, the interface roughness and heterogeneous media have a relative small influence on the inversion results. This is probably due to the use of multi-offset data for calculating the phase-velocity spectrum which is reducing the random noise.

Comments on 'Rapid pulsed microwave propagation' (and reply)

For the original article see ibid., vol.1, no.12, p.374-5 (1991). The commenters point out several errors in a paper by G.C. Giakos and T.K. Ishii in which they claim to have developed conclusive laboratory evidence that demonstrates microwave pulse propagation in waveguides, and in air, at velocities exceeding that of light. The authors defend their observations.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Influence of Wall Losses on Pulse Propagation in Wave Guides

Assuming that the resistivity of wave guide walls is not too large, the influence which energy losses have upon the shape of a pulse propagating along the wave guide is discussed. It is found that pulses are damped with a damping factor equal in the first approximation to that in a steady state.