Nonlinear Propagation Regime Research Articles

Guided Mode Evolution and Ionization Injection in Meter-Scale Multi-GeV Laser Wakefield Accelerators.

We show that multi-GeV laser wakefield electron accelerators in meter-scale, low density hydrodynamic plasma waveguides operate in a new nonlinear propagation regime dominated by sustained beating of lowest order modes of the ponderomotively modified channel; this occurs whether or not the injected pulse is linearly matched to the guide. For a continuously doped gas jet, this emergent mode beating effect leads to axially modulated enhancement of ionization injection and a multi-GeV energy spectrum of multiple quasimonoenergetic peaks; the same process in a locally doped jet produces single multi-GeV peaks with <10% energy spread. A three-stage model of drive laser pulse evolution and ionization injection characterizes the beating effect and explains our experimental results.

Numerical simulation study on distinguishing nonlinear propagation regimes of femtosecond pulses in fused silica

We perform numerical simulations to investigate the nonlinear propagation dynamics of femtosecond Gaussian and vortex beams in fused silica. By analyzing the extent of spectral broadening, we are able to distinguish between the linear, self-focusing, and filamentation regimes. Additionally, the maximum intensity and fluence distribution within the cross-section of the vortex beams are analyzed for different incident laser energies. The results demonstrate a direct correlation between the spectral broadening and the peak intensity of the femtosecond laser pulse. As a result, this provides a theoretical foundation for distinguishing different propagation regimes, and determining critical powers for self-focusing and filamentation of both femtosecond Gaussian and structured beams.

Distinguishing the nonlinear propagation regimes of vortex femtosecond pulses in fused silica by evaluating the broadened spectrum.

The nonlinear propagation dynamics of vortex femtosecond laser pulses in optical media is a topic with significant importance in various fields, such as nonlinear optics, micromachining, light bullet generation, vortex air lasing, air waveguide and supercontinuum generation. However, how to distinguish the various regimes of nonlinear propagation of vortex femtosecond pulses remains challenging. This study presents a simple method for distinguishing the regimes of nonlinear propagation of femtosecond pulses in fused silica by evaluating the broadening of the laser spectrum as the input pulse power gradually increases. The linear, self-focusing and mature filamentation regimes for Gaussian and vortex femtosecond pulses in fused silica are distinguished. The critical powers for self-focusing and mature filamentation of both types of laser pulses are obtained. Our work provides a rapid and convenient method for distinguishing different regimes of nonlinear propagation and determining the critical powers for self-focusing and mature filamentation of Gaussian and structured laser pulses in optical media.

Guided electromagnetic discharge pulses driven by short intense laser pulses: Characterization and modeling

Strong electromagnetic pulses (EMPs) are generated from intense laser interactions with solid-density targets and can be guided by the target geometry, specifically through conductive connections to the ground. We present an experimental characterization by time- and spatial-resolved proton deflectometry of guided electromagnetic discharge pulses along wires including a coil, driven by 0.5 ps, 50 J, 1019 W/cm2 laser pulses. Proton-deflectometry allows us to time-resolve first the EMP due to the laser-driven target charging and then the return EMP from the ground through the conductive target stalk. Both EMPs have a typical duration of tens of ps and correspond to currents in the kA-range with electric-field amplitudes of multiple GV/m. The sub-mm coil in the target rod creates lensing effects on probing protons due to both magnetic- and electric-field contributions. This way, protons of the 10 MeV-energy range are focused over cm-scale distances. Experimental results are supported by analytical modeling and high-resolution numerical particle-in-cell simulations, unraveling the likely presence of a surface plasma, in which parameters define the discharge pulse dispersion in the non-linear propagation regime.

Mode decomposition of multimode optical fiber beams by phase-only spatial light modulator

Multimode optical fibers (MMF) recently attracted a renewed attention, because of their potential for spatial division multiplexing, medical imaging and high-power fiber lasers, thanks to the discovery of new nonlinear optical effects, such as Kerr beam self-cleaning, spatiotemporal mode-locking, and geometric parametric instability, to name a few. The main feature of these effects is that many transverse modes are involved in nonlinear interactions. To advance our understanding, it is necessary to analyse the modal content of beams at the output of MMFs. In this work, based on a computer digital holography method using a phase-only spatial light modulator (SLM) as a correlation filter, we experimentally demonstrate a method of mode decomposition involving a large (≃80) number of fiber modes. To obtain this, we carried out a SLM calibration, and numerically investigated the most critical parameters which affect the fidelity of the decomposition, by comparing experimental and reconstructed beam patterns in both the linear (speckled structures) and in the nonlinear (self-cleaned beams) propagation regime.

High-energy soliton fission dynamics in multimode GRIN fiber.

The process of high-energy soliton fission is experimentally and numerically investigated in a graded-index multimode fiber. Fission dynamics is analyzed by comparing experimental observations and simulations. A novel nonlinear propagation regime is observed, where solitons produced by the fission have a nearly constant Raman wavelength shift and same pulse width over a wide range of soliton energies.

Symmetric and antisymmetric surface plasmon polariton solitons in a metal-dielectric-metal waveguide with incoherent pumping

We propose a scheme to realize stable linear and nonlinear propagation of symmetric and antisymmetric surface plasmon polaritons (SPPs) solitons by doping ladder-type three-level quantum emitters into the middle layer of a metal-dielectric-metal (MDM) waveguide. In linear propagation regime, we show that both symmetric and antisymmetric SPPs can acquire a gain from electromagnetically induced transparency (EIT) effect with an incoherent pumping. The EIT can be used not only to completely compensate the Ohmic loss in the metal but also to acquire a subluminal group velocity for the SPPs. We also show that in nonlinear propagation regime a huge enhancement of Kerr nonlinearity of the symmetric and antisymmetric SPPs can be obtained but with different incoherent pumping intensities. As a result, gain-assisted (1+1)-dimensional symmetric and antisymmetric subluminal surface polaritonic solitons may be produced based on the strong confinement of electric field in the MDM waveguide. Our study may have promising applications in light information processing and transmission at nanoscale level based on MDM waveguides.

Performance of optical amplifiers with regard to the power penalties of a DWDM XGS-PON

Introduction:This article is the product of the research “Simulation level performance analysis of optical amplifiers for a DWDM XGS-PON network environment”, supported by the Research Group in New Technologies in Telecommunications (GNTT) of the University of Cauca during 2019.&#x0D; Problem:For the implementation of XGS-PON optical network architectures, amplification processes are required. These generate power penalties that lead to the presence of non-linear phenomena, significantly degrading network performance.&#x0D; Objective:To analyze the performance of a DWDM / XGS-PON network architecture, respecting the power penalties generated by implementing different types of optical amplifiers in different amplification modes.&#x0D; Methodology:Compilation of relevant bibliography on long-range network architectures with symmetric speeds of 10 Gbps.&#x0D; Results:Not working in the ideal region of amplification affects the performance of a DWDM XGS-PON network, generating a migration of a linear to non-linear propagation regime, due to the presence of power penalties such as FWM.&#x0D; Conclusion:With this investigation it is possible to determine which amplifier should be used according to the needs of the optical network, evaluating the different alternatives available in the market, depending on the cost, configuration and performance of the system.&#x0D; Originality:A new research is carried out for the performance of DWDM XGS-PON state-of-the-art network architectures, regarding the incidence of non-linear propagation phenomena.&#x0D; Restrictions:In some cases, the simulation tool requires a high consumption of machine resources and processing time.

Propagation of megawatt subpicosecond light pulses with the minimum possible shape and spectrum distortion in an air- or argon-filled hollow-core revolver fibre

We have examined the feasibility of undistorted subpicosecond pulse transmission at peak powers of up to 100 MW and a wavelength of 1030 nm in a hollow-core revolver fibre having a core filled with atmospheric air or argon. A hollow-core revolver fibre has been fabricated which has a low-dispersion (β2 ≈ – 0.25 ps2 km−1) transmission band in the spectral range of ytterbium and neodymium lasers. Conditions for the nonlinear (soliton) regime of megawatt picosecond pulse propagation over up to 1-km lengths of revolver fibre in atmospheric air and argon at pressures in the range 10−5 to 1 atm have been found theoretically. We have experimentally demonstrated transmission of pulses of 0.8-ps duration and up to ∼100-MW power over an ∼3-m length of revolver fibre with the minimum possible distortion of their shape and spectrum.

Control of spatial four-wave-mixing efficiency in Bessel beams using longitudinal intensity shaping

Diffraction-free Bessel beams have attracted major interest because of their stability even in regimes of nonlinear propagation and filamentation. However, Kerr nonlinear couplings are known to induce significant longitudinal intensity modulation, detrimental to the generation of uniform plasma or for applications in the processing of transparent materials. These nonlinear instabilities arise from the generation of new spatio-spectral components through an initial stage of continuous spectral broadening followed by four wave mixing. In this paper, we investigate analytically and numerically these processes and show that nonlinear instabilities can be controlled through shaping the spatial spectral phase of the input beam. This opens new routes for suppressing the nonlinear growth of new frequencies and controlling ultrashort pulse propagation in dielectrics.

Dispersion-Engineered Step-Index Tellurite Fibers for Mid-Infrared Coherent Supercontinuum Generation from 1.5 to 4.5 μm with Sub-Nanojoule Femtosecond Pump Pulses

Mid-infrared supercontinuum generation from 1.5 to 4.5 µm with sub-nanojoule femtosecond pump pulses is demonstrated by using a short segment of dispersion-engineered step-index tellurite fiber with very low OH content. Distinct group-velocity dispersion regimes in a simple design of step-index tellurite fiber are also reported, which allows to choose the nonlinear pulse propagation regime according to the required tailoring of the supercontinuum source. Numerical simulations based on the generalized nonlinear Schrödinger equation are used to determine optimized fiber parameters before experimental demonstrations. We also analyse the coherence properties of the resulting supercontinuum sources.

Investigation of thermomechanical motion in a nanomechanical resonator based on optical intensity mapping

In the present study, the acoustic field of a high intensity focused ultrasound (HIFU) transducer in water was measured by using a commercially available needle hydrophone intended for HIFU use. To validate the results of hydrophone measurements, numerical simulations of HIFU fields were performed by integrating the axisymmetric Khokhlov-Zabolotskaya-Kuznetsov (KZK) equation from the frequency-domain perspective with the help of a MATLAB-based software package developed for HIFU simulation. Quantitative values for the focal waveforms, the peak pressures, and the size of the focal spot were obtained in various regimes of linear, quasilinear, and nonlinear propagation up to the source pressure levels when the shock front was formed in the waveform. The numerical results with the HIFU simulator solving the KZK equation were compared with the experimental data and found to be in good agreement. This confirms that the numerical simulation based on the KZK equation is capable of capturing the nonlinear pressure field of therapeutic HIFU transducers well enough to make it suitable for HIFU treatment planning.

Coupler-free surface polariton excitation and propagation with cold four-level atomic medium

The possibility of the direct excitation of surface polaritons (SPs) by the free-space laser fields at the interface of negative-index metamaterial (NIMM) layer and a bottom layer of cold double Lambda-type atomic medium is investigated. The giant field enhancement (up to |E/E0|2≈4.1), together with suppressed ohmic loss of the NIMM layer in a wide transparency window of a double electromagnetically induced transparency, results in the SPs generation. The excitation efficiency of these SPs can be effectively enhanced by applying the unidirectional atomic motion, modulation of the coupling laser characteristics, and using a probe laser with proper incident angle. Based on the special modulation of the driven fields parameters, the group velocity switching of the SPs is observed. In the nonlinear propagation regime of the SPs, a giant Kerr nonlinearity is produced by applying a negative unidirectional atomic velocity. The controllable optical Kerr effect in this coupler-free scheme results in the lossless propagation of the nonlinear SPs. It is revealed that the electromagnetically induced transparencybased system can be used for stable propagation of one-dimensional (1+1)D bright and dark superluminal polaritonic solitons. It is also shown that the proposed scheme could be easily realized in an experiment and hence can be used for various ultrafast optical devices.

Realization of non- PT -symmetric optical potentials with all-real spectra in a coherent atomic system

We present a physical setup for realizing all-real-spectrum optical potentials with arbitrary gain-and-loss distributions in a coherent medium consisting of a cold three-level atomic gas driven by control and probe laser fields. We show that by the interference of Raman resonances and the Stark shift induced by a far-detuned laser field, tunable, non-parity-time (non-$\mathcal{PT})$-symmetric optical potentials with all-real spectra proposed recently by Nixon and Yang [Phys. Rev. A 93, 031802(R) (2016)] can be actualized physically. We also show that when the real parts of the non-$\mathcal{PT}$-symmetric optical potentials are tuned cross certain thresholds, phase transitions---where the eigenspectrum of the system changes from all real to complex---may occur and hence the stability of the probe-field propagation is altered. Our scheme can also be extended to high dimensions and to a nonlinear propagation regime, where stable optical solitons with power of the order of nano-Watts may be generated in the system.

Tailoring supercontinuum generation beyond 2 μm in step-index tellurite fibers.

We report numerical and experimental demonstrations of flexible group-velocity dispersion regimes in step-index tellurite fibers by fine control of the fiber core diameter. Our simple fiber design allowed us to explore various nonlinear propagation regimes beyond 2 μm, which involved careful control of four-wave mixing processes. Combined with the recent development of 2 μm fiber lasers, we present an easy way to tailor supercontinuum generation and related coherence features in the high-demand 1.5-3.5 μm spectral region.

Nonlinear coupled electromagnetic wave propagation: Saturable nonlinearities

Propagation of a sum of two monochromatic transverse electric (TE) waves in a plane dielectric layer filled with nonlinear medium is considered. Nonlinearity in the layer is described by a diagonal tensor with arbitrary functions w.r.t. squared module of the complex amplitudes of an electric field. We look for guided waves that propagate along the boundaries of the layer and decay when they move off from the boundaries. It is proved that a novel nonlinear propagation regime arises, called ‘coupled TE wave.’ It is shown that two TE waves–generating the coupled wave–propagate at different frequencies ω1, ω2 with different propagation constants γ1, γ2, respectively. The wave propagation problem is reduced to a nonlinear 2-parameter transmission eigenvalue problem for Maxwell’s equations. An original analytical method to study the problem is suggested. For a wide class of saturable nonlinearities, it is proved the existence of isolated coupled eigenvalues (that correspond to the coupled propagation modes) and intervals of its localisation are found, zeros of the eigenfunctions are also determined. Theoretical results are illustrated with numerical calculations.

Turning the tables on nonlinearity

The discovery of a new nonlinear light propagation regime in optical fibres paves the way to spectrally brighter lasers and control of signal distortion in communication links.

Nonlinear regime of electrostatic waves propagation in presence of electron-electron collisions

The effects are presented of including electron-electron collisions in self-consistent Eulerian simulations of electrostatic wave propagation in nonlinear regime. The electron-electron collisions are approximately modeled through the full three-dimensional Dougherty collisional operator [J. P. Dougherty, Phys. Fluids 7, 1788 (1964)]; this allows the elimination of unphysical byproducts due to reduced dimensionality in velocity space. The effects of non-zero collisionality are discussed in the nonlinear regime of the symmetric bump-on-tail instability and in the propagation of the so-called kinetic electrostatic electron nonlinear (KEEN) waves [T. W. Johnston et al., Phys. Plasmas 16, 042105 (2009)]. For both cases, it is shown how collisions work to destroy the phase-space structures created by particle trapping effects and to damp the wave amplitude, as the system returns to the thermal equilibrium. In particular, for the case of the KEEN waves, once collisions have smoothed out the trapped particle population which sustains the KEEN fluctuations, additional oscillations at the Langmuir frequency are observed on the fundamental electric field spectral component, whose amplitude decays in time at the usual collisionless linear Landau damping rate.

Surface polaritons in a negative-index metamaterial with active Raman gain

We propose a scheme to realize stable propagation of linear and nonlinear surface polaritons (SPs) by placing a $N\ensuremath{-}\text{type}$ four-level quantum emitters at the interface between a dielectric and a negative-index metamaterial (NIMM). We show that in linear propagation regime SPs can acquire an active Raman gain (ARG) from a pump field and a gain doublet appears in the gain spectrum of a signal field induced by the quantum interference effect from a control field. The ARG can be used not only to completely compensate the Ohmic loss in the NIMM but also to acquire a superluminal group velocity for the SPs. We also show that in the nonlinear propagation regime a huge enhancement of the Kerr nonlinearity of the SPs can be obtained. As a result, ARG-assisted $(1 + 1)\ensuremath{-}$ and $(2 + 1)\ensuremath{-}$ dimensional superluminal surface polaritonic solitons with extremely low generation power may be produced based on the strong confinement of the electric field at the dielectric-NIMM interface.

Filamentation with nonlinear Bessel vortices.

We present a new type of ring-shaped filaments featured by stationary nonlinear high-order Bessel solutions to the laser beam propagation equation. Two different regimes are identified by direct numerical simulations of the nonlinear propagation of axicon focused Gaussian beams carrying helicity in a Kerr medium with multiphoton absorption: the stable nonlinear propagation regime corresponds to a slow beam reshaping into one of the stationary nonlinear high-order Bessel solutions, called nonlinear Bessel vortices. The region of existence of nonlinear Bessel vortices is found semi-analytically. The influence of the Kerr nonlinearity and nonlinear losses on the beam shape is presented. Direct numerical simulations highlight the role of attractors played by nonlinear Bessel vortices in the stable propagation regime. Large input powers or small cone angles lead to the unstable propagation regime where nonlinear Bessel vortices break up into an helical multiple filament pattern or a more irregular structure. Nonlinear Bessel vortices are shown to be sufficiently intense to generate a ring-shaped filamentary ionized channel in the medium which is foreseen as opening the way to novel applications in laser material processing of transparent dielectrics.