Propagation Of Short Laser Pulses Research Articles

Three-dimensional FTn finite volume solution of short-pulse laser propagation through heterogeneous medium

In this paper, 3-D heterogeneous medium, containing small inhomogeneous zones, subjected to a short-pulse laser has been examined by solving the transient radiative transfer equation. Both curved-line advection method and STEP schemes of the FTn finite volume method have been applied. The curved-line advection method predictions proved that a decrease of the false scattering and ray effects are obtained. In fact, there was a good agreement between the curved-line advection method and the Monte Carlo method. However, the STEP results are slightly mismatching the predictions of the aforementioned reference method. Then, the effects of the absorption coefficient, the size, the number and the position of inhomogeneous zone on the transmittance and reflectance signals have been analyzed. The predictions showed that the increase of the size of the inhomogeneity reduces the intensity of radiation. For both homogenous and heterogonous medium, the change of the detector position varies both the broadening of the signal pulse-width and the time with peak reflectance and/or transmittance. That is can be explained by the effects of the distance and the medium property between the laser-incident source and the detector position. Thus, these both parameters are the main factors for determining the peak position and the pulse broadening. Finally, the effects of the absorption coefficient in the inhomogeneity zone on the absolute values of logarithmic slope has been discussed. The results proved that the absolute values of logarithmic slope may be a perfect indicator for detecting any abnormal absorbing zones in the medium.

Pulse Slippage of Resonant Third Harmonic Generation in Electron-Hole Plasma**Supported by a financial grant from CSIR, New Delhi, India, under Project No. 03(1438)/18/EMR-II

Effect of pulse slippage on resonant third harmonic generation of a short pulse laser in electron-hole plasma in the presence of wiggler magnetic field has been investigated. The group velocity mismatch of the third harmonic pulse and the fundamental pulse is significant in electron hole plasma. As the third harmonic pulse has higher group velocity than that of fundamental pulse, therefore, it moves faster than the fundamental pulse. It gets slipped out of the domain of fundamental pulse and its amplitude saturates. Phase matching condition is satisfied by applying wiggler magnetic field, which provides additional angular momentum to the third harmonic photon to make the process resonant. Enhancement in the efficiency of third harmonic generation of an intense short pulse laser in electron-hole plasma embedded with a magnetic wiggler is seen.

Development of a numerical model to characterize laser-induced plasmas in aqueous media

A spatio-temporal model for investigating the characteristics of laser-induced plasmas in aqueous media is developed by modifying the general form of the well-known rate equation and simultaneously accounting for the influences of multiphoton and cascade ionization on the propagation of short laser pulses. In this model, the nonlinear absorption of laser pulse energy is considered to be time and space dependent inside the computational volume. The model is verified by comparing the results of three-dimensional axisymmetric numerical simulations with existing experimental data for laser pulses of 30 ps, 1064 nm at focusing angles between 4° and 28° with energies in the wide range between 0.1 to 6000 μJ. This model could reasonably predict the various characteristics of a laser-induced plasma, such as breakdown threshold, size, shape and energy transmitted through the plasma. Also the transmitted energy data obtained from the model is within 10% of the experimental data at the largest focusing angle and 20% at the smallest angle. To compare the calculations with plasma photographs, electron density values are transformed into a gray scale. The simulated plasma shapes correlate well with the existing experimental observations. The outcomes of the model, such as spatial distribution of plasma energy density, could be used as input for a hydrodynamic model to estimate the strength of the mechanical effects associated with plasma formation.

Study of the Propagation of Short Pulse Laser With Cavity Using Numerical Simulation Software

The purpose of this representation is to show the potentialities (Computational Time, access to the dynamic and feasibility of systematic studies) of the numerical study of the nonlinear dynamics in laser cavity, assisted by software. We will give as an example, one type of cavity completely fibered composed of several elements and then studying the physical parameters of a pulse propagating into this cavity, determining its characteristics at the output. The results are interesting but we also projects to verify them experimentally by making assemblies similar to this type of cavities. Keywords : Optical fiber, short pulse, bandwidth, spectral width

Coherent interactions in nonlinear multilevel media

We propose in this work an alternative method of easier calculation of necessary conditions for lossless propagation of short laser pulses in multilevel atomic media. Method is based on the quasienergy approach and illustrated with example of a five-level system. We also demostrated effective population transfer in this system.

Adiabatic control of atomic population and distortion-free propagation of short laser pulses in multilevel media

A relatively simple technique of analytic calculation based on the method of quasienergies is proposed, which allows all necessary conditions for lossless short pulse propagation in a multilevel atomic medium to be revealed. The method is illustrated for three- and five-level atomic systems. The dynamics of pulse propagation in a multilevel medium is demonstrated for the case of a W-system of levels. The conditions for adiabatic interaction in a medium are obtained.

The study of pulse laser propagation through breast tissues by means of the Green function method

Propagation of a short pulse through human breast tissues is studied by numerically solving the diffuse equation. Different numerical methods, such as Monte Carlo and finite difference time domain, have been used to study the short pulse laser propagation through biological tissues. In this paper, we use boundary integral method to study the laser pulse–tissue interaction. The diffuse equation is used to examine the propagation of laser into biological tissues, and boundary integral method is used to alter this equation to the integral form and the result is solved by using boundary element method. To verify the precision of a boundary element method code, we compared the obtained results with those obtained by finite difference time domain method. In addition, the effects of different optical parameters of breast tissues, i.e., reduced scattering and absorption coefficients, on time evolution of a diffusely reflected pulse are studied.

Finite-Difference Time-Domain Simulation of Light Propagation in 2D Periodic and Quasi-Periodic Photonic Structures

Ultra-short pulse is a promising technology for achieving ultrahigh data rate transmission which is required to follow the increased demand of data transport over an optical communication system. Therefore, the propagation of such type of pulses and the effects that it may suffer during its transmission through an optical waveguide has received a great deal of attention in the recent years. We provide an overview of recent theoretical developments in a numerical modeling of Maxwell's equations to analyze the propagation of short laser pulses in photonic structures. The process of short light pulse propagation through 2D periodic and quasi-periodic photonic structures is simulated based on Finite-Difference Time-Domain calculations of Maxwell’s equations.

Third Harmonic Generation of a Short Pulse Laser in a Tunnel Ionizing Plasma: Effect of Self-Defocusing

Third harmonic generation of a Gaussian short pulse laser in a tunnel ionizing plasma is investigated. A Gaussian short pulse laser propagating through a tunnel ionizing plasma generates third harmonic wave. Inhomogeneity of the electric field along the wavefront of the fundamental laser pulse causes more ionization along the axis of propagation while less ionization off axis, leading to strong density gradient with its maximum on the axis of propagation. The medium acts like a diverging lens and pulse defocuses strongly. The normalized third harmonic amplitude varies periodically with the distance with successive maxima acquiring lower value. The self-defocusing of the fundamental laser pulse decays the intensity of the third harmonic pulse.

Curvature aided long range propagation of short laser pulses in the atmosphere

The pre-filamentation regime of propagation of a short and intense laser pulse in the atmosphere is considered. Spatiotemporal self-focusing dynamics of the laser beam are investigated by calculating the coupled differential equations for spot size, pulse length, phase, curvature, and chirp functions of a Gaussian laser pulse via a variational technique. The effect of initial curvature parameter on the propagation of the laser pulse is taken into consideration. A method relying on the adjustment of the initial curvature parameter can expand the filamentation distance of a laser beam of given power and chirp is proposed.

Time-dependent ionization models designed for intense and short laser pulse propagation in dielectric materials

When an intense and short laser pulse propagates in a dielectric material, significant production of conduction electrons through multiphoton absorption (MPA) may occur. In addition to the laser intensity, the MPA process depends mainly on the laser frequency spectrum which may evolve significantly during the course of laser propagation in the material. Simple models for MPA accounting for possible time-dependent evolution of the laser frequency spectrum (as harmonic generation, chirping or broadening) are addressed. The first model is based on Bloch-Volkov states whereas the second approach relies on the density matrix formalism which has been adapted for the present study. Both models are well adapted for their introduction in a propagation code and are shown to correctly account for the MPA process whatever the characteristics of the laser frequency spectrum. The reliability of these approaches has been studied in two cases of practical interest. First, in the case where a second harmonic is present within the fundamental pulse, calculations show that the ionization rate may be significantly enhanced. Second, in the case of a chirped pulse, models are shown to correctly account for possible change in the multiphoton order during the course of interaction.

Wakefield effects and solitary waves of an intense short laser pulse propagation in a plasma channel

In the presence of relativistic and channel-coupling nonlinearity and wakefield effects, the propagation characteristics and solitary waves of an intense short laser pulse in a preformed plasma channel are investigated. The evolution equation of the laser spot size is derived by using variational technique, the initial laser and plasma parameters for propagation with constant spot size, periodic defocusing and focusing oscillations, and solitary waves are identified. For illustration, some numerical results are also presented. It is found that the laser focusing is enhanced by the wakefield effects that result in a significant reduced focusing power.

From higher-order Kerr nonlinearities to quantitative modeling of third and fifth harmonic generation in argon

The recent measurement of negative higher-order Kerr effect (HOKE) terms in gases has given rise to a controversial debate, fed by its impact on short laser pulse propagation. By comparing the experimentally measured yield of the third and fifth harmonics, with both an analytical and a full comprehensive numerical propagation model, we confirm the absolute and relative values of the reported HOKE indices.

Study of short-pulse laser propagation in biological tissue by means of the boundary element method

Propagation of short pulses of light through biological tissues can be studied by numerically solving the diffusion equation. The boundary integral method was used to convert the differential equation to integral form and the result was solved using the boundary element method. The effects of different optical parameters of the tissue, i.e. scattering, absorption coefficients and anisotropic factor, on temporal evolution of the diffusely reflected pulse were studied. The results were compared with those obtained using the finite difference time domain method and the boundary integral method was found to be more precise and faster than the last method. The method can be used to investigate reflected pulses in the study of cell morphology and tumours in different types of tissue.

Wakefield effects on the evolution of symmetric laser pulses in a plasma channel

AbstractThe present study deals with the propagation of short laser pulses in a parabolic plasma channel. A variational technique is used to obtain simultaneous evolution equations describing the laser pulse length and spot size in the presence of relativistic and ponderomotive nonlinearities. The effect of wakefields (inside the pulse) on the propagation characteristics of the laser beam is studied. Numerical methods are used to graphically analyze the evolution of the laser pulse length and intensity.

Propagation of ultrashort laser pulses in water: linear absorption and onset of nonlinear spectral transformation

We study propagation of short laser pulses through water and use a spectral hole filling technique to essentially perform a sensitive balanced comparison of absorption coefficients for pulses of different duration. This study is motivated by an alleged violation of the Bouguer-Lambert-Beer law at low light intensities, where the pulse propagation is expected to be linear, and by a possible observation of femtosecond optical precursors in water. We find that at low intensities, absorption of laser light is determined solely by its spectrum and does not directly depend on the pulse duration, in agreement with our earlier work and in contradiction to some work of others. However, as the laser fluence is increased, interaction of light with water becomes nonlinear, causing energy exchange among the pulse's spectral components and resulting in peak-intensity dependent (and therefore pulse-duration dependent) transmission. For 30 fs pulses at 800 nm center wavelength, we determine the onset of nonlinear propagation effects to occur at a peak value of about 0.12 mJ/cm(2) of input laser energy fluence.

Resonant third harmonic generation of a short pulse laser in plasma by applying a wiggler magnetic field

We examine the effect of wiggler magnetic field on pulse slippage of short pulse laser-induced third harmonic generation in plasma. The process of third harmonic generation of an intense short pulse laser in plasma is resonantly enhanced by the application of a magnetic wiggler. The laser exerts a ponderomotive force at second harmonic driving density oscillations. The second harmonic oscillations coupled with electron velocity at the laser frequency, produces a non-linear current, driving the third harmonic. Third harmonic pulse generates in the fundamental pulse domain. However, the group velocity of the third harmonic wave is greater than the fundamental wave. Hence, the third harmonic pulse saturates strongly and moves forward from the fundamental pulse at shorter distance than the second harmonic pulse.

Improved Ponderomotive Guiding Center Algorithm

The original implementation of the ponderomotive guiding center algorithm is not suitable for modeling the propagation of short laser pulses over long distances. An improved algorithm is given, which improves the accuracy of the calculation, allowing much longer propagation distances to be modeled.

Experimental study of optical characteristics of homogeneous highly-dispersive biological medium

Specific features of the process of short laser pulse propagation through a highly-dispersive biological medium are considered. Two models are used for the theoretical description of propagation of optical radiation through a highly-dispersive medium: diffusion approximation and non-stationary axial model. Physical characteristics of the dispersive medium are determined in both models. The obtained theoretical dependences are found to be in qualitative agreement with the results of experimental study.

Oval-like hollow intensity distribution of tightly focused femtosecond laser pulses in air

The propagation of a tightly focused femtosecond laser pulse in air has been investigated. Unlike long-distance self-guided propagation of short laser pulses, a novel oval-like hollow distribution of the laser intensity is observed in the experiments and reproduced by the numerical simulations. The formation of the hollow structures can be explained by the interplay between ionization-induced refraction and Kerr self-focusing.