Structuring a laser beam subject to optical Kerr effect for improving its focusing properties

Only recently researchers started to investigate the frequency conversion of high power picosecond pulses produced by regenerative amplification1. The high power densities easily achieved with short, pulses allow efficient generation of parametric fluorescence. However, experiments show that the efficiencies achieved in these systems are not as high as one would expect from a simple calculation, due to the temporal and spatial pulse shape, which has to be considered in a reasonably accurate model2,3. We performed calculations on the efficiency of the parametric conversion for various temporal and spatial pulse profiles and for different arrangements of a picosecond optical parametric generator (OPG) and optical parametric amplifier (OPA) combination. In the numerical simulation and the experiments we used a KTP crystal pumped with the fundamental of an all-solid-state Nd:YAG regenerative amplifier.

Dynamics of Gaussian and Super-Gaussian optical beams in weakly nonlinear nonlocal media

Numerical investigation on laser transformation hardening with different temporal pulse shapes

A volume holographic optical element is designed and fabricated using photopolymer (PQ-PMMA) for super Gaussian beam. A Lee hologram and Digital Micro Mirror Device (DMD) are used for modulating the laser beam to a uniform one all over its beam diameter. The efficiency and uniformity of super Gaussian beams generated through volume holographic optical component is experimentally demonstrated with optimized parameters. Super Gaussian beam generated by volume holographic grating with unique diffractive properties are advantageous for variety of laser applications.

Summary form only given. Some optical systems require both high laser power and high beam quality. The wavefront distortion induced by illuminating an absorbing transmissive optical material with a Gaussian beam is not parabolic in shape, which causes the laser beam quality to degrade. The thermal lens induced by a high-order super-Gaussian or top-hat intensity profile laser beam, however, is parabolic and does not degrade laser beam quality. To avoid laser beam duality degradation when passing through transmissive optics, we create a pair of optics, which convert a Gaussian intensity profile into a super-Gaussian profile for transmission through the absorbing optics, and then convert the super-Gaussian profile back into a Gaussian for subsequent use. We performed ray-optics modeling to determine the spatial phase profile necessary to convert a Gaussian intensity profile into a super-Gaussian intensity profile.

The commercial availability of fiber lasers based on MOPA architectures with arbitrary temporal pulse shaping capabilities offers completely new possibilities for laser material processing. In this study, based on numerical modeling results in the nanosecond regime for the case of silicon at 1064 nm wavelength, we show that not only the single pulse laser ablation efficiency depends on the temporal pulse shape but, we also demonstrate how a stochastic approach can be applied in order to reach an optimized pulse shape maximizing the material vaporization rate for given laser pulse energy and duration. Experimental results are compared to the numerical modeling results, and the discrepancies are discussed in terms of the role played by plasma shielding effects and melt ejection at high intensity.

The plasma-facing components (PFCs) in future fusion devices such as ITER will receive intense transient heat fluxes from plasma instabilities, such as edge localized modes (ELMs) and disruptions, which will limit material lifetime. The energy density and transient pulse duration are typically used to characterize the PFC damage threshold. However, these parameters are not sufficient to define a damage threshold, because different transient pulse shapes with the same energy density and same pulse duration produce different peak surface temperatures (and thus stresses, which ultimately determine material damage). The surface temperature and damage of tungsten (the material to be used for the ITER divertor target armor) in the form of surface roughening and melting are investigated using various temporal pulse shapes from an Nd:YAG laser in the PISCES-B facility. The heat flux factor is examined and shown to be an inadequate parameter to characterize the temperature rise except for square temporal pulse shapes. For ELM-like heat pulses, the long tail in the temporal shape results in a lower peak surface temperature and less damage compared to a symmetric triangle pulse with equal energy density.

We analyze the transfer function of a three-dimensional atomic Bragg beamsplitter formed by two counterpropagating pulsed Gaussian laser beams. Even for ultracold atomic ensembles, the transfer efficiency depends significantly on the residual velocity of the particles as well as on losses into higher diffraction orders. Additional aberrations are caused by the spatial intensity variation and wavefront curvature of the Gaussian beam envelope, studied with (3+1)D numerical simulations. The temporal pulse shape also affects the transfer efficiency significantly. Thus, we consider the practically important rectangular-, Gaussian-, Blackman- and hyperbolic secant pulses. For the latter, we can describe the time-dependent response analytically with the Demkov-Kunike method. The experimentally observed stretching of the $\pi$-pulse time is explained from a renormalization of the simple Pendell\"osung frequency. Finally, we compare the analytical predictions for the velocity-dependent transfer function with effective (1+1)D numerical simulations for pulsed Gaussian beams, as well as experimental data and find very good agreement, considering a mixture of Bose-Einstein condensate and thermal cloud.

We report on the numerical simulation of temporal and spatial flat top pulse pumped 3μm OPCPA. Compared to the results with Gaussian shape pulse pumping, three fold efficiency improvement was found in two-stage 3μm OPCPA with flat top pulse pumping. This simulation result will be a good guideline to our further experiments.

Based on the Collins formula and irradiance moments definition, the propagation of the kurtosis parameter of super-Gaussian beams through a spherically aberrated lens is studied. Detailed numerical results are given. It is shown that, as compared with aberration-free super-Gaussian beams, the profile of spherically aberrated super-Gaussian beams can be leptokurtic, mesokurtic or platykurtic, depending on the aberration and propagation distance. The results for the spherically aberrated Gaussian beams can be dealt with as a special case treated in this paper. Finally, the advantage of our method is pointed out.

We consider the propagation of super-Gaussian monochromatic laser beams in a three-dimensional array of quantum dots coupled by the tunneling effect along one axis. The electron energy spectrum of the system corresponds to the Hubbard model, where the Coulomb interaction of electrons in quantum dots is taken into account. The field of the laser beam is described by the Maxwell equations, from which a nonhomogeneous wave equation for the vector potential is obtained. In the approximation of slowly varying amplitudes and phases, the wave equation is reduced to a phenomenological equation describing the electromagnetic field in an array of chains of quantum dots. We study the influence of the system parameters and the frequency of the laser-beam field on the propagation in the medium by solving numerically the phenomenological equation. We obtain the dependence of the factor characterizing the diffraction blooming of the beam in an array of chains of quantum dots on the parameters of the system’s electron energy spectrum.

Profile cutting of thin sheet bright metals can be achieved in a cost effective manner through the use of a pulsed fiber laser in conjunction with a galvanometric scanhead without co-axial assist gas. This is attractive where many batches of distinct small parts are manufactured in volume, for instance within the jewellery and the micro-electronics industries.In this paper the capabilities of a pulsed fiber laser are considered in detail when processing bright metals such as brass, copper and silver. The effect of key laser parameters such as peak power, temporal pulse shape and laser pulse frequency will be considered along with several process parameters such as pulse overlap and beam wobbling techniques. This is with the aim of producing parts at high speed and/or at high quality and to determine the maximum thickness that can be cut at effective speeds. The results will be compared and contrasted with other metals such as stainless steel and aluminium to determine the range of processing strategies needed to cut these materials.Profile cutting of thin sheet bright metals can be achieved in a cost effective manner through the use of a pulsed fiber laser in conjunction with a galvanometric scanhead without co-axial assist gas. This is attractive where many batches of distinct small parts are manufactured in volume, for instance within the jewellery and the micro-electronics industries.In this paper the capabilities of a pulsed fiber laser are considered in detail when processing bright metals such as brass, copper and silver. The effect of key laser parameters such as peak power, temporal pulse shape and laser pulse frequency will be considered along with several process parameters such as pulse overlap and beam wobbling techniques. This is with the aim of producing parts at high speed and/or at high quality and to determine the maximum thickness that can be cut at effective speeds. The results will be compared and contrasted with other metals such as stainless steel and aluminium to determine the range of processing strategies ne...

This article reports a study of the effect of anions on the optical Kerr effect (OKE) spectra of binary ionic liquid mixtures with one mixture comprising the 3-methyl-1-pentylimidazolium ([C 5mim] (+)) cation and the anions PF 6 (-) and CF 3CO 2 (-) (TFA (-)), and another mixture comprising the [C 5mim] (+) cation and the anions Br (-) and bis(trifluomethanesulfonyl)imide (NTf 2 (-)). The spectra were obtained by the use of optical heterodyne-detected Raman-induced Kerr Effect Spectroscopy at 295 K. The OKE spectra of the mixtures are compared with the calculated mole-fraction weighted sum of the normalized OKE spectra of the neat liquids. The OKE spectra are nearly additive for [C 5mim]Br/[C 5mim][NTf 2] mixtures, but nonadditive for [C 5mim][PF 6]/[C 5mim][TFA] mixtures. In the case of the equimolar [C 5mim][PF 6]/[C 5mim][TFA] mixture, the nonadditivity is such that the experimental OKE spectrum is narrower than the calculated OKE spectrum. The additivity or nonadditivity of OKE spectra for IL mixtures can be explained by assuming ionic liquids are nanostructurally organized into nonpolar regions and ionic networks. The ionic networks in mixtures will be characterized by "random co-networks" for anions that are nearly the same in size (PF 6 (-) and TFA (-)) and by "block co-networks" for anions that differ greatly in size (Br (-) and NTf 2 (-)).

In this paper, the nanostructural organization and subpicosecond intermolecular dynamics in the mixtures of CS(2) and the room temperature ionic liquid (IL) 1-pentyl-3-methylimidazolium bis{(trifluoromethane)sulfonyl}amide ([C(5)mim][NTf(2)]) were studied as a function of concentration using molecular dynamics (MD) simulations and optical heterodyne-detected Raman-induced Kerr effect spectroscopy. At low CS(2) concentrations (<10 mol.% CS(2)/IL), the MD simulations indicate that the CS(2) molecules are localized in the nonpolar domains. In contrast, at higher concentrations (≥10 mol.% CS(2)/IL), the MD simulations show aggregation of the CS(2) molecules. The optical Kerr effect (OKE) spectra of the mixtures are interpreted in terms of an additivity model with the components arising from the subpicosecond dynamics of CS(2) and the IL. Comparison of the CS(2)-component with the OKE spectra of CS(2) in alkane solvents is consistent with CS(2) mainly being localized in the nonpolar domains, even at high CS(2) concentrations, and the local CS(2) concentration being higher than the bulk CS(2) concentration.

One of the nonlinear optics phenomena, discovered after lasers have been put to use, is birefringence of transparent media induced by a laser beam, known as optical Kerr effect. The report presents the development of the technique of optical Kerr effect and a review of the theories of this effect including fundamental formulas and dependencies used for interpretation of experimental data. Recent experimental techniques: nanosecond, picosecond, and femtosecond are discussed and schematic diagrams of most frequently used two- and four- component systems are given and compared. A method, developed by the author, permitting considerable improvement of sensitivity of measuring equipment is described. The application of this method enabled observation of optical Kerr effect in vapors and gases and thus systematic investigation of Kerr effect versus gas pressure and temperature. The exemplary results of optical Kerr effect measurements in liquid and gas media are given as a function of temperature, solution concentration, or gas pressure with particular emphasis put on their cognitive value. Practical applications of Kerr effect are specified. The results of OKE investigation of molecule conformation and local structure of organic liquids, water, and water solution of optically active substances which are a simple model of biological media are presented. Perspectives of further studies of optically induced birefringence of the media using the femtosecond technique are also outlined.