Laser Pulse Characteristics Research Articles

Characteristics of filamentation in ZK7 glass by negatively chirped femtosecond laser pulses

The filamentation characteristics of femtosecond laser pulses in ZK7 glass are investigated experimentally as a function of initial negative chirps. It is found that the filament threshold power grows rapidly and the filament length extends over a long distance with increasing initial temporal chirps. The measurement of supercontinuum reveals that the plasma generation process within filamentation becomes weaker as the initial negative chirp increases, leading to a self-guiding long light channel dominated by Kerr nonlinearity. The interference of transverse rings in multifilamentation of the chirped laser pulses is observed as well. Analyses and discussions give an interpretation of this chirp-induced ionization-free filamentation. These results indicate that initial chirps will play a crucial role in the filament formation of ultrashort laser pulses in transparent media.

Accelerating protons to therapeutic energies with ultraintense, ultraclean, and ultrashort laser pulses

Proton acceleration by high-intensity laser pulses from ultrathin foils for hadron therapy is discussed. With the improvement of the laser intensity contrast ratio to 10(-1) achieved on the Hercules laser at the University of Michigan, it became possible to attain laser-solid interactions at intensities up to 10(22) W/cm2 that allows an efficient regime of laser-driven ion acceleration from submicron foils. Particle-in-cell (PIC) computer simulations of proton acceleration in the directed Coulomb explosion regime from ultrathin double-layer (heavy ions/light ions) foils of different thicknesses were performed under the anticipated experimental conditions for the Hercules laser with pulse energies from 3 to 15 J, pulse duration of 30 fs at full width half maximum (FWHM), focused to a spot size of 0.8 microm (FWHM). In this regime heavy ions expand predominantly in the direction of laser pulse propagation enhancing the longitudinal charge separation electric field that accelerates light ions. The dependence of the maximum proton energy on the foil thickness has been found and the laser pulse characteristics have been matched with the thickness of the target to ensure the most efficient acceleration. Moreover, the proton spectrum demonstrates a peaked structure at high energies, which is required for radiation therapy. Two-dimensional PIC simulations show that a 150-500 TW laser pulse is able to accelerate protons up to 100-220 MeV energies.

Modelling of passively Q-switched lasers with intracavity Raman conversion

We present a model of passively Q-switched Raman lasers by utilizing the rate equations. The intracavity fundamental photon density, Raman photon density and the initial population-inversion density of the gain medium are assumed to be of Gaussian spatial distributions. These rate equations are normalized by introducing some synthetic parameters and solved numerically, and a group of general curves are generated. From these curves we can understand the dependence of the Raman laser pulse characteristics on the parameters about the pumping, the gain medium, the Raman medium and the resonator. An illustrative calculation for a passively Q-switched Nd3+:GdVO4 self-Raman laser is presented to demonstrate the usage of the curves and related formulas.

Examining the Influence of Changing Laser Pulse Repetition Frequencies on Conifer Forest Canopy Returns

The distribution of laser pulses within conifer forest trees and canopies are examined by varying the rate of laser pulse emission and the inherent laser pulse properties (laser pulse energy, pulse width, pulse length, and roll-over or trigger time). In this study, an Optech, Inc. ALTM 3100 airborne lidar is used, emitting pulses at 50 kHz and 100 kHz, allowing for changes in laser pulse characteristics while also keeping all other survey parameters equal. We found that: 1. Pulses and associated characteristics emitted at 50 kHz penetrated further into the canopy than 100 kHz for a significant number of individual trees. 2. At tall tree plots with no understory, pulses emitted at 50 kHz penetrated further into the canopy than 100 kHz for a significant number of plots. 3. For plots with significant understory and shorter trees, pulses emitted at 100 kHz penetrated further into the canopy than 50 kHz. We suspect that this may be due, in part, to canopy openness. Laser pulse energy and character differences associated with different laser pulse emission frequencies are likely a contributing factor in laser pulse penetration through the canopy to the ground surface. Efforts to understand laser pulse character influences on canopy returns are important as biomass and vegetation structure models derived from lidar are increasingly adopted.

Plasma temperature and ion current analysis of gold ablation at different laser power rates

Deconvolution of time-of-flight ion currents recorded with an ion collector and cylindrical ion analyser spectra were applied to characterization of production of Auq+ ions (1⩽q⩽52) by focusing laser-beam intensity of 5×1014W/cm2 onto a bulk target. The deconvolution of TOF spectra indicates up to eight current peaks carried by ion groups having different mean ionisation state and different energy. The nature of the laser-produced ion groups depends on the laser pulse characteristics, the focusing conditions and on the non-linear phenomena induced by the interaction of the focused laser pulse with the plasma.Comparison of recovered ion current peaks of multiplies charged Auq+ ions (1⩽q⩽9) produced with laser intensity of 3×1010W/cm2 with reconstructed currents of single charge-states is presented. The multi-peak structure of the current of a single charge-state Iq(t) is discussed with regard to charge recombination.

Nonlinear Characteristics of an Intense Laser Pulse Propagating inPartially Stripped Plasmas**The project supported byNational Natural Science Foundation of China under Grant No. 10276002

The nonlinear optic characteristics of an intense laserpulse propagating in partially stripped plasmas are investigatedanalytically. The phase and group velocity of the laser pulse propagationas well as the three general expressions governing the nonlinear opticbehavior, based on the photon number conservation, are obtained byconsidering the partially stripped plasma as a nonlinear optic medium. Thenumerical result shows that the presence of the bound electrons in partiallystripped plasma can significantly change the propagating property of theintense laser pulse.

Numerical solution of Boltzmann tranport equation for TEA CO 2 laser having nitrogen-lean gas mixtures to predict laser characteristics and gas lifetime

Selective laser isotope separation by TEA CO 2 laser often needs short tail-free pulses. Using laser mixtures having very little nitrogen almost tail free laser pulses can be generated. The laser pulse characteristics and its gas lifetime is an important issue for long-term laser operation. Boltzmann transport equation is therefore solved numerically for TEA CO 2 laser gas mixtures having very little nitrogen to predict electron energy distribution function (EEDF). The distribution function is used to calculate various excitation and dissociation rate of CO 2 to predict laser pulse characteristics and laser gas lifetime, respectively. Laser rate equations have been solved with the calculated excitation rates for numerically evaluated discharge current and voltage profiles to calculate laser pulse shape. The calculated laser pulse shape and duration are in good agreement with the measured laser characteristics. The gas lifetime is estimated by integrating the equation governing the dissociation of CO 2. An experimental study of gas lifetime was carried out using quadrapole mass analyzer for such mixtures to estimate the O 2 being produced due to dissociation of CO 2 in the pulse discharge. The theoretically calculated O 2 concentration in the laser gas mixture matches with experimentally observed value. In the present TEA CO 2 laser system, for stable discharge the O 2 concentration should be below 0.2%.

TEA CO 2 Laser Simulator: A software tool to predict the output pulse characteristics of TEA CO 2 laser

“TEA CO 2 Laser Simulator” has been designed to simulate the dynamic emission processes of the TEA CO 2 laser based on the six-temperature model. The program predicts the behavior of the laser output pulse (power, energy, pulse duration, delay time, FWHM, etc.) depending on the physical and geometrical input parameters (pressure ratio of gas mixture, reflecting area of the output mirror, media length, losses, filling and decay factors, etc.). Program summary Title of program: TEA_CO2 Catalogue identifier: ADVW Program summary URL: http://cpc.cs.qub.ac.uk/summaries/ADVW Program obtainable from: CPC Program Library, Queen's University of Belfast, N. Ireland Computer: P.IV DELL PC Setup: Atomic Energy Commission of Syria, Scientific Services Department, Mathematics and Informatics Division Operating system: MS-Windows 9x, 2000, XP Programming language: Delphi 6.0 No. of lines in distributed program, including test data, etc.: 47 315 No. of bytes in distributed program, including test data, etc.:7 681 109 Distribution format:tar.gz Classification: 15 Laser Physics Nature of the physical problem: “TEA CO 2 Laser Simulator” is a program that predicts the behavior of the laser output pulse by studying the effect of the physical and geometrical input parameters on the characteristics of the output laser pulse. The laser active medium consists of a CO 2–N 2–He gas mixture. Method of solution: Six-temperature model, for the dynamics emission of TEA CO 2 laser, has been adapted in order to predict the parameters of laser output pulses. A simulation of the laser electrical pumping was carried out using two approaches; empirical function equation (8) and differential equation (9). Typical running time: The program's running time mainly depends on both integration interval and step; for a 4 μs period of time and 0.001 μs integration step (defaults values used in the program), the running time will be about 4 seconds. Restrictions on the complexity: Using a very small integration step might leads to stop the program run due to the huge number of calculating points and to a small paging file size of the MS-Windows virtual memory. In such case, it is recommended to enlarge the paging file size to the appropriate size, or to use a bigger value of integration step.

Measurement of femtosecond laser pulses using PG frequency-resolved optical gating

A frequency_resolved optical gating (PG_FROG) with polarization gating technique is developed to diagnosis the characteristics of laser pulses generated from a 20_TW femtosecond laser amplifier. Based on the data of the correlation signal in time and frequency domains, we obtained the detail information on pulse shape, bandwidth, pulse duration and phase by using the retrieval algorithm. The measured results with different phase compensation are also presented and discussed.

Single-shot dynamics of pulses from a gas-filled hollow fiber

We present measurements of the performance characteristics of few-cycle laser pulses generated by propagation through a gas-filled hollow fiber. The pulses going into the fiber and the compressed pulses after the fiber were simultaneously fully characterized shot-by-shot by using two kHz SPIDER setups and kHz pulse energy measurements. Output-pulse properties were found to be exceptionally stable and pulse characteristics relevant for non-linear applications like high-harmonic generation are discussed.

Mathematical modeling of the intracavity Q-switched Nd-glass laser

The temporal characteristics of the Q-switched Nd-glass laser have been numerically investigated. A mathematical model describing the dynamic emission and different physical processes has been adapted. This model allows the investigation of the nonlinear saturable absorber effects on the mode characteristics of the Nd-glass laser, and studying the affects of the laser input parameters on the output laser pulse characteristics. Numerical solutions of a nonlinear rate equation system predict the generation of nanosecond pulses of Q-switched Nd-glass laser. The solutions estimate the laser density and the relative population inversion of the Nd-glass laser rod and saturable absorber for different emission regimes. The estimated results of the laser output pulse characteristics are in a good agreement with the other calculated and experimental results.

Influence of Temperature Phase Grating Relaxation Times on the Lasing Characteristics of Q-Switched and Mode-Locked Nd:YAG Laser-Pumped Distributed Feedback Dye Lasers

This work presents a theoretical explanation and experimental evidence of pulses- intensity displacement and pulse- broadening profiles of a Q-switched and mode-locked Nd:YAG laser- pumped distributed feedback dye laser (DFDL). Experimental results followed by a theoretical simulation indicate that DFDL pulses peaked noticeably later than their corresponding Nd:YAG laser pulses. The DFDL pulse corresponding to the peak- power pump laser pulse occurs few pulses later than the expected location. The observed and simulated delays were found to depend upon the inter-pulse periods of the pumping Q-switched and mode-locked laser. We report the theoretical basis for the influence of temperature grating relaxation times on the dye laser's pulse characteristics, and apply the transient grating technique to measure the refractive index of a lasing dye solution. Results indicate that the thermal effects of preceding pulses integrate the gain for receding pulses. These results provide a good understanding of temperature grating relaxation times and their impact on the lasing characteristics of DFDL.

Saturated Ni-like Ag x-ray laser at 13.9 nm pumped by a single picosecond laser pulse

A demonstration of a transient collisional x-ray laser pumped by only a single profiled picosecond laser pulse is reported. Saturated operation on the $4d\ensuremath{-}4p$ transition in Ni-like silver at 13.9 nm is demonstrated using a pump energy of less than 3 J, i.e., more than halved in comparison to the best achievements in the double-pulse scheme. The pump laser pulse characteristics, traveling excitation wave, and x-ray output are discussed as well.

Uniform theory of the Talbot effect with partially coherent light illumination.

A uniform formulation for the self-imaging of gratings with any kind of partially coherent illumination is developed in terms of the cross mutual spectral density of the partial coherence theory. The formulation includes the time diffractive intensity distribution and the averaged diffractive intensity distribution at self-imaging distances and can be applied to both continuous and temporal illuminations with any kind of spectra. It is found that the averaged intensity distribution is related only to the intensity spectrum of illumination. The continuous polychromatic illumination and the ultrashort laser pulses with or without frequency chirp are then studied by a numerical stimulation. It is shown that the ultrashort laser pulse and the continuous polychromatic illuminations have similar averaged self-image distributions. Thus the Talbot effect may help in the study of the temporal and spectral characteristics of ultrashort laser pulses. An experiment with an LED is given, as well.

An alternative approach to temporal laser-wing effects in saturated laser-induced fluorescence

Temporal laser-wing effects are associated with the unavoidable time characteristics of laser pulses. To remove their contribution in saturated laser-induced fluorescence, time sampling is commonly adopted. In the present work, an alternative approach is formulated. It is based on power measurements obtained from the whole fluorescence response. Numerical simulations with a four-level dynamical model are described to test the method against the traditional time-sampling procedure. A greater amount of measured signal and independence from the fast electronic sampler are the main advantages of the proposed method.

Ultrafast-laser-radiation transfer in heterogeneous tissues with the discrete-ordinates method.

Here light propagation and radiation transfer of ultrafast laser pulses in heterogeneous biological tissues are simulated by use of the discrete-ordinates method (DOM). Formulations for solving the time-dependent radiation-transfer equation are deduced for three-dimensional geometries incorporating the Fresnel specularly reflecting boundary condition and characteristics of ultrafast laser pulses. The present method can treat both the incident laser intensity and the scattered radiation intensity from the walls of the targeted tissue as two components, i.e., a diffuse part and a specular part. Reflectivity at the tissue-air interface is calculated by use of Snell's law and the Fresnel equation. The high-order S10 DOM method is found to be adequate for describing the propagation and transfer of ultrafast laser radiation in heterogeneous tissues. The time-dependent radiation field in the tissue as well as the temporal radiation intensity profiles at the boundaries can be obtained simultaneously. The absolute values of the logarithmic slope of the temporal reflectance and transmittance at various detector positions are found to converge to a constant value in a homogeneous tissue model. With the inclusion of a small inhomogeneity, such a value will change in line with the property of the embedded inhomogeneity. The orientation of heterogeneity of the tissues also substantially affects the radiation intensity at the boundaries. The effect of the Fresnel boundary in the modeling is pronounced. The simulated transmitted signals are broadened and amplified under specularly reflecting boundary condition as compared with those under diffusely reflecting boundary conditions.

Fabrication of a miniaturized electron lens system and laser micro-machining condition for silicon membrane

For a miniaturized electron beam system application, miniaturized electrostatic electron lenses and their assembly, called a microcolumn, have been fabricated using an active Q-switched 1.06 μm Nd:YAG pulsed laser beam. The laser micro-machining condition for 20 μm thick Si membrane has been investigated by varying the pulse width and the effect of power density will be discussed. The geometrical structures of laser micro-machined electron lenses such as diameter, the roundness of the aperture wall and the degree of the thermal distortion are thought to be dependent on the characteristics of the individual laser pulse. A successful alignment of electron lenses using a simple laser diffraction pattern will be reported, also.

Ultrashort laser pulse beam shaping.

We calculated the temporal and spatial characteristics of an ultrashort laser pulse propagating through a diffractive beam-shaping system that converts a Gaussian beam into a beam with a uniform irradiance profile that was originally designed for continuous waves [Proc. SPIE 2863, 237(1996)]. The pulse front is found to be considerably curved for a 10-fs pulse, resulting in a temporal broadening of the pulse that increases with increasing radius. The spatial intensity distribution deviates significantly from a top-hat profile, whereas the fluence shows a homogeneous radial distribution.

Influence of pulse duration on ultrashort laser pulse ablation of biological tissues.

Ablation characteristics of ultrashort laser pulses were investigated for pulse durations in the range of 130 fs-10 ps. Tissue samples used in the study were dental hard tissue (dentin) and water. We observed differences in ablation crater morphology for craters generated with pulse durations in the 130 fs-1 ps and the 5 ps-10 ps range. For the water experiment, the surface ablation and subsequent propagation of stress waves were monitored using Mach-Zehnder interferometry. For 130 fs-1 ps, energy is deposited on the surface while for longer pulses the beam penetrates into the sample. Both studies indicate that a transition occurs between 1 and 5 ps.

Remote Measurement of Turbulent Wind Spectra by Heterodyne DopplerLidar Technique

Heterodyne Doppler lidars (HDLs) are used to monitor atmospheric wind field and wind turbulence at remote distance. This last application calls for the derivation of wind spectra, which can be characterized by the dissipation rate and the κ-spectral peak (or outer scale of turbulence). However, the HDL technique may suffer two problems. First, HDL measurements result in spatial averaging of the true wind velocity along the line of sight, because of the laser pulse duration and windowing effect on processed signals. Second, even at high signal-to-noise ratio, the retrieved turbulent velocity field may be contaminated by errors due to speckle fluctuations. It is shown that both spatial averaging and error contribution to the wind spectra can be modeled starting from the transmitted laser pulse characteristics and signal processing parameters, so that their effect can be predicted. The rms difference between the estimated and predicted turbulent spectra is minimized in order to infer the turbulence parameters. This procedure is tested on simulated signals and validated on actual data taken by a 10-μm HDL during a field campaign in 1995. The data collected during two periods of two consecutive days (9 and 10 March and 13 and 14 March 1995) are analyzed. On these days, moderate to light winds prevailed. The stability parameter zi/LMO indicated slightly unstable conditions with sometimes probable convection. The HDL measured energy dissipation rates ranging between 0.7 × 10−3 and 8 × 10−3 m2 s−3 in good agreement with sonic anemometer measurements. The κ-spectral peak ranged between 200 and 600 m.