Flat-top Laser Research Articles

Simple scheme for ultraviolet time-pulse shaping

We present a method to generate high-energy flat-top UV laser pulses such as the ones needed to optimally drive high-brightness radio-frequency photoinjectors. In this scheme we believe to be novel, the longitudinal profile of a laser pulse from a Ti:sapphire master oscillator power amplifier system is controlled using a mechanical mask in the Fourier plane of a 4f stretcher located after the harmonic conversion crystals. Such a scheme allows us to overcome many of the difficulties faced by current state-of-the-art pulse-shaping designs. These are in fact based on various versions of preamplifier infrared shapers and hence suffer from the limitations set by the nonlinearities of chirped-pulse amplification and harmonic conversion. Beyond the clear advantages of simplicity and robustness, the proposed solution offers the possibility to deliver a pulse with very short rise and fall times and to freely change the output pulse length. We also note that, after proper calibration between spectral and temporal profiles, the shaper optical setup offers the possibility to retrieve the longitudinal profile of the laser pulse on a shot-to-shot basis.

Effects of sidelobes of focused flat-topped laser beams on vacuum electron acceleration

Using three-dimensional test particle simulations, we investigated electrons accelerated by a focused flat-top laser beam at different intensities and flatness levels of the beam profile before focusing in vacuum. The results show that the presence of sidelobes around the main focal spot of the focused flat-top laser beam influences the optimum (as far as electron acceleration is concerned) initial momentum (and incident angle) of electrons for acceleration. The difference of initial conditions between laser beams with and without sidelobes becomes evident when the laser field is strong enough (a0>10, corresponding to intensities I>1×1020 W/cm2 for the laser wavelength λ=1 μm, where a0 is a dimensionless parameter measuring laser intensity). The difference becomes more pronounced at increasing a0. Because of the presence of sidelobes, there exist three typical CAS (capture and acceleration scenario) channels when a0≥30 (corresponding to I>1×1021 W/cm2 for λ=1 μm). The energy spread of the outgoing electrons is also discussed in detail.

Steady state ion acceleration by a circularly polarized laser pulse

The steady state ion acceleration at the front of a cold solid target by a circularly polarized flat-top laser pulse is studied with one-dimensional particle-in-cell (PIC) simulation. A model that ions are reflected by a steady laser-driven piston is used by comparing with the electrostatic shock acceleration. A stable profile with a double-flat-top structure in phase space forms after ions enter the undisturbed region of the target with a constant velocity.

Simulations on Generating Long Flat-Top Laser Pulses for Fast Ignition of Laser Fusion

A modified input–output algorithm is first applied to generate long flat-top pulses using a phase-only pulse shaper for fast ignition of laser fusion. More accurate long (10–20 ps) flat-top pulses can be obtained within several hundred iterations that take much shorter simulation time compared with “trial-and-error” algorithms. Also the flat-top pulses generated by the input–output algorithm are free from noise in the wings of the main pulse, thus higher pulse contrast with reduced pedestal can be obtained, which is very important for the fast ignition of laser fusion. Simulations also show that flatness on the flat-top of the pulse is fairly insensitive to perturbations of input signals.

Design and construction of a prototype of a flat top beam interferometer and initial tests.

A non-Gaussian, flat-top laser beam profile, also called Mesa Beam Profile, supported by non spherical mirrors known as Mexican Hat (MH) mirrors, has been proposed as a way to depress the mirror thermal noise and thus improve the sensitivity of future interferometric Gravitational Wave detectors, including Advanced LIGO [1]. Non-Gaussian beam configurations have never been tested before [2] hence the main motivation of this project is to demonstrate the feasibility of this new concept. A 7m rigid suspended Fabry-Perot (FP) cavity which can support a scaled version of a Mesa beam applicable to the LIGO interferometers has been developed. The FP cavity prototype is being designed to prove the feasibility of actual MH mirror profiles, determine whether a MH mirror cavity is capable of transforming an incoming Gaussian beam into a flat top beam profile, study the effects of unavoidable mirror imperfections on the resulting beam profile and gauge the difficulties associated with locking and maintaining the alignment of such an optical cavity. We present the design of the experimental apparatus and simulations comparing Gaussian and Mesa beams performed both with ideal and current (measured) mirror profiles. An overview of the technique used to manufacture this kind of mirror and initial results showing Mesa beam properties are presented.

Characterizing flat-top laser beams using standard beam parameters

We examine the correspondence between various models describing flat-top laser beam profiles using two standard parameters; namely, the M2 factor and the kurtosis parameter. Numerical expressions for M2, based on the second moment of the beam irradiance distribution in the near and far fields and for the kurtosis parameter, k, based on the fourth moment at the near field, are obtained. Plots of k in the near field versus M2 demonstrate the similarities between the different analytical models used to describe flat-top profiles. Using the Padé approximation, a relationship between k and M2, a new reference formula, is derived that predicts the values of M2 to within less than a percent for these flattened beams. This method is then extended to define numerical expressions relating the beam parameters (i.e., M2 and k) and the parameters describing the beam characteristic in each analytical model (model parameters). The results obtained using the Padé method are used to describe the output beam profiles for a high power copper vapour laser fitted with an unstable resonator.PACS Nos.: 42.55.–f, 42.55.Lt, 42.60.–v

Desorption/Ionization Fluence Thresholds and Improved Mass Spectral Consistency Measured Using a Flattop Laser Profile in the Bioaerosol Mass Spectrometry of Single Bacillus Endospores

Bioaerosol mass spectrometry is being developed to analyze and identify biological aerosols in real time. Mass spectra of individual Bacillus endospores were measured with a bipolar aerosol time-of-flight mass spectrometer in which molecular desorption and ionization were produced using a single laser pulse from a Q-switched, frequency-quadrupled Nd:YAG laser that was modified to have an approximately flattop profile. The flattened laser profile allowed the minimum fluence required to desorb and ionize significant numbers of ions from single aerosol particles to be determined. For Bacillus spores, this threshold had a mean value of approximately 1 nJ/microm(2) (0.1 J/cm(2)). Thresholds for individual spores, however, could apparently deviate by 20% or more from the mean. Threshold distributions for clumps of MS2 bacteriophage and bovine serum albumin were subsequently determined. Finally, the flattened profile was observed to increase the reproducibility of single-spore mass spectra. This is consistent with the general conclusions of our earlier paper on the fluence dependence of single-spore mass spectra and is particularly significant because it is expected to enable more robust differentiation and identification of single bioaerosol particles.

Thomson backscattering X-rays from ultra-relativistic electron bunches and temporally shaped laser pulses

The process of Thomson scattering of an ultra-intense laser pulse by a relativistic electron bunch has been proposed as a way to obtain a bright source of short, tunable and quasi-monochromatic X-ray pulses. The real applicability of such a method depends crucially on the electron-beam quality, the angular and energetic distributions playing a relevant role. In this paper we present the computation of the Thomson-scattered radiation generated by a plane-wave, linearly polarized and flat-top laser pulse, incident on a counterpropagating electron bunch having a sizable angular divergence and a generic energy distribution. Both linear and nonlinear Thomson-scattering regimes are considered and the impact of the rising front of the pulse on the scattered-radiation distribution has been taken into account. Simplified relations valid for long laser pulses and small values of both scattering angle and bunch divergence are also reported. Finally, we apply the results to the cases of backscattering with electron bunches typically produced with both standard radio-frequency-based accelerators and laser–plasma accelerators.

Improvements in ion signal reproducibility obtained using a homogeneous laser beam for on-line laser desorption/ionization of single particles.

A major factor limiting on-line single particle mass spectrometry techniques from becoming more quantitative is the large shot-to-shot variability in ion intensities observed in the laser desorption/ionization (LDI) mass spectra.1,2 In previous work, lab-generated particles showed fluctuations of up to 152% in the absolute ion intensities in averaged spectra of 200-300 'identical' particles.2 Most of these fluctuations were attributed to inhomogeneities in the laser beam profile, leading to significant differences in the power each particle encountered depending on the position in the LDI laser beam where it underwent analysis. The goal of the work presented herein is to determine whether a fiber optic actually reduces the observed variability in single particle LDI mass spectral data. Initial results are presented for individual single component organic particles composed of 2,4-dihydroxybenzoic acid (2,4-DHB) analyzed using a low-power flat-top laser beam profile created by sending an ultraviolet (266 nm) DI laser through a fiber optic. Relative standard deviations of the total ion intensities for peaks in individual spectra are reduced to 31%. Single particle spectra, compared with and without laser homogenization at the same nominal laser fluence, show a marked enhancement. Specifically, the ion signal patterns of the 2,4-DHB particle spectra obtained using a homogenous LDI beam look identical to one another (i.e. only one particle type was produced with a commonly used neural network grouping algorithm), whereas without beam homogenization 25 different particle types (based on ion intensity patterns) were obtained. Future publications will explore more particle types and matrices but the initial results described herein are quite encouraging.

Status of the SPARC project

In this paper we report on the final design of the SPARC FEL experiment which is under construction at the Frascati INFN Laboratories by a collaboration between INFN, ENEA, ELETTRA, Un. of Rome (Tor Vergata), CNR and INFM. This project comprises an advanced 150 MeV photo-injector aimed at producing a high brightness electron beams to drive a SASE-FEL experiment in the visible using a segmented 12 m long undulator. The project, finally approved and funded early this year, has a 3 year time span, with the final goal of reaching saturation on the fundamental of the SASE-FEL and studying the resonant non-linear generation of harmonics. Peculiar features of this project are the optimized design of the photo-injector to reach minimum emittances by using flat-top laser pulses on the photocathode, and the use of an uncompressed electron beam of 100 A peak current at very low emittance to drive the FEL. Results of start-to-end simulations carried out to optimize the performances of the whole system are presented, as well as the status of the construction and assembly of the system components. Activities planned for a second phase of the project are also mentioned: these are mainly focused on velocity bunching experiments that will be conducted with the aim to reach higher peak currents with preservation of low transverse emittances.

Line-focused laser ablation for depth-profiling analysis of coated and layered materials.

The performance features of line-focused laser ablation for the characterization of interfaces in layered materials by laser-induced plasma spectrometry (LIPS) have been compared with the point-focusing method in terms of signal precision, signal-to-noise ratio, ablation rates, and surface sensitivity. In both optical configurations a pulsed Nd:YAG laser beam operating at 532 nm, with a homogeneous energy distribution (flattop laser), is used to generate point and microline plasmas on the sample surface. Subsequent light from the plasma is spectrally resolved and detected with an imaging spectrograph and an intensified charge-coupled-device detector that is binned along the slit-height direction. Line-focusing LIPS permits much higher laser power input while maintaining relatively low laser fluence, thus yielding better surface sensitivity and improved detection power. Values of the signal-to-noise ratio are improved by a factor of 6. In addition the ablation rate is 9 nm/pulse with the microline approach compared with 23 nm/pulse obtained with the point-focusing method. The results demonstrate that the microline-focusing approach is suitable for the depth analysis of coated and layered materials.

Chemical maps of patterned samples by microline‐imaging laser‐induced plasma spectrometry

AbstractThe potential of a microline‐imaging laser‐induced plasma spectrometry (LIPS) system for surface and depth analysis of heterogeneous solid samples in air at atmospheric pressure has been demonstrated. A pulsed Nd : YAG laser beam operating at 532 nm, with a homogeneous energy distribution (flat top laser), was used to generate a microline plasma on the sample surface. Subsequent light from the microline plasma was resolved spectrally and spatially and detected with an imaging spectrograph and an intensified charged‐coupled device detector. A patterned metal sample was chosen as the most appropriate for this study. Three‐dimensional chemical maps of Ni and Cu from the edge connectors of a printed circuit board have been obtained. With this experimental configuration, the lateral resolution (limited by crater width) was 42 µm and the spatial resolution along the spectrometer slit was 17.4 µm. The results illustrate the capability of microline imaging for fast mapping of large‐area samples and for depth profiling purposes. Copyright © 2003 John Wiley & Sons, Ltd.

Pulsed optoacoustic characterization of layered media

Thermoelastic waves generated by absorption of short laser pulses are used to characterize the layer structure of materials. The method is based on the analysis of the distribution of absorbed laser energy from temporal profiles of recorded acoustic signals. Particularly in view of noninvasive medical applications, optoacoustic front surface transducers are investigated in this study, where irradiation of the surface and detection of the acoustic wave take place on the same side of the sample. Front surface detection of optoacoustic waves is studied theoretically and experimentally, with special emphasis on acoustic diffraction and the differences between measurements in the acoustic near and far field. In the experiments, samples with stepwise and continuously varying depth profiles of absorption coefficient were irradiated with laser pulses of 6–8 ns duration. For the detection of the acoustic waves either an optical ultrasound sensor or an annular piezoelectric film was used. Generating the optoacoustic waves with a flat top laser beam profile and detecting the signals in the acoustic near field yields optimal conditions for direct measurements of the distribution of absorbed energy and the absorption coefficient in the medium. Far field measurements are advantageous for detecting and imaging layer boundaries at large depths in the sample.

Influence of the laser fluence in infrared matrix-assisted laser desorption/ionization with a 2.94 microm Er : YAG laser and a flat-top beam profile.

The dependence of the signal intensity of analyte and matrix ions on laser fluence was investigated for infrared matrix-assisted laser desorption/ionization (IR-MALDI) mass spectrometry using a flat-top laser beam profile. The beam of an Er : YAG laser (wavelength, 2.94 microm; pulse width, 90 ns) was coupled into a sapphire fiber and the homogeneously illuminated end surface of the fiber imaged on to the sample by a telescope. Three different laser spot sizes of 175, 350 and 700 microm diameter were realized. Threshold fluences of common IR matrices were determined to range from about 1000 to a few thousand J m(-2), depending on the matrix and the size of the irradiated area. In the MALDI-typical fluence range, above the detection threshold ion signals increase strongly with fluence for all matrices, with a dependence similar to that for UV-MALDI. Despite the strongly different absorption coefficients of the tested matrices, varying by more than an order of magnitude at the excitation laser wavelength, threshold fluences for equal spot sizes were found to be comparable within a factor of two. With the additional dependence of fluence on spot size, the deposited energy per volume of matrix at threshold fluence ranged from about 1 kJ mol(-1) for succinic acid to about 100 kJ mol(-1) for glycerol.

Transverse emittance measurements from a photocathode RF gun with variable laser pulse length

The gun test facility (GTF) at SSRL was started in 1996 to develop an appropriate injector for the proposed linac coherent light source (LCLS) at SLAC. The LCLS design requires the injector to produce a beam with at least 1 nC of charge in a 10 ps or shorter pulse with no greater than 1π mm mrad normalized rms emittance. The photoinjector at the GTF is 1.6 cell S-band symmetrized gun and emittance compensation solenoid. Emittance measurements, reported here, were made as function of laser pulse width using Gaussian longitudinal pulses. The lowest achieved emittance to date with 1 nC of charge is 5.6π mm mrad and was obtained with a pulse width of 5 ps (FWHM) and is in agreement with simulation. There are indications that the accelerator settings for these results may not have been optimal. Simulations also indicate that a normalized emittance meeting the LCLS requirement can be obtained using appropriately shaped transverse and temporal laser/electron beam pulses. Work has begun on producing temporal flat top laser pulses which combined with transverse clipping of the laser is expected to lower the emittance to approximately 1π mm mrad for 1 nC with optimal accelerator settings.

Investigations of 2,5-DHB and succinic acid as matrices for IR and UV MALDI. Part: I UV and IR laser ablation in the MALDI process

UV and IR laser ablation under MALDI conditions is described for two typical solid UV and IR matrices. 2,5-Dihydroxy benzoic acid (2,5-DHB) as a UV and IR matrix and succinic acid (SA) as an IR matrix were investigated systematically by light and electron microscopy, and by mass spectrometry. Large single crystals of 2,5-DHB and SA with and without protein incorporated were used for the experiments. The UV MALDI experiments were performed with a flat-top laser beam profile. Within a limited fluence range, these exposures resulted in the formation of typical cone structures, occurring after several hundred laser shots onto a given spot. Such structures had originally been described for materials processing of polymers and ceramics with excimer lasers. For the IR exposures, a Gaussian laser beam profile was used. The much lower absorption of matrix compounds at IR wavelengths compared with the absorption of matrix compounds at UV wavelengths results in a much larger penetration depth of the IR laser light into the matrix solid and consequently in a much higher ablation depth and amount of ablated material. This large volume of material, ablated per single exposure, prevents the formation of specific surface structures in IR MALDI. The amount of matrix material ablated per laser shot was measured with a laser profilometer to about 10 000 μm 3. This determines the amount of consumed protein per laser shot to about 1 fmol under typical IR MALDI conditions.

Nearly flat-top laser beams from unstable resonators with internal spatial filtering

We theoretically show that negative branch confocal unstable resonators with an aperture at the confocal plane can be used to generate nearly flat-top laser beams at large Fresnel numbers. Such a behavior is due to the fact that these resonators can be viewed as image relay systems that provide magnified versions of the internal aperture. Modal discrimination increases with magnification and, for most conditions, should provide single transverse mode oscillation. Other unstable resonators with an internal aperture can be used to generate nearly flat-top laser beams provided they satisfy a simple analytical criterion.

On the phase velocity of plasma waves in a self-modulated laser wake-field accelerator

The properties of the wake field excited by a flattop laser pulse with a sharp leading edge and a power below the critical one for relativistic self-focusing are studied analytically and numerically with emphasis on the phase velocity of the plasma wave. The paraxial model describing modulation of the pulse spot size is shown to provide a good quantitative description, including preexponential factors, of the wake field during the linear stage of the resonant self-modulation instability. Though the phase velocity of the excited plasma wave is shown to be close to the group velocity of the laser pulse, the relativistic γ-factor corresponding to the phase velocity, which is a crucial parameter for particle acceleration, can be substantially less than that of the group velocity. Fortunately, for the considered three-dimensional regime of self-modulation, the noticeable difference between these two γ-factors takes place only in the initial stage of the instability and vanishes in the later stages.