Flat-top Profile Research Articles

Effect of driving pulse properties on the performance of sub-terawatt laser wakefield acceleration

By introducing a sub-terawatt (TW) laser pulse into a high-density gas target, the self-focusing effect and the self-modulation instability can greatly enhance the laser peak power to a level capable of driving the laser wakefield acceleration (LWFA) of electrons. A particle-in-cell model has been developed to study the scheme in which 1030-nm pulses produced from a diode-pumped laser system are introduced into a gas cell with a flat-top density profile, allowing the LWFA to be operated at high frequencies. Because 1030-nm lasers are typically produced with a long duration >200 fs, a spectral broadening technique can be applied to reduce the pulse duration, from which a greater ponderomotive force is acquired to drive LWFA. To understand the dependence of LWFA performance on the driving pulse duration, selected durations, ranging from 200 fs to 10 fs, are assigned for 0.5-TW, 1030-nm pulses in a series of simulations. Results show that a duration around 50 fs can provide the optimal LWFA results, as a compromise between the weak ponderomotive force available from a long pulse >100 fs and the depletion effect which can rapidly diminish a short pulse <25 fs in a dense plasma. When a low laser peak power of 0.25-TW is available, the pulse depletion can be significant at a high target density and render LWFA ineffective. Using a laser pulse with a longer wavelength >2 μm represents a viable route to realize the LWFA with a low laser peak power; in this way, an appropriately selected target density which allows the laser peak power PL ∼ 1.25Pcr of self-focusing critical power is favourable for realizing an efficient LWFA process.

Experimentally-validated computational model for temperature evolution within laser heated fiber-reinforced polymer matrix composites

Comprehensive experimental and computational studies have been conducted to accurately measure and predict the temperature evolution of polymer matrix composite material subjected to laser heating. Plain-woven 16-ply composites with T650 carbon fibers in a bismaleimide matrix were irradiated with a continuous wave fiber laser (1.0692 μm wavelength) with a 3.0 cm diameter flat-top beam profile. The spatial and temporal evolution of temperature for the front (laser-exposed side) and back (unexposed) surfaces of the composite specimens were monitored and recorded with calibrated infrared cameras. Additionally, internal temperature evolution was measured with thermocouples embedded in the midplane of the composite specimens. A nonlinear transient finite element (FE) analysis, based on conductive, convective, and radiative heat transfer, was conducted using ANSYS to predict the temperature history during heating (laser exposure) and cooling (after exposure). For all investigated laser irradiances (i.e., 0.71–1.95 W cm−2), the numerical prediction demonstrated excellent agreement with experimental data.

Photobiomodulation with 808-nm diode laser light promotes wound healing of human endothelial cells through increased reactive oxygen species production stimulating mitochondrial oxidative phosphorylation.

Photobiomodulation of cells using near-infrared (NIR) monochromatic light can affect cell functions such as proliferation, viability, and metabolism in a range of cell types. Evidence for the effects of near-infrared light on endothelial cells has been reported, but the studies were mainly performed using VIS light emitted by low-energy lasers, because NIR wavelengths seemed negatively stimulate these cells. Cell viability, free radical-induced oxidative stress, NF-κB activation, nitric oxide release, mitochondrial respiration, and wound healing repair were assessed in human endothelial cells (HECV) irradiated with 808-nm diode laser light (laser setup = 1W/cm2, 60s, 60J/cm2, CW vs measured energy parameter = 0.95W/cm2, 60s, 57J/cm2, mode CW) emitted by an handpiece with flat-top profile. No difference in viability was detected between controls and HECV cells irradiated with 808-nm diode laser light for 60s. Irradiated cells demonstrated higher proliferation rate and increased migration ability associated to moderate increase in ROS production without a significant increase in oxidative stress and oxidative stress-activated processes. Near-infrared light stimulated mitochondrial oxygen consumption and ATP synthesis in HECV cells. Short near-infrared irradiation did not affect viability of HECV cells, rather led to a stimulation of wound healing rate, likely sustained by ROS-mediated stimulation of mitochondrial activity. Our results demonstrating that near-infrared led to a shift from anaerobic to aerobic metabolism provide new insight into the possible molecular mechanisms by which photobiomodulation with 808-nm diode laser light protects against inflammation-induced endothelial dysfunction, seemingly promising to enhance their therapeutic properties.

Focus shaping of the radially polarized Laguerre-Gaussian-correlated Schell-model vortex beams.

In this paper, we introduce a new kind of partially coherent vector beam with special correlation function and vortex phase named radially polarized Laguerre-Gaussian-correlated Schell-model (LGCSM) vortex beam as a natural extension of scalar LGCSM vortex beam. The realizability conditions for such beam are derived. The tight focusing properties of a radially polarized LGCSM vortex beam passing through a high numerical aperture (NA) objective lens are investigated numerically based on the vectorial diffraction theory. We find that not only the transverse component but also the longitudinal component of the focal field distributions can be shaped by regulating the structures of the correlation functions, which is quite different from that of the conventional radially polarized partially coherent beam. Moreover, a series of wildly used focal field with novel structure, e.g., focal spot, flat-topped or doughnut beam profiles, needle-like focal field and controllable three-dimensional (3D) optical cage, were obtained. These results indicate that the focus shaping can be achieved by combining the regulation of the structures of the correlation functions with the regulation of beam parameters effectively. Our results may be useful for potential applications in optical trapping, optical high-resolution microscopy and optical data storage.

Average intensity and spreading of a radially polarized multi-Gaussian Schell-model beam in anisotropic turbulence

Analytical expressions of average intensity and long-term beam width for a radially polarized multi-Gaussian Schell-model (MGSM) beam in anisotropic atmospheric turbulence are developed based on the extended Huygens-Fresnel principle. The influences of anisotropic non-Kolmogorov turbulence and source parameters on the average intensity, polarization and long-term beam spreading of radially polarized MGSM beam are examined in detail. The source correlations, free-space diffraction and turbulent effect jointly affect both the intensity and polarization profiles of the beam propagated in the atmosphere. As a radially polarized MGSM beam propagates in the atmosphere, increasing the associated beam order can lower the evolution rate from its flat-topped beam profile into a Gaussian beam profile because of the turbulent effect. Variations in the average intensity and long-term beam width are closely related to the propagation distance, power spectrum index, inner-scale, initial spatial coherence, and beam width. The atmospheric turbulence-induced beam spreading is generally suppressed with the enhanced anisotropy coefficient.

Flat-field illumination for quantitative fluorescence imaging.

The uneven illumination of a Gaussian profile makes quantitative analysis highly challenging in laser-based wide-field fluorescence microscopy. Here we present flat-field illumination (FFI) where the Gaussian beam is reshaped into a uniform flat-top profile using a high-precision refractive optical component. The long working distance and high spatial coherence of FFI allows us to accomplish uniform epi and TIRF illumination for multi-color single-molecule imaging. In addition, high-throughput borderless imaging is demonstrated with minimal image overlap.

Record fifth-harmonic-generation efficiency producing 211 nm, joule-level pulses using cesium lithium borate.

The fifth harmonic of a pulsed Nd:YLF laser has been realized in a cascade of nonlinear crystals with a record efficiency of 30%. Cesium lithium borate is used in a Type-I configuration for sum-frequency mixing of 1053 and 266nm, producing 211nm pulses. Flat-topped beam profiles and pulse shapes optimize efficiency. The energies of the fifth harmonic up to 335mJ in 2.4ns pulses were demonstrated.

Experimental study: Underwater propagation of polarized flat top partially coherent laser beams with a varying degree of spatial coherence

We report on experiments where spatially partially coherent laser beams with flat top intensity profiles were propagated underwater. Two scenarios were explored: still water and mechanically moved entrained salt scatterers. Gaussian, fully spatially coherent beams, and Multi-Gaussian Schell model beams with varying degrees of spatial coherence were used in the experiments. The main objective of our study was the exploration of the scintillation performance of scalar beams, with both vertical and horizontal polarizations, and the comparison with electromagnetic beams that have a randomly varying polarization. The results from our investigation show up to a 50% scintillation index reduction for the case with electromagnetic beams. In addition, we observed that the fully coherent beam performance deteriorates significantly relative to the spatially partially coherent beams when the conditions become more complex, changing from still water conditions to the propagation through mechanically moved entrained salt scatterers.

All-fiber laser with flattop beam output using a few-mode fiber Bragg grating.

We report a novel method of beam shaping in an all-fiber laser. Flattop beams are first generated from the all-fiber laser using a few-mode fiber Bragg grating by superposing the fundamental mode (LP01) with a solid central beam distribution, and the second-order mode (LP11) gives rise to a donut-shape beam distribution. Two modes are obtained in the fiber laser simultaneously, and the wavelengths of the two modes are 1053.7nm and 1055.4nm, respectively. An experiment has been carried out, and a high-quality flattop profile is obtained for the normalized root-mean-square of the light intensity less than 5% under no numerical aperture illumination, consistent with the theoretical prediction.

The earthworm Dendrobaena veneta (Annelida): A new experimental-organism for photobiomodulation and wound healing.

Photobiomodulation (PBM) is a manipulation of cellular behavior using non-ablative low intensity light sources. This manipulation triggers a cascade of metabolic effects and physiological changes resulting in improved tissue repair, of benefit in the treatment of tissue injury, degenerative or autoimmune diseases. PBM has witnessed an exponential increase in both clinical instrument technology and applications. It is therefore of benefit to find reliable experimental models to test the burgeoning laser technology for medical applications. In our work, we proposed the earthworm Dendrobaena veneta for the study of nonablative laser-light effects on wound healing. In our preliminary work, D. veneta has been shown to be positively affected by PBM. New tests using D. veneta were set up to evaluate the effectiveness of a chosen 808 nm-64 J/cm2–1W-CW laser therapy using the AB2799 hand-piece with flat-top bean profile, on the wound healing process of the earthworm. Effective outcome was assimilated through examining the macroscopic, histological, and molecular changes on the irradiated posterior-segment of excised-earthworms with respect to controls. Three successive treatments, one every 24 hours, were concluded as sufficient to promote the wound healing, by effects on muscular and blood vessel contraction, decrement of bacteria load, reduction of inflammatory processes and tissue degeneration. D. veneta was demonstrated to be a reliable experimental organism that meets well the 3Rs principles and the National Science Foundation statement. Through their genetic and evolutionary peculiarity, comparable to those of scientifically accredited models, D. veneta allows the effect of laser therapies by multidisciplinary methods, at various degree of complexity and costs to be investigated.

Novel gas target for laser wakefield accelerators.

A novel gas target for interactions between high power lasers and gaseous medium, especially for laser wakefield accelerators, has been designed, manufactured, and characterized. The gas target has been designed to provide a uniform density profile along the central gas cell axis by combining a gas cell and slit nozzle. The gas density has been tuned from ∼1017 atoms/cm3 to ∼1019 atoms/cm3 and the gas target length can be varied from 0 to 10 cm; both changes can be made simultaneously while keeping the uniform gas profile. The gas density profile inside the gas cell has been measured using interferometry and validated using computational fluid dynamics.

Tight focusing properties of spirally polarized LG (1,1)* beam with High NA Parabolic mirror

The tight focusing of double ring shaped spirally polarized beam with high NA Parabolic mirror system is analyzed numerically based on the vector diffraction theory. Calculation results show that intensity distribution in focal region can be altered considerably on changing the spiral parameter (C) and pupil to beam ratio (β) of the incident double ring shaped beam. Many novel focal patterns such as focal spot, dark hollow focus and flat top profile of sub wavelength scale are achieved by proper tuning β and C.

Transport-of-intensity-based phase imaging to quantify the refractive index response of 3D direct-write lithography.

Precise direct-write lithography of 3D waveguides or diffractive structures within the volume of a photosensitive material is hindered by the lack of metrology that can yield predictive models for the micron-scale refractive index profile in response to a range of exposure conditions. We apply the transport of intensity equation in conjunction with confocal reflection microscopy to capture the complete spatial frequency spectrum of isolated 10 μm-scale gradient-refractive index structures written by single-photon direct-write laser lithography. The model material, a high-performance two-component photopolymer, is found to be linear, integrating, and described by a single master dose response function. The sharp saturation of this function is used to demonstrate nearly binary, flat-topped waveguide profiles in response to a Gaussian focus.

Radially polarized multi-Gaussian Schell-model beam and its tight focusing properties

In this paper, we introduce a new kind of vector multi-Gaussian Schell-model (MGSM) beam named radially polarized MGSM beam as an extension of recently introduced scalar MGSM beam [Opt. Lett.37, 2970 (2012)], and we obtain the realizability conditions of such beam. The tight focusing properties of a radially polarized MGSM beam passing through a high numerical aperture objective lens are investigated numerically based on the vectorial diffraction theory. It is interesting to find that the intensity distributions of both transverse and longitudinal fields near the focus display flat-top beam profiles due to the special correlation functions of the incident beam. Our results clearly show that engineering the structures of the correlation functions of a radially polarized partially coherent beam provides a novel way for shaping both transverse and longitudinal fields distributions, which have potential applications in particle acceleration and laser machining. Finally, we report experimental generation of a radially polarized MGSM beam with the help of a spatial light modulator and a radial polarization converter.

Compact conductively cooled electro-optical Q-switched Nd:YAG laser

We report on a compact conductively cooled high-repetition-rate nanosecond Nd:YAG laser. The oscillator was an laser diode side-pumped electro-optical (EO) Q-switched Nd:YAG rod laser adopting unstable cavity with a variable reflectivity mirror. A pulse train of 142 mJ with duration of 10 ns, repetition rate of 80 Hz at 1064 nm has been achieved. Maximum pulse energy was obtained at the pump energy of 1380 mJ, corresponding to the optical–optical conversion efficiency of 10.3%. The peak power was deduced to be 14.2 MW. The near-field pattern demonstrated a nearly super Gaussian flat top profile. To our knowledge, this is the highest repetition rate operation for a conductively cooled EO Q-switched Nd:YAG rod laser.

Beam shaping diffractive wave plates [Invited

We present and discuss opportunities opened up by a new generation of beam shaping optical elements that combine capabilities of digital spatial light polarization converters and diffractive properties of thin liquid crystalline films with patterned orientation of anisotropy axis (diffractive wave plates). Several functions of the new generation beam shapers are demonstrated, among them converting a laser beam of a Gaussian profile into a ring profile in the far field, a flattop profile, and into complex images. We also describe electrically controlled beam shaping optical elements which can be turned off and on within milliseconds by applying a low external voltage. Optical, morphological, and electro-optical properties of the components are characterized.

Be discs in coplanar circular binaries: Phase-locked variations of emission lines

The first results of radiative transfer calculations on decretion discs of binary Be stars are presented. A smoothed particle hydrodynamics code computes the structure of Be discs in coplanar circular binary systems for a range of orbital and disc parameters. The resulting disc configuration consists of two spiral arms, and can be given as input into a Monte Carlo code, which calculates the radiative transfer along the line of sight for various observational coordinates. Making use of the property of steady disc structure in coplanar circular binaries, observables are computed as functions of the orbital phase. Orbital-phase series of line profiles are given for selected parameter sets under various viewing angles, to allow comparison with observations. Flat-topped profiles with and without superimposed multiple structures are reproduced, showing, for example, that triple-peaked profiles do not have to be necessarily associated with warped discs and misaligned binaries. It is demonstrated that binary tidal effects give rise to phase-locked variability of the violet-to-red (V/R) ratio of hydrogen emission lines. The V/R ratio exhibits two maxima per cycle; in certain cases those maxima are equal, leading to a clear new V/R cycle every half orbital period. This study opens a way in identifying binaries and in constraining the parameters of binary systems that exhibit phase-locked variations induced by tidal interaction with a companion star.

Propagation of short pulses in a non-uniform gas-jet induced plasma

A formalism describing the propagation of short duration (sub-picosecond) Gaussian pulses in a preformed axially non-uniform collisionless plasma has been established; a plasma inhomogeneity consistent with the gas jet induced plasma as super-Gaussian (flat-top) profile has been considered herein. In order to specify the pulse dynamics using nonlinear Schrödinger wave equation (NLSE) in paraxial regime, a set of coupled equations characterizing the transverse focusing (in space) and temporal compression (in time) of the laser pulse has been derived, and further solved numerically to investigate the propagation characteristics of the pulse as it advances through gas jet induced plasma. The effect of plasma inhomogeneity parameters on the characteristic features of the pulse propagation have been analysed and illustrated graphically. The focusing and axial intensity of the propagating pulse is observed to be sensitive to the plasma profile (gas jet parameters), and the focusing length can be tuned up to desired extent by suitable choice of the inhomogeneity parameters.

Cosmic ray modulation and radiation dose of aircrews during the solar cycle 24/25

AbstractWeak solar activity and high cosmic ray flux during the coming solar cycle are qualitatively anticipated by the recent observations that show the decline in the solar activity levels. We predict the cosmic ray modulation and resultant radiation exposure at flight altitude by using the time‐dependent and three‐dimensional model of the cosmic ray modulation. Our galactic cosmic ray (GCR) model is based on the variations of the solar wind speed, the strength of the heliospheric magnetic field, and the tilt angle of the heliospheric current sheet. We reproduce the 22 year variation of the cosmic ray modulation from 1980 to 2015 taking into account the gradient‐curvature drift motion of GCRs. The energy spectra of GCR protons obtained by our model show good agreement with the observations by the Balloon‐borne Experiment with a Superconducting magnetic rigidity Spectrometer (BESS) and the Payload for Antimatter Matter Exploration and Light‐nuclei Astrophysics (PAMELA) except for a discrepancy at the solar maximum. Five‐year annual radiation dose around the solar minimum at the solar cycle 24/25 will be approximately 19% higher than that in the last cycle. This is caused by the charge sign dependence of the cosmic ray modulation, such as the flattop profiles in a positive polarity.

2.36 J, 50 Hz nanosecond pulses from a diode side-pumped Nd:YAG MOPA system

We report on a high-energy high-repetition-rate nanosecond Nd:YAG main oscillator power amplifier (MOPA) system. Maximum output pulse energy of 2.36J with duration of 9.4ns at 50Hz has been achieved. The master oscillator was a LD side-pumped electro-optical Q-switched Nd:YAG rod laser adopting unstable cavity with variable reflectivity mirror (VRM). It delivered a pulse train with energy up to 180mJ and pulse duration of 10.7ns. The near-field pattern demonstrated a nearly super Gaussian flat top profile. In the amplification stage, the pulse was boosted via double-pass two Nd:YAG rod amplifiers. Maximum pulse energy was obtained at the peak pump power of 37.5kW, corresponding to an optical-optical conversion efficiency of 25.2%. The correlative peak power was deduced to be 251MW. We also presented the result of 100Hz nanosecond laser with average output power of >100W.