Gaussian Beam Profile Research Articles

Improving periodic uniformity of area-type LIPSS on Si wafer using a flat-top beam femtosecond NIR laser

Fabrication of line- and area-type laser-induced periodic surface structures (LIPSS) induced by a flat-top beam provides improved periodic uniformity in structure compared to fabrication with a Gaussian beam. Improved periodic uniformity induces high reproducibility and high performance through signal uniformity. Especially, this characteristic makes LIPSS induced by flat-top beam can be applied in structural color, hydrophobicity control and surface-enhanced Raman scattering. To demonstrate the improved periodic uniformity, the periods of the LIPSSs induced by Gaussian and flat-top beams are analyzed. Period uniformity is evaluated by applying the fast Fourier transformation to atomic force microscopy data. Improved periodic uniformity is observed in the area-type LIPSS induced by a flat-top beam. This is because of the peak of line width induced by the Gaussian beam profile characteristic. Consequently, area-type LIPSSs induced by a flat-top beam exhibit improved periodic uniformity.

Vortex Dynamics and Structured Darkness of Laguerre-Gaussian Beams Transmitted Through Q-Plates Under Weak Axial-Asymmetric Incidence

We describe the rich vortex dynamics at normal and near-normal or off-centered Laguerre-Gaussian beam transmission through the q-plate – a waveplate capable of generating vortex beams from circularly polarized incident beams. First, we explore the basics of the spin-to-orbit angular momentum conversion and show the vortex constellations under normal incidence for the case of quarter-wave retardation. Second, we analyze the optical Gaussian and vortex beam profile transformations accompanying the giant beam shifts under axial-asymmetric incidence. Namely, we consider the near-normal (for the Fourier-space q-plate) and off-centered (for the real-space q-plate) incidence, when the incident angle becomes comparable with beam angular divergence and off-center distance becomes comparable with beam width, respectively. In this regime, one can control the positions of the single-charge vortices with the specified polarization components of the beam, including those residing in its structured darkness. All mentioned effects arise due to the extreme spin-orbit coupling. The results obtained form a novel platform for a plethora of optical vortex transformation and structured light phenomena.

Microstructure and electrical characterizations of CuO/Fe2O3 ceramics sintered by CO2 laser beams

Layers of CuO/Fe2O3 mixture powders in a Cu/Fe molar ratio of 1:1 irradiated by continuous-wave CO2 laser beams in air transformed into face-centered-cubic Cu0.67Fe2.33O4 or CuFe5O8. The uneven or wavy surface with precursor residues after laser sintering was attributed to the intrinsic Gaussian beam profile and the temperature-induced tension gradient in the melt pool. The center valleys of the laser tracks exhibited larger faceted particles and more circular voids than those in the edge peaks. The nonstoichiometric phase transformation to Cu0.67Fe2.33O4 or CuFe5O8 grains and the formation of Cu2O along the grain boundaries can be seen in the X-ray diffraction patterns and transmission electron microscopic images. Microstructure characterizations via selected area electron diffraction revealed a preferred growth direction of <111> and coherent interfaces between the CO2 laser sintered Cu0.67Fe2.33O4 or CuFe5O8 grains and precipitated Cu2O due to a small lattice mismatch. The electrical voltage sweep measurements showed the resistivity in the range of 1.5 × 10−5–1.5 × 10−4 Ω ∙ m in the center valleys, indicating their potential applications on sensors, thermistor, and thermoelectric materials.

Study on usage of optical vortices in laser beam position estimation

In many practical applications, it is necessary to accurately and precisely estimate the position of a laser beam. This task becomes particularly problematic in industrial applications when the beam travels many meters in a turbulent atmosphere. In this paper, we show that the use of a specially shaped beam profile together with a dedicated computational algorithm makes it possible to reduce the influence of air refraction fluctuations on the quality of beam position estimation. The required beam shape can be easily obtained using the phenomenon of optical vortices and ready-to-use spiral phase plates. The presented solution, together with dedicated computational algorithms, provides better beam position estimation accuracy by about 20% compared to solutions using a Gaussian beam profile.

Influence of Curvature Feature on Laser Heating during Tape Placement Process for Carbon Fiber Reinforced Polyether Ether Ketone Composite

The curvature feature makes the irradiance and absorptivity change, resulting in an uneven power density distribution, which affects the quality of composite parts. In this study, a theoretical model-based Super-Gaussian profile beam in the laser irradiation area was established to obtain the heat flux distribution on the curved surface. The effect of curvature on the surface scattering reflection, temperature distribution, and surface morphology were investigated and verified the validity of the theoretical model. Furthermore, the influence of the laser intensity distribution, laser inclination and curvature radius on the power density distribution and distribution uniformity were studied. Research indicated that the power density increases as the distance from the origin increase resulting from the variation of the irradiance and absorptance along the circumference. The flatter the intensity distribution of the laser beam in the height direction, the less uniform the power density distribution. Accordingly, the typical Gaussian profile beam significantly ameliorates the power density distribution. This research provides a novel understanding of using heat sources during laser heating thermoplastic tape placement.

Focus on lasers, imaging, microscopy, and nanoscience

Focus on lasers, imaging, microscopy, and nanoscience

Generation of ultrafast laser with 11 MW peak power from a gain-managed nonlinear tapered fiber amplifier

A gain-managed nonlinear (GMN) tapered fiber amplifier with a monolithic configuration is experimentally demonstrated. Both the forward and backward pumping scheme were investigated, and the results indicate that the latter is advantageous in suppressing the unwanted stimulated Raman scattering and the resulting mode distortion while boosting the pulse energy with a shortest pulse width of sub-50 fs. Specifically, a maximum single pulse energy of 707 nJ and a peak power of 11 MW with a fundamental Gaussian distribution beam profile were respectively realized without temporally stretching the seed pulse. In addition, the backward pumped amplifier was further examined through changing its seeding power, and it is found that a relatively low input power is preferable for obtaining high performance laser pulses with shorter duration while without compromise on the output single pulse energy. It is believed that the combination of the GMN amplification and the large mode area tapered fiber provides a promising scheme for realizing high energy ultrafast lasers with a monolithic and concise structure.

Time-resolved dose rate measurements in pencil beam scanning proton FLASH therapy with a fiber-coupled scintillator detector system.

The spatial and temporal dose rate distribution of pencil beam scanning (PBS) proton therapy is important in ultra-high dose rate (UHDR) or FLASH irradiations. Validation of the temporal structure of the dose rate is crucial for quality assurance and may be performed using detectors with high temporal resolution and large dynamic range. To provide time-resolved in vivo dose rate measurements using a scintillator-based detector during proton PBS pre-clinical mouse experiments with dose rates ranging from conventional to UHDR. All irradiations were performed at the entrance plateau of a 250MeV PBS proton beam. A detector system with four fiber-coupled ZnSe:O inorganic scintillators and 20μs temporal resolution was used for dose rate measurements. The system was first characterized in terms of precision and stem signal. The detector precision was determined through repeated irradiations with the same field. The stem signal contribution was quantified by irradiating two of the detector probes alongside a bare fiber (fiber without a coupled scintillator). Next, the detector system was calibrated against an ionization chamber (IC) with all four detector probes and the IC placed in a water bath at 2cmdepth. A scan pattern covering 9.6×9.6cmwas used. Multiple irradiations with different requested nozzle currents provided instantaneous dose rates at the detector positions in the range of 7-1270Gy/s.The correspondence of the detector signal (in Volts) to the instantaneous dose rate (in Gy/s) was found. The instantaneous dose rate was calculated from the beam current and the spot-to-detector distance assuming a Gaussian beam profile at distances up to 8mmfrom the spot. Afterwards, the calibrated system was used in vivo, in mouse experiments, where mouse legs were irradiated with a constant dose and varying field dose rates of 0.7-87.5Gy/s.Theinstantaneous dose rate was measured for each delivered spot and the delivered dose was determined as the integrated instantaneous dose rate. The spot dose profile and PBS dose rate map were calculated. The dose contamination to neighbouring mice were measured together with the upper limit of the dose to the mouse body. The detectors showed high precision with ≤0.4% fluctuations in the measured dose. The stem signal exceeded 10% for spots <5mmfromthe optical fiber and >18mmfromthe scintillator. It contributed up to 0.2% to the total dose, which was considered negligible. All four detectors showed a non-linear relation between signal and instantaneous dose rate, which was modelled with a polynomial response function. In the mouse experiments, the measured scintillator dose showed 1.8% fluctuations, independent of the field dose rate. The in vivo measured spot dose profile had tails that deviated from a Gaussian profile with measurable dose contributions from spots up to 85mm from the detector. Neighbour mouse irradiation contributed ∼1% of the total mouse dose. The upper limit of the mouse body dose was 6% of the mouse leg dose. A fiber-coupled inorganic scintillator-based detector system can provide high precision in vivo measurements of the instantaneous dose rate if correction for the non-linear dose rate dependency is applied.

Spot size estimation of flat-top beams in space-based gravitational wave detectors

Motivated by the necessity of a high-quality stray light control in the detection of the gravitational waves in space, the spot size of a flat top beam generated by the clipping of the Gaussian beam (GB) is studied. By adopting the mode expansion method (MEM) approach to simulating the beam, a slight variant of the definition of the mean square deviation (MSD) spot size for the MEM beam is proposed. This enables us to quickly estimate the spot size for arbitrary propagation distances. Given that the degree of clipping is dependent on the power ratio within the surface of an optical element, the power ratio within the MSD spot range is used as a measure of spot size. The definition is then validated in the cases of simple astigmatic Gaussian beam and nearly-Gaussian beam profiles. As a representative example, the MSD spot size for a top-hat beam in a science interferometer in the detection of gravitational waves in space is then simulated. As in traditional MSD spot size analysis, the spot size is divergent when diffraction is taken into account. A careful error analysis is carried out on the divergence and in the present context, it is argued that this error will have little effect on our estimation. Using the results of our study allows an optimal design of optical systems with top-hat or other types of non-Gaussian beams. Furthermore, it allows testing the interferometry of space-based gravitational wave detectors for beam clipping in optical simulations. This work will serve as a useful guide in the future system design of the optical bench and the sizes of the optical components.

Process modeling of laser scanning vat-photopolymerization operating under intermittent exposure conditions

Process modeling is an important tool in additive manufacturing (AM) because it elucidates the relationship between processing parameters and material properties. Ultraviolet laser scanning vat-photopolymerization (VPP-UVL) is a common additive manufacturing technique, with a strong physical modeling based on the Jacobs work curve framework. The work curve model was developed for continuous UV exposure (CE) systems, however most VPP-UVL systems to date operate under intermittent exposure (IE) conditions due to the wide adoption of solid-state pulsed-laser sources. Despite their fundamental differences, the Jacobs work curve model is still successfully employed in IE systems under high overlap conditions between pulses, however there are few investigations regarding the limiting conditions for the applicability of the Jacobs work curve model to describe the total exposure profile in intermittent exposure (IE) setups. To address this issue, a model describing a VPP-UVL-IE process was developed. The VPP-UVL-IE model presented here has the same logarithmic dependence as the Jacobs work curve model, but with the addition of a multiplying factor carrying information on the spatial distribution of energy, that is, the overlap of individual pulses. The model is adaptable for non-gaussian beam profiles and can be solved for bidimensional analysis representing layer fabrication. To test the model, a custom-built VPP-UVL-IE additive manufacturing system was used, and structures were printed and characterized using SEM and stylus profilometry. The model`s reliability was investigated under constant overlap and exposure conditions. The model is very robust under constant overlap conditions, with good agreement between experimental data and simulation. For constant exposure, there is good qualitative agreement, but discrepancies occur due to insufficient modeling of the spatial distribution of energy. The 3D printing of complex structures is demonstrated, and the model indicates that VPP-UVL-IE can offer, theoretically, faster printing compared to VPP-UVL-CE for large beam radius (>275 μm), under the assumption of Gaussian beam profile, constant irradiance during the pulses, and dwell time longer than beam travel time. Conversely, same levels of exposure can be achieved at higher scan speed using CW laser sources instead of pulsed sources for small beam radius (<275 μm). • The work curve equation can model VPP-UVL with pulsed light sources. • Pulsed light VPP-UVL can print faster than continuous scanning. • Single pulse energy dose and spatial distribution of energy dose are independent.

Thermal effects of beam profiles on X-ray photon correlation spectroscopy at megahertz X-ray free-electron lasers.

X-ray free-electron lasers (XFELs) with megahertz repetition rates enable X-ray photon correlation spectroscopy (XPCS) studies of fast dynamics on microsecond and sub-microsecond time scales. Beam-induced sample heating is one of the central concerns in these studies, as the interval time is often insufficient for heat dissipation. Despite the great efforts devoted to this issue, few have evaluated the thermal effects of X-ray beam profiles. This work compares the effective dynamics of three common beam profiles using numerical methods. Results show that under the same fluence, the effective temperatures increase with the nonuniformity of the beam, such that the Gaussian beam profile yields a higher effective temperature than the donut-like and uniform profiles. Moreover, decreasing the beam sizes is found to reduce beam-induced thermal effects, in particular the effects of beam profiles.

Laser-Induced Crystallization of Copper Oxide Thin Films: A Comparison between Gaussian and Chevron Beam Profiles.

The use of a laser with a Gaussian-beam profile is frequently adopted in attempts of crystallizing nonsingle-crystal thin films; however, it merely results in the formation of polycrystal thin films. In this paper, selective area crystallization of nonsingle-crystal copper(II) oxide (CuO) is described. The crystallization is induced by laser, laser-induced crystallization, with a beam profile in the shape of a chevron. The crystallization is verified by the exhibition of a transition from a nonsingle-crystal phase consisting of small (∼100 nm × 100 nm) grains of CuO to a single-crystal phase of copper(I) oxide (Cu2O). The transition is identified by electron back scattering diffraction and Raman spectroscopy, which clearly suggests that a single-crystal domain of Cu2O with a size as large as 5 μm × 1 mm develops. The transition may embrace several distinctive scenarios such as a large number of crystallites that densely form grow independently and merge, and simultaneously, solid-state growth that takes place as the merging proceeds reduce the number of grain boundaries and/or a small number of selected crystallites that sparsely form grow laterally, naturally limiting the number of grain boundaries. The volume fraction of the single-crystal domain prepared under the optimized conditions─the ratio of the volume of the single-crystal domain to that of the total volume within which energy carried by the laser is deposited─is estimated to be 32%. Provided these experimental findings, a theoretical assessment based on a cellular automaton model, with the behaviors of localized recrystallization and stochastic nucleation, is developed. The theoretical assessment can qualitatively describe the laser beam geometry-dependence of vital observable features (e.g., size and gross geometry of grains) in the laser-induced crystallization. The theoretical assessment predicts that differences in resulting crystallinity, either single-crystal or polycrystal, primarily depend on a geometrical profile with which melting of nonsingle-crystal regions takes place along the laser scan direction; concave-trailing profiles yield larger grains, which lead to a single crystal, while convex-trailing profiles result in smaller grains, which lead to a polycrystal, casting light on the fundamental question Why does a chevron-beam profile succeed in producing a single crystal while a Gaussian-beam profile fails?

Position sensitive detectors in proton therapy: online monitoring of the beam position

Spot-scanning is a highly dynamic treatment method in proton therapy, tailored to each tumor shape individually. By superimposing many single spots in all three spatial axes, a prescribed dose is applied to the tumor volume. To minimize dose inhomogeneity across this volume, tight constraints on the beam position accuracy apply: for a dose inhomogeneity below 1%, a longitudinal beam position accuracy of the order of 1 mm is necessary, whereas in the lateral plane, the accuracy needs to be roughly one order of magnitude better. Longitudinal position control is achieved through selecting the beam energy; laterally, this is achieved by two sweeper magnets, allowing position changes within milliseconds. Such dynamics and accuracy constraints require an online measurement of the beam position to enable and maintain high treatment quality assurance. Gantry 2 at the Center for Proton Therapy at PSI operates a plane parallel strip ionization chamber for this purpose as the final beamline element before the patient. The foil-based detector design is optimized for an in-situ placement in the beam axis and keeps beam disturbance at a minimum. A strip pitch of 2 mm allows to reconstruct the Gaussian shape beam profile with the desired accuracy. These beam profiles are analyzed (and verified) during a treatment on a spot-by-spot basis, before the next spot is applied, introducing a dead time after every spot. Applying 50, 000 spots in a treatment introduces the challenge to data acquisition and — processing in terms of keeping treatment times reasonable. Data pileup and long dead times are mitigated by optimized front-end electronics and early-stage data processing, without compromising data quality and the accuracy of the measured beam position.

Femtosecond Laser Modification of Silica Optical Waveguides for Potential Bragg Gratings Sensing.

The optimum femtosecond laser direct writing of Bragg gratings on silica optical waveguides has been investigated. The silica waveguide has a 6.5 × 6.5 µm2 cross-sectional profile with a 20-µm-thick silicon dioxide cladding layer. Compared with conventional grating inscribed on fiber platforms, the silica planar waveguide circuit can realize a stable performance as well as a high-efficiency coupling with the fiber. A thin waveguide cladding layer also facilitates laser focusing with an improved spherical aberration. Different from the circular fiber core matching with the Gaussian beam profile, a 1030-nm, 400-fs, and 190-nJ laser is optimized to focus on the top surface of the square silica waveguide, and the 3rd-order Bragg gratings are inscribed successfully. A 1.5-mm long uniform Bragg gratings structure with a reflectivity of 90% at a 1548.36-nm wavelength can be obtained. Cascaded Bragg gratings with different periods are also inscribed in the planar waveguide. Different reflection wavelengths can be realized, which shows great potential for wavelength multiplexing-related applications such as optical communications or sensing.

Enhanced spectral-domain optical coherence tomography (SD-OCT) using in situ ultrasonic virtual tunable optical waveguides.

A conventional optical lens can enhance lateral resolution in optical coherence tomography (OCT) by focusing the input light onto the sample. However, the typical Gaussian beam profile of such a lens will impose a tradeoff between the depth of focus (DOF) and the lateral resolution. The lateral resolution is often compromised to achieve a mm-scale DOF. We have experimentally shown that using a cascade system of an ultrasonic virtual tunable optical waveguide (UVTOW) and a short focal-length lens can provide a large DOF without severely compromising the lateral resolution compared to an external lens with the same effective focal length. In addition, leveraging the reconfigurability of UVTOW, we show that the focal length of the cascade system can be tuned without the need for mechanical translation of the optical lens. We compare the performance of the cascade system with a conventional optical lens to demonstrate enhanced DOF without compromising the lateral resolution as well as reconfigurability of UVTOW for OCT imaging.

Design and commissioning of an e-beam irradiation beamline at the Upgraded Injector Test Facility at Jefferson Lab

The Upgraded Injector Test Facility (UITF) at Jefferson Lab is a continuous-wave superconducting linear accelerator capable of providing an electron beam with energy up to 10 MeV. A beamline for electron-beam irradiation has been designed, installed and successfully commissioned at this facility, aimed at the degradation study of 1,4-dioxane and per- and polyfluoroalkyl substances (PFAS) in wastewater treatment. A solenoid with a peak axial magnetic field of up to 0.28 T and a set of raster coils were used to obtain a Gaussian beam profile with a transverse standard deviation of ∼15.0mm at the target location. Monte-Carlo simulations using FLUKA were carried out to calculate the total absorbed dose and the dose distribution in the sample volume inside the target cell. The simulations were benchmarked experimentally by dosimetry mapping using opti-chromic dosimeters. The results of the irradiation experiments showed a ∼95% reduction of 1,4-dioxane in ultra-pure water for a dose of 1 kGy, demonstrating the potential of electron-beam irradiation towards addressing growing challenges in environmental remediation.

Diagnostic study of interface materials of co-deposition simulating impurity relevant to international thermonuclear experimental reactor by using calibration-free laser-induced breakdown spectroscopy

The quantitative analysis and depth profiling of a co-deposition simulating the impurity of the international thermonuclear experimental reactor (ITER) has been performed by using laser-induced breakdown spectroscopy (LIBS). In the depth profile analysis of a co-deposition of Al/W/Mo, the materials were resolved across the interfaces of layers using four different values of laser fluence. The variation in concentration of materials across the interfaces was observed for seven laser shots with each fluence value. The calibration-free LIBS analysis provided elemental abundance across the interfaces. The decay and rise occurred slowly in the evolution of the elemental percentage of the two materials across the interfaces as material ablated by the laser pulse having a Gaussian beam profile. The quantitative and depth profiling results are fully agreed in material analysis across the interfaces of layers. The results obtained through depth profiling and quantitative analysis of co-deposition evidenced that LIBS is a promising technique for the diagnosis of unknown materials of impurity deposition in the fusion devices.

Study of Laser Backside Ohmic Contact Formation of SiC-Ni Interface to Evaluate the Process Influence on the Electrical Characteristics

Within this paper, we will present the results of a study on the ohmic contact formation process with nanosecond (ns) pulsed UV lasers. For the study we compared two laser processes: The base line process with a 100-300 ns pulsed laser with Gaussian beam profile and the 3D-Micromac AG process with a 50-100 ns pulsed laser with top hat beam profile. The forward voltage characteristics at wafer level was analyzed and proves a clear benefit of the top hat laser process. Besides, the forward voltage characteristics of a second run was performed to analyze the influence of increasing energy density to the electrical characteristic of heat sensitive front side structures. Also with high energy density no negative influence could be detected.

Laser-atom interaction simulator derived from quantum electrodynamics

A laser-atom interaction simulator derived from quantum electrodynamics (LASED) is presented, which has been developed in the python programming language. LASED allows a user to calculate the time evolution of a laser-excited atomic system. The model allows for any laser polarization, a Gaussian laser beam profile, a rotation of the reference frame chosen to define the states, and an averaging over the Doppler profile of an atomic beam. Examples of simulations using LASED are presented for excitation of calcium from the $4^{1}S_{0}$ state to the $4^{1}P_{1}$ state, for excitation from the helium $3^{1}D_{2}$ state excited by electron impact to the $10^{1}P_{1}$ state, and for laser excitation of caesium via the ${D}_{2}$ line.

Yb-fiber-pumped high-average-power picosecond optical parametric oscillator tunable across 1.3−1.5 μm

We report the generation of high-repetition-rate picosecond pulses in the 1.3-1.5µm spectral range by internal second harmonic generation (SHG) of an idler-resonant optical parametric oscillator (OPO) based on MgO-doped periodically-poled LiNbO3 (MgO:PPLN), synchronously pumped by ∼20 ps pulses at 80 MHz using an Yb-fiber laser at 1.064 µm. By taking advantage of the high spatial quality of the resonant idler beam in the 2503-3030 nm wavelength range and using a second MgO:PPLN crystal with fanout grating structure for intracavity SHG, we have achieved spectral coverage across 1272-1515 nm with up to 1.23 W average power. The second harmonic output exhibits a power stability of 3% rms over 1 hour in pulses of 8.3 ps with Gaussian beam profile. The described approach overcomes the spectral limitation of 1.064 µm-pumped OPOs based on MgO:PPLN and other oxide-based nonlinear crystals, where signal generation below ∼1.45 µm is precluded by multi-phonon absorption of idler radiation above ∼4µm.