Gaussian-shaped Beam Research Articles

Microstructural control in metal laser powder bed fusion additive manufacturing using laser beam shaping strategy

Additive manufacturing (AM) promises to revolutionize manufacturing by producing complex parts with tailored mechanical properties through local microstructure control. The main challenge is to control or prevent columnar (elongated) growth morphology which is prevalent in AM parts. Here, we elucidate mechanisms of microstructure control that promote favorable equiaxed grains (aspect ratio close to 1) using a laser beam shaping strategy. This requires an accurate thermal profile that is only captured using advanced predictive simulation that couples full laser ray tracing, ultra-fast hydrodynamic melt flow and the cellular automata method for grain growth. We investigate columnar to equiaxed microstructure transition during single-track laser powder bed fusion processing of 316 L stainless steel using Gaussian (circular) and elliptical (transverse and longitudinal) laser beam shapes. We demonstrate that the propensity to produce equiaxed grains through nucleation events correlates with large beam width as delivered by an elliptical transverse laser beam. In addition, we reveal different microstructure evolution mechanisms during transient states such as at start and end of a scan track when the laser is respectively turned on and off. Columnar growth is hard to prevent at the start of a track and the growth morphology in the absence of heat input is dictated by the melt pool width and depth achieved and the degree of thermal undercooling. We expect this fundamental understanding of the physics of local beam shaping for microstructural control would have implications on future complex beam shape designs as well as beam modulation.

Circular Airy Beam Shaping by Annular Arrayed-Core Fiber

We propose and theoretically demonstrate a beam shaping method using a novel all-fiber device composed of a single mode fiber, a tapered annular-core fiber, and an annular arrayed-core fiber. By using the proposed device, a circular Airy beam with outward acceleration can be generated using a Gaussian beam shaping and the autodefocusing Airy beam can be converted into an autofocusing Airy beam through a cone-frustum shaped fiber end. The properties of wave propagation in fiber and surrounding media are studied by using coupled-mode theory and angular spectrum method. The calculated results show that the output beam from the proposed device has a circular Airy-like pattern, which experiences autodefocusing (or autofocusing) during propagation, and forms a Bessel-like beam after the focal point. The tolerances of the beam generation device are discussed and we show that the focused output beam from the cone-frustum shaped fiber end can be used for multi-point trap to form an optical trap array.

Fast and Controllable Beam Switching in a Pulsed Fiber Laser

In this letter, we demonstrate a fast fiber mode converter in a short piece of special passive fiber. The switching mechanism relies on selective wavelength resonance between the core and a high index surrounding ring in the fiber. Beam switching at 530 kHz from a Gaussian beam shape to an expanded beam with a ring with corresponding M <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> from 1.4 to 3.5 is demonstrated in a modified industrial pulsed fiber laser at power levels in the excess of 15 W.

Direct-current current transformer for the measurement of an electric propulsion ion beam.

An important task in the development of electric propulsion devices is ground-based testing. In order to characterize electric thrusters, their generated ion beams are regarded as figures of merit and have to be well-known and understood. We present an approach to measure DC ion beam currents generated by electric thrusters by means of an inductive direct-current current transformer. In contrast to commonly used devices like Faraday cups, this device can measure the beam current non-intrusively by means of magnetic field coupling rather than charge collection. This paper shows the development of a prototype sensor which can measure currents in the typical range for electric thrusters with high resolution and satisfactory accuracy. A detailed SPICE model to assist during hardware development is also introduced and verified by test case measurements. The prototype has been tested and validated with a radio-frequency ion thruster. Its readout shows very good agreement with the output of an analytical model which computes a Gaussian-shaped ion beam in the far-field based on experimental input data.

Theoretical study on 252 terahertz Gaussian beam shaping based on an off-axis transmissive all-dielectric metasurface

An off-axis transmissive all-dielectric square metasurface with a side length of 55 mm is designed and studied in this paper. It can shape the 2.52 terahertz Gaussian beam into a rigorous plane wave (we define the required field-quality of the rigorous plane wave region: peak-to-peak amplitude ripple less than 1 dB and phase ripple less than 10°). The metasurface is composed of silicon cylindrical resonators whose phase modulation can almost span over the entire 2π range and the poly (4-methyl-1-pentene) substrate. The diameters of the resonators are from 11 to 35 μm and the heights of them are all 60 μm. The working range of the obtained plane wave is calculated by the physical optics integral formula when the off-axis angle is 22°. The results show that when the off-axis propagation distance is 1050 mm, the area satisfying the plane wave condition is 17×20 mm2, and the diffraction efficiency (we define the diffraction efficiency: the ratio of the total light field energy on the target plane to the total energy of light field that is just after the metasurface along the propagation of incident light) of the rigorous plane wave region cross-section is 23.4%. In addition, a random silicon cylinder diameter fabrication error within ± 4.5 μm is taken into account for the actual fabrication condition. The error satisfies the normal distribution with a mean of 0 μm and a standard deviation of 3 μm. The influence of the fabrication error on the shaping result is studied. We anticipate that the whole study will have certain pragmatic applications in some fields, such as enlarging the capture area of terahertz optical tweezers.

Measuring the ablation threshold fluence in femtosecond laser micromachining with vortex and Bessel pulses.

Femtosecond laser micromachining holds significant promise for advanced manufacturing, however uptake has been limited by the low processing speed. Altering the beam shape from its typical Gaussian profile has been attempted to improve efficiency, however virtually all reliable methods for quantifying the efficiency assume a Gaussian beam shape. Here, we describe an approach for quantifying the ablation threshold fluence - the key parameter for comparing efficiency - suitable for weakly focused non-Gaussian beams. We successfully demonstrate this method for Bessel and vortex beams, finding that the ablation threshold depends not just on the material, but the beam shape as well.

Vortex fibers for STED microscopy

Super-resolution imaging using the principles of stimulated emission depletion (STED) microscopy requires collinear excitation of a sample with a Gaussian-shaped excitation beam and a donut-shaped depletion beam whose spin (polarization) and orbital angular momentum (OAM) signs are aligned. We leverage recent advances in stable OAM mode propagation in optical fibers for telecom applications to design, fabricate, and validate the utility of a vortex fiber as the beam shaping device at visible and near-IR wavelengths for STED microscopy. Specifically, using compact UV-written fiber-gratings yielding high purity mode conversion (98.7%), we demonstrate the simultaneous generation of Gaussian and OAM beams at user-defined wavelengths. Point spread function measurements reveal a depletion beam with &amp;gt;17.5-dB extinction ratio, a naturally co-aligned Gaussian beam, and a setup in which these characteristics are maintained even as the fiber is bent down 6-mm radii. The proof-of-concept of all-fiber STED microscopy realized using this fiber device is used to image fluorescent bead samples yielding a sub-diffraction-limited resolution of 103 nm in the lateral plane. This opens the door to performing fiber-based STED microscopy with a setup that is not only resistant to environmental perturbations but also facilitates the development of endoscopic STED imaging.

Paraxial expansion of the wave kinetic equation for electron cyclotron beams in turbulent plasmas

The paraxial WKB (beam tracing) method has proven to be very powerful for the computation of electron cyclotron (EC) beams for heating, current-drive and diagnostics applications in smooth plasma equilibria. However, fluctuation-induced beam broadening with possible concerns for ITER application has raised interest on the effect of edge density fluctuations. This issue was recently tackled through a new approach based on the wave kinetic equation (WKE) and a representation of the beam in phase-space. This method has been implemented in the WKBeam code and employed to assess the impact of fluctuations under ITER conditions.In this work we propose to apply the paraxial technique to the wave kinetic equation. On the one hand this allows a comparison to the pWKB approach on the level of equations, clarifying the physical meaning of the WKE in phase-space and the limitations of a standard Gaussian beam in physical pace. On the other hand, we achieve a remarkable speed-up compared to WKBeam: Evolution of the beam is a direct result of a system of 11 ordinary differential equations whereas in WKBeam typically 105 rays are traced (Monte-Carlo approach).In its present formulation the paraxial method applies to situations in which turbulence conserves the Gaussian beam shape, which is the case in the diffusive scattering regime. For beam and turbulence parameters chosen in accordance with this requirement we achieve good agreement with the well-benchmarked WKBeam code.

106 W, picosecond Yb-doped fiber MOPA system with a radially polarized output beam

We report the generation of high average output power, high peak power, and high pulse energy radially polarized picosecond pulses from a compact gain-switched laser-diode-seeded Yb-doped fiber master oscillator power amplifier (MOPA) system. A q-plate was employed as a mode converter prior to the final power amplifier to efficiently convert the linearly polarized Gaussian-shaped beam into a donut-shaped radially polarized beam. The desired vector beam was efficiently amplified yielding ∼110 ps pulses with a maximum output pulse energy of ∼30.7 μJ and a peak power of ∼280 kW at a repetition rate of 1.367MHz. The average power was scaled up to 106W by increasing the repetition rate to 5.468MHz.

Flat-Field Illumination Microscopy for Large Field-of-View Quantitative Imaging

Fluorescence imaging is a powerful tool for understanding of molecular mechanisms of cellular processes and functions. It is desirable to have a robust imaging method that enables us to obtain quantitative information. However, in laser-based fluorescence imaging, the nonuniform illumination distribution by the Gaussian-shaped beam results in severe problems for quantitative analysis of images. Additionally, it limits the field-of-view and leads to rapid photobleaching at the center of the beam. The solution for the uneven distribution is to have an even illumination beam. The method proposed is the reshaping of the Gaussian illumination profile into a Flattop illumination profile by using refractive optics. Our results showed that the flat-field illumination yielded far more leveled intensities and much wider field-of-view compared to Gaussian profile. We demonstrated epi- and TIRF illumination with multiple wavelengths. Our method is likely to combine with high-throughput imaging as well.

New receiving line for the remote-steering antenna of the 140 GHz CTS diagnostics in the FTU Tokamak

A new receiving antenna for collecting signals of the Collective Thomson Scattering (CTS) diagnostics in FTU Tokamak has been recently installed. The squared corrugated section and the precisely defined length make it possible to receive from different directions by remotely steering the receiving mirrors. This type of Remote-Steering (RS) antennas, being studied on FTU for the DEMO Electron Cyclotron Heating (ECH) system launch, is already installed on the W7- X stellarator and will be tested in the next campaign. The transmission of the signal from the antenna in the tokamak hall to the CTS diagnostics hall will be mainly realized by means of oversized circular corrugated waveguides carrying the hybrid HE11 (quasi-gaussian) waveguide mode, with inclusion of a special smooth-waveguide section and a short run of reduced-size square-corrugated waveguide through the tokamak bio-shield. The coupling between different waveguide types is made with ellipsoidal focusing mirrors, using quasi-optical matching formulas between the gaussian-shaped beams in input and output to the waveguides. In this work, after a complete study of feasibility of the overall line, a design for the receiving line will be proposed, in order to realize an executive layout to be used as a guideline for the commissioning phase.

基于球面-非球面柱透镜的圆光斑高斯光束整形

基于球面-非球面柱透镜的圆光斑高斯光束整形

Dipole saturated absorption modeling in gas phase: Dealing with a Gaussian beam

With the advent of new accurate and sensitive spectrometers, cf. combining optical cavities (for absorption enhancement), the requirement for reliable molecular transition modeling is becoming more pressing. Unfortunately, there is no trivial approach which can provide a definitive formalism allowing us to solve the coupled systems of equations associated with nonlinear absorption. Here, we propose a general approach to deal with any spectral shape of the electromagnetic field interacting with a molecular species under saturation conditions. The development is specifically applied to Gaussian-shaped beams. To make the analytical expressions tractable, approximations are proposed. Finally, two or three numerical integrations are required for describing the Lamb-dip profile. The implemented model allows us to describe the saturated absorption under low pressure conditions where the broadening by the transit-time may dominate the collision rates. The model is applied to two specific overtone transitions of the molecular acetylene. The simulated line shapes are discussed versus the collision and the transit-time rates. The specific collisional and collision-free regimes are illustrated, while the Rabi frequency controls the intermediate regime. We illustrate how to recover the input parameters by fitting the simulated profiles.

Efficient method for the measurement of lifetime optical damage performance of thin film coatings from laser damage size analysis.

A laser damage test method based on damage size analysis (DSA) is described that simplifies the derivation of the lifetime optical damage threshold of film materials critical in the design of devices used in high-repetition-rate, high-power laser systems. The DSA method presented here is solely based on imaging to measure the damage site size produced from exposure to a known Gaussian-shaped beam with a fixed, systematically selected fluence well above the ablation threshold. The method locates the damage boundary produced from repeated exposures, using images with a high contrast, and maps it to the beam profile to extract a lifetime laser damage fluence threshold value. We validate the DSA approach using a few relevant transparent film material systems and by comparing it to the standard S/1 laser damage test method. The DSA method can be more efficient and accelerate materials development and validation necessary to support the design of high-power repetition-rated lasers and optoelectronic devices.

Laser processing parameter optimization and tribological characteristics of different surface treatment

The laser surface processing parameters and tribological properties of a textured surface filled with Gr-MoS2-PI-carbon nanotube nanocomposite solid lubricant under oil lubrication were investigated. Micro-dimples were fabricated on GCr15-bearing steel material using Gaussian-shaped beam profile nanosecond semiconductor sound and light-pumped Nd:YAG laser machining equipment. Friction and wear tests were performed using MMW-1A testing machine with the flat-on-flat configuration under different conditions in air. The results show that micro-dimple morphology is affected strongly by the laser head to sample surface distance, pulse repetition frequency, and single pulse energy. Additionally, the textured surface filled with solid lubricant (TSL) exhibited an enhanced tribological performance as compared to the textured (T) and smooth (S) surfaces under dry friction condition. In the oil medium, the efficiency of the textured surface (TO) was 18%, while the textured surface filled with solid lubricant (TSO) was 22% as compared to the plateau smooth surface (SO). Furthermore, under different conditions of load and rotational speed, the solid-oil composite lubricating film effectively reduces the friction and wear on the surface of the sample. The research results identified that the good lubrication and anti-wear properties of the TSO sample were mainly attributed to the double-action lubrication system and synergistic lubricating effect generated by the composite solid lubricant and oil.

Postfabrication Phase Error Correction of Silicon Photonic Circuits by Single Femtosecond Laser Pulses

Phase errors caused by fabrication variations in silicon photonic integrated circuits are an important problem, which negatively impacts device yield and performance. This paper reports our recent progress in the development of a method for permanent, postfabrication phase error correction of silicon photonic circuits based on femtosecond laser irradiation. Using beam shaping technique, we achieve a 14-fold enhancement in the phase tuning resolution of the method with a Gaussian-shaped beam compared to a top-hat beam. The large improvement in the tuning resolution makes the femtosecond laser method potentially useful for very fine phase trimming of silicon photonic circuits. We also show that femtosecond laser pulses can directly modify silicon photonic devices through a SiO2 cladding layer, making it the only permanent post-fabrication method that can tune silicon photonic circuits protected by an oxide cladding.

Simulation of Kerr Lens Modelocking Behavior in Sagittal Plane

Femtosecond oscillator is first step to generate femtosecond laser pulse. Simulation of the laser beam behavior in the oscillator cavity, especially in the gain medium is essential to realize and optimize the oscillator output pulse characteristics. In this work, at first we simulate the laser beam propagation in a folded cavity with curved mirrors in symmetric and asymmetric designs using the ABCD ray tracing technique for a Gaussian beam shape to achieve influence of Kerr lens modelocking in sagittal plane. We use split-step method to obtain the beam intensity changes in the ti:sa rod and shooting method to calculate the beam spot size on the first cavity mirror. In following the influence of the gain medium position changes on the size and position of the beam waist and also on the misalignment sensitivity parameter for the sagittal plane is simulated. Furthermore the misalignment sensitivity parameter and the size and position of the beam waist are not identical for the beams in the sagittal plane, therefore to actualize an optimum design it is necessary to simulate the beam propagation in plane which is not usually considered in early design because of complication of the simulation.

高斯光束整形非球面透镜的粒子群优化设计方法

高斯光束整形非球面透镜的粒子群优化设计方法

Effects of beam irregularity on uniform scanning

An active scanning beam delivery method has many advantages in particle beam applications. For the beam is to be successfully delivered to the target volume by using the active scanning technique, the dose uniformity must be considered and should be at least 2.5% in the case of therapy application. During beam irradiation, many beam parameters affect the 2-dimensional uniformity at the target layer. A basic assumption in the beam irradiation planning stage is that the shape of the beam is symmetric and follows a Gaussian distribution. In this study, a pure Gaussian-shaped beam distribution was distorted by adding parasitic Gaussian distribution. An appropriate uniform scanning condition was deduced by using a quantitative analysis based on the gamma value of the distorted beam and 2-dimensional uniformities.

利用汉克尔变换设计高斯光束整形衍射元件的应用研究

利用汉克尔变换设计高斯光束整形衍射元件的应用研究