Sheldon Wu

Hydrocarbon-free resonance transition 795-nm rubidium laser

For what we believe to be the first time, an optical resonance transition rubidium laser (52P1/2-->52S1/2) has been demonstrated with a hydrocarbon-free buffer gas. Prior demonstrations of alkali resonance transition lasers have used ethane as either the buffer gas or a buffer gas component to promote rapid fine-structure mixing. However, our experience suggests that the alkali vapor reacts with the ethane producing carbon as one of the reaction products. This degrades long term laser reliability. Our recent experimental results with a

Direct measurements of temperature-dependent laser absorptivity of metal powders

A compact system is developed to measure laser absorptivity for a variety of powder materials (metals, ceramics, etc.) with different powder size distributions and thicknesses. The measured results for several metal powders are presented. The results are consistent with those from ray tracing calculations.

Metal powder absorptivity: modeling and experiment

We present results of numerical modeling and direct calorimetric measurements of the powder absorptivity for a number of metals. The modeling results generally correlate well with experiment. We show that the powder absorptivity is determined, to a great extent, by the absorptivity of a flat surface at normal incidence. Our results allow the prediction of the powder absorptivity from normal flat-surface absorptivity measurements.

Temperature-dependent 780-nm laser absorption by engineering grade aluminum, titanium, and steel alloy surfaces

Abstract. The modeling of laser interaction with metals for various applications requires a knowledge of absorption coefficients for real, commercially available materials with engineering grade (unpolished, oxidized) surfaces. However, most currently available absorptivity data pertain to pure metals with polished surfaces or vacuum-deposited thin films in controlled atmospheres. A simple laboratory setup is developed for the direct calorimetric absorptivity measurements using a diode-array laser emitting at 780 nm. A scheme eliminating the effect of convective and radiative losses is implemented. The obtained absorptivity results differ considerably from existing data for polished pure metals and are essential for the development of predictive laser-material interaction models.

Simulation Of The Main Physical Processes In Remote Laser Penetration With Large Laser Spot Size

A 3D model is developed to simulate remote laser penetration of a 1mm Aluminum metal sheet with large laser spot size (∼ 3x3cm2), using the ALE3D multi-physics code. The model deals with the laser-induced melting of the plate and the mechanical interaction between the solid and the melted part through plate elastic-plastic response. The effect of plate oscillations and other forces on plate rupture, the droplet formation mechanism and the influence of gravity and high laser power in further breaking the single melt droplet into many more fragments are analyzed. In the limit of low laser power, the numerical results match the available experiments. The numerical approach couples mechanical and thermal diffusion to hydrodynamics melt flow and accounts for temperature dependent material properties, surface tension, gravity and vapor recoil pressure.

Use of High-Power Diode Laser Arrays for Pre- and Post-Weld Heating during Friction Stir Welding of Steels

In this research a high-power diode laser array was used to preheat HY-80 steel to determine the efficacy of using a diode laser array for preheating prior to friction stir welding in order to reduce frictional forces thereby reducing tool wear and increasing welding speeds. Using instrumented plates the temperature profile using diode heating alone was determined in order to validate theoretical models. A high-power diode laser is proposed as a more cost effective and efficient means of preheating compared to other hybrid friction stir welding techniques. Parameters of the diode array were easily controllable and resulted in a preheated area that very closely matches the typical stir zone observed in friction stir welds. A proposed diode laser assisted friction stir welding system is presented, and it is hypothesized that the addition of diode laser preheating will improve tool life and/or increase welding speeds on steels.

Probing melt pool dynamics and particle ejection using high speed optical diagnostics

We present experimental results and modeling of melt pool dynamics and particle ejection associated with selective laser melting. High speed optical diagnostics are used to probe morphological changes for SS316L. Data is compared to simulations.

Resonance Transition 795-nm Rubidium Laser Using 3 He Buffer Gas

Demonstration of 795-nm Rubidium laser using a buffer gas consisting of pure3He is reported. The use of3He yields enhanced mixing of Rb fine-structure levels and enables efficient lasing at reduced buffer gas pressures.

Polarization effects associated with thermal processing of HY-80 structural steel using high-power laser diode array

Abstract. Localized heating of roughened steel surfaces using highly divergent laser light emitted from high-power laser diode arrays was experimentally demonstrated and compared with theoretical predictions. Polarization dependence was analyzed using Fresnel coefficients to understand the laser-induced temperature rise of HY-80 steel plates under 383- to 612-W laser irradiation. Laser-induced, transient temperature distributions were directly measured using bulk thermocouple probes and thermal imaging. Finite-element analysis yielded quantitative assessment of energy deposition and heat transport in HY-80 steel using absorptivity as a tuning parameter. The extracted absorptivity values ranged from 0.62 to 0.75 for S-polarized and 0.63 to 0.85 for P-polarized light, in agreement with partially oxidized iron surfaces. Microstructural analysis using electron backscatter diffraction revealed a heat affected zone for the highest temperature conditions (612 W, P-polarized) as evidence of rapid quenching and an austenite to martensite transformation. The efficient use of diode arrays for laser-assisted advanced manufacturing technologies, such as hybrid friction stir welding, is discussed.

Laser Pulses for Compton Scattering Light Sources

Nuclear Resonance Fluorescence (NRF) is an isotope specific process in which a nucleus, excited by gamma-rays, radiates high energy photons at a specific energy. This process has been well known for several decades, and has potential high impact applications in homeland security, nuclear waste assay, medical imaging and stockpile surveillance, among other areas of interest. Although several successful experiments have demonstrated NRF detection with broadband bremsstrahlung gamma-ray sources1, NRF lines are more efficiently detected when excited by narrowband gamma-ray sources. Indeed, the effective width of these lines is on the order of 6 / ~ 10 E E   . NRF lines are characterized by a very narrow line width and a strong absorption cross section. For actinides such as uranium, the NRF line width is due to the intrinsic line width