W. P. Latham

Scaling laws for thick‐section cutting with a chemical oxygen–iodine laser

Almost all laser‐assisted materials processing involves melting, vaporization and plasma formation which affect the utilization of laser energy for materials processing. To account for the effect of these phases, an effective absorptivity is defined, and a simple mathematical model is developed for the cutting of thick‐section stainless steel using a high power chemical oxygen—iodine laser (COIL). The model is based on an overall energy balance, and it relates the cutting depth with various process parameters that can be used to predictively scale the laser materials processing performance to very thick sections. The effects of various process parameters such as laser power, spot size, cutting speed and cutting gas velocity on the cutting depth are discussed. The results of the mathematical model are compared with experimental data. Such a comparison provides a means of determining the effective absorptivity during laser materials processing.

Characteristic analysis of a refractive axicon system for optical trepanning

An annular beam provides a new laser drilling mechanism, which we refer to as optical trepanning. A refractive axicon system has been designed to transform an input Gaussian laser beam into a collimated annular beam. The diffractive effects of the axicon system and a convex lens focusing the collimated annular beam have been studied using the Fresnel diffraction integral. The theoretical diffraction patterns are compared with the patterns measured with a laser-beam analyzer. The results show that the refractive axicon system can produce Gaussian-like annular beams with the capability of easily adjusting the size of the annular beam.

Lumped parameter model for multimode laser cutting

A lumped parameter mathematical model is developed to relate the cut depth to the laser cutting parameters and material properties. The model takes into account the threshold power of the incident laser beam for the initiation of cutting and modifies an earlier cutting model so that it applies to a wide set of process parameters ranging from low to high laser powers and slow to fast cutting speeds. Plain steel is taken as an example to show the effects of various process parameters such as the laser power, spot size and cutting speed on the cut depth. Special emphasis is given to the effect of laser mode structure on its cutting capability.

6.5 kW, Yb:YAG Ceramic Thin Disk Laser

The operation of a 1030 nm, single, thin disk laser which produced 6.5 kW of laser output power with 57 percent slope efficiency is reported. The Yb:YAG ceramic gain element is 200 μm thick, bonded to a 1 mm, undoped, ceramic YAG cap. The gain element is pumped by diodes at 940 nm. The maximum incident pump intensity was 5 kW/cm2, which yielded an output intensity of 2.6 kW/cm2 of multimode laser radiation. Rigrod analysis suggests that the laser operates with inhomogeneous gain saturation. This is attributed to the enhanced, spatial‐hole‐burning effect when the gain element is adjacent to a mirror. The pump threshold and output intensities were independent of pump spot size, which validates area scaling. Observed thermal lensing contributions include thermal‐expansion‐induced disk flexure, pump‐edge‐induced temperature profile and a strong thermal imprint of the cooling nozzle due to the direct jet impingement on the high reflection (HR) coated side. Weak absorption of the 1030 nm intracavity intensity i...

Two-dimensional model for melting and vaporization during optical trepanning

Annular laser beams provide a drilling mechanism that can be referred to as optical trepanning. In this paper an analytical two-dimensional model is developed for optical trepanning. The analysis accounts for conduction in the solid, vaporization, and convection due to the melt flow caused by an assist gas. Based on the model, the influences of pulse duration, laser-pulse length, pulse repetition rate, intensity profiles, and beam radius are investigated to examine their effects on the recast layer thickness, hole depth, and taper. Deeper cavity depth, thicker recast layer, and larger taper are obtained with the increase in the laser intensity. By using different types of intensity profiles, the nature of the hole taper can be modified, i.e., convergent or divergent holes can be produced. The effects of the inner radius of annular beams are more significant than other laser parameters. An increase in the inner radius reduces the hole taper and produces thinner recast layer and deeper cavity depth.

ASE in thin disk lasers: theory and experiment

We derive equations for the ASE intensity, decay time, and heat load. The crux of our development is frequency integration over the gain lineshape followed by a spatial integration over the emitters. These integrations result in a gain length that is determined from experiment. We measure the gain as a function of incident pump power for a multi-pass pumped Yb:YAG disk doped at 9.8 at.% with an anti-ASE cap. The incident pump powers are up to 3kW. Our fit to the measured gain is within 10% of the measured gain up to pump powers where the gain starts to flatten out and roll over. In this comparison we extract the gain length that turns out to be 43% of the pump spot size of 7mm.

Temperature distributions due to annular laser beam heating

Optical trepanning is a new laser drilling method using an annular beam. Since laser heating of the substrate occurs first due to heat conduction, this article investigates the temperature distribution in the workpiece due to pulsed annular laser beams by solving an axisymmetric transient heat conduction equation. The annular beams allow numerous irradiance profiles to supply laser energy to the workpiece and thus provide more flexibility in affecting the hole quality than a traditional circular laser beam. Such profiles include half Gaussian with maximum intensities at the inner and outer radii of the annulus, respectively, and full Gaussian with maximum intensity within the annulus. In addition to this spatial beam shaping, the temporal profile of the laser pulse can be shaped to improve the hole quality. The Hankel and Laplace transforms have been used to obtain an analytic solution for the temperature distribution in a semi-infinite workpiece. The effects of the temperature distribution on laser drill...

Cutting performance of a chemical oxygen-iodine laser on aerospace and industrial materials

ABS TRAC T A chemical oxygen-iodine laser (COIL) was used for cutting aluminum, titanium,inconel and copper plates. The laser was operated with a stable resonator having an intracavity aperture to produce a circular COIL beam with very few transverse modes. The multimode focal spot diameter was calculated to be 0.24 mm. The new aluminum cut was of particularly high kerf edge quality. These COIL cutting data are compared with an existing theoretical laser cutting model. Using thermophysical data for aluminum, titanium, inconel and copper, this theory agrees very well with the data. To test the versatility of the model, the effects of different assumptions are examined; different assumptions produced very little effect on model predictions at high cutting speeds and a small difference at very slow cutting speeds. Overall, the theoretical model provides good agreement with experiments for a wide variety of metals.

Thermal and stress characterization of various thin disk laser configurations at room temperature

Operational performance of kilowatt-class thin-disk ceramic and single crystal Yb:Yag lasers is presented. High pump power is applied to various thin-disk assemblies on two different test beds. The assemblies are composed of ASE caps, 200μm gain media, and heat sinks made of SiC, sapphire, or diamond. A novel mounting and cooling process is described. FEA modeling of the assemblies is performed using COMSOL stress and thermal computations to understand and quantify thermal and stress effects on beam quality and laser output power. Under increased pump power, the thin-disk can deform 5-10 μm in the center, destroying cavity stability. This is observed experimentally. The results of this work indicate that a single thin-disk laser could simultaneously produce high beam quality and high power if novel thermal management techniques are employed.

Cryogenic ceramic 277 watt Yb:YAG thin-disk laser

A ceramic ytterbium:yttrium aluminum garnet (Yb:YAG) thin-disk laser is investigated at 15°C (288 K) and also at 80 K, where it behaves as a four-level laser. We introduce a new two-phase spray cooling method to cool the Yb:YAG. One system relies on R134a refrigerant while the other uses liquid nitrogen (LN 2 ). The use of two systems allows the same disk to be tested at the two temperatures. When the Yb:YAG is cooled from room to cryogenic temperatures, the lasing threshold drops from 155 W to near 10 W, while the slope efficiency increases from 54% to a 63%. A 277 W laser with 520 W of pump is demonstrated. We also model the thermal and structural properties at these two temperatures and estimate the beam quality.