Isaac L. Bass

High-average-power dye laser at Lawrence Livermore National Laboratory.

The copper-laser-pumped dye laser system developed at the Lawrence Livermore National Laboratory (LLNL) is now capable of sustained, efficient, and reliable operation at total powers exceeding 2500 W and single amplifier chain powers exceeding 1300 W. Wavelength center frequency stability is maintainable to < 50 MHz. Laser dyes developed at LLNL permit tunability from 550 to 650 nm. Wave-front quality is < lambda/4 peak to valley. The system is operated remotely with the aid of a comprehensive set of diagnostics. Besides supporting its primary atomic-vapor-laser-isotope-separation mission, the system is being used in alternate applications such as materials processing and the generation of artificial guide stars.

An Improved Method of Mitigating Laser Induced Surface Damage Growth in Fused Silica Using a Rastered, Pulsed CO2 Laser

A new method of mitigating (arresting) the growth of large (>200 m diameter and depth) laser induced surface damage on fused silica has been developed that successfully addresses several issues encountered with our previously-reported5,6large site mitigation technique. As in the previous work, a tightly-focused 10.6 m CO2 laser spot is scanned over the damage site by galvanometer steering mirrors. In contrast to the previous work, the laser is pulsed instead of CW, with the pulse length and repetition frequency chosen to allow substantial cooling between pulses. This cooling has the important effect of reducing the heat-affected zone capable of supporting thermo-capillary flow from scale lengths on the order of the overall scan pattern to scale lengths on the order of the focused laser spot, thus preventing the formation of a raised rim around the final mitigation site and its consequent down-stream intensification. Other advantages of the new method include lower residual stresses, and improved damage threshold associated with reduced amounts of redeposited material. The raster patterns can be designed to produce specific shapes of the mitigation pit including cones and pyramids. Details of the new technique and its comparison with the previous technique will be presented.

In situ monitoring of surface postprocessing in large-aperture fused silica optics with optical coherence tomography

Optical coherence tomography (OCT) is explored as a method to image laser-damage sites located on the surface of large aperture fused silica optics during postprocessing via CO2 laser ablation. The signal analysis for image acquisition was adapted to meet the sensitivity requirements for this application. A long-working-distance geometry was employed to allow imaging through the opposite surface of the 5 cm thick optic. The experimental results demonstrate the potential of OCT for remote monitoring of transparent material processing applications.

Mitigation of growth of laser initiated surface damage in fused silica using a 4.6-micron wavelength laser

Surface damage caused by high fluence, 351 nm light to fused silica optics can adversely affect the performance of fusion class laser systems like that of the National Ignition Facility (NIF). It is typically initiated as a small pit and grows in both diameter and depth during normal operation with cracks that extend into the bulk. Mitigation of this growth has been previously reported using a 10.6 micron CO2 laser. Here, we report growth mitigation with the 4.6 micron light from a frequency-doubled, 9.2 micron CO2 laser. The motivation for using 4.6 microns is >25 times longer absorption length in fused silica at room temperature compared to that at 10.6 micron. Mitigation of subsurface cracks at 10.6 micron required ablation of material to the depth of the cracks. In contrast, it was possible to mitigate the subsurface cracks using 4.6 micron light without significant ablation of material. Damage sites as large as 500 microns in diameter with cracks extending to 200 microns in depth were successfully mitigated with 4.6 microns.

Metallic burn paper used for in situ characterization of laser beam properties.

In situ ablation of thin metal films on fused silica substrates by picosecond class lasers was investigated as a method of characterizing the beam at the sample plane. The technique involved plotting the areas enclosed by constant fluence contours identified in optical microscope images of the ablation sites versus the logs of the pulse energies. Inconel films on commercially available neutral density filters as well as magnetron sputtered gold films were used. It was also shown that this technique could be used to calibrate real-time beam profile diagnostics against the beam at the sample plane. The contours were shown to correspond to the boundary where part or all of the film was ablated.

Study of CO2 laser smoothing of surface roughness in fused silica

Small micrometer-sized roughness on optical surfaces, caused by laser damage and/or redeposition of laser ablated material, can cause local electric field intensification which may lead to damage initiation both on the optics and/or downstream. We examined the smoothing of etched periodic surface structures on SiO2 substrate with 10.6μm CO2 laser using atomic force microscopy. The characteristic surface tension driven mass flow of the glass under different laser parameters were simulated using computational fluid dynamics and correlated with experimental results. We found that during CO2 laser polishing the estimate viscosity of the silica glass appears to be higher than typical literature values measured at a temperature similar to the laser heating conditions. This discrepancy can be explained by the observation that at high temperature, a significant portion of the hydroxyl content in the layer of heated silica glass can diffuse out resulting in a much stiffer glass.

Optical damage performance of conductive widegap semiconductors: spatial, temporal, and lifetime modeling

The optical damage performance of electrically conductive gallium nitride (GaN) and indium tin oxide (ITO) films is addressed using large area, high power laser beam exposures at 1064 nm sub-bandgap wavelength. Analysis of the laser damage process assumes that onset of damage (threshold) is determined by the absorption and heating of a nanoscale region of a characteristic size reaching a critical temperature. This model is used to rationalize semi-quantitatively the pulse width scaling of the damage threshold from picosecond to nanosecond timescales, along with the pulse width dependence of the damage threshold probability derived by fitting large beam damage density data. Multi-shot exposures were used to address lifetime performance degradation described by an empirical expression based on the single exposure damage model. A damage threshold degradation of at least 50% was observed for both materials. Overall, the GaN films tested had 5-10 × higher optical damage thresholds than the ITO films tested for comparable transmission and electrical conductivity. The route to optically robust, large aperture transparent electrodes and power optoelectronics may thus involve use of next generation widegap semiconductors such as GaN.

Characterization of laser-induced damage by picosecond pulses on multi-layer dielectric coatings for petawatt-class lasers

We investigate the optical damage performance of multi-layer dielectric (MLD) coatings suitable for use in high energy, large-aperture petawatt-class lasers. We employ small-area damage test methodologies to evaluate the damage resistance of various coatings as a function of deposition methods and coating materials under simulated use conditions. In addition, we demonstrate that damage initiation by raster scanning at lower fluences and growth threshold testing are required to estimate large-aperture optics’ performance.

High-resolution 3-D imaging of surface damage sites in fused silica with Optical Coherence Tomography

In this work, we present the first successful demonstration of a non-contact technique to precisely measure the 3D spatial characteristics of laser induced surface damage sites in fused silica for large aperture laser systems by employing Optical Coherence Tomography (OCT). What makes OCT particularly interesting in the characterization of optical materials for large aperture laser systems is that its axial resolution can be maintained with working distances greater than 5 cm, whether viewing through air or through the bulk of thick optics. Specifically, when mitigating surface damage sites against further growth by CO2 laser evaporation of the damage, it is important to know the depth of subsurface cracks below the damage site. These cracks are typically obscured by the damage rubble when imaged from above the surface. The results to date clearly demonstrate that OCT is a unique and valuable tool for characterizing damage sites before and after the mitigation process. We also demonstrated its utility as an in-situ diagnostic to guide and optimize our process when mitigating surface damage sites on large, high-value optics.

Apparatus and techniques for measuring laser damage resistance of large-area, multilayer dielectric mirrors for use with high energy, picosecond lasers

We present techniques for measuring the damage performance of a variety of optical components with ps laser pulses, introduce a novel beam diagnostic technique, and explore the sensitivity of damage resistance to laser spot size for the case of high-reflectivity, multilayer dielectric (MLD) mirrors.