Christopher J. Stolz

NIF Optical Materials and Fabrication Technologies: An Overview

The high-energy/high-power section of the NIF laser system contains 7360 meter-scale optics. Advanced optical materials and fabrication technologies needed to manufacture the NIF optics have been developed and put into production at key vendor sites. Production rates are up to 20 times faster and per-optic costs 5 times lower than could be achieved prior to the NIF. In addition, the optics manufactured for NIF are better than specification giving laser performance better than the design. A suite of custom metrology tools have been designed, built and installed at the vendor sites to verify compliance with NIF optical specifications. A brief description of the NIF optical wavefront specifications for the glass and crystal optics is presented. The wavefront specifications span a continuous range of spatial scale-lengths from 10 μm to 0.5 m (full aperture). We have continued our multi-year research effort to improve the lifetime (i.e. damage resistance) of bulk optical materials, finished optical surfaces and multi-layer dielectric coatings. New methods for post-processing the completed optic to improve the damage resistance have been developed and made operational. This includes laser conditioning of coatings, glass surfaces and bulk KDP and DKDP and well as raster and full aperture defect mapping systems. Research on damage mechanisms continues to drive the development of even better optical materials.

Reactive evaporation of low-defect density hafnia

Motivation for this work includes observations at Lawrence Livermore National Laboratory of a correlation between laser damage thresholds and both the absorption and the nodular-defect density of coatings. Activated oxygen is used to increase the metal-oxidation kinetics at the coated surface during electron-beam deposition. A series of hafnia layers are made with various conditions: two µ-wave configuations, two sources (hafnium and hafnia), and two reactive oxygen pressures. Laser damage thresholds (1064-nm, 10-ns pulses), absorption (at 511 nm), and nodular-defect densities from these coatings are reported. The damage thresholds are observed to increase as the absorption of the coatings decreases. However, no significant increase in damage thresholds are observed with the coatings made from a low nodular-defect density source material (hafnium). Hafnia coatings can be made from hafnium sources that have lower nodular-defect densities, lower absorption, and damage thresholds thatare comparable with coatings made from a conventional hafnia source.

Laser intensification by spherical inclusions embedded within multilayer coatings

The initiation of laser damage within optical coatings can be better understood by electric-field modeling of coating defects. The result of this modeling shows that light intensification as large as 24x can occur owing to these coating defects. Light intensification tends to increase with inclusion diameter. Defects irradiated over a range of incident angles from 0 to 60 deg tend to have a higher light intensification at a 45 deg incidence. Irradiation wavelength has a significant effect on light intensification within the defect and the multilayer. Finally, shallow, or in the case of 45 deg irradiation, deeply embedded inclusions tend to have the highest light intensification.

Alignment and wavefront control systems of the National Ignition Facility

The National Ignition Facility (NIF) at the Lawrence Livermore National Laboratory is a stadium-sized facility containing a 192-beam Nd glass laser. Its 1.053-µm output is frequency converted to produce 1.8-MJ, 500-TW pulses in the ultraviolet. Refer to the companion overview articles in this issue for more information. High-energy-density and inertial confinement fusion physics experiments require the ability to precisely align and focus pulses with single-beam energy up to 20 KJ and durations of a few nanoseconds onto millimeter-sized targets. NIFs alignment control system now regularly provides automatic alignment of the four commissioned beams prior to every NIF shot in approximately 45 min, and speed improvements are being implemented. NIF utilizes adaptive optics for wavefront control, which significantly improves the ability to tightly focus each laser beam onto a target. Multiple sources of both static and dynamic aberration are corrected. This article provides an overview of the NIF automatic alignment and wavefront control systems, and provides data to show that the facility is expected to meet its primary requirements to position beams on the target with an accuracy of 50-µm rms over the 192 beams and to focus the pulses into a 600-µm spot.

Photothermal characterization of optical thin film coatings

Photothermal techniques are widely used in thin film characterizations and are particularly useful in studying laser-induced damage in optical coatings. The specific applications include measuring weak absorption, characterizing thermal conductivity, detecting local defects, and monitoring laser-interaction dynamics and determining laser damage thresholds as well as thermal impedance at boundaries of multilayers. We take an overview of the principle of photothermal techniques, the various detection methods, and the progress made during the last decade in applying these techniques to optical thin films. The further potential and limitations of the techniques will also be discussed, with emphasis on {ital in situ} studies of laser interaction with thin films and local defects. {copyright} {ital 1997 Society of Photo-Optical Instrumentation Engineers.}

The National Ignition Facility: the world's largest optics and laser system

The National Ignition Facility, a center for the study of high energy density plasma physics and fusion energy ignition, is currently under construction at the Lawrence Livermore National Laboratory. The heart of the NIF is a frequency tripled, flashlamp-pumped Nd:glass laser system comprised of 192 independent laser beams. The laser system is capable of gen-erating output energies of 1.8MJ at 351nm and at peak powers of 500 TW in a flexible temporal pulse format. A descrip-tion of the NIF laser system and its major components is presented. We also discuss the manufacture of nearly 7500 pre-cision large optics required by the NIF including data on the manufactured optical quality vs. specification. In addition, we present results from an on-going program to improve the operational lifetime of optics exposed to high fluence in the 351-nm section of the laser.

Light intensification modeling of coating inclusions irradiated at 351 and 1053 nm

Electric-field modeling provides insight into the laser damage resistance potential of nodular defects. The laser-induced damage threshold for high-reflector coatings is 13x lower at the third harmonic (351 nm) than at the first harmonic (1053 nm) wavelength. Linear and multiphoton absorption increases with decreasing wavelength, leading to a lower-third harmonic laser resistance. Electric-field effects can also be a contributing mechanism to the lower laser resistance with decreasing wavelength. For suitably large inclusions, the nodule behaves as a microlens. The diffraction-limited spot size decreases with wavelength, resulting in an increase in intensity. Comparison of electric-field finite-element simulations illustrates a 3x to 16x greater light intensification at the shorter wavelength.

Effects of vacuum exposure on stress and spectral shift of high reflective coatings

Coating stress and spectral shift are affected by changing from ambient to vacuum environments. This change can affect optical systems that are aligned in air and used in a vacuum or in a dry environment. Spectral shifts up to 3% and reflected wave-front changes up to 0.35 waves peak to valley are reported for conventional electron-beam deposition and ion-assisted deposition. Alternatively, ion-beam sputtered coatings have virtually no changes between different pressure environments.

BDS thin film damage competition

A laser damage competition was held at the 2008 Boulder Damage Symposium in order to determine the current status of thin film laser resistance within the private, academic, and government sectors. This damage competition allows a direct comparison of the current state-of-the-art of high laser resistance coatings since they are all tested using the same damage test setup and the same protocol. A normal incidence high reflector multilayer coating was selected at a wavelength of 1064 nm. The substrates were provided by the submitters. A double blind test assured sample and submitter anonymity so only a summary of the results are presented here. In addition to the laser resistance results, details of deposition processes, coating materials, and layer count will also be shared.

A comparison of nodular defect seed geometeries from different deposition techniques

A focused ion-beam milling instrument, commonly utilized in the semiconductor industry for failure analysis and IC repair, is capable of cross-sectioning nodular defects. Utilizing the instruments scanning on beam, high-resolution imaging of the seeds that initiate nodular defect growth is possible. In an attempt to understand the origins of these seeds, HfO2/SiO2 and Ta2O5/SiO2 coatings were prepared by a variety of coating vendors and different deposition processes including e-beam, magnetron sputtering, and ion beam sputtering. By studying the shape, depth, and composition of the seed, inferences of its origin can be drawn. The boundaries between the nodule and thin film provide insight into the mechanical stability of the nodule. Significant differences in the seed composition, geometry of nodular growth and mechanical stability of the defects for sputtered versus e-beam coatings are reported. Differences in seed shape were also observed from different coating vendors using e-beam deposition of HfO2/SiO2 coatings.