NIF Optical Materials and Fabrication Technologies:    An Overview
J. H. Campbell, R. Hawley-Fedder, C. J. Stolz, J. A. Menapace, M. R. Borden, P. Whitman, J. Yu, M. Runkel, M. Riley, M. Feit, and R. Hackel
SPIE 2004 Photonics West, San Jose, California, January 24-29, 2004
February 23, 2004谭维维道歉
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NIF optical materials and fabrication technologies:
An overview
J. H. Campbell*, R. A. Hawley-Fedder, C. J. Stolz, J. A. Menapace, M. R. Borden,  P. K. Whitman, J. Yu, M. Runkel, M. O. Riley, M. D. Feit and R. P. Hackel University of California, Lawrence Livermore National Laboratory, 7000 East Avenue, L-491
Livermore, CA 94550
ABSTRACT
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 sp
ecifications 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.
1. INTRODUCTION
The NIF contains 7360 large aperture optics (~0.5 to 1.0 m) making it not only the largest laser but also the largest optical system ever constructed[1]. Table 1 summarizes the number and types of optics installed in the NIF’s 192-beamlines.  To manufacture the NIF optics required an extensive optical materials and process development effort that began in 1995. This was followed by several years of activities for design and construction of manufacturing facilities and start-up (“pilot”) operation.  To accomplish this, LLNL has partnered with a group of well-known optical materials and optics fabrication companies located around the world. At the time of this article (Jan 2004) we are e
ntering our tenth year since the start of the NIF optics effort. More than 40% of the large-aperture optics required for the NIF have been completed and are in the warehouse ready for installation. Many of the remaining 60% are in the manufacturing cycle and are on schedule for meeting the NIF project completion.
Table 1. Summary of the number and types of large aperture (~0.5 to 1.0 m) optics used on the NIF
material(s)
Key
Optic Number
required
关于友情的歌Amplifier slabs 3072 Phosphate glass
Mirrors and polarizers 1600 HfO2/SiO2 coating on BK-7
Windows and lenses 1728 SiO2
Crystals 576 DKDP and KDP因为寂寞才想你
SiO2
Gratings 192
Debris shields 192 SiO2
Total: 7360
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2. OVERVIEW OF THE NIF OPTICAL LAYOUT
丢了幸福的猪吉他谱The optical layout of one of the NIF’s 192 identical beamlines is shown schematically in Figure 1. Each beamline contains a series of flashlamp-pumped, Nd-glass amplifiers operating at 1053 nm. These amplifiers are located in two sections in the main laser: the power amplifier, and the multi-pass or main amplifier; these sections respectively contain five and eleven Nd-glass laser slabs per
beamline. In total, the 192 beamline NIF contains 3072 such slabs, each weighing about 42 kilogram and measuring 46 cm × 81 cm × 4 cm. The slabs are mounted at Brewster’s angle to minimize Fresnel surface reflection losses and to enhance the pump efficiency by the surrounding flashlamps.
A plasma-electrode pockels cell (PEPC) is used in combination with a large-aperture (1-m) polarizer to control the number of passes through the amplifier sections[2-4]. The NIF utilizes four passes through the 11-slab-long amplifier section within the main laser cavity, in other words, two round trips. Each of the PEPC apertures contains a 1-cm thick potassium dihydrogen (KDP) plate cut normal to the crystal z-axis. The crystal plate is installed in a vacuum cell between two fused silica windows.  Glow-discharge plasmas are ignited in the evacuated spaces on the two faces of the crystal. The plasmas have an electron density low enough not to absorb, reflect or diffract the beam as it passes through the cell yet sufficient to provide the conductivity needed to charge the opposing crystal faces to the switching voltage (~15kV) in about 100 nanoseconds.
Fig. 1. Schematic representation of the optical layout of one of the 192 identical laser beam-lines that comprise the NIF.
Two large confocal spatial filters (nominally 25 and 60m in length) use pinholes as low-pass filters to remove high spatial frequency noise and image relay the propagating beams to the target chamber[1,5]. The equi-convex lenses used at both ends of the spatial filters have a slight aspheric correction applied to one side.  The lenses are fabricated from fused silica and have back-focal lengths of 11.6 and 29.7 m, respectively, measured to an accuracy of about one part in 10,000.  Multi-layer high-reflectivity dielectric coatings deposited on BK-7 substrates serve as mirrors throughout the NIF. The coating layers are HfO2/SiO2 deposited by e-beam evaporation[6-8]. The mirrors are installed in two main locations on the NIF. The first location is the multi-pass cavity where two mirrors comprise the ends of the cavity and a third is paired
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with the polarizer in a periscope assembly (Fig. 1). The mirror located on the amplifier end of the cavity is a deformable mirror composed of a matrix of 39 precision actuators installed on the rear surface of the substrate[9].  These actuators can modify the surface topography of the mirror (i.e. coating) to correct for system optical aberrations. The second cavity-end mirror consists of the same multi-layer coating design but deposited on a static substrate.  The periscope mirror is used in conju
nction with the optical switch (PEPC and polarizer) to direct the beam in and out of the multi-pass cavity.
The second major location where mirrors are installed is in two large bays called the “switch yards” (Fig. 1)[1]. These mirrors are used to transform the 2-dimensional, planar geometry of the 192 output beams from the main laser into the 3-dimensional spherical configuration required for target illumination. These mirrors also use HfO2/SiO2 multi-layer stacks but the designs vary somewhat to account for the different use angles[10,11].
All SiO2 and KDP transmissive optics in the NIF beamlines are coated with quarter-wave anti-reflection coatings. The coatings consist of a porous layer of silica nano-particles deposited by either a dip or spin-coating process from a sol-containing alcohol/water solution[12-14]. The coatings are about 50% porous with an effective refractive index of 1.23+0.02 and have a single-surface transmission of >99.5% on SiO2 (n=1.45) and KDP (n=1.5).
The NIF target area houses a 10-meter diameter vacuum target chamber that contains 48 laser entry ports. Each laser port allows four laser beams (a “quad”) to enter the chamber[1,15].  Mounted on each entry port is a precision optical assembly containing four separate optical cells (one for each
beam).  Each cell in turn holds several precision optical elements (Fig. 2). A thick fused silica window provides vacuum isolation of the target interior from the surrounding ambient atmosphere.  Next, there is a dual-crystal frequency converter (comprised of KDP and deuterated-KDP) that converts the fundamental 1053 nm laser output to 351 nm. The converted light enters the final focusing lens that is purposely wedged to refract any unconverted light (at 1053 and 526.5 nm) away from the target.  Also included in this optical assembly is a full-aperture diffraction grating (one per beam), used to diffract about 0.2% of the beam energy to a diagnostic package. The last optic in each NIF beamline is a relatively low-cost fused silica debris shield that blocks target debris from hitting the more expensive optics contained in the final optics assembly.
Fig. 2. Schematic drawing of the Final Optics Assembly (FOA) showing the key optical elements and their relative locations when fully assembled. Note the FOA is designed to accommodate four separate NIF beams (a “quad”).