Frequency Conversion of the

Arrays of potassium dihydrogen phosphate (KDP) crystals are used to convert the infrared light of the Nova laser to shorter wavelengths in the visible and near­ ultraviolet for improved target performance.

For further information contact he performance of inertial­ coherent and virtually lossless. The Mark A. Summers (415) 423-2861. confinement fusion (ICF) generation of harmonics in optically T targets is known to improve at nonlinear media is the best laser wavelengths shorter than 111m, understood frequency-conversion for example near 0.5 or 0.3 11m (visible technique, the one that provides most green or near-ultraviolet light). At control over beam quality, and the these shorter wavelengths, ICF targets one adopted at LLNL. absorb more light and the deleterious In the process of harmonic preheating of the fusion fuel is generation, very intense laser light reduced. However, most high-power incident on a transparent, optically considered for fusion nonlinear medium interacts with the applications operate either near 10 11m material's atomic structure to generate (the carbon dioxide laser) or near electromagnetic radiation with 111m (Nd:glass lasers). Therefore, frequencies that are multiples of the to produce the desired shorter frequency of the incident light (see the wavelengths, we have two choices. box1 on p. 4). Such multiples are Either we can develop a completely known as harmonics of the new short-wavelength laser, or we can fundamental frequency. These convert the fundamental frequency of harmonics are commonly designated an existing I-11m laser using a as lw, 2w, 3w, etc., where lw is the technique called . fundamental frequency, 2w is the Frequency conversion of Nd:glass second harmonic, 3w is the third lasers has been pursued at LLNL and harmonic, etc. For Nd:glass lasers, elsewhere as the more reliable and lw corresponds to a wavelength of cost-effective approach to obtaining 1.05 11m, in the near-infrared; 2w shorter wavelengths. corresponds to a wavelength of In principle, any physical 0.527 11m, green near the spectral peak phenomenon that changes the for visible light; and 3w corresponds frequency of light is a candidate for to 0.351 11m, in the near-ultraviolet frequency conversion. In practice, just beyond the visible spectrum. however, since we also require a focusable output, a high conversion istory efficiency, and a high damage Frequency conversion of laser threshold, frequency conversion must H light was first demonstrated be based on a phenomenon that is in the 1960s, when experimenters

2 INERTIAL FUSION

passed the deep red light of a ruby By 1980, frequency conversion as laser through a quartz crystal, applied to ICF laser systems was well producing a harmonic in the established. The nonlinear material of ultraviolet range.2 At about the same choice, KDP, had been used to time, other researchers demonstrated demonstrate conversion efficiencies conversion of the infrared output of greater than 50% on high-power laser an Nd:YAG laser to an ultraviolet systems around the world at aperture harmonic by illuminating a crystal diameters as large as 15 cm. The next of potassium dihydrogen phosphate challenge was to scale up the (KDP) .3 frequency-conversion systems to the This frequency-conversion Nova size (74-cm aperture). capability greatly increased the Construction of the Nova flexibility of -based laser frequency-conversion system has media (e.g., Nd:YAG, Nd:YLF, and required considerable technology Nd:glass). In fact, the potential for development. Over the last five years, high performance at short the most significant achievements in wavelengths was one reason we the technology of large-aperture selected the Nd:glass laser system frequency conversion have been: over the much-longer-wavelength • A new geometry for KDP crystals carbon dioxide laser for ICF research, that efficiently produces both 2m and since the benefits of target irradiation 3m light. with short wavelengths had been • Methods for fabricating large anticipated.4 plates of KDP crystals accurately Through the 1970s, frequency­ aligned in relation to their conversion techniques were applied to crystallographic axes. Nd:glass laser systems in this country • Techniques for mounting the and abroad. The research team at KDP plates in large arrays. KMS Fusion, Inc. (Ann Arbor, • A high-damage-threshold Michigan), was the first to conduct antireflection coating for KDP. physics experiments using the second harmonic (2m) of an Nd:glass DP Geometry laser. They used a KDP crystal with a The Nova frequency­ clear aperture approximately 10 cm in K conversion system is an diameter.s At Ecole Poly technique in extremely simple and flexible device France, ICF laser-plasma interaction that can efficiently produce 2m or 3m characteristics were measured at the light. Third-harmonic (3m) light is second (2m) and fourth (4m) generated using a method first harmonics of Nd:glass, again using developed at the University of KDP.6 Researchers at the University Rochester's LLE,7 where two KDP of Rochester's Laboratory for Laser crystals are mounted in series or Energetics (LLE) characterized the "cascade." The first KDP crystal plasma conditions at the third converts some of the 1m light to 2m harmonic (3m) using KDP,7 light, with a conversion efficiency of Here at Livermore, the approximately 67%. A second KDP system was frequency-converted to crystal, oriented differently, mixes the study plasma conditions at 2m, 3m, 2m light with residual 1m light from and 4m.8 With the Novette laser, the the first crystal to produce 3m light, advantage of ICF implosions using with an intrinsic conversion efficiency shorter wavelengths was clearly approaching 90%. demonstrated at the multikilojoule At Livermore, we recognized that a level, where, by irradiating targets modification of an LLE design would with visible (2m) light, higher fuel efficiently and conveniently produce densities were achieved than had 2m as well as 3m light. Two KDP been possible with Shiva experiments crystals, optimized for 3m conversion, using the infrared fundamental (1m) are oriented so that they produce two wavelength.9 components of 2m light, orthogonally

3 HarlDonic Generation

When a low-intensity light wave propagates through Laser pulses typically have a spectral brightness a transparent medium, it drives the electron cloud of the (power density per steradian-hertz) many orders of constituent atoms into oscillation (a). This oscillating magnitude greater than that of incoherent sources. It is electric charge constitutes a polarization wave that gives this property that enables a laser pulse to produce a rise to a radiated wave. The radiated wave represents more vigorous oscillation of the electron cloud. The the usual "linear" response of the medium to the electrons may be displaced from their equilibrium incident light. Because the oscillatory motion of the position by a distance many times larger than the truly electrons follows the vibratory motion of the driving minute excursion induced at low intensity. As a result, field, the radiated wave is of the same frequency as the electrons come closer to neighboring atoms, probing the incident wave. The wave thus emerges from the their repulsive potential with each oscillatory cycle. This transparent medium with its frequency unchanged but process enables the structure of the medium to influence with a phase delay that depends on the material's the frequency of light radiated by the oscillating refractive index. This applies to all low-intensity light electrons. If the strength of the repulsion experienced by waves passing through transparent media. an electron driven by intense laser light is asymmetric (different in opposite directions), its motion will be biased (b). If we decompose an electron's motion into its frequency components, they appear at multiples of the (a) input frequency, 1m (c). For a centrosymmetric site, the components appear at odd multiples (mostly 3m and, to Atomic site ~." h,,. La.. ,. , • • ",,;00 a vastly smaller extent, Sm). For an asymmetric site, we obtain both odd and even multiples. By far the strongest ;/;;:\f ) of these is 2m and, in order of rapidly decreasing rv-v+ ( .' ..... ~ .' rv-v+ strength, 3m, 4m, and higher frequencies. (There is also Incident wave " 1/ Transmitted wave a zero-frequency, or dc, polarization term.) Each of the oscillatory motions gives rise to a radiated wavelet (a r Electric field amplitude harmonic) at multiples of the input frequency. , .... , / ' -" , -, " - In generating second harmonics, for example, each / \ ~ ~ // . wavelet at the second-harmonic frequency, 2m , radiates ~\ \ I ~\ . T'm. ' ... ~' '-" , ~", / Electron motion

(b) (c)

Symmetric repulsive potential SymmetriC electron motion ~ ~ (centric site) 1w ' "--.-/ A A Ar)3W ~ ~ 5wJV\JV\f\f\N

Asymmetric electron motion 1W ~ ~ ....--...... ~ r)2W ~ \IV 3w ~ 4w'\.../VVV\. Asymmetric repulsive potential (acentric site) 5w'\J\MMf\l\..

4 INERTIAL FUSION

inKDP

outward at the phase velocity of the medium for that orient a crystal of KDP so that the propagation velocity frequency, which is the vacuum speed of light divided of incident light waves produces exact phase matching. by the refractive index at 2w (V2w = c/nzw). The The optical axis of KDP is such that light polarized fundamental wave, however, propagates with a different perpendicular to the axis experiences an angle­

phase velocity ( VIW = c/nlw). Because the fundamental independent (ordinary) refractive index, nO' whereas wave imposes its phase on each of the electron light of the other polarization experiences an angle­ oscillators, and thus on the radiated 2w wavelets, the dependent (extraordinary) refractive index, n~ see net radiated wave at 2w is the sum of many out-of­ figure (e). The extraordinary index ne(e) at the second phase wavelets from the host of oscillators throughout harmonic is equal to the ordinary index at the first the material. We describe this situation by saying that harmonic at a particular angle e, the phase-matching the process is not phase matched (d). The result is a angle of incident light. This condition, called type-l very weak conversion of the laser energy from the phase matching, produces phase matching for the fundamental to the desired frequency,2w. second harmonic. To achieve the phase-matched condition, in which Phase matching may also occur when the second the 2w wavelets will add coherently, we must arrange harmonic is matched with two fundamental waves for the phase velocity of the fundamental wave to equal polarized in orthogonal directions. This is called type-II that of the second-harmonic wave. This can be done by phase matching. In this way, a KDP crystal provides a choosing a material with anisotropic characteristics that high conversion efficiency in one particular orientation compensate for the variation in phase velocity with but a low efficiency in any other orientation. We have wavelength. The wavelets from all the oscillators in demonstrated experimentally this conversion technique, such a material then will reinforce one another (d), called angle-tuned harmonic generation, with the resulting in a high conversion efficiency. KDP crystals laboratory's Argus and other laser facilities. This display this property. technique requires precise orientation of the KDP The phase velocity of light in a transparent medium crystal, typically to within 100.urad. is inversely proportional to its refractive index, which, in tum, depends on the spatial orientation of the material and on the wavelength of the light. It is possible to

(e)

(d)

Not phase matched c .2 t) ~ ~ Input wave at ~w ",_ .... , ,, __ , ,, __ '0 >< , / , " , / Q) '-'" '-~ '-~ "C £ r------~~~~------~=-r_------~

Phase matched ---

Wavelength

5 polarized with respect to each other; crystals, with input and output surface "quadrature" is used to refer to this normals approximately parallel to the mode of operation. This design led to phase-matching direction. We found the two-crystal architecture, referred to that the input and output surfaces as Quadrature 2w/Cascade 3w (or could be made very parallel Doubler/Mixer). As shown in Fig. 1, ( <5 .urad), but the accuracy with the cascaded crystals may also be which the surface normal could be oriented to pass lw light unchanged. aligned to the phase-matching With this design, crystals in each direction was very poor (> 300 .urad). Nova beam can efficiently produce 2w Upon investigation, we discovered or 3w light. The change from 2w to that the lack of orientation accuracy 3w conversion is accomplished by was a result of the x-ray diffraction small rotations in two axes, permitting measurement technique and a rapid changeover. This design thus inaccurate crystal machining. offers the great advantages of Fabricated in this manner, each Nova simplicity of operation and crystal would have to be individually commonality of parts between aligned to the Nova beam to obtain Fig. 1 optimized 2w and 3w crystal arrays. high conversion efficiency. The The frequency-conversion arrays are located complexity of providing individual at the output of each Nova beam line, just be­ abrication of KDP alignment adjustments on such a large fore the target focusing lenses. Simple rota­ Crystals number of crystals made this a very tion adjustments of the arrays provide Nova When we first started working unattractive approach. with the flexibility to irradiate targets with F three wavelengths of light: infrared (1w), visi­ with KDP crystals, we polished them Therefore, we took a different ble green (2w), or near-ultraviolet (3w). using techniques similar to those approach to controlling the fabrication developed for glass polishing. Our of KDP crystals. Our object was to Doubler goal was to produce optically flat improve the orientation accuracy so rMixer that alignment adjustments of Lens , array individual crystals in an array were not required. Instead of using x-ray diffraction to determine the phase­ matching direction, the crystal under 1.05 JllTl 1.05 JllTl test is used to frequency-convert a small infrared beam of the same wavelength as produced by the Nova laser. The orientation for peak conversion efficiency is measured by Doubler comparison with a standard crystal array "\ rMixer and the angular correction needed to , array Lens Rotate 45° properly locate the surface normal is determined. The next challenge was to 0.527 JllTl accurately machine the surface of the 1.05 JllTl T/2., > 75% KDP crystal, which is very soft (about the hardness of chalk). Therefore, we turned to the highly precise technology of single-point diamond machining. We successfully developed Doubler the methods and tooling for Rotate 35° rMixer machining KDP to the exacting , array Lens Tilt 1/4° angular tolerances required for Nova. In fact, this machining technique is so precise that it can produce the final 0.351 JllTl surface finish required «A!4, 1.05 JllTl 'bw > 70% < 20 nm peak-to-valley surface roughness), allowing us to bypass conventional polishing techniques.

6 INERTIAL FUSION

Several crystal plates are cut from a process was extremely time Fig. 2 single boule (Fig. 2) using a "water­ consuming and costly; finishing Large KDP crystal boule (grown by Cleveland and-string saw" (KDP dissolves in accounted for approximately 50% of Crystals, Inc.) will yield four or five plates water). These rough-cut plates are the cost of the array. We realized that, measuring 1 x 27 x 27 cm. The smaller crys­ "edged" on the diamond turning with this array design, the Nova tal is the size of the typical boule produced machine. They are then mounted, 20)130) arrays would require more before large-aperture frequency-conversion stress free, with an elastomer onto a than 500 individual KDP crystal technology using KDP was required. The insert photograph shows finished KDP crystal rubber-gel chuck, and a flat reference plates, and fabrication would take plates (5 x 5 cm, 15 x 15 cm, and surface is machined. Using the crystal more time than the schedule 27 x 27 cm in dimension). orientation measurement system permitted. Therefore, we scaled up the (CaMS), the angular correction is determined, and the crystal is then remachined to produce the final orientation and finish (Fig. 3). One of the 27 x 27-cm Nova KDP crystals being machined on the DL-l diamond lathe is shown in Fig. 4. rystal Arrays During construction of the C Nova laser, single KDP crystals large enough to convert the 74-cm -diameter beam were not available, and it was not feasible to scale up the crystal technology in time to meet the Nova deadlines. In fact, the largest high-quality KDP crystals available at that time measured only 15 x 15 cm. Therefore, we were forced to design an array of KDP crystals. Our first array, tested on the Novette laser system, was a 5 x 5 matrix of 25 individual KDP crystals, each precisely machined so that its plate surface normal was within 30 ,urad of the phase-matched direction. However, because of the accuracy and careful handling required, this machining

Standard~

,...- E)=4= 81 Rubber- Rubber- - COMS ~ gel ~ gel ~ c:b:!-; chuck chuck chuck t.J ' "2 -0-0- tt t-t 2 1 1 2 L....-----'\ \ Boule Plates Edges Rough Cut Cut Phase match 2 1 grown cut finished crystal face 1 face 2 obtained

Fig. 3 Initial stages of fabrication of KDP crystals. We first cut the KDP boule ment system (COMS), the angular correction required for optimum con­ into plates. The plates are edge-finished and mounted on a rubber-gel version efficiency is determined. Finally (not shown), the KDP plates are chuck so that one side can be machined flat. Then the plate is reversed put back on the diamond-turning machine to correct the surface orienta­ and the other side is machined. Using the crystal orientation measure- tion and to machine the surface to its final optical finish.

7 size of each KDP crystal plate to Building an array of KDP crystals 27 x 27 cm, since crystals of this size for frequency conversion is difficult could be grown in time. This allowed because of the fine angular alignment construction of a 3 x 3 KDP array for required between the Nova lev pump the 74-cm aperture. This design was beam direction and the KDP less expensive and could be fabricated crystallographic axis. In previous in time to meet the 1984 Nova work using a single KDP crystal, the completion deadline. conversion efficiency would be maximized by manually adjusting the crystal alignment to reach the Fig. 4 optimum orientation. With an array of A KDP crystal plate (1 x 27 x 27 cm) im­ crystals, each element of the array mersed in an oil shower (for temperature sta­ must be aligned to the beam bility) as it is machined on the DL-1 diamond individually. An approach using lathe. We use single-point diamond turning to orient the surface normal relative to the inter­ individual crystal adjustments, nal crystal structure and to machine the final although feasible, would be very optical finish. complex and is of questionable practicality on a large, complex system like Nova. To avoid adjustments of individual crystals and align only the overall array, each of the crystals in the Nova arrays is precisely mounted so that when the Nova beam passes through the assembly, each crystal is aligned to within < 100 .urad of the optimum orientation. It was fairly straightforward to bolt the precisely fabricated KDP plates to a support structure without complex alignment adjustments. However, this "egg crate" support structure, itself, had to be machined extremely flat so that when mounted, all the KDP crystals would lie in a plane to within better than 30.urad. The Quadrature/ Cascade design required that both sides of the egg crate be flat to this tolerance, one side for the doubler array and the other side for the mixer array. One very important feature of the egg-crate design is the extremely thin (3-mm) webs that provide the crystal supports. The webs are thin so they do not obscure a large fraction of the beam aperture. To provide the required stiffness and yet maintain the alignment tolerances, the webs are deep (100 mm). The egg crate is fabricated from a single piece of aluminum using electric-discharge Fig. 5 machining (EDM) technology to prevent warping of the thin webs. Ten A 3 x 3 deep-web "egg crate," mounted on the DL-3 diamond lathe. As the egg crate spins on the lathe, the single-point diamond tool traverses of the Nova egg crates were fabricated it, producing an extremely flat «10 JlITI) surface. Both sides of the Nova on the DL-3 lathe, one for each beam egg-crate must be finished in this manner. line (see Fig. 5). In addition to the egg

8 INERTIAL FUSION

crates, special crystal installation coating design and a high damage fixtures were made to assist with the threshold. l1 It can be easily applied loading of the 18 individual KDP by a spinning technique similar to crystals. The final assembly was the photoresist deposition technique performed in a class-IOO clean room, used in semiconductor technology.12 after which the crystal array was Figure 6 shows the coating tank used tested interferometrically for to coat each of the Nova KDP crystals. Fig. 6 wavefront distortion and crystal We optimized the spectral The sol-gel antireflection coating is applied to alignment, before it was installed on characteristics of this sol-gel coating the Nova KDP crystals in the crystal-coating the Nova system. for each position in the array and tank shown here. The crystal is mounted on a plate in the center of the tank. The crystal is coated all KDP crystals before they spun at about 380 rpm while the coating solu­ were installed in the egg crate. The tireflection Coating tion (0.75% Si02 in ethanol) is applied from a The final step in the final assembly is shown in Fig. 7. syringe mounted above the crystal. X development of the Nova frequency-conversion array was the invention of an antireflection coating for KDP. Uncoated KDP crystals mounted in a 2m/3m architecture lose approximately 16% in performance as a result of Fresnel reflections at each crystal surface. In previous frequency­ conversion systems, this loss was reduced by immersing the crystals in an index-matching fluid. In fact, the first 2m array built for the Novette laser was a fluid-filled array. However, this method proved to be less than satisfactory.lO The fluid-filled gaps must be very narrow-a few microns-and the width is difficult to control. Also, the fluid-filled cells are subject to damage initiated by the interaction of the high-intensity light with microscopic contaminants in the fluid. The light heats the particles and induces local decompositions of the • fluid. Slight opacities then develop, which absorb energy on successive shots and grow progressively larger, eventually obscuring a large fraction of the aperture. During the initial Nova design, as Fig. 7 a backup design to the low-loss but damage-prone fluid-filled cells, we An assembled, 74- cm-aperture, KDP built a bare crystal, deep-web egg­ crystal array. Indi­ crate design and successfully tested it vidual KDP crystals in a 4m converter for Novette target are mounted on experiments. However, with this both sides of a backup design, we would have to deep-web "egg crate." All pieces of accept a 16% lower performance. this array-each During this time, we developed an crystal and the egg antireflection coating that could be crate itself-have applied at room temperature to KDP. been machined flat This porous sol-gel coating of silicon so that the array performs like a dioxide has the spectral characteristics single, monolithic of a classic quarter-wave antireflection crystal.

9 erformance We identified the problem as an The crystal arrays perfonned in tensi ty -dependen t depolarization P well at low intensity during of the 1m pump beam due to a tests on the Nova laser. However, combination of stress-induced when we increased the beam depolarization in the optical intensity, the frequency-conversion components and a nonlinear efficiency unexpectedly decreased. propagation phenomenon, called 2 Above 1.5 GW/ cm , the conversion polarization ellipse rotation, that efficiency from 1m to 3m departed appears at high intensity. To solve this radically from our theoretical problem, we added a polarizer to the estimates and from the results of end of a Nova laser chain, before the small-scale experiments conducted frequency-conversion array, to prevent prior to the Nova/ Novette trials. The the "contamination" of the beam by 2m perfonnance also fell off at the weakly depolarized light. This same pump intensity. Closer increased the conversion efficiency examination of the early Novette 2m, significantly (Fig. 8). Residual 3m , and 4m experiments revealed that, alignment errors introduced by the in all cases, the measured conversion prototype 74-cm-aperture polarizer efficiency departed from our are believed to be the source of the predictions when the pump intensities slightly low conversion efficiency and 2 exceeded -1.5 GW/ cm . can be eliminated by improving the Fig. 8 In small-scale, well-characterized optical quality of the polarizer Third-harmonic conversion efficiency vs input experiments conducted with Nova substrate. beam intensity observed on Nova beam 10, production KDP crystals, 2m and 3m before and after the addition of a 74-cm polar­ izer. The predicted efJiciences are shown for conversion results were in close onc1usion a flat and a temporally Gaussian input pulse. agreement with our theoretical The development over the The curves with data pOints are the measured calculations up to intensities as high last five years of large­ 2 C conversion efficiencies, with and without the as 4 GW/ cm . Thus, it was apparent aperture frequency-conversion arrays polarizer. The polarizer eliminates depolariza­ that some beam parameter important for the Nova laser has propelled this tion contamination of the 3m conversion pro­ cess and allows the efficiency to approach to the frequency-conversion process technology to a new level of the levels predicted and measured in small­ was strongly dependent upon beam perfonnance. Frequency conversion scale studies. intensity. now incorporates advanced crystal architectures, precise crystal orientation, optical finishing 1.0 ,------, techniques, antireflection coatings, and precision mounting techniques. Just Nova beam 10 one beam of the Nova laser has Flat temporal already produced record levels of 0.8 0.351-,um ultraviolet light (> 5 kJ, -- 1 ns). All ten Nova beams together § Gaussian can produce > 50 kJ of ultraviolet light a M temporal in 1 ns and even more energy at S 0.6 longer pulse widths. At this stage in u>­ c: the development of large-aperture 'uQ) frequency-conversion arrays, there is ~ no fundamental limitation on the c: 0.4 o continued scaling of the aperture in 'iiia; order to increase the output energy >c: o of future laser systems. ~ () 0.2

Key Words: egg-crate array; frequency conversion; harmonic generati on- 1m, 2w, 3w; 2 3 4 laser-Nova, Novette; potassium dihydrogen Intensity at 1m, GW/cm2 phosphate (KDP); precision machining.

10 INERTIAL FUSION

Notes and References High Efficiency Third Harmonic Conversion of High Power Nd:Glass Laser 1. D. Eimerl, "Laser Fusion with Green and Radiation," Optics Communications, 34 (3), Blue Light," Energy and Technology 469 (1980). Review, Rept. UCRL-52000-82-8 (August 8. G. J. Linford et aI. , "Large Aperture 1982), p. 19-29. Harmonic Conversion Experiments at 2. P. A. Franken et al., Phys. Rev. Lett. 7, 118 Lawrence Livermore National Laboratory," (1961). AppI. Opt. 21 (20), 3633 (1982); see also G. 3. J. A. Giordmaine, "Mixing of Light Beams J. Linford et aI. , "Large Aperture Harmonic in Crystals," Phys. Rev. Lett. 8, 19 (1962); Conversion Experiments at Lawrence see also P. D. Maker et al., "Effects of Livermore National Laboratory: Dispersion and Focusing on the Production Comments," Appl. Opt. 22 (13), 1957 of Optical Harmonics," Phys. Rev. Lett. 8, (1983). 21 (1962). 9. J. F. Holzrichter et al., "Research with 4. J. Nuckolls et al. , "Laser Compression of High-Power Short-Wavelength Lasers," Matter to Super-High Densities: Science 229 (4718), 1045 (1985). Thermonuclear (CTR) Applications," 10. M. A. Summers et al., "Nova Frequency Nature 239, 139 (1972). and Focusing System," Laser Program 5. 1977 Annual Report on Laser Fusion Annual Report, Lawrence Livermore Research, KMS Fusion, Inc., Ann Arbor, National Laboratory, Rept. UCRL-50021-83 Michigan, Rept. KMSF-U762, pp. 2-17 (1983), p. 2-8. (1977). 11. D. Milam and 1. M. Thomas, "Sol-Gel 6. F. Amiranoff et aI. , "Interaction Coatings," Laser Program Annual Report, Experiments at Various Laser Lawrence Livermore National Laboratory, Wavelengths," presented at the Twenty­ Rept. UCRL-50021-84 (1984), p. 6-39; see First Annual APS Meeting Division of also 1. M. Thomas, "Optical Coatings by Plasma Physics, Boston, Massachussetts, the Sol-Gel Process," Energy and November 12-16, 1979. Technology Review, UCRL-52000-85-1O 7. R. S. Craxton, "Theory of High Efficiency (October 1985), p. 8-14. Third Harmonic Generation of High 12. "Frequency Conversion for High Power Power Nd:Glass Laser Radiation," Optics Lasers," Technology Transfer Symposium, Communications, 34 (3), 474 (1980); see Lawrence Livermore National Laboratory also W. Seka et al., "Demonstration of aune 13-14, 1985).

11