Photonics Work at Lawrence Livermore National Laboratory, USA

Research HighlightsPhotonics Work atLawrence Livermore National LaboratoryTony Ladran, Deputy of Operations, Photon Science and Applications Program,Lawrence Livermore National LaboratoryTHIS WORK PERFORMED UNDER THE AUSPICES OF THE U.S.DEPARTMENT OF ENERGY BY LAWRENCE LIVERMORE NATIONALLABORATORY UNDER CONTRACT DE-AC52-07NA27344The National Ignition Facility (NIF) & PhotonScience Directorate at Lawrence Livermore NationalLaboratory comprises five programs, each focused on oneor more aspects of NIF's missions. They are:National Ignition FacilityThe National Ignition Facility Project isresponsible for the construction and operationof NIF, a 192-beam experimental laserfacility. This unique facility is the world'slargest and highest-energy laser, capable of creating temperaturesand pressures similar to those that exist only in thecores of stars and giant planets and inside nuclear weapons.National Ignition CampaignThe National Ignition Campaign (NIC)encompasses all of the experiments, hardwareand infrastructure needed to carry outthe initial ignition experiments on NIF beginning in2010 and to continue research on ignition in the followingyears. NIC is a key element of the National NuclearSecurity Administration.Photon Science & ApplicationsThe Photon Science & Applications (PS&A)Program provides advanced solid-state laser andoptics technologies to the laboratory, government, and industry forimportant national needs. The primary activities of PS&A inrecent years have been: (1) to complete the laser technology ddevelopment and laser component testing for the U.S.Department of Energy's NIF project, (2) to develop advancedsolid-state laser systems and optical components for theDepartment of Defense and Department of Energy and (3) toaddress the needs of other government agencies and U.S. industry.Inertial Fusion EnergyThe Inertial Fusion Energy Program isexploring a variety of approaches to usinginertial confinement fusion, NIF's coretechnology to achieve energy gain and helplay the groundwork for the eventual use of fusion energyas a clean, safe, virtually limitless source of electricity.Science at the ExtremesNIF will become a premier internationalcenter for experimental science early in thenext decade. The extreme temperaturesand pressures that will be created insidethe NIF target chamber will enable scientists from aroundthe world to conduct unprecedented experiments in highenergy density science and to gain new insights into suchmysterious astrophysical phenomena as supernovae, giantplanets, and black holes.NIF: The ‘Crown Joule’ of Laser ScienceThe National Ignition Facility (NIF) is the world'slargest laser. See Figure 1. NIF's 192 intense laser beamswill deliver to its target more than 60 times the energy ofany previous laser system. When all 192 beams are oper-Figure 1. Three football fields could fit inside the NIF Laser andTarget Area Building.4 IEEE LEOS NEWSLETTER June 2008

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ational in 2009, NIF will direct nearly two million joulesof ultraviolet laser energy in billionth-of-a-second pulsesto the target chamber center.When all that energy slams into millimeter-sized targets,it can generate unprecedented temperatures andpressures in the target materials – temperatures of morethan 100 million degrees and pressures more than 100billion times Earth's atmosphere. These conditions aresimilar to those in the stars and the cores of giant planetsor in nuclear weapons; thus one of the NIF & PhotonScience Directorate's missions is to provide a betterunderstanding of the complex physics of nuclear weapons.Researchers can also explore basic science, such as astrophysicalphenomena, materials science and nuclear science.NIF's other major mission is to provide scientistswith the physics understanding necessary to create fusionignition and energy gain for future energy production.Figure 2. Technicians adjust the target positioner inside the NIFTarget Chamber.A Variety of ExperimentsNot all experiments on NIF need to produce fusion ignition.Researchers are planning many other types of experimentsthat will take advantage of NIF's tremendous energyand flexible geometry in non-ignition shots. Non-ignitionexperiments will use a variety of targets to derive abetter understanding of material properties under extremeconditions. These targets can be as simple as flat foils orconsiderably more complex. By varying the shock strengthof the laser pulse, scientists can obtain equation-of-statedata that reveal how different materials perform underextreme conditions for stockpile stewardship and basic science.They also can examine hydrodynamics, which is thebehavior of fluids of unequal density as they mix.NIF experiments also will use some of the beams toilluminate "backlighter" targets to generate an X-rayflash. This allows detailed X-ray photographs, or radiographs,of the interiors of targets as the experimentsprogress. In addition, moving pictures of targets taken atone billion frames a second are possible using sophisticatedcameras mounted on the target chamber. These diagnosticscan freeze the motion of extremely hot, highlydynamic materials to see inside and understand the physicalprocesses taking place As construction of the 48"quads" of four beams each proceeded, many shots werealready being performed using the first quad of beams.Experiments beginning in the winter of 2007-2008 willtake advantage of additional quads as they come online.New Technologies Make NIF PossibleAmplifying NIF's beams to record-shattering energies,keeping the highly energetic beams focused, maintainingcleanliness all along the beam's path, and successfullyoperating this enormously complex facility – all requiredNIF's designers to make major advances in existing lasertechnology as well as to develop entirely new technologies.Innovations in the design, manufacture, and assemblyof NIF's optics were especially critical.The Seven Wonders of NIFWhile construction of the football-stadium-sizedNational Ignition Facility was a marvel of engineering,NIF is also a tour de force of science and technologydevelopment. To put NIF on the path to ignition experimentsin 2010, scientists, engineers and technicians hadto overcome a daunting array of challenges.Working closely with industrial partners, the NIFteam found solutions for NIF's optics in rapid-growthcrystals, continuous-pour glass, optical coatings and newfinishing techniques that can withstand NIF's extremelyhigh energies. The team also worked with companies todevelop pulsed-power electronics, innovative control systemsand advanced manufacturing capabilities. Seventechnological breakthroughs in particular were essentialfor NIF to succeed:1.Faster, Less Expensive Laser Glass ProductionLaser glass is the heartof the NIF laser system;it's the material thatamplifies the laser lightto the very high energiesrequired for experiments.NIF's laserglass is a phosphateglass that contains achemical additive withatoms of neodymium.Figure 3. NIF Glass laser slab inproductionThe NIF laser system uses about 3,070 large plates oflaser glass. Each glass plate is about three feet long andabout half as wide. If stacked end-to-end, the plateswould form a continuous ribbon of glass 1.5 miles long.To produce this glass quickly enough to meet constructionschedules, NIF uses a new production method developedin partnership with two companies – HoyaCorporation, USA and Schott Glass Technologies, Inc. –that continuously melts and pours the glass. See Figure3. Once cooled, the glass is cut into pieces that are polishedto the demanding NIF specifications.6 IEEE LEOS NEWSLETTER June 2008

2. Large Aperture Optical SwitchesA key element of the amplifier sectionof NIF's laser beampath is an opticaldevice called a plasma electrode Pockelscell, or PEPC, that contains a plate ofpotassium dihydrogen phosphate(KDP). See Figure 4. This device, inconcert with a polarizer, acts as a switch– allowing laser beams into the amplifierand then rotating its polarization totrap the laser beams in the amplifiersection. A thin plasma electrode that istransparent to the laser wavelength allows a high electric field tobe placed on the crystal, which causes the polarization to rotate.The trapped laser beams can then increase their energy muchmore efficiently using multiple passes back and forth throughthe energized amplifier glass. After the laser beams make fourpasses through the amplifiers, the optical switch rotates theirpolarization back to its normal configuration, letting themspeed along their path to the target chamber.3. Stable, High-Gain PreamplifiersNIF uses 48 preamplifiermodules, or PAMs,each of which provideslaser energy for fourNIF beams. The PAMreceives a very lowenergy (billionth of ajoule) pulse from themaster oscillator roomand amplifies the pulseby a factor of about amillion, to a millijoule. It then boosts the pulse onceagain to a maximum of about ten joules by passing thebeam four times through a flashlamp-pumped rod amplifier.To perform the range of experiments needed on NIF,the PAMs must perform three kinds of precision spatial,spectral and temporal shaping of the input laser beams.4. Deformable MirrorsThe deformable mirroris an adaptive optic thatuses an array of actuatorsto bend its surface tocompensate for wavefronterrors in the NIFlaser beams. There is onedeformable mirror foreach of NIF's 192beams. Advances inadaptive optics in theFigure 4. PlasmaElectrode Pockels CellFigure 5. 10 Joule PreAmpliermoduleFigure 6. 40 cm x 40 cm full aperturedeformable mirroratomic vapor laser isotope separation (AVLIS) program atLawrence Livermore National Laboratory demonstratedthat a deformable mirror could meet the NIF performancerequirement at a feasible cost. Livermore researchers developeda full-aperture (40-centimeter-square) deformablemirror that was installed on the Beamlet laser in early1997. Prototype mirrors from two vendors were also testedin the late 1990s. The first of NIF's deformable mirrorswere fabricated, assembled and tested at the University ofRochester's Laboratory for Laser Energetics and installedand successfully used on NIF to correct wavefronts for thefirst beams sent to target chamber center.5. Large, Rapid-Growth CrystalsNIF's KDP crystals servetwo functions: frequencyconversion and polarizationrotation. The developmentof the technologyto quickly grow highqualitycrystals shown inthis photo, were a majorundertaking and is perhapsthe most highlypublicized technologicalsuccess of the NIF project. NIF laser beams start out asinfrared light, but the interaction of the beams with thefusion target is much more favorable if the beams are ultraviolet.Passing the laser beams through plates cut from largeKDP crystals converts the frequency of their light to ultravioletbefore they strike the target. The rapid-growth processfor KDP, developed to keep up with NIF's aggressive constructionschedule, is amazingly effective: Crystals that wouldhave taken up to two years to grow by traditional techniquesnow take only two months. In addition, the size of the rapidgrowthcrystals is large enough that more plates can be cutfrom each crystal, so a smaller number of crystals can provideNIF with the same amount of KDP.6. Target FabricationTo meet the needs of NIFexperiments, NIF's millimeter-sizedtargetsmust be designed andfabricated to meet precisespecifications for density,concentricity and surfacesmoothness. When a newFigure 7. NIF rapid-growth KDPcrystalFigure 8. Target Assemblymaterial structure is needed, materials scientists create thenecessary raw materials. Fabrication engineers then determinewhether those materials – some never seen before – canbe machined and assembled. Manufacturing requirements forall NIF targets are extremely rigid. Components must bemachined to within an accuracy of one micrometer, or onemillionthof a meter. In addition, the extreme temperaturesand pressures the targets will encounter during experimentsmake the results highly susceptible to imperfections in fabrication.Thus, the margin of error for target assembly, whichvaries by component, is strict. Throughout the designprocess, engineers inspect the target materials and componentsusing nondestructive characterization methods toensure that target specifications are met and that all componentsare free of defects. Together, this multidisciplinary teamtakes an experimental target from concept to reality.8 IEEE LEOS NEWSLETTER June 2008

7. Integrated Computer Control SystemFulfilling NIF's promiserequires one of the mostsophisticated computercontrol systems in governmentservice or privateindustry. Every NIFexperimental shot requiresFigure 9. NIF Control Roomthe coordination of complex laser equipment. In theprocess, some 60,000 control points for electronic, highvoltage, optical and mechanical devices – such as motorizedmirrors and lenses, energy and power sensors, videocameras, laser amplifiers, pulse power and diagnosticinstruments – must be monitored and controlled. SeeFigure 9. The precise orchestration of these parts byNIF's integrated computer control system will result inthe propagation of 192 separate nanosecond (billionth ofa second)-long bursts of light over a one-kilometer pathlength. The 192 separate beams must have optical pathlengthsequal to within nine millimeters so that thepulses can arrive within 30 picoseconds (trillionths of asecond) of each other at the center of a target chamberten meters in diameter. Then they must strike within 50micrometers of their assigned spot on a target measuringless than one centimeter long – an accuracy comparableto throwing a pitch over the strike zone from 350miles away.Developing the State-of-the-artfor National SecurityThe Photon Science and Applications (PS&A) program pursuesnational security missions by developing state-of-the-art opticsand laser technology. Through research and development,PS&A is developing technologies that advance the frontiers ofdiode-pumped laser technology, high-peak power lasers andscience, generation and application of bright, high-energy photonsand fabrication of precision, meter-scale optics.Tailored-Aperture Ceramic LaserTo improve the run time and beam quality of large, highaverage-power(HAP), diode-pumped solid-sate laser(DPSSL) systems, PS&A has developed a new slab laser technologycalled the tailored-aperture ceramic laser (TACL). Ahigher-performance descendent of the solid-state heat-capacityThe TACLLaser (SSHCL) system shown, TACL in Figure10 uses composite ceramic Nd3+:YAG/Sm3+:YAG slabsthat are edge pumped.By smoothing the high spatial frequency pump-deposition ripplesthat result from face pumping, edge pumping can improve systembeam quality and runtime considerably. New diode-pumparrays are also employed in TACL that use high-performancemicrolens conditioning and microchannel cooling. The microchannelcooling of the arrays allows the diode packages to be run with ahigh duty cycle – even continuously – while generating high averageoptical pump power. As a result, TACL designs can use as litx(−1)Figure 10. Edge pumping is made possible by use of a ceramic Sm3+:YAG frame that is integral to the Nd3+:YAG portion of the slab.Sm3+:YAG absorbs at the laser wavelength, but is transparent at the pump wavelength. The Sm3+:YAG frame suppresses amplified spontaneousemission without affecting passage of the pump light. This recent development proceeds from the concerted work of LLNL and theKonoshima Chemical Company and promises to revolutionize the design of large HAP laser systems by significantly improving beam quality.10 IEEE LEOS NEWSLETTER June 2008

tle as one-tenth of the diode arrays employed by the SSHCL systemwhile achieving the same power output capability.Solid-State Heat-Capacity LaserPS&A is developing a high-average-power (100-kW-class),diode-pumped, solid-state heat-capacity laser (SSHCL) suitable foruse in military weapons. A mobile, compact, and lightweightSSHCL laser system capable of being deployed on a variety of platformsis also under development. Potential military applications ofsuch a system include the targeting and destruction of short-rangerockets, guided missiles, artillery and mortar fire, unmanned aerialvehicles and improvised explosive devices, or IEDs.In 2006, PS&A achieved a major accomplishment when theSSHCL produced 67 kilowatts of power – a 50 percent increase inthe world-record-setting power level achieved the previous year.See Figure 11. This class of power demonstrates that tabletop-sizedsolid-state lasers have come of age and can fulfill the performancerequirements for their use in tactical weapon applications.Additionally, improvements to the SSHCL's laser optics, bothin material selection and geometric architecture, have greatlyenhanced temperature profile uniformity throughout the lasingcavity, yielding a beam quality two times the diffraction limit forfive seconds of run time in PS&A's unstable resonator. See Figure12. Beam quality control is integral to PS&A's laser systemdevelopment, and results like these portend the use of directedenergyweapons on the battlefield where they can effect "speedof-light"engagement in compact, mobile packages.The extremely high fracture toughness of ceramicYAG:Nd3+, along with its facility for use in composinglarger slabs, makes it the ideal material for laser gain media.Output Power (kW)806040200ReducedApertureThreeThree Four FiveNumber of SlabsCalculatedMeasured @10%Duty CycleMeasured @20%Duty CycleFigure 11. The 67 kilowatts of average power was achieved usingfive ceramic neodymium-doped yttrium aluminum garnet(YAG:Nd3+) laser-gain media slabs; model calculations were validatedby experimental results as depicted in the graph.Laser-Material InteractionPS&A's SSHCL test bed has afforded execution of extensivelaser-material interaction experiments using a wide varietyof materials such as steels, aluminums, titaniums andorganic composites. Varying the spot size of the laser beamfrom a few millimeters to a 162-centimeter locus while sustainingan air flow rate of 100 meters per second at thepoint of laser-material interaction loosely simulates a targetflying in the atmosphere. See Figure 13. All tests were conductedat the nominal 25 kilowatts of average laser power.June 2008 IEEE LEOS NEWSLETTER 11

These test results reveal the functioning of several mechanismswhen the laser is used to destroy a target: thermalheating of the high explosive to ignition; increased materialremoval due to combustion of certain materials from the“Times Diffractioon Limited” BeamQuality8765432100Fall 2005Beam QualityFall 2005200 400 600 800 1000Shot Number (200 Shots = 1 Second)BK7 Window, No DiffusersFS Window, with Diffusers, Run #2Fused Silica Window, No Diffusers FS Window, with Diffusers, Run #3FS Window, with Diffusers, Run #1 FS Window, with Diffusers, Run #4Figure 13. SSHCL experimental test setup, using 25-kilowattaveragelaser power, 132-cm laser beam spot size, and 100-meterper-secondair flow.Figure 14. Concept of a 100-kW, movable, fully self-containedTACL/SSHCL ready for live target testing.Spring 2006No Diffusers, Power ~12 kW, with Diffusers, Power ! 10 kW, Unstable ResonatorFigure 12. A beam quality of two times the diffraction limit – for five seconds – wasachieved on the SSHCL testbed.increased oxygen during air flight; and aerodynamic forcesthat literally rip the membrane from the annealed andweakened target, making it aerodynamically unstable.Potential PlatformsThe development of a prototype platformusing SSHCL TACL technologysuitable for testing at a proving groundor test range is the next step in the evolutionof directed-energy weapons. SeeFigure 14.The transition from a laser-technologydemonstration device in the laboratoryto a fully-operational, directedenergyweapon capable of engaging livetargets under battlefield conditions is akey PS&A objective.LLNL's work on the SSHCL heat capacityconcept has set the stage for a new generationof ceramic lasers and high-powerlaser architectures which will be capable ofrunning continuously at high efficiencyand with exceptional beam quality.Diode-Pumped Alkali Laser:A New CombinationSince the advent of lasers more thanfour decades ago, solid-state and gaslasers have followed largely divergentdevelopment paths. Gas lasers are based primarily ondirect electrical discharge for pumping (energizing),while solid-state lasers are pumped by flashlamps andsemiconductor diode laser arrays.The alkali-vapor laser's intrinsically high efficiencyand its compatibility with today's commercially availablediode arrays enable fast-track development paths to tacticalsystems, with mass-to-power ratios that far exceedwhat is possible with today's other laser systems.Building on alkali-vapor laser research done by Z.Konefal, PS&A's Directed Energy Systems andTechnology (DEST) program element recently developeda new class of laser that combines features of both gas andsolid-state lasers, based on diode excitation of atomicalkali vapors. The defining features of the diode-pumpedalkali laser (DPAL) are its ability to be incoherentlypumped and its compatibility with diode arrays havingseveral-nanometer-wide spectral emissions. These characteristicsdistinguish DPALs from previous demonstrationsof alkali-based lasers that used narrow-band, coherentpumping to demonstrate lasing.DEST's extensive laser modeling capability, anchored toexperimental laboratory demonstrations, supports extremepower scaling with good efficiency and beam quality.DEST is the world leader in the development of thisnew class of laser; the first demonstration took place atLawrence Livermore National Laboratory in 2002, seeFigure 15, and it has been followed by many otherdemonstrations and developments.12 IEEE LEOS NEWSLETTER June 2008

Fusion Energy Systems and ScienceFusion is the process by which the sun and stars produceenergy, and it is also responsible for much of the power ofthermonuclear weapons. It also has the potential to be asource of unlimited, environmentally friendly energy forhumankind.The National Ignition Facility (NIF) is designed todemonstrate inertial confinement fusion, ICF, thatinvolves the rapid compression of small fuel capsules, asshown in figure 15, to reach densities and temperaturesgreater than those in the core of the sun; when a sufficientenergy density is reached, the fuel capsule ignites andthen burns while confined by its own inertia. NIF isexpected to demonstrate fusion ignition – the release ofmore energy via fusion burn than the laser energy used toinitiate ignition – early in the next decade, but will onlybe able to do so at a rate of one experiment every fewhours. For inertial fusion energy (IFE) production tobecome practical, targets will have to be ignited at a rateof several shots per second.The mission of the Fusion Energy Systems and Science(FESS) program element of PS&A is to develop the laserdriver technology necessary to make IFE production practical– including the development of low-cost, high-energy,high-efficiency laser drivers and components for repetitionrates of several times a second. Concurrent with thedevelopment of the National Ignition Facility is anotherambitious Livermore laser project named Mercury.The Mercury laser project is an important part of theFESS mission. It is designed to produce 100-joule pulsesat a ten-per-second repetition rate with one kilowatt ofaverage power, using an architecture that could be scaledto IFE-relevant size. Mercury is a single-beam laser system,as shown in figure 16 that has developed capabilitiesthat will build on NIF's accomplishments. As currentlydesigned, NIF's 192 beams can fire simultaneously onlyonce every few hours. After each shot, the thousands ofoptics must be given a chance to cool down to ensure thatthey can operate correctly for the next shot.Mercury has developed a method of continuously coolingthe optics, while at the same time allowing the laserto fire rapidly over extended periods.The current technology propels high-velocity helium gasacross the optics to keep them cool, while laser pulses passthrough the optics at a sustained rate of ten shots a second.Unlike NIF, which uses seven-foot tall flashlamps toenergize the laser amplifiers, Mercury relies on diodelasers – similar to those in commercial read/write CDplayers – which give off one-third as much heat as flashlamps.Mercury's beam is amplified as it passes throughslabs of specially grown ytterbium-strontium flouroapatitecrystals, as opposed to NIF's neodymium-dopedphosphate laser glass. More advanced amplifier media,such as transparent ceramics, are also being developed.As of mid-2008, Mercury has been able to run continuouslyfor several hours (300,000 shots), firing ten timesa second at more than 50 joules per shot, each shot lastingjust 15 nanoseconds (billionths of a second).The project, which beganin 1996 and was initiallyfunded through LLNL'sLaboratory Directed Researchand Development (LDRD)office, has already been awardedthree R&D 100 Awards,most recently for developing aunique frequency conversioncrystal. Earlier awards were forthe original design of Mercury'sdiode array and for its Pockelscell, a unique light-switchingtechnology.The long-term goal is alaser system capable producingof NIF's energy output,with Mercury's ability toFigure 15. NIF beam path intoa Hohlraumrapidly fire shots, and ignite inertial fusion targets forelectrical power generationHigh-Average-Power Laser-Diode ArraysFor 100-kW diode arrays to become a common reality, twoelements of the technology must be realized: high-performance,reliable diode bars, and heat sinks that can sustain superiorthermal management and precision-diode bar mounting.Figure 16. The Mercury laser system, a gas-cooled, diode-pumped, solidstatelaser, operates in a facility about the size of a handball court.June 2008 IEEE LEOS NEWSLETTER 13

Figure 17. The first demonstrationof a resonance-transitionalkali laser using Rb vaporoccurred at LLNL in the winterof 2002.Lawrence Livermore NationalLaboratory (LLNL) has developedjust such a package fordiode bars, using silicon – themainstay of the semiconductorindustry.LLNL uses photolithographyand etching techniquesto produce tens ofthousands of 30-μm-widechannels in silicon substratesthat carry coolingwater. The water flowingthrough these microchannelssignificantly cools the laser-diode bars, which aremounted on the silicon less than 200 μm from the channels.Combining ten diode bars onto a single heat sinkyields a ten-bar package (referred to as a "tile"), whichconstitutes the unit cell from which large, two-dimensionaldiode arrays can be built up through tiling.The tiles used to make up these large arrays shown inFigure 18 are called silicon monolithic microchannels(SiMMs). Considerations that drove the SiMM packagedesign included ease of fabrication and the ability to constructlarge laser-diode arrays with output power capabilitiesof ten to 100 kW. Of paramount importance in thedesign of the SiMM package was incorporation of thesame aggressive heat removal capability that characterizedLLNL's original, rack-and-stack silicon microchannel-cooledpackage. This was accomplished by placing themicrochannels into the silicon directly below the locationof the attached laser-diode bars. Like the rack-and-stacksilicon microchannel cooler, the SiMM design maintains avery tight thermal circuit, with just 177 μm of siliconseparating the heat-generating laser-diode bars and themicrochannel fins that define the cooling channels.The SiMM laser-diode array is a packaging technology forproducing the smallest, most powerful and most inexpensivelaser-diode pumps ever. Each package of ten laser-diode barsintegrates the electrical, optical and hydraulic requirementsnecessary for high-average-power lasers. To date, arrays of up to100 kW have been fabricated at LLNL, and arrays of up to 100kW have been fabricated by SiMMtec, a commercial licensee ofthe technology.With the relatively low cost of silicon, large arrays ofthese precision microchannels can be fabricated inexpensivelyusing standard photolithography and etching techniques.Moreover, with silicon as the base material, individualdiode bars can be precisely soldered to a package;each laser diode bar is connected to an LLNL-patentedmicrolens, giving the SiMM package its unsurpassedoptical brightness.Silens FrameMicro LensLightOutputCuRibbonMetalNotchDiodeMicro-ChannelsWater InletGlassManifoldFigure 18. A 41-kW array made up of 28 SiMM packages. The sketch illustrates a portion of a SiMM laser-diode package.14 IEEE LEOS NEWSLETTER June 2008