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渐露峥嵘的激光防御武器系统

译自:美国信号杂志2008年3月刊
作者:罗伯特.K.阿克曼
编译:全球防务(知远/若水)
当激光技术从实验实走向战场时候,梦想通过激光击毁以超音速飞行的来袭导弹就变得更加接近现实了。今天,多种类型激光技术的研究正取得突破性进展,可以预见,在不久的将来,士兵和平民就可能免受从像迫击炮那样简单到像核装药洲际弹道导弹那样复杂的各种威胁了。

图1:波音公司的机载激光系统采用自适应光学激光技术,以兆瓦级的激光束瞄准弹道导弹目标并予以摧毁。化学激光发生器将能在弹道导弹的助推段将其摧毁。
在激光武器名册里顶级装备当属机载激光系统了。去年夏天,该系统用低能量激光发生器演示了跟踪、瞄准和照射空中目标的强大功能。到2009年秋天,一个完整的激光系统就可以使用兆瓦级的激光发生器随时准备击落弹道导弹了。
“机载激光器是第一种任何人都可以使用的空中定向能激光武器系统。”机载激光武器系统项目主任、美国空军劳伦斯.A.多伯罗特上校说,“这将使我们的作战样式发生革命性改变。在光速的战场上我们还有许多没有遇到过的事情,因而,机载激光器是指明未来前进方向的探路者。”
多伯罗特上校在过去的两年里参与了这个已经取得重大进展的项目。在去年夏天的试验中,该系统使用红外跟踪系统和激光跟踪系统锁定了一个失控的目标---KC-135飞机。该系统的激光信标照射目标并测定在大气中的变形量。这些数据依次不断地被系统主激光发生器接收并修正大气失真,尔后对目标实施打击。主激光发生器采用的是低能量光束,以防止靶机被摧毁。
该系统的主高能光束是由化学激光发生器激发,具有自适应光学系统。当激光信标接收到信号时,系统在激光束发射出去之前通过透镜进行一系列的变形,使之成为具有杀伤力的光束。实际上,该系统可以通过调整透镜来补偿大气引起的变形。经过镜头变焦后,然后发出一道最集中最有效的预调光束来打击目标。
主激光器的能量全部来自于化学反应和涡轮泵,而涡轮泵燃油反应的能量又是来自于过氧化物气体发生器。该系统很少有激光器是由飞机自带的引擎能量来推动的。它不需要额外的能量单元,上校解释说。
在过去的几年里,克服了最难的技术整合问题,多伯罗特上校说。该系统的许多技术直接来自于实验室。一个独立的高能激光发生器由六个能产生兆瓦级能量的激光模块组成。在此之前,还没有一个人能依次激发这六个模块,上校解释说。在加利福尼亚爱德华空军基地的一个机库内,为这种结构而建造的试验台是由一架老式波音747飞机客舱和机身组成。工程师们在经过早期调试后可以持续激发激光器近七十次。
自适应光学系统由地基天文学家进行测试,他们能消除星光失真—就是肉眼所见的星光闪烁—产生高强度光点来对抗那些哈勃太空望远镜。工程师们努力消除大气光束的失真。
信息处理技术实际上已经超出了预期,上校说,在这项漫长的项目中,工程师们能处理各种各样的过渡信号。
当前,高能激光发生器正在往飞行器上安装。地面测试将在今年夏天进行。其它空中试验用来评估打击比亚音速飞机更快的目标。多伯罗特上校解释说,已经进行了一次对在急速爬升过程中超音速飞行的F-16战机的跟踪和瞄准试验。当激光瞄准塔回转并保持在目标范围内时,低速目标和超音速目标在速度上的不同并不能影响激光系统跟踪目标的能力,上校提示道。

图2:诺期罗普格鲁曼公司空间技术部门的一名技术人员正在检测通常用于监控红外激光束激发的诊断仪器,该公司实验室正在为美国军用联合高能固态激光发生器(JHPSSL)项目做演示。实验的成功为固态激光发生器能量从27千瓦到最终目标100千瓦铺平了道路。
如果地面测试成功的话,通过扩大系统包络等措施,2009年8月的测试将随之进行。工程师们将试图找出飞机携带一个兆瓦级激光发生器的极限能力。这方面的努力包括测试不同类型目标,空中飞行器和导弹。第二个装备激光发生器的飞机也将建成并投入使用。
最大的潜在陷阱可能不是技术问题,而是预算。明年将迎来一个新的政府和国会,没有人能预测他们的优先预算将是什么。
一些地基定向能系统会得到财政支持。其中主要的是通过在火箭弹头聚集热能使之爆炸从而摧毁火箭的红外激光系统,而不是建造一个采用突发的能量来摧毁弹丸的系统。他们中大都采用固态技术,这使得产生激光能量不需要像化学激光器那样大量的后勤支援。
    由雷声公司研发的激光区域防御系统或LADS,是建立在用于保护美国海军舰艇的密集阵近距防空系统之上的。
亚利桑那州图森市雷声公司负责先进导弹防御系统和定向能武器系统的副总裁迈克尔.布恩解释说,激光区域防御系统的研制工作大约开始于两年前。最初的目标是保护在巴格达、绿区的美国军队免遭迫击炮袭击。为了很快建造一个激光防御系统,公司寻求采用场外现成的技术在六个月内生产出一台原型机。
工程师们选择了一个由空军研究实验室研制的20千瓦固态光纤激光发生器,尔后再由公司激光区域防御系统研发部进行重新设计改装。这个激光器能发射一条1微米的红外光束来加热来袭弹头,直至弹头爆炸。在最后的六个月期限内,工程师们在正式的试验中在500米距离上打下了两枚迫击炮弹。
    用密集阵武器系统所进行的试验显示出激光系统可以摧毁来袭迫击炮弹或火箭,因此一些陆基密集阵系统被部署到伊拉克和阿富汗。为部署激光区域防御系统,公司计划用装备20千瓦激光发生器的20毫米密集阵武器系统替换传统的20毫米密集阵武器系统。激光系统将使用原有的密集阵系统的雷达和指挥与控制系统。今年晚些时候的试验安排将全面评估激光区域防御系统抗击来袭迫击炮弹的能力。如果这些试验成功了,该系统将随时进行部署,布恩说。
    布恩指出,20千瓦激光发生器同传统密集阵武器系统一样有一个射程的问题。但是,先进的固态激光发生器功能正在逐步增加,这将极大地延伸激光防御系统的射程。这将使激光系统比密集阵系统更加有效。而且,激光系统将是“弹药无限”。布恩说,因为它只需要源源不断的电力来将光子送到来袭目标上。
     “我们正进入一个真正拥有可部署并能为今天而战斗的激光系统时代。”布恩强调说。
当强大的激光系统因其可用而逐步被接纳时,现有的20千瓦单元就有能力击落喀秋莎火箭弹,也包括迫击炮弹。即使是无人飞行器,特别是其影像传感器将成为可用的目标。
这些强大的固态激光系统可能出现在两个由陆军资助的独立研究团队中。它是联合高能固态激光系统(JHPSSL)项目的一部分,两个研究的目标都是发展100千瓦固态激光系统,这种系统可用于击落来袭炮弹和小火箭。
其中一项研究团队,在加利福利亚州诺格公司领导下,在去年早些时候已经完成它的第一个里程碑。建设一个4千瓦的增益操作模块,并将这些模块整合进激光链,然后依次整合其余部分从而制造出激光系统。诺格公司联合高能固态激光系统工程第三期项目负责人杰伊.马默解释说。
这个系统有效地将几个独立的激光光束连贯地整合成一个单一激光光束。这条光束是一条1.06毫米的红外光束。
激光链通常为15千瓦,马默继续解释说。组合这些链将使设计师得到自己想要的任何光束能量。在二期工程,双链技术产生了能量为27千瓦的光束。这显示了多链模块激发激光的能力和为获得足够质量的光束而进行波前校正的必要性。
随着激光链试验的成功,公司的工程师们也自信地认为他们能增强系统的能量。工程师正在努力使这一技术更加紧凑和更好地演示,马默说。实现这些目标将可以从双链系统中产生出质量更高的30千瓦光束。这也就是说加入更多的激光链就可以达到产生出100千瓦光束的目标。年内就可能达到,他认为。
马默认为大部分的技术障碍已经被克服。剩下的主要是大量的工程问题。当这些问题都被克服时,它有可能扩大规模,制造出能量超过100千瓦光束的目标。最后,工程师们将达到电力极限,马默承认,但是这一限制尚未确定。
第一次使用的将可能是移动式地基激光系统,他认为。舰艇也可能为近程防御系统安装这种类型的激光系统,最后,这个系统可能被安装在飞行器上。
联合高能固态激光系统工程的另一个研究团队是由马萨诸塞州威尔明顿的特克斯通防御系统公司组成。这项工作围绕名为ThinZag的公司专利技术进行的,这项专利技术由特克斯通防御系统公司激光系统主任丹.泰勒博士研发。
在此技术支持下,厚板被制造成设计薄板,以便热量能轻易提取。它的Z字型格局有助于均衡不一致的格式。这个几何形状允许在一个小型单一模块中使用多个大型板块。单一模块是基础,然后多个模块进行排列作为功率振荡器。
板块也是用陶瓷钕、钇铝石榴石(YAG)制成。这便能造出比用水晶材料制造大得多的板块。这些板块结构可达到几个厘米高,几十个厘米长。
特克斯通防御系统公司技术副总裁约翰.珀利斯声称,这种理念允许只用一个简易的配置就可建造大型激光装置。“它相当简单,而且,基于这种基本原理,不用花费上千个组成部分,你就能建造很大的激光发生器。”他强调说,它也只能产生一束光,因为这个系统没有打算处理多光束融合。
只用一个模块,这个设计就已经产生出20千瓦激光束。泰勒解释说,公司已经配置了两个单元来发电,并且下一步将增加第三个单元。为了达到100千瓦目标,激光发生器将要求有六个模块。“以前我们用一个模块,而其它的都还未用。”他说,“如果你将两个模块放在一起并当作功率振荡器来运行他们,那么将六个连在一起就没有什么新的工程或物理学问题了。”
珀利斯警告说,当它扩展到六个模块时,该公司不能指望这条道路上“毫无新意”。然而,该公司预计,它的进展是不断的重复。泰勒认为,这儿对模块数量有一个限制问题,特别是工程师积累了比他们能掌控的更多的增益时候。那就有可能克服建造大型和更强大模块来产生高能光束,尽管数量很少。物理和制造技术都是潜在的限制。
制造技术作为支撑帐篷长柱子中的一根,珀利斯强调。激光特有的精密光学元件和资源受限的国外材料。这些任务长期地左右和影响着进程。因为设计的改变可能是一个主要的问题。另一个方面是异国材料的供应问题,特别是那些来自海外的资源。
控制光束是研发中最具挑战性的任务了,泰勒承认。固态激光发生器的失效和必需的热量管理产生需要控制的相位误差。他声称,公司在这个领域已经取得成功。
国家试验室继续进行固态激光发生器的研究,固态激光发生器已经更新了好几代。这种激光发生器采用五个陶瓷YAG激光二极管就能产生出67千瓦的激光光束。这标志着能量和材料都在发生着变化。实验室正在采用四个由陶瓷钕、钇铝石榴石(YAG)制成的二极管板块,但是它在两年前转用采用陶瓷YAG了。
固态热能激光项目主管/首席工程师罗伯特.扬曼莫托解释说,实验室改变材料有几个原因。YAG激光块容易制造而且大小适中,这也能转化为更大的激光功率。钕GGG只能被制成六英寸大小。而且,水晶材料比水晶钕材料更抗压。
    透明陶瓷板块也允许有更大的灵活性。实验室里板块掺杂YAG激光,而这提供了更好的均匀温度,改善了激光光束质量。该实验室目前购买YAG激光板块,该板块由日本化学公司生产。但在透明陶瓷的激光增益介质研究方面,它也有其自身的内部研究和开发力度。
扬曼莫托表示,该实验室还没有得到进行最后攻坚所需的资金。它将继续努力以现有的设备来改善光束质量。实验室希望进一步验证其边泵结构作为关键,来显著改善光束质量,这将又可能导致需要更多的资金。
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原文

Laser Defense Outlook Brightens

美国信号杂志3月期
By Robert K. Ackerman
  

Boeing’s  Airborne Laser system uses adaptive optics to focus a megawatt-class beam on a ballistic missile target, destroying it. The chemical-pumped laser soon will be able to destroy a ballistic missile in its boost phase.
Longtime research, new technologies bring reality closer.
The dream of zapping incoming missiles traveling at supersonic speeds into nonexistence is becoming closer to reality as laser science transitions from the laboratory to the field. Research into several different laser technologies is bearing fruit, and soon warfighters and civilians may be protected from threats as simple as mortar rounds or as complex as nuclear-armed intercontinental ballistic missiles.
At the top of the laser roster is the Airborne Laser system. Last summer, that system demonstrated the ability to track, target and illuminate an airborne target with a low-power laser. By August 2009, the complete system should be ready to shoot down a ballistic missile using a megawatt-class laser.
“The airborne laser is the first airborne directed-energy laser weapon produced by anyone,” declares Col. Laurence A. Dobrot, USAF, acting program director for the Airborne Laser program. “That’s going to revolutionize the way we conduct those types of operations. Battle at the speed of light is something that we have not dealt with before, and the airborne laser is a pathfinder to that future.”
Col. Dobrot shares that the program has made significant progress in the past two years. In last summer’s test, the system locked onto an uncooperative target vehicle—a KC-135 aircraft with a missile painted on its side—using its infrared tracking system and its tracking laser. The system’s beacon laser then illuminated the target to determine the degree of atmospheric distortion. That data in turn allowed the system’s main laser to compensate for the distortion and to strike the target. The main laser was represented by a low-power beam to prevent destruction of the target aircraft.
The system’s main high-energy beam is a chemical-pumped laser that features adaptive optics. When cued by the beacon laser, the system deforms a series of mirrors to pre-distort the killing beam before it leaves the aircraft. In effect, the system compensates for atmospheric distortion by treating the atmosphere as an optical lens. It then emits a pre-adjusted beam that will strike its target with maximum efficiency and concentration after undergoing that lens effect.
The main laser is powered entirely by its chemical reaction, and the turbopumps that fuel that reaction are powered by a peroxide-powered gas generator. The system’s lesser lasers draw power from the aircraft’s own engine generation. No extra power units are needed, the colonel explains.
The toughest hurdles that had to be overcome over the past few years involved technology integration, Col. Dobrot offers. Many of the system’s technologies came directly out of the laboratory. The high-energy laser alone comprises six laser modules generating megawatt-class laser power. No one ever had fired those six modules in sequence before this program, the colonel notes. The testbed for that construct was an old Boeing 747 aircraft cabin and fuselage in a hangar at Edwards Air Force Base, California. Engineers were able to fire the laser almost 70 times with consistent performance after early tuning, he recounts.
The adaptive optics were tested by ground-based astronomers who were able to remove starlight distortion—twinkling, to the naked eye—to produce sharp images that rivaled those of the Hubble Space Telescope. Engineers applied that work to eliminate atmospheric beam distortion.
Information processing technology actually exceeded expectations, the colonel says. Over the length of the program, engineers were able to transition signal processing from multiple boxes to single cards.
The high-energy laser currently is being installed aboard its aircraft. Ground testing should begin this summer with activation tests. Other airborne tests will evaluate performance against faster targets than a piloted subsonic aircraft. Col. Dobrot notes that one test already evaluated tracking and targeting capabilities against a supersonic F-16 in a zoom climb. The difference in speed between a slow-moving object and a supersonic one does not affect the system’s ability to track a target, as the laser’s aiming turret slews to keep the target in frame, the colonel offers.
  

A technician at Northrop Grumman’s Space Technology sector checks diagnostic instruments used to monitor infrared laser beams fired by the company’s laboratory demonstrator for the U.S. military’s Joint High Power Solid-State Laser (JHPSSL) program. Successful tests have paved the way for scaling up the solid-state laser from 27 kilowatts to an eventual goal of 100 kilowatts.
If successful, the August 2009 test would be followed by efforts to expand the system’s envelope. Engineers would try to find limits to the capability of an aircraft carrying a megawatt-class laser. This effort will include tests against different types of targets, both airborne and ballistic. A second laser-equipped aircraft also would be built with enough design improvements to make it operational.
The biggest potential pitfall may not be technical but budgetary. Next year will bring a new administration and Congress, and no one can predict what their budget priorities will be.
Some land-based directed energy systems show promise. These largely are infrared lasers that destroy incoming rockets by focusing heat on their warheads causing them to explode, instead of destroying projectiles with a burst of energy. Many of these exploit solid-state technology, which generates laser energy without the large logistics footprint of chemical lasers.
One system is the Laser Area Defense System, or LADS. Under development by Raytheon Company, this laser builds on the Phalanx close-in air defense system that protects U.S. Navy ships.
Michael Booen, vice president for advanced missile defense and directed energy weapons at Raytheon in Tucson, Arizona, explains that work on LADS began about two years ago. The original aim was to protect U.S. forces in the Baghdad, green zone from mortar attacks. To build a laser defense system quickly, the company sought off-the-shelf technologies that could produce a prototype within six months.
Engineers selected a 20-kilowatt solid-state fiber laser from the Air Force Research Laboratory, which then partnered with the company on LADS’ development. The laser emits a 1-micron infrared beam that heats incoming warheads until they detonate. Within their six-month deadline, engineers were able to blow up two mortar shells in static tests at a range of 500 meters (1,650 feet).
Tests with the Phalanx gun system showed that it could destroy incoming mortar shells and rockets, so some of these land-based Phalanx systems were deployed to Iraq and Afghanistan. To deploy LADS, the company plans to swap out the conventional 20-millimeter Phalanx gun with the 20-kilowatt laser. The laser would operate using the existing Phalanx radar and command and control system. Tests scheduled for later this year will evaluate the full LADS system against incoming mortar rounds. If these tests are successful, the system will be ready for deployment, Booen says.
Booen offers that the 20-kilowatt laser has a range similar to that of the kinetic-kill Phalanx system. But, advances in solid-state lasers are increasing beam power, which will extend the range of the laser defense system. This will make it more effective than the Phalanx gun. And, the laser will have an “unlimited magazine,” Booen says, as it will need only a steady flow of electricity to keep pumping photons onto incoming targets.
“We’re entering an era where we actually have systems that can be deployed and that are ready today,” Booen emphasizes.
While stronger lasers can be incorporated incrementally as they become available, the existing 20-kilowatt unit is capable of shooting down Katyusha rockets in addition to mortar rounds. Even unmanned aerial vehicles, particularly their imaging sensors, would be viable targets.
Those stronger solid-state lasers may emerge from two separate efforts funded by the U.S. Army. Part of the Joint High Power Solid-State Laser (JHPSSL) program, both aim to develop a 100-kilowatt solid-state laser that can be used to shoot down incoming artillery and small rockets.
One effort, led by Northrop Grumman Corporation, Redondo Beach, California, completed its first milestone early last year. This entailed developing a gain module operating at 4 kilowatts. These gain modules will be combined into a laser chain that in turn will be combined with others to produce the laser, explains Jay Marmo, JHPSSL Phase III program manager at Northrop Grumman.
This system effectively combines several individual laser beams coherently into a single beam in phase. The beam is an infrared laser operating at 1.06 microns.
The laser chains are nominally 15 kilowatts, Marmo continues. Combining these chains will lead to whatever power designers want to attain. In Phase II, two-chain technology generated about 27 kilowatts. This demonstrated both the ability of multiple chain modules to generate a laser and the necessary wavefront correction for adequate beam quality, he says.
With the laser chain phasing being demonstrated successfully, company engineers feel confident that they can increase the power of the system. The engineers are striving to make this technology more compact and better performing, Marmo says. Achieving these goals would generate 30 kilowatts of laser output from a two-chain system with better beam quality. This also will allow the addition of more laser chains to reach the 100-kilowatt goal. That should be attained this year, he notes.
Marmo allows that most of the technological hurdles have been overcome. What remains are largely engineering issues. As these are overcome, it may be possible to scale up the laser beyond the 100-kilowatt goal. Ultimately, engineers will reach a ceiling in scalable power, Marmo admits, but that limit has not been determined yet.
First uses likely would be in mobile ground-based laser systems, he allows. Ships also could install this type of laser for close-in defense, and ultimately the system might be placed on aircraft.
Another effort in the JHPSSL program is run by Textron Defense Systems, Wilmington, Massachusetts. This work is built around a proprietary company technology it calls ThinZag, reports Dr. Dan Trainor, director for laser systems at Textron Defense Systems Corporation.
In this technology, the slab is fabricated in a thin design so that heat can be extracted easily. Its zigzag pattern helps average out non-uniformities. This geometry allows the use of multiple large slabs in a single module. A single module acts as a base, and then multiple modules are arrayed in series to act as a power oscillator.
The slabs also are made of ceramic neodymium:yttrium-aluminum-garnet (YAG), which enables the fabrication of larger slabs than those made with crystalline material. These slab constructs can be multiple centimeters high and tens of centimeters long.
John Boness, vice president for applied technology at Textron Systems, states that this concept allows the construction of large laser devices with a simple configuration having fewer parts. “It’s simpler, and you can build very large lasers based on this fundamental concept without getting into thousands of parts,” he emphasizes. It also generates only one beam, so the system does not have to deal with the challenge of combining multiple beams.
The design already has generated a 20-kilowatt laser beam using one module. Trainor explains that the company has configured two modules to generate power, and the next step will be to add a third module in late spring. To reach the 100-kilowatt target, the laser will require six modules. “Once we’ve done one [module], all the rest are exactly the same,” he says. “If you put two together and operate them as a power oscillator successfully, it follows that putting six together entails no new engineering or physics.”
Boness warns that the company cannot expect “no surprises” along the way as it scales up to six modules. Yet, the firm does expect its progress to be fairly repetitious. Trainor notes that there is a limit to the number of modules that can be configured, particularly as engineers accumulate more gain than they can handle. That might be overcome by building larger and more powerful modules that can generate a stronger beam despite being fewer in number. Both physics and manufacturability pose potential limitations.
One of the long poles in the tent involves manufacturability, Boness notes. The laser features precise optical components and exotic materials with limited sources. Those mandate long lead and fabricability schedules, so design changes can be a major problem down the line. Other constraints may involve the availability of these exotic materials, particularly those that must come from sources overseas.
Controlling the beam phase has been the most significant challenge faced in development, Trainor allows. The inefficiency of solid-state lasers and necessary thermal management generates phase errors that required control. He states that the company has been successful in that realm.
Research continues at Lawrence Livermore National Laboratories in a solid-state laser that has been in development for some time (SIGNAL Magazine, April 2005). This laser has achieved an output of 67 kilowatts by using five ceramic YAG diode blocks. This represents a change both in power—up from 30 kilowatts—and in material. The laboratory had been using four diode slabs made of neodymium:gatalinium-gallium-garnet (GGG) crystal, but it switched to ceramic YAG a couple of years ago.
Robert Yamamoto, program manager/chief engineer for the Solid-State Heat-Capacity Laser program, explains that the laboratory switched materials for several reasons. YAG blocks are easier to fabricate and scale up in size, which also translates to greater laser power. Neodymium:GGG can be made only in sizes up to six inches. And, the ceramic material resists cracking better than the crystalline neodymium:GGG.
The transparent ceramic slabs also allow greater versatility in the pumping architecture. Instead of pumping off of the face of the neodymium:GGG slab, YAG slabs enables pumping off of the four edges of the slab. The laboratory’s slabs are framed with samarium-doped YAG, and this provides better uniformity of temperature, which improves laser beam quality. The laboratory currently is buying its YAG blocks from Konoshima Chemical Company in Japan, but it also has its own internal research and development effort in transparent ceramics for laser-gain media, Yamamoto notes.
Yamamoto shares that the laboratory has not received funding for the final push to achieve a 100-kilowatt beam with a solid-state laser. It continues to strive to improve beam quality with its existing construct. This will increase the distance a beam can travel through the atmosphere. The laboratory hopes to further validate its edge-pumping architecture as a key to significant beam quality improvement, which in turn may lead to renewed funding.
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