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Gen 3 High Energy Laser Completes Beam Quality Evaluation HEL System Designed for Land, Sea, and Airborne Platforms SAN DIEGO – 8 April 2015 – General Atomics Aeronautical Systems, Inc. (GA-ASI), a leading manufacturer of Remotely Piloted Aircraft (RPA) systems, radars, and electro-optic and related mission systems solutions, today announced that an independent measurement team contracted by the U.S. Government has completed beam quality and power measurements of GA-ASI’s Gen 3 High Energy Laser System (HEL) using the Joint Technology Office (JTO) Government Diagnostic System (GDS). “These measurements confirm the exceptional beam quality of the Gen 3 HEL, the next-generation leader in electrically-pumped lasers,” said Claudio Pereida, executive vice president, Mission Systems, GA-ASI. The new laser represents the third generation of technology originally developed under the High Energy Liquid Laser Area Defense System (HELLADS, Gen 1) program. The Gen 3 Laser employs a number of upgrades resulting in improved beam quality, increased electrical to optical efficiency, and reduced size and weight. The recently certified Gen 3 laser assembly is very compact at only 1.3 x 0.4 x 0.5 meters. The system is powered by a compact Lithium-ion battery supply designed to demonstrate a deployable architecture for tactical platforms. The Gen 3 HEL tested is a unit cell for the Tactical Laser Weapon Module (TLWM) currently under development. Featuring a flexible, deployable architecture, the TLWM is designed for use on land, sea, and airborne platforms and will be available in four versions at the 50, 75, 150, and 300 kilowatt laser output levels. The GDS was employed by an independent measurement team to evaluate the beam quality of the Gen 3 system over a range of operating power and run time. According to JTO’s Jack Slater, “The system produced the best beam quality from a high energy laser that we have yet measured with the GDS. We were impressed to see that the beam quality remained constant with increasing output power and run-time.” With run time limited only by the magazine depth of the battery system, beam quality was constant throughout the entire run at greater than 30 seconds. These measurements confirm that the exceptional beam quality of this new generation of electrically-pumped lasers is maintained above the 50 kilowatt level. Following this evaluation, the independent team will use the GDS again to conduct beam quality measurements of the GA-ASI HELLADS Demonstrator Laser Weapon System (DLWS). The HELLADS DLWS includes a 150 kilowatt class laser with integrated power and thermal management. The goal of the High Energy Liquid Laser Area Defense System (HELLADS) program is to develop a high-energy laser weapon system (150 kW) with an order of magnitude reduction in weight compared to existing laser systems. With a weight goal of <5 kg/kW, HELLADS will enable High-Energy Lasers (HELs) to be integrated onto tactical aircraft and will significantly increase engagement ranges compared to ground-based systems. The HELLADS program has completed the design and demonstration of a revolutionary subscale high-energy laser that supports the goal of a lightweight and compact high-energy laser weapon system. An objective unit cell laser module with integrated power and thermal management is being designed and fabricated and will demonstrate an output power of >34 kW. A test cell that represents one-half of the unit cell laser has been fabricated and used to characterize system losses and diode performance and reliability. The test cell is being expanded to a unit cell. Based on the results of the unit cell demonstration, additional laser modules will be fabricated to produce a 150 kW laser that will be demonstrated in a laboratory environment. The 150 kW laser then will be integrated with an existing beam control capability to produce a laser weapon system demonstrator. The capability to shoot down tactical targets such as surface-to-air missiles and rockets will be demonstrated. Features/Benefits:
 General Atomics: Third-Gen Electric Laser Weapon Now Ready While fashions in high-energy lasers have changed as technology progresses, from gas to diode and now fiber, General Atomics Aeronautical Systems (GA-ASI) has stayed its course over more than a decade and believes its third generation of electric laser weapon is ready for prime time. The company has responded to an Office of Naval Research (ONR) solicitation for a 150-kw laser weapon suitable for installation on DDG-51-class destroyers to counter unmanned aircraft and small boats using only ship power and cooling. Under ONR’s Solid-State Laser Technology Maturation program, the weapon is to be demonstrated in 2018 on the USS Paul Foster, a decommissioned Spruance-class destroyer that now serves as the U.S. Navy’s ship-defense test vessel at Port Hueneme in California. GA-ASI has proposed its Gen 3 High-Energy Laser (HEL) system, which recently completed independent beam-quality and power testing for the U.S. government. The Gen 3 system is the third generation of electrically pumped laser using the architecture developed for Darpa’s Hellads program.  Under development since 2003, the 150-kw Hellads will be tested this summer at White Sands Missile Range in New Mexico. A smaller, lighter and more efficient Gen 2 system was built and tested in 2010-12 for the Pentagon’s HEL Joint Technology Office (JTO), says Jim Davis, director of laser weapons. Gen 3 has increased electrical-to-optical efficiency, improved beam quality and further reduced size and weight, says GA-ASI. A mockup of the Tactical Laser Weapon Module was displayed for the first time at the Sea-Air-Space show on April 13-15 in Washington. The module includes high-power-density lithium-ion batteries, liquid cooling for the laser and batteries, one or more laser unit cells and optics to clean up and stabilize the beam before it enters the platform-specific beam-director telescope, says Davis. The unit cell is a laser oscillator that produces a single 75-kw beam. Modules can be ganged together to produce a 150- or 300-kw beam. There is no beam-combining, Davis says, as there is in systems that use multiple lower-power fiber lasers. The Pentagon and several other manufacturers have shifted focus to fiber lasers because they are a commercial technology and have higher electrical-to-optical “wallplug” efficiency than diode lasers previously demonstrated at power levels exceeding 100 kw.  But the Gen 3’s efficiency is at the level of fiber lasers, Davis says, adding that the company has worked for several years to improve beam quality and achieved “excellent quality” in the latest tests. Adaptive optics adjust the beam to compensate for atmospheric distortion. In the independent unit-cell tests, beam quality was measured over a range of operating power and run time, which is limited only by the “magazine depth” of the battery system. “Beam quality was constant throughout the entire run of greater than 30 sec.,” says GA-ASI. “Fiber lasers are interesting, but it is a matter of maturity,” says Davis. “We are where fiber may be in five years. We have built several versions of this laser over the last 10 years, and we believe [the Gen 3 system] is affordable as is.” In addition to the ONR program, GA-ASI is eyeing the U.S. Army’s Boeing High Energy Laser Mobile Demonstrator (HEL MD). Live-fire tests of the HEL MD used a 10-kw industrial fiber laser and the Army intends to upgrade the system to a 60-kw Lockheed Martin fiber laser. The next step is a 120-kw laser, planned for testing in the early 2020s, and for which GA-ASI plans to propose the Gen 3 system. The Air Force Research Laboratory, meanwhile, is interested in a podded laser weapon, although there is no formal program yet. Davis
says the Gen 3’s size enables an airborne laser module in the 150-kw
range to be carried by GA-ASI’s Avenger unmanned aircraft. The UAV has
sufficient onboard power to recharge the module’s batteries in flight.
“That’s the utility; you don’t need to go back to reload,” Davis says. ========== HELLADS (High Energy Liquid Laser Area Defense System) 是在DARPA支持下在研的最为保密的一项高能激光计划，其目标是研制出150 kW的高能激光系统，并对激光器的重量和体积有非常严格的要求。 从2002至2007年，通用原子公司是HELLADS计划唯一的合同承担者，他们采用的技术方案从未公开报道过，甚至没有谈及是何种介质类型的激光器，只是宣称该技术方案结合了固体激光器的高储能密度和液体激光器的流动热管理技术。经调研和分析，认为通用原子公司很可能采用了分布式增益的薄片YLF激光器专利技术，其单元模块的结构如图4所示，一连串厚度为毫米量级(甚至有可能是亚毫米) 的Nd:YLF薄片(图中标号14)固定在激光腔中(标号30和32为一对腔镜)，冷却剂(标号34)沿Z方向流过增益介质薄片之间的区域并带走热量，抽运源(标号18)沿Y方向入射到增益介质内部。这种技术方案的要点是，在增益介质很薄的情况下，介质内部的温度场比较均匀，介质不会因为热应力而损伤，且由于温度场的梯度主要在X方向和激光传输方向一致，造成的波前畸变也较小，保证了激光束的光束质量，这与薄片激光器类似。若要实现更高功率的输出，可将几个模块串联使用。  通用原子公司2004年验证了千瓦级输出，2007年将功率水平定标放大至15 kW。2010年，该公司接受新的合同，转入最后一个研究阶段，按照合同要求应在2012年年底实现150 kW的地面演示验证激光器系统。 2007年9月，达信公司也进入HELLADS计划，在DARPA支持下利用“薄Z字形”陶瓷板条的专利技术实现高能固体激光输出。达信公司的液体冷却激光器从本质上讲也属于板条激光器构型，但是与诺格板条方案不同之处在于激光板条浸泡在冷却液之中，激光沿“Z”字形光路通过板条和冷却液。如图5所示，板条介质固定于两石英窗口之间，冷却液在窗口内流动，抽运光垂直入射板条大表面，信号光则通过特殊光楔导入，在石英窗体之间沿“Z”字形光路前进。达信公司先后开展了基于ThinZag技术的板条激光器定标试验，完成了板条数量?抽运面积和抽运功率的定标，先后实现了1.2、5.6、15 kW的单谐振腔输出，但是15 kW板条固体激光器的光束质量一直未公开报道。他们采用腔内自适应光学?腔外调节补偿和自适应补偿技术改善光束质量，但光束质量还是随输出功率增加明显下降。尽管如此，达信公司还是采用6个17 kW的模块串联单谐振腔方式实现了100 kW激光输出，但未能实现第三个里程节点的光束质量指标。值得特别指出的是达信方案中使用了新型的陶瓷板条取代传统的单晶板条，陶瓷板条在大尺寸?高掺杂方面具有独特的优势。2010年之后，未见到达信公司进入HELLADS下一阶段研制任务的报道。  液冷激光器的缺点主要在于冷却液包含在激光光路之中，这带来一系列技术难题和工程化问题。无论是通用原子公司的技术方案还是达信公司的方案，强激光都会经过流动的冷却液，冷却液的存在对激光性能产生了严重的影响：1) 冷却液的折射率和固体增益介质的折射率须严格匹配，由于固液界面很多，很小的折射率失配就会带来很大的激光损耗；2) 冷却液对抽运光和激光要足够透明，而现有的折射率匹配冷却液体(如CS2和CCL4的混合液)对1μm波段的激光辐射具有一定的吸收(0.1/m的量级)，在光路较长时会引起较大的吸收损耗；3) 即使同时满足了折射率匹配条件和光学透明条件，由于液体的热光系数通常比固体YAG材料大两个数量级，很小的温度变化将引入较大的激光波前畸变，再加上流体的涡流状态也会影响到激光的传输特性。因此，这种激光器的光束质量不容乐观，很难在100 kW量级达到两倍衍射极限的要求。 摘自《高能固态激光器技术路线分析》，《中国激光》，2013年第6期 |
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