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平面平行波导激光综合了光纤激光和块状固体激光的优点,同时避免两者的缺点。光纤激光的优点在于结构紧凑、效率高、废热导致的光束质量退化较小,缺点在于由于激光亮度极高,非线性效应直接限制最高输出功率;块状固体激光定标放大时则基本不存在非线性效应问题,缺点在于废热导致的光束质量退化严重,同时结构难以实现小型紧凑化。典型的平面波导激光结构如图1所示。其芯层为Nd:YAG或Yb:YAG;内包层为无掺杂的YAG,用以耦合泵浦光;外包层为蓝宝石,利用它的高导热性能以及极高的热断裂极限(3倍于YAG)来移除废热。激光在平面波导芯层传播时,在平面波导宽度方向以平行光方式传播,在平面波导厚度方向的光场分布则由导波效应主导,因此可以在很大程度上降低冷却带来的温度梯度造成的严重光束质量退化。
尽管平面波导激光具有很多优点,雷神公司宣称将其定标到100 kW、甚至MW级也不存在技术障碍。但平面波导激光的技术难点同样突出:(1)是材料的生长、加工、键合等工艺存在较大的技术挑战,特别是芯层和包层之间大尺寸表面的光学级别键合难度极大;(2)是非导波方向(宽度方向)的泵浦不均匀性极易产生严重的热致相差,导致光束质量严重退化[6]。此外,由于波导芯层很薄(μm量级),将前级放大器输出的高功率激光通过空间光路高效率地耦合进入芯层的难度很大,多块平面波导实现多级放大则难度更大(基于多级放大的光纤激光器快速走向工业化很大程度上依赖于光纤激光熔接技术问题的解决)。
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雷神公司是高能平面波导激光技术的主要推动者。2006年,该公司利用一块平面波导放大器实现了16 kW输出,但光束质量较差[7]。2011年,雷神公司获得RELI计划支持,开始大力解决复杂结构的平面波导材料制备技术。2013年,雷神公司在5 kW水平验证了AO技术对该相差的校正能力(图2),校正后光束质量达到近衍射极限水平[8]。但更高功率水平的AO校正尚未见报道。同时,雷神公司宣称已经完成了可满足RELI计划第一阶段目标——输出功率大于25 kW,电光效率大于30%——的平面波导激光设计。此后该技术转由美海军陆战队支持,为其研制30 kW级轻小型低空防御激光武器,并已安装在越野吉普上进行了外场测试(图3)[9]。
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相对于其他类型激光器,平面波导激光增益模块可以很高。以Yb:YAG晶体(芯层)、无掺杂YAG晶体(内包层)和蓝宝石晶体(外包层)键合组成的5层结构平面波导增益模为例,利用无穷大平板的弹性力学方程,其最大应力为:
$$ \begin{split} \sigma _{\max }^{{\text{(YAG)}}} =& \frac{{{\alpha _{{\text{YAG}}}}{D^2}}}{{8{k_{{\text{YAG}}}}l}} - \frac{{{E_{{\text{YAG}}}}{{Q}}d}}{{1 - {\nu _{{\text{YAG}}}}}}\left( {1 - \frac{D}{l}} \right) \times \\ & \begin{array}{*{20}{c}}{ \left[ {({\alpha _{{\text{YAG}}}} - {\alpha _{{\text{Sph}}}})\dfrac{1}{{2h}} + (2{\alpha _{{\text{YAG}}}} - {\alpha _{{\text{Sph}}}})\dfrac{{l - D}}{{8{k_{{\text{Sph}}}}}}} \right]} \end{array} \\ \end{split} $$ (1) 式中:l、D和d分别为平面波导的全厚度、内包层厚度和芯层厚度;Q为芯层热负载功率密度;EYAG和vYAG分别为YAG晶体的杨氏模量和泊松比;αYAG和αSph分别为YAG晶体和蓝宝石的热膨胀系数;κYAG和κSph分别为YAG晶体和蓝宝石的热导率;h为表面换热系数。以全尺寸120 mm×30 mm×1 mm、内包层厚度0.4 mm的双端泵浦平面波导为例,其安全泵浦负载可达到200 kW。
因此,在同等功率水平下,高能平面波导激光的体积、质量原则上可以做到最小,因此美海军陆战队支持了该技术的后续发展和应用。但平面波导激光在高功率状态下的多级放大难题和非导波方向光束质量控制困难是制约其走向高功率的技术瓶颈。高能平面波导激光实现更高功率的一种可能技术措施是,直接使用一块平面波导激光输出100 kW级激光(在200 kW的泵浦功率下保守的假设光光效率只有50%),这样通过沿导波方向进行空间合束就可以将功率提升至满足更多战术应用场景的要求。但这要求平面波导的制备工艺获得大幅进步,并且解决满足极高功率密度的光学薄膜、光束整形和AO技术。
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文中主要讨论了平面波导激光器、固体薄片激光器、浸入式液冷固体激光器、碱金属蒸汽激光器、相干合成光纤激光器等新型电驱动高能激光光源的技术发展现状、关键技术、以及未来作为战术激光武器应用的潜力。相对固体板条激光和光纤激光,这些新型激光在功率定标能力、体积重量等方面具有潜在优势,因此近年来在国际上获得较广泛关注,部分技术体制还获得相当大的突破。特别是HEL-JTO支持下的RELI项目,支持了平面波导激光器、固体薄片激光器、浸入式液冷固体激光器、共孔径相干合成光纤激光器等多种新型激光技术。
表1所示为不同技术体制激光的优缺点与关键技术对比。具体到新型电驱动高能激光的武器级应用,应结合装载平台自身的特性。目前看来,在这些新型激光中,浸入式液冷固体激光作为战术激光武器的主战光源具有很好的应用前景,但需要提升其电光转换效率,延长激光器的可持续工作时间。其他新型激光中,平面波导激光在高功率状态下的多级放大难题和非导波方向光束质量控制困难是制约其走向高功率的技术瓶颈,平面波导的制备工艺获得大幅度进步前,预期平面波导激光的输出功率难以满足多数作战场景要求。目前尚看不到固体薄片激光定标到100 kW以上功率水平的可行性,因此其为战术激光武器主战光源的前景并不看好。碱金属蒸汽激光的输出功率应该能够提升至100 kW以上,但其是否能实现高光束质量产生、以及工程化应用是否存在问题尚待观察。拼接孔径相干合成的光纤激光看起来突破100 kW并不存在大的技术困难,但全系统的轻小型化仍然是制约其作为战术激光武器应用的关键问题。
表 1 各种新型激光优缺点及关键技术
Table 1. Advantages, disadvantages and key technologies of various new lasers
Technical system Advantage Shortcoming Key technology Planar waveguide laser High light to light conversion efficiency; No non linear effect; High output power of single module Material preparation is difficult; Multi level amplification is difficult; Difficult to control beam quality in non waveguide direction Fabrication technology of planar waveguide; Efficient uniform pump coupling technology; High efficiency seed injection technology Sheet laser High light to light conversion efficiency; High thermal management efficiency Sheet welding is difficult; Beam quality control is difficult; Small energy storage of single module Low deformation welding technology of thin sheet; Multi pass pump coupling technology; High efficiency laser extraction technology; Beam quality control technology Immersion liquid-cooled solid state laser Small size and light weight; High energy storage of single gain module;Easy power scaling and amplification Beam quality control is difficult; The coolant is toxic Solid liquid interface loss suppression technology; Laminar cooling technology; Pressure equalizing and shunt technology; Beam quality control technology Alkali metal vapor laser Small size and light weight; High light to light conversion efficiency; Efficient thermal management Low pump light absorption efficiency; Alkali metal vapor is easy to cause pollution of optical devices Pump band broadening Technology; LD wavelength locking technology; Steam cycle technology Coherent synthesis of fiber laser High light to light conversion efficiency; Easy power scaling and amplification Sub laser output power is limited; Complex system and difficult integration High power single frequency fiber laser technology; Large scale compact integration technology
Status and development trend of overseas new type electric drive high-energy laser technology
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摘要: 主要讨论了国外平面波导激光器、固体薄片激光器、浸入式液冷固体激光器、碱金属蒸汽激光器、相干合成光纤激光器等新型电驱动高能激光光源的技术发展现状、关键技术以及未来作为激光武器应用的潜力。它们至少在原理上可以解决当前高能固体激光或光纤激光面临的一些难题,但因为某些缺点或者面临一些待解决技术问题,使其输出功率、光束质量或体积、重量等指标暂时达不到典型高能固体激光或光纤激光的水平。详细讨论了这些新型电驱动高能激光的优缺点,并对其技术发展前景进行初步分析判断。Abstract: The status and development trend of overseas new type electric drive high-energy lasers, such as PWG laser, thin-disk laser, liquid immersed solid-state laser, alkali vapor laser and coherent beam combining of fiber lasers, are discussed in this paper. The applied potential of these new type lasers used as light source of laser weapon is analyzed. They can at least solve some problems faced by high-energy bulk solid-laser or fiber laser. Because of some shortcomings or problems to be solved, its output power, beam quality, or volume and weight cannot reach the level of a typical high-energy solid-state laser or fiber laser. The advantages and disadvantages of these new electrically driven high energy lasers are discussed in detail, and their technical development prospects are preliminarily analyzed and judged.
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表 1 各种新型激光优缺点及关键技术
Table 1. Advantages, disadvantages and key technologies of various new lasers
Technical system Advantage Shortcoming Key technology Planar waveguide laser High light to light conversion efficiency; No non linear effect; High output power of single module Material preparation is difficult; Multi level amplification is difficult; Difficult to control beam quality in non waveguide direction Fabrication technology of planar waveguide; Efficient uniform pump coupling technology; High efficiency seed injection technology Sheet laser High light to light conversion efficiency; High thermal management efficiency Sheet welding is difficult; Beam quality control is difficult; Small energy storage of single module Low deformation welding technology of thin sheet; Multi pass pump coupling technology; High efficiency laser extraction technology; Beam quality control technology Immersion liquid-cooled solid state laser Small size and light weight; High energy storage of single gain module;Easy power scaling and amplification Beam quality control is difficult; The coolant is toxic Solid liquid interface loss suppression technology; Laminar cooling technology; Pressure equalizing and shunt technology; Beam quality control technology Alkali metal vapor laser Small size and light weight; High light to light conversion efficiency; Efficient thermal management Low pump light absorption efficiency; Alkali metal vapor is easy to cause pollution of optical devices Pump band broadening Technology; LD wavelength locking technology; Steam cycle technology Coherent synthesis of fiber laser High light to light conversion efficiency; Easy power scaling and amplification Sub laser output power is limited; Complex system and difficult integration High power single frequency fiber laser technology; Large scale compact integration technology -
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