综述|回顾与展望:世界范围内拍瓦和艾瓦级激光器

朱坪

“我认为这是一篇优秀的论文,反映了超高强度激光科学、技术和工程领域的快速发展。”

——李儒新院士,上海光机所所长

“这篇综述论文非常及时,因为我们正处于高功率激光发展的三岔路口:一方面为基础物理研究提供更高的峰值功率,另一方面在现实应用中将重复率大幅提高到每秒数千次。”

——Colin Danson教授,《High Power Laser Science and Engineering》主编

“该领域的科学家以及学生都会从这篇全面的综述中受益匪浅……这篇论文提供了一个独特的全景,介绍了世界范围内此类激光器独特的功能与不断提升的性能,并展示了该领域巨大的潜力……这无疑是一个新的里程碑。”

——Leonida Gizzi教授,意大利国家光学研究所主任

还记得2015年英国著名激光物理学家Colin Danson教授领衔的国际激光物理科学家团队在High Power Laser Science and Engineering上发表的综述文章《Petawatt Class laser Worldwide》吗?此文全面介绍了当时世界上高功率激光装置的性能参数与应用,获得广大研究者好评,是Web of Science核心集中的高被引论文。2018年,啁啾脉冲放大技术的发明者Gerard Mourou教授和Donna Strickland教授荣获诺贝尔物理学奖。2019年,Colin Danson教授团队再次发表综述论文《Petawatt and Exawatt Class laser Worldwide》,介绍了世界各地超高功率激光器现状,并展望了下一代高功率激光技术的突破方向。

历史背景与里程碑

自1960年激光发明以来,激光技术经历了调Q(Q-switching)、锁模(mode-locking)等发展阶段,在实验室中实现了前所未有的高能量密度物质状态。1985年,啁啾脉冲放大技术(CPA)的提出使得激光输出功率实现了飞跃性的发展。从上世纪90年代开始,世界上陆续出现拍瓦(1015 W)级激光系统,包括NOVA Petawatt、Vulcan、OMEGA-EP、J-KAREN、BELLA、ELI等里程碑式的激光装置,同时钕玻璃放大、钛宝石放大、光参量放大等技术也得到了长足发展。

图1 高功率激光系统装置与技术发展历史

发展现状与地缘分布

最新综述的第二部分从地理布局的全新角度介绍了当前世界超高功率激光装置的发展现状。目前,超过50个高功率激光装置分布在北美洲、欧洲、亚洲的多个国家与地区,其中许多装置已经建成并提供实验运行,部分装置正在建设之中,还有不少更大规模的高功率激光装置计划开建。未来,亚洲将成为超高功率激光系统发展的重心。文中提到,全球激光输出能力的提升与装置的广泛分布,得益于大装置零部件从国家实验室到学术界的转移与再利用,从早年的美国NOVA装置和法国Phebus装置中衍生出了德国PHELIX、英国Vulcan PW、美国Texas PW、法国LULI-2000等当前稳定运行的高功率激光装置。

图2 高功率激光器的地理分布。圆的直径与激光峰值功率成对数关系;正在运行的为实线圆圈,过去运行但现在停用的为实线八角形,正在建设的为虚线圆圈;颜色表示最终放大器使用的激光介质:钛宝石(红色)、钕玻璃(灰色)、Yb:X(橙色)、Cr:X(黄色)、光参量放大(紫蓝)、气体(粉红)。

未来技术发展展望

更高峰值功率的激光装置不但可以用于探索更高能量密度物质的科学,还可以应用于医学诊断和治疗、射线照相、无损检测、特殊核材料探测、对撞粒子加速器、惯性聚变能源等领域,这些应用目前已完成了原理性验证,未来真正应用则需要激光驱动器具有更高的重复频率。综述文章指出,为了实现更短脉宽、更大能量、更高重复频率的激光输出,需要在以下几个技术发展方面上进行不断突破:

1、面向艾瓦(1018 W)级的光参量啁啾脉冲放大(OPCPA)技术、CafCA非线性压缩技术、等离子体放大技术等超高峰值功率技术发展;

2、高平均功率气冷技术、万瓦级拍瓦激光技术、低温高平均功率激光技术、相干合成技术、时域脉冲合成技术、温度不敏感OPCPA技术等高平均功率技术发展;

3、中红外激光技术、脉冲信噪比提升技术、等离子体光学技术、先进光学元器件、激光诊断技术、等离子体诊断技术、制靶技术等具有挑战性的关键技术发展。

图3 CafCA非线性压缩技术。

图4 HAPLS激光装置。

图5 耶拿16通道相干合束系统。

图6 用于实验的复杂气体靶。

综上,最新综述着眼于超高功率激光的历史背景、发展现状与未来方向。当前各大洲的各国家和地区拥有什么样高功率激光装置?未来高功率激光发展中有哪些关键问题与关键技术?想要了解更多细节,欢迎大家阅读在High Power Laser Science and Engineering上发表的这篇最新综述文章:《Petawatt and Exawatt Class laser Worldwide》。

On the Cover of HPL: Petawatt and exawatt class lasers worldwide

Rory Penman and Colin Danson

An international team of scientific experts has gathered to examine the current status of ultra-high-powered lasers around the world and look to the future to predict what the next generation of laser systems will offer. The culmination of their work is a major review paper ‘Petawatt and Exawatt Class Lasers Worldwide’, which looks at the historical context of this technology, its current and future use, and direction.

These ultra-high power lasers are capable of creating some of the most extreme conditions found in our solar system, if only for a tiny fraction of a second. They are used for understanding fundamental physics and producing secondary sources such as x-rays, electrons, protons and neutrons.

The paper takes a geographical tour of Asia, Europe and North America documenting operational facilities and those under construction. This is followed by a focus on future technologies where the authors have identified two main laser development streams: ultra-high-power and high-average-power. The first of which describes how scientists will produce exawatt class lasers, whilst the latter looks at how we can produce petawatt pulses at repetition rates at and beyond kHz repetition rates. The paper then concludes with a view to the future and what grand challenges need to be overcome in order to achieve the ultra-high-power and high-average-power lasers.

These two development streams are both working on some really exciting laser physics at the moment:

In the ultra-high power stream teams of scientists all over the world are working towards smashing current power records. Using the OPCPA (Optical Parametric Chirped Pulsed Amplification) technique a team at SIOM, China are planning to deliver 100 petawatts in 2023. This is particularly impressive when you consider that in the year 2000 scientists using the OPCPA technique were only capable of delivering 1.6 terawatts; that’s a 62,500 times increase in output power in just 23 years.

In delivering high repetition rate systems at ultra-high powers the review paper identifies those milestone events: the BELLA facility producing petawatt pulses at 1 Hz in 2013; and the HAPLS laser producing petawatt pulses at 10 Hz in 2018. This incredible progress will have to continue in the next decade to deliver the kHz petawatt class lasers of the future for industrial / medical applications.

Geographic distribution of high-peak-power lasers (top). Diameter of circle is logarithmically proportional to peak laser power and circle colour is chosen for graphical clarity. Evolution of high-peak-power lasers ( > 100 GW) in the world over the last fifty years (bottom).