激光相干合成的研究进展:2011—2020

周朴; 粟荣涛; 马阎星; 马鹏飞; 吴坚; 李灿; 姜曼

doi:doi:10.3788/CJL202148.0401003

中国激光, 2021, 48 (4): 0401003, 网络出版: 2021-02-24

激光相干合成的研究进展:2011—2020 下载： 2828次封底文章特邀综述

Review of Coherent Laser Beam Combining Research Progress in the Past Decade

论文大纲

周朴 ^*粟荣涛马阎星马鹏飞吴坚李灿姜曼

作者单位

国防科技大学前沿交叉学科学院, 湖南长沙 410073

激光器相干合成高能激光光纤激光固体激光半导体激光 lasers coherent beam combining high energy laser fiber laser solid state laser diode laser

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摘要

本文回顾了过去10年激光相干合成领域取得的重要进展,总结了可合成激光模块性能提升的标志性成果,梳理了相干合成使能技术取得的重要突破,展示了不同类型激光相干合成取得的成果及其多样化应用,并对激光相干合成领域的发展进行了展望。

Abstract

Significance Coherent beam combining (CBC) of lasers is an effective technical approach for scaling the power of a laser while maintaining good beam quality. In 2009, Northrop Grumman Aerospace Systems demonstrated the world's first 100 kW solid-state laser system, in which seven 15 kW master oscillator-power amplifier laser chains were coherently combined. It was an important milestone in the history of laser development. In the past 10 years (2011—2020), CBC has developed rapidly, and many representative results have been produced. In this paper, the research progress of coherent laser beam combining in the past decade is reviewed.

Progress First, the output power of fiber, solid-state, and semiconductor lasers has increased significantly, which has provided high-power combinable laser elements for CBC systems. For example, the output power levels of single-frequency and narrow-line-width fiber lasers have reached 500 W and 4 kW, respectively; the average power of femtosecond fiber lasers has exceeded 1 kW; and Nd∶YAG and Yb∶YAG solid-state lasers have both generated over 20 kW in output power.

Second, enabling technologies for coherent laser beam combining have been developed, including phase control, tip-tilt control, polarization control, optical path difference control, high-order aberration control, and aperture?filling. More than 100 fiber lasers have been phase-locked based on active phase control technology. High-power adaptive fiber optic collimators have been designed that can be used for tip-tilt control of fiber lasers with kilowatt-level output power. Active polarization control of a kilowatt-level fiber amplifier has been realized, which has been employed to increase the combination efficiency of CBC systems. High-precision optical path difference real-time control systems have been designed and used for CBC of broad-spectrum and ultrafast lasers. Mode control technologies and adaptive optics methods have been employed for flexible mode manipulation. Aperture-filling technologies, such as microlens arrays, diffractive optical elements, and polarization beam combiners, have also been developed to increase efficiency of combination.

Third, representative results of the coherent combining of various kinds of lasers have been produced. For semiconductor lasers, coherent combining of 218 elements, with total output power of 38.5 W, has been realized. For solid-state lasers, pulse energy of 15.3 J has been obtained by the coherent combing of 6 solid-state lasers, and peak power of 3.7 GW has been obtained by the coherent combining of 2 ultrafast Yb∶YAG lasers. For fiber lasers, the number of laser channels has been increased to 107, and 16-kW output power has been obtained by the coherent combining of 32 fiber lasers. For ultrafast lasers, 61-fs fiber lasers have been coherently combined, and 10.4 kW average power has been generated by the coherent combining of 12 fs fiber lasers. In order to increase pulse energy, divided-pulse amplification and coherent temporal pulse-stacking technologies have been developed. Based on divided-pulse amplification, femtosecond pulses with energy of 23 mJ have been generated; using temporal pulse-stacking technology, 81 pulses were coherently combined to be one pulse with 10 mJ energy.

Fourth, coherent laser beam combining has been employed in versatile applications. For nonlinear frequency conversion applications, high pump brightness is required. A 600 W, 520 nm laser (second harmonic) and a 300 W, 347 nm laser (third harmonic) were obtained based on 1040 nm with kilowatt output power generated through a CBC system. Similarly, by coherently combining narrow-line-width lasers, sufficient output power was obtained for applications such as laser guide star and laser radar. By controlling the optical parameters of a coherent laser array, structural light fields with special spatial distribution can be obtained, such as vortex beams carrying orbital angular momentum. In recent years, research plans for large scientific facilities based on coherent laser beam combining have been proposed. For example, LIGO needs a low-noise single-frequency laser with hundreds of watts of output power; researchers have demonstrated the feasibility of obtaining such a laser source through coherent combining of fiber lasers. The International Coherent Amplification Network project has been proposed to provide a laser source for the next generation of particle accelerators.

Prospects In future work, increasing the number of coherent laser array elements and scaling the power of single-channel lasers will still be the development tendency. Lasers can be extended to almost arbitrary gain medium and wavelength band (visible light, mid-infrared, and even terahertz). Moreover, with the rapid development of computing technology and computational power, artificial intelligence techniques may be used for CBC-enabling technologies such as phase control and tip-tilt control. In the case of multiple-parameter control for massive laser arrays, high-power laser phased arrays will be realized.

1 引言

对多束激光进行相干合成是实现提升激光功率的同时保持光束质量的有效技术途径,它的发展基本和激光技术的发展同步,并已经应用在几乎各种类型的激光器中^[1]。2009年,美国Northrop Grumman公司以光纤激光作为种子源,利用主动相位控制实现了7路万瓦级板条放大器的相干合成,获得超过百千瓦级固体激光输出^[2],成为激光发展史上的重要里程碑事件。近10年来,随着可合成激光模块的性能不断提升和相干合成使能技术不断发展,各种类型的激光相干合成成果不断涌现^[3-6],并发展了大量基于相干合成的大型研究计划^[7-11]。本文在文献[ 1]的基础上,梳理近10年(2011—2020)来激光相干合成领域的关键技术突破和代表性研究成果,总结激光相干合成技术已经实现的重要应用,呈现该领域的发展态势和全貌,展望激光相干合成技术的未来发展。

2 可合成激光模块的性能不断提升

尽管相干合成具有提升输出功率的同时提升光束质量的效果,但从以往的发展历程分析^[1],系统并非对参与合成的单束激光性能没有要求。为保证相干合成系统的输出性能,一般会对参与合成的单束激光提出模块化、紧凑化、高效率等需求。近年来,以光纤激光、固体激光、半导体激光为代表的各种类型的激光器性能均取得了显著提升,为相干合成系统的研制提供了高性能单元模块。

2.1 光纤激光

光纤激光是目前相干合成系统采用最多的激光器类型^[4-5,12]。自从1988年发明双包层光纤以来,单束光纤激光的输出功率不断提升;与此同时,受限于非线性效应和模式不稳定效应等因素的影响,单束光纤激光的输出功率存在物理极限^[13-17],对多束激光进行相干合成是构建高功率光纤激光系统的重要技术途径^[8,18]。相干合成对单元激光的结构和参数有较为严格的要求,并非所有类型的光纤激光都能用来相干合成。由于单元激光的性能、多束光纤激光控制能力等因素的限制,在21世纪前10年,基于光纤激光的相干合成系统性能并不十分突出,千瓦级相干合成系统直到2010年底才首次实现^[19],与当时基于固体激光的百千瓦级相干合成系统相比有着明显差距^[2]。

近10年来,受益于先进制造(如飞秒激光加工)、大科学工程(如引力波探测)等应用领域的驱动和激光材料制备、非线性效应调控等技术的不断突破,单元光纤激光的性能得到了显著提升,并正在朝着三个“任意”:任意功率、任意波长和任意输出模式的方向发展^[20]。比如单频光纤激光的输出功率由2008年的200瓦级提升至2020年的500瓦级^[21],窄线宽光纤激光的输出功率由2010年的千瓦级提升至2020年的4千瓦级^[22-23],飞秒光纤激光的平均功率突破了千瓦级^[24];此外,超高电光转换效率(大于50%)光纤激光^[25]和紧凑型光纤激光^[26]等高性能工业级激光的推出为构建模块化的激光阵列提供了技术基础。与此同时,相干合成技术也被应用于多种类型的光纤激光系统中,如纳秒激光/皮秒激光/飞秒激光、1.0 μm/1.5 μm/2.0 μm等^[27-32],大大加速了光纤激光技术的发展。

2.2 固体激光

在21世纪初期,以Nd∶YAG板条激光为基本合成单元的固体激光相干合成是激光技术领域的研究热点^[33-34],美国诺格公司先后实现2路、4路、7路Nd∶YAG板条激光相干合成,并基于相干合成技术在国际上率先实现百千瓦级固体激光系统^[35]。但由于当时单元激光电光效率提升、光束质量保持等存在较大技术挑战,板条固体激光相干合成的后续报道并不多见;文献[ 36]报道的万瓦级板条激光的光光转换效率约为30%、光束质量约为7倍衍射极限,这与同期万瓦级光纤激光的指标相比有较大差别。近年来,研究人员通过腔外光束净化和引入Yb∶YAG介质等方式,显著提升了光束质量和光光效率。2018年,中国科学院理化技术研究所和清华大学课题组合作实现了万瓦级(10.8 kW)Nd∶YAG板条激光2.6倍衍射极限输出^[37];中国科学院理化技术研究所和中国科学院光电技术研究所课题组合作实现了2万瓦级(24.6 kW)Nd∶YAG板条激光1.5倍衍射极限输出^[38]。由于具有更小的量子亏损和更低的产热^[39-43],采用Yb∶YAG介质有望比Nd∶YAG介质实现更好的光束质量和光光效率。2016年,中国工程物理研究院实现了3千瓦级(3.54 kW)Yb∶YAG激光,光光转换效率41%^[39];2018年,中国工程物理研究院等单位实现了2万瓦级(22.3 kW)Yb∶YAG板条激光高功率输出,光束质量为3.3倍衍射极限,提取效率为36%并有望提升到46% ^[40];2020年,中国科学院理化技术研究所和清华大学课题组合作实现了2万瓦级(21.2 kW)Yb∶YAG板条激光1.94倍衍射极限输出、光光转换效率为30%^[43]。综上,通过采用光束净化、Yb∶YAG介质和温度管理^[44]等方式,高功率固体激光的光束质量和效率等参数取得了明显进展,若再次与相干合成技术结合,有望获得新的突破。

2.3 半导体激光

半导体激光具有效率高、体积紧凑、寿命长、可靠性好等优点,在通信、制造、医疗、科研等多个领域得到了广泛应用^[45-47]。与光纤激光、固体激光相比,高功率半导体激光的光束质量相对较差,因此常被用作光纤激光和固体激光的泵浦源。提升输出功率的同时保证光束质量成为高功率半导体激光技术领域的研究热点,半导体激光相干合成就是其中的一种重要技术途径^[48]。尽管半导体激光的合成路数很早就取得了巨大突破^[49],但由于合成单元的性能(效率、功率、光束质量等)限制,基于相干合成的半导体激光系统并没有实质性进展。近年来,随着芯片设计、材料生产和器件制备等技术的不断发展,半导体激光的性能不断提升,为半导体激光的相干合成提供了新的机遇。基于内腔光反馈技术和外腔光反馈技术的半导体激光已经分别实现了低于10 kHz和低于100 Hz的窄线宽输出^[45],德国FBH研究所、美国nLight公司等单位通过半导体光放大器实现了高功率输出的同时保持较好光束质量^[50-51]。此外,半导体激光的亮度和效率等方面也取得了显著进步^[46]:JDSU公司于2015年报道了输出功率高达29.5 W的950 nm单管^[52],Fujikura公司2017年报道了输出功率为27 W、电光效率大于60%的915 nm单管^[53],IPG公司2017年报道了室温连续条件下转换效率高达73%的976 nm单管^[54],Fujikura公司2019年报道了室温连续条件下转换效率72.5%的975 nm单管(20 W输出功率时转换效率仍高达66.7%)^[55]。类比2.1小节总结的光纤激光单元性能提升的代表性成果,可以对类似光纤激光经典结构^[54]的半导体激光相干合成系统的构建提供重要启发。值得注意的是,基于光谱合成的高功率半导体激光系统的亮度已经接近或者达到相同功率量级的固体激光系统^[56-57]。

3 相干合成使能技术不断发展

相干合成通过各路激光的参量控制和激光阵列的孔径填充,实现激光阵列的同相位、高占空比输出,达到激光阵列亮度提升的目的。典型的相干合成系统结构如图1所示,种子激光由分束器分为多路,每一路激光经过相位调制器(PM)、延迟线(DL)、放大器(AMP)和激光准直器(CO)等器件后,再利用合束装置实现各路激光的高占空比合成。为了对合成效果进行评价,同时为参量控制提供反馈信号,通常利用光束分光镜(BS)提取小部分激光进行误差探测,同时提取小部分光经过透镜形成远场光斑。由图1可以看出,对各路激光的相位、倾斜、偏振、光程和高阶像差进行控制以及对阵列激光进行孔径填充获得高占空比是获得良好的合成效果的关键技术。近年来,上述使能技术均取得了重要进展,为获得高性能相干合成激光系统奠定了技术基础。

图 1. 主动相位控制相干合成系统的结构示意图

Fig. 1. Schematic setup of active phase control coherent beam combining system

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3.1 相位控制技术

根据相位控制的物理机理,可以将其分为被动相位控制和主动相位控制两类。被动相位控制是通过一定的能量耦合机制或者非线性相互作用实现各路激光相位起伏的自动补偿。与主动相位控制相比,无需复杂的相位控制系统,结构相对简单。被动锁相方法主要有外腔法^[58-62]、倏逝波耦合法^[63-64]、全光纤自组织法^[65-67]和相位共轭法^[68-69]等。2011年,中国科学院上海光学精密机械研究所采用单模光纤滤波环形腔实现了8路光纤放大器的相干输出,总功率达到1.09 kW^[70]。

主动相位控制利用相位检测和控制系统对各路激光的相位起伏进行补偿。根据相位误差的探测方式,可以将其分为直接探测和间接探测两类。直接探测采用外差法^[34,71]、Hänsch-Couillaud (H-C)偏振探测^[72-73]、剪切干涉^[74]、参考光干涉^[75]和干涉图像^[76]等方法获取各路激光之间的活塞相位误差,具有算法迭代步数较小、控制带宽较高等优点,但是误差探测系统较为复杂。间接探测利用光电探测器提取合成光束的远场中央主斑能量,利用随机并行梯度下降(SPGD)算法^[77-78]、多抖动算法^[79-80]和单抖动算法^[81]等进行优化迭代,使其达到最大值。间接探测具有结构简单的优点,但是对算法的收敛速度提出了较高要求。近十年来,主动相位控制的激光路数和总功率得到了有效提升,已经实现了最高百路级路数和万瓦级功率的激光阵列相位锁定,代表性研究结果如表1所示。

表 1. 相干合成主动相位控制代表性研究成果

Table 1. Representative research results of active phase control coherent beam combining

Method	Year	Institution	Result		Reference
SPGD	2011	Massachusetts Institute of Technology		8 fiber lasers with 4 kW overall power		[78]
	2014	National University of Defense Technology		32 low power fiber lasers		[82]
	2016	University of Dayton		21 fiber lasers with 0.5 kW overall power, target-in-the-loop over 7 km		[83-84]
	2019	National University of Defense Technology		60 low power fiber lasers		[85]
	2020	National University of Defense Technology		7 fiber lasers with 8 kW overall power		[86]
	2020	National University of Defense Technology		107 low power fiber lasers		[87]
Method	Year	Institution	Result		Reference
Multi-frequencydithering	2011	Air Force Research Laboratory		16 fiber lasers with kW level overall power		[88]
	2014	Northrop Grumman		3 fiber lasers with 2.4 kW overall power		[80]
	2016	Air Force Research Laboratory		5 fiber lasers with 4.9 kW overall power		[89]
	2018	Friedrich Schiller University Jena		4 pulsed fiber lasers with 3.5 kW overall power		[90]
	2020	Friedrich Schiller University Jena		12 pulsed fiber lasers with 10.4 kW overall power		[91]
Single frequencydithering	2010	National University of Defense Technology		9 fiber lasers with 1.08 kW overall power		[92]
	2016	National University of Defense Technology		4 fiber lasers with 5.02 kW overall power		[93]
	2017	China Academy of Engineering Physics		30 low power fiber lasers		[94]
Quadriwave lateralshearing interferometry	2011	Thales Research & Technology		64 low power fiber lasers		[74]
Reference beaminterference	2014	Thales Research & Technology		16 low power fiber lasers		[75]
Reference beaminterference	2020	Thales Research & Technology		61 low power pulsed fiber lasers		[95]
Phase-intensitymapping	2017	Université de Limoges		37 fiber lasers with 30 W overall power		[96]

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为了进一步提高相位控制带宽,研究人员从多个层面上进行了研究:1)对现有算法进行改进,提出了自适应SPGD算法、正交编码抖动算法等算法^[97-100];2)探索新的相位探测与控制方法,提出了基于倾斜调干涉和人工智能算法等的相位误差解算方法^[101-105];3)从体系上进行改进,利用分组控制、级联控制等方法提升控制带宽^[106-108]。

3.2 倾斜控制技术

为了使各路激光在目标处进行有效重叠,需要控制各路激光的倾斜像差。静态倾斜像差控制可以利用高精度的光学调节架实现。由于热效应的影响,高功率激光的输出光斑通常存在动态抖动;在需要对激光进行远距离传输的应用中,大气湍流也会产生随机的倾斜误差。因此,为了在上述场景中获得较好的相干合成效果,必须进行动态的倾斜控制。最常见的倾斜控制方法就是使用快速倾斜镜,倾斜镜通过外加压电控制信号,改变反射镜面的整体倾斜量。2012年,国防科技大学采用快速倾斜镜对2路百瓦级光纤激光进行倾斜控制,实现了350 W光纤激光有效相干合成^[109]。倾斜镜具有精度高、响应快、技术成熟等优点,但是当阵列数目较多时,系统光路将过于复杂。

为了实现光纤激光阵列倾斜控制的紧凑化,研究人员提出了自适应光纤准直器(AFOC)的技术方案。AFOC通过压电陶瓷驱动输出光纤,改变其在准直透镜焦平面处的位置,实现准直光束的倾斜控制和激光准直的一体化设计,具有惯性小、谐振频率高、结构紧凑等优势,适合应用于阵列光纤激光系统。2011年、2016年,美国Dayton大学的Vorontsov课题组在此前研制的AFOC^[110]的基础上,构建基于AFOC的相干阵列系统,在7 km传输距离上先后开展了7路^[111]和21路^[83]目标在回路实验。2011年,中国科学院光电技术研究所成功研制了AFOC,并实现了3路光纤激光的相干合成^[112];随后,基于自研的AFOC搭建光纤激光阵列,开展了目标在回路实验研究^[113]。千瓦级的光纤激光一般采用光纤端帽输出,这就需要AFOC的能动器件具有更大的推力,2014年,国防科技大学研制了基于柔性铰链和光纤端帽的AFOC^[114],并开展了实验研究^[115-116]。基于AFOC的相干合成倾斜控制代表性成果如表2所示。

表 2. 基于AFOC的相干合成倾斜控制代表性成果

Table 2. Representative research results of AFOC-based tilt control in coherent beam combining

Year	Institution	Result	Reference
2011	University of Dayton	7 low power fiber lasers, target-in-the-loop over 7 km	[111]
2011	Institute of Optics and Electronics	3 low power fiber lasers	[112]
2012	Northrop Grumman	5 low power fiber lasers	[117]
2013	Institute of Optics and Electronics	7 fiber lasers with 10 W level overall power	[118]
2014	National University of Defense Technology	2 fiber lasers with 10 W level overall power	[115]
2016	University of Dayton	21 fiber lasers with 0.5 kW overall power, target-in-the-loop over 7 km	[83-84]
2017	National University of Defense Technology	7 fiber lasers with 10 W level overall power	[116]
2018	Institute of Optics and Electronics	7 fiber lasers with 10 W level overall power, target-in-the-loop over 200 m	[113]
2018	National University of Defense Technology	6 fiber lasers with 10 W level overall power, target-in-the-loop over 800 m	[119]

查看所有表

3.3 偏振控制技术

为提升合成效率,一般要求各路激光的偏振方向相同。根据偏振控制施加的位置,可以将其分为末端偏振控制和前端偏振预补偿两大类^[120-121]。末端偏振控制就是在激光输出端利用半波片^[29,122]、偏振合束器^[123-124]、马赫-曾德尔(M-Z)干涉仪^[125]等方式对激光的偏振方向进行控制;由于直接对输出的激光进行操作,控制直接有效,灵活度较高,但是输出功率和控制带宽均受限于偏振控制器件。在光纤激光相干合成系统中,通常采用保偏光纤放大器进行功率放大,使激光保持较好的偏振特性,再在输出端利用半波片进行静态的偏振调节,从而达到良好的相干合成效果。但是,保偏器件价格昂贵,保偏光纤中模式不稳定性等效应更强,功率提升更加困难。一条有效的途径就是搭建非保偏放大器,在放大器的前端利用偏振控制器进行偏振预补偿,从而实现高功率的线偏振激光输出^[126]。采用该方法,美国Northrop Grumman公司、国防科技大学、中国工程物理研究院等单位实现了千瓦级窄线宽线偏振激光输出^[127-129],并在相干合成系统中验证了该技术的可行性^[130-131]。图2所示为国防科技大学课题组的千瓦级高功率光纤放大器偏振控制系统结构。系统由窄线宽激光器种子和三级光纤放大器组成,通过白噪声源对单频光纤激光器进行相位调制产生窄线宽激光种子,主放大器最大功率为1.43 kW(受到泵浦功率限制)。利用基于SPGD算法的偏振控制技术对种子激光的注入偏振态进行控制,在最大输出功率下激光束的偏振消光比大于11 dB,线性偏振激光约为总功率93%。美国Northrop Grumman公司在2012年验证5路光纤激光相干合成系统中的预补偿偏振控制技术的基础上^[130],与美国空军实验室开展合作,利用该技术在2014年和2016年先后实现了3路2.4 kW和5路4.9 kW的非保偏光纤放大器高效相干合成^[80,89]。

图 2. kW级窄线宽光纤放大器中的偏振控制系统结构^[129]

Fig. 2. Schematic diagram of polarization control system for narrow linewidth fiber amplifier^[129]

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3.4 光程控制技术

各路激光之间光程差会引起群延时效应,降低相干合成效率。为了使合成效率达到99%,各路激光的光程差均方根值应小于0.1L_coh,L_coh表示相干长度^[132-133]。对于脉冲激光相干合成系统,除了群延时外,光程差还会引起时域误差和非线性相移误差^[134-136]。当激光线宽较窄、脉宽较宽时,对光程差的要求并不高。如线宽在GHz量级、脉宽在ns量级时,只需将各路激光的光程差控制到厘米量级就能获得良好的相干合成效果^[135]。此时,采用空间光路调节^[27]和被动光纤匹配熔接^[28]等方法就能达到光程控制要求。随着线宽的增加,系统对光程控制的精度要求也随之提高,通常采用动态光纤延迟线^{[29,75,78,89]}、压电陶瓷高精度空间位移器^[91,137]、光纤拉伸器^[133,138]等高精度光程控制器件。当线宽增加到10 nm量级时,需要将光程精度控制到10 μm量级^[136]。此时,需要对各路激光直接的光程差进行实时控制,以获得理想的合成效果^{[91,133,137-139]}。

图3为国防科技大学课题组设计的适用于高精度实时控制光程差的实验系统^[137]。首先,调节空间光路,即准直器到偏振合束器的距离,使自由空间传输部分光程基本一致;其次,熔接被动光纤,使光纤传输部分的光程基本一致,同时起到色散补偿的作用,上述两项操作的精度大约可以控制在厘米量级;第三,调节光学延迟线(ODL)以消除固定光程差:ODL调节精度高(约为0.3 μm)、范围广(0~180 mm),但响应速度慢(3 μm/s),适用于补偿固定光程差,但不足以达到光程慢漂移自适应补偿的应用要求。最后,通过压电陶瓷相位调制器(FS)实现光程差漂移的自适应补偿:FS控制精度高(0.035 μm/V)、响应速度快(1阶谐振频率约为50 kHz),可以满足光程差慢漂移自适应控制的要求。

图 3. 高精度光程差实时控制系统^[139]

Fig. 3. High precision optical path difference real-time control system^[139]

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3.5 高阶像差控制技术

从像差的角度而言,3.1节、3.2节介绍的相位控制和倾斜控制指的是单元激光的低阶像差。为了获得理想的相干合成效果,通常要求各路激光具有良好的光束质量且光斑形态保持一致;为此一般追求单元激光输出光束为基模。由于热效应等因素的影响,高功率单元光束(包括半导体激光、固体激光、光纤激光等)往往含有高阶模成分^[47,140-141];对于光纤激光而言,还存在从波导到自由空间的元件引入的像差因素^[142-143]。一般需要引入模式控制技术或者自适应光学方法进行控制或校正^[144-147]。图4(a)为国防科技大学课题组设计的大功率离焦补偿型准直系统的原理图^[142],其中Endcap为光纤端帽,F1 和F2 为两个组合式透镜,用来实现光束的扩束、准直。为了实现离焦补偿功能,在准直器机械装配时设计透镜F1 和F2 之间距离可调;F1 和F2 组合式透镜的等效焦距将具备可调节的功能。通过改变组合式透镜的等效焦距,便可以实现离焦像差的补偿。图4(b)为该大功率离焦补偿型准直系统的激光输出测试结果,可以看出,随着输出功率的增加,透镜窗口组建的准直系统基本不会引起光束质量的退化,可以用于大功率相干合成系统中^[143]。

图 4. 大功率离焦补偿型准直系统。(a) 工作原理^[142];(b) 实验结果^[143]

Fig. 4. High power defocus compensation collimation system. (a) Principle schematic^[142]; (b) experimental results^[143]

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此外,由于模体积引入的功率提升潜力,近年来也有不少产生纯净高阶模式激光的报道。以光纤激光为例,通过光栅选模等方式,研究人员已经实现了百瓦级高阶模式激光输出^[148-151]。

3.6 孔径填充技术

为了提高阵列光束远场光斑的能量集中度,通常还需要利用合束器提升阵列光束的占空比。其技术方案可以分为分孔径合成和共孔径合成两类。分孔径合成通过压缩阵列光束的占空比,来减少阵列光束远场光斑中旁瓣的能量占比,主要方法有准直器直接拼接、功率合束器、空间光路拼接和微透镜阵列等。共孔径合成通过使各路激光在空间上完全重合,从而形成一束激光输出,主要方法有M-Z干涉仪、自成像光波导^[152]、相干偏振合成和衍射光学元件(DOE)等。分孔径合成是对各路激光的紧密拼接,合成过程中不存在能量损失,但是远场光斑的能量会向旁瓣分散,导致中央主斑的能量损失。分孔径合成的关键就是要保持各路激光在远场的重合度,尽可能压缩占空比,提升远场光斑中央主斑的能量占比,通常用桶中功率(PIB)作为评价指标。共孔径合成是将各路激光在近场就合为一束,因此在远场不存在旁瓣,但是在合束过程中存在能量损失。共孔径合成的关键就是保持各路激光在近场和远场的重合度,减少合成过程中的能量损失,提升合成效率。近年来,在路数和功率提升等方面具有较大提升的方法有:准直器直接拼接、微透镜阵列、衍射光学元件和相干偏振合成等,代表性结果如表3所示。

表 3. 孔径填充的代表性研究成果

Table 3. Representative research results of aperture-filling

Method	Year	Institution	Result	Reference
Collimatorsarray	2017	Université de Limoges	37 fiber lasers with 30 W overall power, PIB was 36%	[96]
	2017	China Academy ofEngineering Physics	30 low power fiber lasers,PIB was 41.65%	[94]
	2019	National University ofDefense Technology	60 low power fiber lasers, PIB was 34.7%	[85]
	2020	National University ofDefense Technology	7 fiber lasers with 8 kW overall power, PIB was 18.76%	[86]
	2020	National University ofDefense Technology	107 low power fiber lasers, PIB was 22.47%	[87]
Microlensarray	2011	Massachusetts Institute of Technology	8 fiber lasers with 4 kW overall power, PIB was 58%	[78]
	2011	Thales Research &Technology	64 low power fiber lasers, PIB was 34%	[74]
	2020	Thales Research &Technology	61 low power pulsed fiber lasers, PIB was 48%	[95]
Fiber bundle	2020	Civan Advanced Technologies	32 fiber lasers with 16 kW overall power	[153]
DOE	2012	Massachusetts Institute ofTechnology	5 fiber lasers with 1.93 kW overall power, M² was 1.1, combination efficiency was 79%	[154]
	2014	Northrop Grumman	3 fiber lasers with 2.4 kW overall power, M² was 1.2, combination efficiency was 80%	[80]
	2016	Air Force ResearchLaboratory	5 fiber lasers with 4.9 kW overall power, M² was 1.1, combination efficiency was 82%	[89]
Method	Year	Institution	Result	Reference
CPBC	2012	National University ofDefense Technology	8 low power fiber lasers, combination efficiency was 92%	[155]
	2013	Friedrich Schiller University Jena	4 pulsed fiber lasers with 530 W overall power, combination efficiency was 93%,M² was 1.2	[156]
	2014	National University ofDefense Technology	4 fiber lasers with 680 W overall power, combination efficiency was 75.2%	[157]
	2016	Friedrich Schiller University Jena	8 pulsed fiber lasers with 1 kW overall power, combination efficiency was 91%	[158]
	2017	Friedrich Schiller University Jena	16 pulsed fiber lasers with 1.83 kW overall power, combination efficiency was 82%	[72]
	2017	National University ofDefense Technology	4 fiber lasers with 5.02 kW overall power, combination efficiency was 93.8%	[93]
M-Zinterferometer	2016	Laboratoire ARTEMIS	2 fiber lasers with 80 W overall power, combination efficiency was 96%	[159]
	2018	Friedrich Schiller University Jena	4 pulsed fiber lasers with 3.5 kW overall power, combination efficiency was 88.2%	[90]
	2020	Friedrich Schiller University Jena	12 pulsed fiber lasers with 10.4 kW overall power, combination efficiency was 96%	[91]

查看所有表

3.7 小结

上述使能技术并不是相互独立的,在实际相干合成系统中往往是相互耦合或者同步实现的。文献[ 116]报道的AFOC设计就同时具备倾斜控制和孔径填充功能,基于该AFOC阵列实现了接近50%的主瓣能量占比。文献[ 160]提出了一种基于四象限探测器的同时实现倾斜和相位控制的方法,光束指向控制精度达到0.19 μrad、相位锁定残差为0.7°,合成总体效率达到99%。此外,由于光程与相位的关系,如能实现对光程的高速高精度控制,则有望一并实现相位的实时控制。

4 相干合成研究成果不断涌现

4.1 半导体激光相干合成

2011年以来,美国麻省理工学院、Oak Ridge国家实验室、Clemson大学、Corcoran 公司,新加坡南洋理工大学,法国Paris-Saclay大学、Thales研究所,德国FBH研究所,中国科学院长春光学精密机械与物理研究所等单位^[161-174]均开展了半导体激光相干合成研究。其中比较有代表性的结果有:2011年美国麻省理工学院报道通过主动相位控制的方法实现218单元激光相干合成,总输出功率38.5 W^[163];2019年法国Paris-Saclay大学报道通过主动相位控制的方法实现4单元激光相干合成,总输出功率22.7 W时保持接近衍射极限的光束质量^[173]。

虽然目前半导体相干合成的输出功率并没有呈现出明显的优势,但在中红外半导体激光输出功率提升等特殊领域方面表现出了巨大潜力^[161-162]。2011年,法国Thales研究所实现了5束4.65 μm中红外激光半导体激光被动相位控制相干合成,实验原理如图5所示^[162]。合成后的激光输出功率为550 mW,光束质量为 $M_{x}^{2}$ <1.6、 $M_{y}^{2}$ <1.2(与合成单元的光束质量接近),合成效率为66%。

图 5. 5束半导体激光相干合成结构示意图^[162]

Fig. 5. Schematic setup of coherent beam combining of five diode lasers ^[162]

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4.2 固体激光相干合成

如2.2节所述,21世纪前10年固体激光相干合成领域取得了巨大成就,实现了首个百千瓦级固体激光系统。近10年来,并没有进一步提升平均功率的结果报道,研究人员将重点转至相位控制方法、脉冲激光相干合成等领域^[175-181]。相位控制方法方面,中国工程物理研究院^[175]和韩国科学技术研究院^[177]的研究人员通过方波信号抖动主动相位控制和基于受激布里渊散射被动相位控制的方法,实现了2路板条固体激光和4路高重复频率固体激光相干合成。2018年,中国科学院半导体研究所的科研人员基于级联Michelson复合腔的方法实现3束Nd∶YAG激光相干合成,合成前单束激光的功率约为65 W、光束质量因子M²约为5.5,合成后的激光输出功率为124.4 W、光束质量因子约为1.36^[181]。脉冲激光相干合成方面,2013年,原武汉军械士官学校的研究人员运用角锥作为合成元件,实现了6束脉冲固体激光相干合成,总输出能量为15.3 J、输出激光重复频率为10 Hz、合成效率达95.6%,实验系统如图6所示^[176]。2014年,德国耶拿大学的研究人员实现两路Yb∶YAG单晶棒状固体激光放大器相干合成,合成后的光束能量为3 mJ、峰值功率为3.7 GW、脉宽为695 fs,合成效率达94%^[178]。2019年,中国科学院上海光学精密机械研究所的科研人员首次实现了两路钛宝石啁啾脉冲激光相干合成,合成激光的重复频率为1 Hz,合成效率达90%^[179]。固体激光是超强激光的重要实现方式,对于高重复频率高平均功率超强激光和低重复频率超高峰值功率超强激光系统而言,相干合成都是重要发展方向^[182]。

图 6. 6路固体激光相干合成系统^[176]

Fig. 6. Schematic setup of coherent beam combining of 6 solid state lasers^[176]

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4.3 光纤激光相干合成

近年来,向大阵元数目扩展是光纤激光相干合成的重要发展方向之一。法国Thales研究所、美国Dayton大学、国防科技大学、中国工程物理研究院等单位都实现了数十路光纤激光的相干合成。2011年法国Thales研究所利用剪切干涉技术实现了64路光纤激光的相干合成^[74]。2014年,国防科技大学利用并行梯度下降(SPGD)算法实现了32路光纤激光的相位锁定^[82]。2016年,美国Dayton大学实现了21路光纤激光的目标在回路相干合成(图7)^[83];中国工程物理研究院利用方波扰动算法实现了30路光纤激光的相干合成^[94]。2019年,国防科技大学利用SPGD算法实现了60路光纤激光的相干合成^[85]。2020年,法国Thales研究所基于参考光干涉技术实现了61路飞秒光纤激光的相干合成^[183]。国防科技大学实现了107路光纤激光相干合成,为目前公开报道的光纤激光相干合成最高路数^[184]。

图 7. 21路光纤激光的目标在回路相干合成实验结构^[83]

Fig. 7. Schematic setup of target-in-the-loop coherent beam combining of 21 fiber lasers^[83]

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高功率系统论证是相干合成光源走向实用的关键。2011年,国防科技大学基于单抖动算法实现了9路1.08 kW光纤激光相干合成,首次突破了千瓦输出功率^[92];同年,中国科学院上海光学精密机械研究所采用单模光纤滤波环形腔实现了8路1.09 kW总功率光纤激光相干合成^[70];美国空军实验室利用多抖动算法实现了16路输出功率为1.4 kW的相干合成;美国林肯实验室基于SPGD算法实现了8路光纤放大器相干合成,总输出功率为4 kW^[78]。2014年,美国Northrop Grumman公司基于DOE实现了3路2.4 kW总功率的共孔径相干合成,合成效率80%^[80]。2016年,美国空军实验室基于DOE实现了5路千瓦级光纤放大器的相干合成,总输出功率4.9 kW,合成效率82%^[89]。2017年,国防科技大学实现了4路5.02 kW总功率的相干偏振合成,合成效率93.8% ^[93]。2019年,国防科技大学实现了7路光纤激光的相干合成,总输出功率为8 kW量级^[86],系统在开环和闭环时的远场光斑如图8所示。2020年,以色列Civan公司实现了37路输出功率为16 kW的光纤激光相干合成,为光纤激光相干合成公开报道的最高功率^[153]。

图 8. 8 kW级高占空比相干合成系统的远场光斑曝光图样^[86]。(a)开环;(b)闭环

Fig. 8. Far-field spot exposure patterns of coherent beam combining system with 8 kW level high duty cycle^[86]. (a) Open loop; (b) closed loop

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除了以上两类代表性成果之外,相干合成技术还被大量应用于2 μm波段光纤激光的相干合成中^[185-189]。2013年,捷克科学院等单位的科研人员采用被动相位控制的方法实现了2路激光相干合成、总输出功率为20 W^[186]。2014年,国防科技大学课题组采用主动相位控制方法实现2路单频光纤放大器相干合成,总输出功率突破百瓦级^[187]。2015年,德国耶拿大学等单位科研人员实现了2路超短脉冲掺铥光纤放大器相干合成,合成光束的峰值功率达25 MW、脉宽为830 fs,合成效率大于90%^[188]。2018年,澳大利亚**科技研究组等单位的科研人员采用主动相位控制的方法实现了4路掺钬光纤放大器激光相干合成^[189]。近年来,2 μm波段光纤激光取得了显著进展,全光纤结构单频掺铥激光输出功率突破了300 W^[190]、掺钬光纤激光输出功率突破了407 W^[191];若与相干合成技术结合,有望实现更高性能的2 μm光纤激光系统。

4.4 超快激光相干合成

随着先进制造等领域对高功率超快激光需求的不断提升,超快激光相干合成已成为近十年来的研究热点之一,德、法、美、中等国的研究人员开展了深入研究,其中德国耶拿大学的研究最具有代表性。该单位研究人员于2011年实现了2路飞秒脉冲相干合成,平均功率为30 W、脉冲能量为3 mJ、峰值功率为5.4 GW、合成效率为89%^[73];2013年实现了4路飞秒脉冲相干合成,平均功率为530 W、脉冲能量为1.3 mJ、峰值功率为1.8 GW、合成效率为93%^[156];2014年实现了4路平均功率为230 W、脉冲能量为5.7 mJ、峰值功率为22 GW、合成效率为88%的相干合成^[192];2016年将合成路数扩展到8路,在重复频率为996 kHz的条件下,平均功率达到1 kW^[158];2017年将合成路数进一步扩展到16路,实现了平均功率为1.83 kW、脉冲能量为2.3 mJ、合成效率为82%的相干合成^[72]。2018年,他们采用分光镜作为合束器件,实现了4路、平均功率为3.5 kW的飞秒脉冲相干合成^[90];2020年,他们将路数扩展到12路,实现了平均功率为10.4 kW的飞秒脉冲相干合成(图9)^[91],这在飞秒激光领域是一件具有里程碑意义的重要事件。

图 9. 10.4 kW平均功率飞秒脉冲光纤激光相干合成实验结构和结果^[91]

Fig. 9. Experimental setup and results for coherent beam combining of femtosecond pulsed fiber lasers with 10.4 kWaverage power^[91]

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法国Thales研究所近年来也持续开展了飞秒脉冲相干合成研究,他们采用参考干涉法相位控制、微透镜阵列光束合成的技术方案,2017年实现了19路飞秒脉冲相干合成的实验验证^[117];2018年实现了7路飞秒脉冲相干合成,平均功率为71 W、脉冲宽度为216 fs^[193];2020年,实现了61路飞秒脉冲相干合成的实验验证^[95],实验结构如图10所示。此外,法国南巴黎大学^[194]、美国密歇根大学^[195]、俄罗斯科学院^[196-197]等单位也开展了相关研究,合成路数一般为2~4路。

图 10. 61路飞秒脉冲光纤激光相干合成实验结构^[95]

Fig. 10. Schematic setup of coherent beam combining of 61 femtosecond pulsed fiber lasers^[95]

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国内开展飞秒脉冲相干合成的单位主要有国防科技大学、上海理工大学、中国科学院上海光学精密机械研究所、天津大学和中国工程物理研究院等单位。2016年,国防科技大学实现了2路飞秒脉冲光纤激光的相干偏振合成,平均功率为313 W^[198];上海理工大学实现了 2路、30.4 W平均功率的飞秒脉冲相干偏振合成^[199];中国工程物理研究院实现了2路飞秒固体激光的相干合成^[200];2017年,中国科学院上海光学精密机械研究所实现了2路100 Hz重复频率的飞秒固体激光的相干合成^[201]。

随着相干合成使能技术的进步,飞秒脉冲光纤激光相干合成已经从传统的空域合成向相干时域合成和相干光谱合成发展^[3,202]。相干时域合成是对高重复频率的脉冲序列进行功率放大后,再通过时序合成,降低激光的重复频率,提升输出激光的峰值功率。相干光谱合成是为了克服单路光纤激光的增益带宽限制以及脉冲放大过程中的增益窄化效应,通过对不同频谱的激光进行相干合成,实现宽光谱、窄脉宽的激光输出。

相干时域合成主要有脉冲分割放大(DPA)和环形腔脉冲堆叠两种技术途径。德国耶拿大学^[203]、法国南巴黎大学^[204]、美国Cornell大学^[205]等单位开展了DPA技术研究。2016年,德国耶拿大学在8路相干合成的基础上再进行了4脉冲分割放大,获得了平均功率为700 W、脉冲能量为12 mJ、脉冲宽度为262 fs的激光输出,合成效率为78%^[206];2019年,他们采用12路相干合成、8脉冲分割放大,实现了23 mJ的飞秒脉冲输出^[207]。奥地利维也纳理工大学^[208]、美国密歇根大学^[209]等单位采用吉莱-图努瓦干涉仪共振腔(GTI)的技术方案。美国密歇根大学于2015年验证了5个脉冲的GTI脉冲堆叠^[209],并于2017年实现了81个脉冲的堆叠,获得了10 mJ的脉冲能量^[210]。德国耶拿大学开展了基于“堆叠-导出腔”(SnD,全称stack-and-dump)的脉冲堆叠研究^[9],于2016年实现了65个脉冲的堆叠^[211]。

相干光谱合成主要有两种技术方案,一是采用多个放大器对同一种子激光的不同谱段进行放大,再进行相干光谱合成,二是直接对多个锁模激光器进行相位锁定。采用第一种技术方案,2011年,美国麻省理工学院对2路光参量啁啾脉冲放大器(OPCPA)进行了相干光谱合成,合成脉冲时间抖动小于250 as^[212];2013年,法国南巴黎大学实现了2路光纤放大器的相干光谱合成^[213],美国密歇根大学通过对3路光纤放大器进行相干光谱合成,使脉宽相对于合成前压窄了1/2~1/3^[214],法国利摩日大学通过12芯光纤放大器的相干光谱合成,使脉冲宽度仅从种子激光的228 fs展宽到放大后的280 fs^[215];2014年,德国电子同步加速器研究所(DESY)通过对三路光参量放大器(OPA)的相干光谱合成,获得了脉冲宽度1.9 fs的超短脉冲激光^[216]。采用第二种技术方案,2012年,美国麻省理工学院通过对一路Ti:蓝宝石激光器和一路光纤超连续谱激光器进行相干光谱合成,获得了3.7 fs的超短脉冲激光^[217];2016年,天津大学实现了对两台飞秒激光器的脉冲序列与载波包络相位同步^[218],两路激光相干光谱合成后的光谱干涉图对比度约为58%^[219]。值得注意的是,近年来固体介质(如薄片)的超快激光技术也取得了迅速进展,若与相干合成技术结合,有望进一步提升系统效能。

4.5 变频激光相干合成

变频激光相干合成是获得特殊波长或者极端光场的有效技术途径,目前已有来自美国、俄罗斯、法国、中国等多个国家的科研人员开展相关研究工作^[220-223]。俄罗斯规划用于极端光学研究的艾瓦中心(XCELS)拟实现200 PW 峰值功率,待建激光装置包含12束功率为15 PW、脉冲宽度为25 fs 超强激光,利用相干合成技术来输出200 PW强光^[10]。2014年,科研人员实现了2路激光相干合成,实验结构和实验结果图11所示^[197],通过2束变频激光的相干合成获得了100 mJ脉冲能量、23 fs脉宽的强激光输出,合成效率达95%。同年,法国科研人员报道了通过基频光相位控制实现变频光相干合成的实验结果^[221]:实验中两束连续波1.55 μm的光纤激光通过周期极化铌酸锂(PPLN)晶体倍频至775 nm,运用电光调制器(EOM)控制其中1束1.55 μm基频光的相位,实现了两路775 nm激光的相位控制,残差约为λ/19.5。

图 11. 两束变频激光的相干合成实验结构和实验结果^[197]

Fig. 11. Experimental setup and results of coherent beam combining of two variable frequency lasers^[197]

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5 应用领域不断拓展

5.1 非线性频率变换

非线性频率变换技术是实现高功率紫外、可见光、中红外等波段激光输出的典型技术手段^[224-226],相干合成技术是解决非线性频率变换系统泵浦亮度的重要技术途径。2017年,立陶宛Center for Physical Sciences & Technology的科研人员报道了4路合成光纤激光在三硼酸钡(LBO)晶体中的频率上转换实验结果,获得了平均功率为29 W的532 nm激光输出^[227-228],转换效率为51%。同年,日本Osaka大学的科研人员报道了基于相干合成光纤激光泵浦的高功率亚纳秒脉冲激光系统,实验结构如图12所示^[229];通过8束激光合成(2套相干合成模块+偏振合成)实现了千瓦级1040 nm基频脉冲激光输出,重复频率为10 MHz、脉宽为285 ps;经过频率变换系统,获得了平均功率为600 W的520 nm激光输出(二次谐波)和平均功率为300 W的347 nm激光输出(三次谐波);对于520 nm激光和347 nm激光而言,上述平均功率都是非常先进的技术指标。

图 12. 600 W绿光和300 W紫外光产生系统的实验结构与输出功率^[229]

Fig. 12. Experimental setup and output power for the 600 W green and 300 W ultraviolet light system^[229]

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2019年,法国 Paris Saclay 大学等单位的科研人员报道了3束相干合成激光频率变换的结果^[230]。通过3路锥形半导体激光放大器,实现了对976 nm单频激光的放大;再通过主动相位控制的方法实现3路激光相干合成,总输出功率为12.9 W、输出功率大于65%。相干合成光束通过周期极化PPLN晶体实现二次谐波变换,最终获得输出功率为2 W的488 nm激光输出。

5.2 雷达与导星

光纤激光具有热管理方便、结构紧凑等优势,是钠导星、激光雷达等应用领域的理想光源。但是,钠导星、相干雷达等需要激光具有较窄的线宽,由于受激布里渊散射等非线性效应的限制,单路光纤激光的功率通常难以满足应用需求。近年来,研究人员利用相干合成技术,将光纤激光的应用范围拓展到了这些领域。2010年,欧洲南方天文台对3路1178 nm的单频拉曼放大器进行相干合成后再进行倍频,获得了50 W的589 nm钠黄光输出,成功应用于激光导星^[231]。2013年,美国罗切斯特大学成功将相干合成技术运用到多通道光纤慢光雷达系统中,使其具备了二维扫描能力^[232]。2015年,法国航空实验室对两路单频纳秒脉冲进行相干合成,将其应用到测风雷达系统中,使激光雷达的精度和量程都得到了大幅提升(图13)^[233]。

图 13. 两路纳秒脉冲光纤激光相干合成的实验结构与实验结果^[233]

Fig. 13. Experimental setup and result for coherent beam combining of two nanosecond pulsed fiber lasers ^[233]

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5.3 光场调控

近年来,随着激光技术的迅速发展,振幅、相位、偏振态及相干结构等具有特殊空间分布的结构光场相继提出,这些结构光场展现出新颖的物理效应和现象,拓展了激光技术的应用,在基础研究和工程应用等方面均引起了国内外研究人员的广泛关注,成为了光学与光电子学领域的研究热点^[234-235]。新型的结构光场通过光场调控产生,主要包括:对光场的相位空间分布进行调控,可以产生具有螺旋波前结构、携带轨道角动量(OAM)的涡旋光束;对光场的偏振态空间分布进行调控,可以产生具有奇异聚焦特性的矢量光束;对光场的振幅和相位空间分布进行联合调控,可以产生具有无衍射传输特性和自加速、自聚焦特性的艾里光束等。通过阵列光场调控是一种实现结构光场的重要方法^[236],近年来取得了一系列重要进展,产生了涡旋光束、贝塞尔高斯光束、矢量光束和涡旋阵列光束等^[237-244],并对大气传输特性进行了研究^[245]。图14(a)、(b)分别为阵列光束产生OAM光束的概念示意图,图14(c)、(d)分别为6束激光产生OAM光束的实验结果。

图 14. 相干合成产生OAM光束的原理与实验结果^[243]。(a)(b)阵列光束产生OAM光束的概念示意图;(c)(d) 6束激光产生OAM光束的实验结果

Fig. 14. Principle and experimental results for producing OAM beams by coherent beam combining^[243]. (a)(b) Schematic diagram of OAM beam produced by array beam; (c)(d) experimental results of OAM beam produced by 6 lasers

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5.4 激光通信

相比传统的微波通信波段,激光波段频率高、波长短,具有载波频带宽、指向性好、抗电磁干扰能力强等优势,以及广阔的应用前景。但是,上述特点也给激光通信技术的发展带来一定挑战。

首先,大气湍流会对相干激光通信的性能产生影响。为了缓解大气湍流引起的深度衰减、探测效率下降,可以在接收望远镜中加入自适应光学系统。另一条有效的途径就是采用多口径接收,西班牙Catalonia大学^[246-247]、美国林肯实验室^[248-249]和中国科学院光电技术研究所^[250-251]等单位开展了相关研究。多口径接收技术能在不增加单个接收望远镜口径的前提下提升总接收功率,还能通过数字合成^[246-249]或者光学合成^[250]的方式缓解大气湍流的影响。图15为中国科学院光电技术研究所研制的7单元阵列激光传输控制实验中激光发射端和回光接收端的实物照片^[113]。

图 15. 7单元阵列激光传输控制实验中激光发射端和回光接收端的实物照片^[113]

Fig. 15. Real photo of laser beam transmitter and receiver in the 7-channel laser-array transmission and control experiment^[113]

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其次,需要配备高精度的跟瞄发射系统来实现激光发射。采用阵列光束相干合成发射是一条有效的技术途径,长春理工大学^[252-253]、中国科学院光电技术研究所^[251]等单位开展了相关研究。采用阵列光束发射具有许多优点:1)通过增加激光路数来增加发射功率,同时增加了空间传输距离;2)通过相位控制实现高精度、无机械式的光束扫描^[252,254];3)通过多载波调制^[253],增加了系统的传输效能。

5.5 大科学装置

激光发明60年来,研究人员不断追求着更高的激光亮度。强激光为人类提供了前所未有的实验手段和极端物理条件,激光大科学装置成为科学技术发展的重要标志^[255]。随着国民经济和科学技术的发展,我国目前已经建成多个指标先进的激光大科学装置^[256-260]。光纤激光以其高效率、易维护、可柔性操作等优点,在工业等领域得到广泛应用。近年来,基于光纤激光的大科学装置也受到广泛关注。例如,法国的研究人员为了应对LIGO对数百瓦低噪声单频激光的需求,演示了两路40 W单频光纤激光的长时间、低噪声相干合成实验,论证了基于光纤激光相干合成为LIGO提供光源的可行性^[159]。

国际相干放大网络工程(简称ICAN)是光纤激光大科学装置计划的典型代表。该计划由诺贝尔物理学奖获得者Mourou教授领导,其目标是对数以万计的超短脉冲进行相干合成,为下一代粒子加速器提供驱动源^{[8-9,261-264]},如图16所示。如此高功率的光源,在激光推进^[265]、粒子束产生^[266]、医疗^[267]、原子能^[268]、激光雷达和空间碎片清理^[269]等方面也有广阔的应用前景。由于经费原因,目前启动了一个由巴黎综合理工学院牵头的XCAN项目,初步完成了61路光纤激光合成的技术验证实验^[95,270]。

图 16. 相干放大网络原理图^[8]

Fig. 16. Principle of a coherent amplifier network^[8]

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6 结束语

相干合成是21世纪以来激光技术领域的研究前沿和热点。近10年来,随着可合成激光模块的性能提升和相干合成使能技术发展,激光相干合成领域取得了以12路10.4 kW超快光纤激光和107路大阵元数目光纤激光相干合成为典型代表的标志性成果,并在激光导星、激光雷达、激光变频等方面得到了成功应用,进一步推进了激光器及相关领域的技术进步。目前,在相干合成单元技术和连续光纤激光相干合成方面,国内外发展水平相当;对于超快激光、固体激光和半导体激光的相干合成,国内在系统指标方面与国外还存在一定差距。从发展趋势来看,全电驱动的可合成固态激光将继续向更高功率、更高效率、更轻重量和更小体积发展;相干合成技术将从阵元数目和单路激光功率等方面继续提升系统指标;且将拓展至任意增益介质和任意波段(可见光、中红外甚至太赫兹)的激光器,从而提升各种类型激光系统的整体性能;此外,随着计算技术和计算能力的飞速进步,将人工智能相关技术引入相位控制、倾斜控制等相干合成使能技术部分,有望成量级地提升控制能力,使得大阵元多参量高速高精度控制变为可能,从而推动光频段的相控阵——激光相控阵逐步走向现实。

致谢感谢博士研究生侯天悦、常琦对本文提出的意见建议。

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周朴, 粟荣涛, 马阎星, 马鹏飞, 吴坚, 李灿, 姜曼. 激光相干合成的研究进展:2011—2020[J]. 中国激光, 2021, 48(4): 0401003. Pu Zhou, Rongtao Su, Yanxing Ma, Pengfei Ma, Jian Wu, Can Li, Man Jiang. Review of Coherent Laser Beam Combining Research Progress in the Past Decade[J]. Chinese Journal of Lasers, 2021, 48(4): 0401003.

激光相干合成的研究进展:2011—2020 下载： 2828次封底文章特邀综述

1 引言

2 可合成激光模块的性能不断提升

2.1 光纤激光

2.2 固体激光

2.3 半导体激光

3 相干合成使能技术不断发展

图 1. 主动相位控制相干合成系统的结构示意图

Fig. 1. Schematic setup of active phase control coherent beam combining system

3.1 相位控制技术

表 1. 相干合成主动相位控制代表性研究成果

Table 1. Representative research results of active phase control coherent beam combining

3.2 倾斜控制技术

表 2. 基于AFOC的相干合成倾斜控制代表性成果

Table 2. Representative research results of AFOC-based tilt control in coherent beam combining

3.3 偏振控制技术

图 2. kW级窄线宽光纤放大器中的偏振控制系统结构[129]

Fig. 2. Schematic diagram of polarization control system for narrow linewidth fiber amplifier[129]

3.4 光程控制技术

图 3. 高精度光程差实时控制系统[139]

Fig. 3. High precision optical path difference real-time control system[139]

3.5 高阶像差控制技术

图 4. 大功率离焦补偿型准直系统。(a) 工作原理[142];(b) 实验结果[143]

Fig. 4. High power defocus compensation collimation system. (a) Principle schematic[142]; (b) experimental results[143]

3.6 孔径填充技术

表 3. 孔径填充的代表性研究成果

Table 3. Representative research results of aperture-filling

3.7 小结

4 相干合成研究成果不断涌现

4.1 半导体激光相干合成

图 5. 5束半导体激光相干合成结构示意图[162]

Fig. 5. Schematic setup of coherent beam combining of five diode lasers [162]

4.2 固体激光相干合成

图 6. 6路固体激光相干合成系统[176]

Fig. 6. Schematic setup of coherent beam combining of 6 solid state lasers[176]

4.3 光纤激光相干合成

图 7. 21路光纤激光的目标在回路相干合成实验结构[83]

Fig. 7. Schematic setup of target-in-the-loop coherent beam combining of 21 fiber lasers[83]

图 8. 8 kW级高占空比相干合成系统的远场光斑曝光图样[86]。(a)开环;(b)闭环

Fig. 8. Far-field spot exposure patterns of coherent beam combining system with 8 kW level high duty cycle[86]. (a) Open loop; (b) closed loop

4.4 超快激光相干合成

图 9. 10.4 kW平均功率飞秒脉冲光纤激光相干合成实验结构和结果[91]

Fig. 9. Experimental setup and results for coherent beam combining of femtosecond pulsed fiber lasers with 10.4 kWaverage power[91]

图 10. 61路飞秒脉冲光纤激光相干合成实验结构[95]

Fig. 10. Schematic setup of coherent beam combining of 61 femtosecond pulsed fiber lasers[95]

4.5 变频激光相干合成

图 11. 两束变频激光的相干合成实验结构和实验结果[197]

Fig. 11. Experimental setup and results of coherent beam combining of two variable frequency lasers[197]

5 应用领域不断拓展

5.1 非线性频率变换

图 12. 600 W绿光和300 W紫外光产生系统的实验结构与输出功率[229]

Fig. 12. Experimental setup and output power for the 600 W green and 300 W ultraviolet light system[229]

5.2 雷达与导星

图 13. 两路纳秒脉冲光纤激光相干合成的实验结构与实验结果[233]

Fig. 13. Experimental setup and result for coherent beam combining of two nanosecond pulsed fiber lasers [233]

5.3 光场调控

图 14. 相干合成产生OAM光束的原理与实验结果[243]。(a)(b)阵列光束产生OAM光束的概念示意图;(c)(d) 6束激光产生OAM光束的实验结果

Fig. 14. Principle and experimental results for producing OAM beams by coherent beam combining[243]. (a)(b) Schematic diagram of OAM beam produced by array beam; (c)(d) experimental results of OAM beam produced by 6 lasers

5.4 激光通信

图 15. 7单元阵列激光传输控制实验中激光发射端和回光接收端的实物照片[113]

Fig. 15. Real photo of laser beam transmitter and receiver in the 7-channel laser-array transmission and control experiment[113]

5.5 大科学装置

图 16. 相干放大网络原理图[8]

Fig. 16. Principle of a coherent amplifier network[8]

6 结束语

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图 2. kW级窄线宽光纤放大器中的偏振控制系统结构^[129]

Fig. 2. Schematic diagram of polarization control system for narrow linewidth fiber amplifier^[129]

图 3. 高精度光程差实时控制系统^[139]

Fig. 3. High precision optical path difference real-time control system^[139]

图 4. 大功率离焦补偿型准直系统。(a) 工作原理^[142];(b) 实验结果^[143]

Fig. 4. High power defocus compensation collimation system. (a) Principle schematic^[142]; (b) experimental results^[143]

图 5. 5束半导体激光相干合成结构示意图^[162]

Fig. 5. Schematic setup of coherent beam combining of five diode lasers ^[162]

图 6. 6路固体激光相干合成系统^[176]

Fig. 6. Schematic setup of coherent beam combining of 6 solid state lasers^[176]

图 7. 21路光纤激光的目标在回路相干合成实验结构^[83]

Fig. 7. Schematic setup of target-in-the-loop coherent beam combining of 21 fiber lasers^[83]

图 8. 8 kW级高占空比相干合成系统的远场光斑曝光图样^[86]。(a)开环;(b)闭环

Fig. 8. Far-field spot exposure patterns of coherent beam combining system with 8 kW level high duty cycle^[86]. (a) Open loop; (b) closed loop

图 9. 10.4 kW平均功率飞秒脉冲光纤激光相干合成实验结构和结果^[91]

Fig. 9. Experimental setup and results for coherent beam combining of femtosecond pulsed fiber lasers with 10.4 kWaverage power^[91]

图 10. 61路飞秒脉冲光纤激光相干合成实验结构^[95]

Fig. 10. Schematic setup of coherent beam combining of 61 femtosecond pulsed fiber lasers^[95]

图 11. 两束变频激光的相干合成实验结构和实验结果^[197]

Fig. 11. Experimental setup and results of coherent beam combining of two variable frequency lasers^[197]

图 12. 600 W绿光和300 W紫外光产生系统的实验结构与输出功率^[229]

Fig. 12. Experimental setup and output power for the 600 W green and 300 W ultraviolet light system^[229]

图 13. 两路纳秒脉冲光纤激光相干合成的实验结构与实验结果^[233]

Fig. 13. Experimental setup and result for coherent beam combining of two nanosecond pulsed fiber lasers ^[233]

图 14. 相干合成产生OAM光束的原理与实验结果^[243]。(a)(b)阵列光束产生OAM光束的概念示意图;(c)(d) 6束激光产生OAM光束的实验结果

Fig. 14. Principle and experimental results for producing OAM beams by coherent beam combining^[243]. (a)(b) Schematic diagram of OAM beam produced by array beam; (c)(d) experimental results of OAM beam produced by 6 lasers

图 15. 7单元阵列激光传输控制实验中激光发射端和回光接收端的实物照片^[113]

Fig. 15. Real photo of laser beam transmitter and receiver in the 7-channel laser-array transmission and control experiment^[113]

图 16. 相干放大网络原理图^[8]

Fig. 16. Principle of a coherent amplifier network^[8]