二极管泵浦重复频率纳秒高能固体激光器研究进展

High Energy Diode-Pumped Rep-Rated Nanosecond Solid-State Laser

二极管泵浦重复频率纳秒高能固体激光器(主要指单脉冲能量10 J以上,脉冲重复频率10 Hz以上)在重大基础和应用研究领域中发挥了重要的作用,是科学研究的前沿热点之一。本文以叠片、激活镜、之字形板条三种放大构型为线索,详细介绍了二极管泵浦重复频率纳秒高能固体激光器的代表性成果和研究进展,分析了激光器的优选技术路线,并对未来的发展前景进行了展望。

Significance High-energy rep-rated nanosecond diode-pumped solid-state lasers(DPSSLs),mainly referring to nanosecond lasers with pulse energies greater than 10 Jand repetition rates greater than 10 Hz,are crucial in major fundamental and applied research domains and are emerging as one of the hot topics at the frontier of scientific research.This study first analyzes the preferred technical paths of high-energy rep-rated nanosecond DPSSLs in terms of the gain medium and amplifier geometry and then reviews the representative achievements and research progress of high-energy rep-rated nanosecond DPSSLs in detail.Furthermore,the prospects of future development of DPSSLs are discussed herein.Progress Favored for its moderate saturation fluence and high thermal conductivity,Yb∶YAG at the cryogenic temperature and Nd∶LuAG at room temperature have been proven to be the most promising gain media in achieving rep-rated nanosecond DPSSLs with even higher energy.Conversely,gain medium with high saturation fluence at room temperature,such as Yb∶YAG,and that with low saturation fluence,such as Nd∶YAG,is not suitable for high energy lasers primarily owing to the defect of high pump threshold from the quasi-three-level structure and high passive loss from too many stages.In addition,the three preferred amplifier geometries are the multislab,active mirror,and zigzag slab(Fig.1),as categorized by the representative achievements of high energy rep-rated nanosecond DPSSLs summarized in Table 2.For the multislab geometry,the Mercury system developed by Lawrence Livermore National Laboratory(LLNL)produced a nanosecond output with the pulse energy of 61 Jat the repetition rate of 10 Hz based on Yb∶S-FAP multislabs,with ultralow wavefront aberration using a new high-speed gas cooling technology at room temperature(Fig.2),a classical approach that was then widely used.Using similar gas cooling technology but operating at cryogenic temperature,the DiPOLE system based on Yb∶YAG ceramic achieved 105 J,10 Hz,and 10 ns in 2017,which was the world’s first demonstration of a kW-level high energy DPSSL(Fig.4).Researchers from STFC Rutherford Appleton Laboratory and HiLASE solved scientific and engineering problems in efficiency optimization,thermal effect management,depumping suppression,and other aspects.In the same year,LLNL reported the output level at 97 J,3.3 Hz of a nanosecond Nd∶glass multislab laser for pumping the petawatt-level HAPLS system,using high-power intelligent laser diode system(HILADS),the highest peak power and brightest pulsed diode light delivery system in the world(Fig.8).For the active mirror mode,the LUCIA system reached 13.9 Jat 2 Hz in 2013,using the Yb∶YAG laser head at room temperature at the pump intensity of 11 kW/cm2 by focusing on the mirror(Fig.12).To improve the energy above the 30 Jlevel,the researchers invented a static helium gas cooling technology and plan to use a cryogenically cooled cosintered Yb∶ YAG ceramic,which may suffer from much stronger thermal lensing and higher depolarization losses than the crystal counterpart.Total-reflection active-mirror(TRAM)and multi-TRAM structures have been proposed by researchers at Osaka University,which achieved 1 J,100 Hz laser amplification in 2015,despite unstable operation.Later this year,they released a new configuration of the conductive-cooled active-mirror amplifier(CcAMA)and reported the 9.3 J,33.3 Hz laser scaling,suppressing the wavefront distortion by an elaborate heat sink design(Fig.16).In 2016,our group at Tsinghua University demonstrated excellent compatibility of the Nd∶YAG seeder and Nd∶LuAG booster(Fig.17),and then proposed a new concept called distributed active mirror amplifier chain(DAMAC)to disperse the gain and thermal deposition among several gain modules,thus achieving in 2019 aroom temperature 10.3 J,10 Hz,10 ns laser from a large-aperture Nd∶YAG-Nd∶LuAG active mirror hybrid chain(Fig.18),its output was recently raised to 100 J,10 Hz at room temperature.For the zigzag slab design,Hamamatsu developed the HALNA system,which demonstrated an output of 21.3 J,10 Hz,8.9 ns in 2008,with an optical-optical efficiency of 11.7%.The beam quality was controlled as 1.8 times diffraction limit,combining a thermally edge-controlled zigzag slab(TECS)design(Fig.22)and a stimulated Brillouin scattering(SBS)mirror.In addition,the Chinese Academy of Sciences built an Nd∶YAG system in 2017(Fig.23),which generated pulse energy of 5 Jat 1064 nm with a pulse duration of 6.6 ns and a repetition rate of200 Hz,while the output energy stability was 4.9% peak-to-valley over 6000 shots.It was verified that the beam quality could be improved to 1.7 times the diffraction limit by an SBS mirror or by a deformable mirror.Conclusions and Prospects Over the past two decades,extensive efforts have been made into achieving the first milestone,that is,the output target of 100 J,10 Hz,and 10 ns,which has been achieved in the development of high energy rep-rated nanosecond DPSSLs with breakthroughs in both cryogenic and room temperature.In the next two decades,as new geometry,new gain medium,and new technical approach will inevitably emerge,the main trend expected will be the continuous upgrade in beamlet pulse energy(beyond kJ level),repetition rate(hundreds to kilohertz),and plug efficiency(over 20%),whereas potential directions of development may include system miniaturization,open and flexible access to other operating mechanisms,such as chirped pulse amplification,and programmable control over temporal,spatial,and frequency tuning.