Letter Vol. 45, No. 24 / 15 December 2020 /Optics Letters 6839

High-power, low-coherence laser driver facilityYanqi Gao,1,†,* Lailin Ji,1,† Xiaohui Zhao,1,† Yong Cui,1 Daxing Rao,1 Wei Feng,1Lan Xia,1 Dong Liu,1 Tao Wang,1 Haitao Shi,1 Fujian Li,1 Jia Liu,1 Du Pengyuan,1Xiaoli Li,1 Jiani Liu,1 Tianxiong Zhang,1 Chong Shan,1 Yilin Hua,1 Weixin Ma,1Zhan Sui,1 Jian Zhu,1 Wenbing Pei,1 Sizu Fu,1 Xun Sun,2 AND Xianfeng Chen31Shanghai Institute of Laser Plasma, China Academy of Engineering Physics, 1129Chenjiashan Road, Shanghai 201800, China2State Key Laboratory of Crystal Materials, ShandongUniversity, Jinan 250100, China3School of Physics and Astronomy, Shanghai Jiao TongUniversity, 800 Dongchuan Road, Shanghai 200240, China*Corresponding author: liufenggyq@siom.ac.cn

Received 13 October 2020; revised 22 November 2020; accepted 23 November 2020; posted 24 November 2020 (Doc. ID 412197);published 15 December 2020

We report the first (to the best of our knowledge) high-power, low-coherence Nd:glass laser delivering kilojoulepulses with a coherent time of 249 fs and a bandwidth of13 nm, achieving the 63%-efficiency second-harmonic con-version of the large-aperture low-coherence pulse and goodbeam smoothing effect. It provides a new type of laser driverfor laser plasma interaction and high energy density physicsresearch. ©2020Optical Society of America

https://doi.org/10.1364/OL.412197

Laser-plasma instability (LPI) is one of the most challengingissues that hinders the successful ignition of laser driven inertialconfinement fusion (ICF) [1]. To suppress the LPI when intenselaser transmits through a surrounding plasma, the operatingmode of laser drivers has evolved from near-infrared to ultravi-olet (third-harmonic wave), narrowband pulse to modulatedbroadband pulse, while various beam smoothing techniqueshave been developed. However, plenty of experiments andanalysis indicate that, the adverse effects of LPI are difficult toeffectively overcome under current laser driver conditions [2],and one of the major obstacles is the hot-electron preheat [3,4].After 2016, the possibility of low-coherence drivers (broadbandwidth and low phase correlation) has been extensively dis-cussed in the fields of laser plasma physics and laser technology[5–9]. It is expected to alleviate the problem of hot-electronpreheat by suppressing stimulated Raman scattering (SRS), andbecomes a strong competitor for next-generation laser drivers.

High-power low-coherence laser has attracted widespreadinterest. Many laboratories try to build laser facilities for thestudy of low-coherence light, such as the GEKKO XII in Japan[10], and the PHEBUS in France [11], the PHAROS-III inthe United States [12], and the KANAL-2 in Russia [13]. Theformer one utilizes an amplified spontaneous emission (ASE)source as the front end, and the latter three use a broadbandphosphate cavity oscillator. However, the coherence of thesesources is not low enough or the outputs have multilongitudinalmode structures. On the one hand, the small spectral bandwidth(1 nm) and the high correlation between spectral and phase

make it difficult to exert low-coherence characteristics andachieve the effect of suppressing LPI. On the other hand, it willlead to serious self-phase modulation and self-focusing effectsduring transmission and amplification, making it difficultto achieve high fluence and high intensity output. Previoustechnical approaches have barriers in four aspects: effectiveamplification, efficient frequency conversion, high fluence andhigh intensity output, and effective beam smoothing techniquesof low-coherent pulses. Recently, researchers have begun toexplore new technical approaches for low-coherence drivers.

Here, we demonstrate the results for high-power low-coherence laser named KUNWU which delivering 1-kJ,adjustable ns-level pulses with a coherent time of 249 fs anda bandwidth of 13 nm. The fluence is up to 5 J/cm2 corre-sponding to a intensity of 1.7 GW/cm2. The pulse waveformand spectral distribution can be independently and preciselyadjusted, showing an instantaneous broadband characteristic.Utilizing a large-aperture partially deuterated KDP crystal as thenonlinear medium, the second-harmonic generation (SHG)efficiency of low-coherence pulse can be up to 63%. The low-coherence smoothing method, which combines a continuousphase plate (CPP) with the induced spatial incoherence (ISI)is demonstrated, and a 14.7% rms nonuniformity of the focalspot is achieved. This facility successfully demonstrates thegeneration, amplification, transmission, nonlinear conversion,and system integration technologies of the broadband low-coherence laser driver. It has great potential in the field of ICFand high-energy density physics research.

The schematic of high-power low-coherence laser is shownin Fig. 1. The light source is an Yb-fiber pulse amplifier whichis seeded by a superluminescent diode (SLD) with a 50-nmbandwidth at the center wavelength of 1050 nm. A wave guideamplitude modulator (AM) and an arbitrary waveform gener-ator (AWG) are combined to chop continuous wave to pulsedlight at a repetition rate of 1 kHz, and adjust the pulse dura-tion, waveform, and repetition rate. To reduce the ASE in fiberamplifier (FA), a 25-nm band-pass filter and acousto-opticmodulators (AOMs) are introduced. A birefringent filter (BF)

0146-9592/20/246839-04 Journal © 2020Optical Society of America

mailto:liufenggyq@siom.ac.cnhttps://doi.org/10.1364/OL.412197https://crossmark.crossref.org/dialog/?doi=10.1364/OL.412197&domain=pdf&date_stamp=2020-12-15

6840 Vol. 45, No. 24 / 15 December 2020 /Optics Letters Letter

SLD AM

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Fig. 1. Schematic of the kilojoule low-coherence broadband Nd:glass laser facility: FE, frond end; SLD, superluminescent diode; AWG, arbitrarywaveform generator; AM, amplitude modulator; AOM, acoustic optical modulator; FA, fiber amplifier; BF, birefringent filter; PA, preamplifier; FR,Faraday rotator; Nd rod, Nd:glass rod; BE, beam expander; LCSM, liquid crystal spatial modulator; SF, spatial filter; MA, main amplifier; TR, targetrange; DM, dichroic mirrors; LA, lens array; ISI, induced spatial incoherence.

Chirped

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A B C DFig. 2. Distributions of spectrum versus time and phase versus spectrum for high-coherence pulse, such as A, narrowband laser; B, chirped andtransform-limited pulse; C, modulated pulse; and D, low-coherent pulse.

is employed for precise spectral shaping, and the spectral distri-bution can be modified to compensate for the gain narrowing ofthe following Nd:glass amplifiers. The typical output of frontend (FE) unit has 1 kHz repetition rate, 25 nm bandwidth,adjustable ns-level pulse duration, and µJ-level pulse energywith an instability less than 1% rms over 4 h [14]. The spectrumof laser pulse is smooth without longitudinal mode structure.Different from high-coherence laser pulses, such as narrowbandlaser, chirped pulse, transform-limited pulse, and the mod-ulated pulse, the spectral phase of the low-coherence pulse israndomly distributed, which can realize uncorrelated tunableoutput of pulse waveform and spectral shape, and the frequencycomponents have a wide distribution at any time within thepulse duration (shown in Fig. 2). The spectrum and temporalwaveform of the typical FE output are shown in Fig. 3.

Through a Pockels cell, the pulse train is converted to single-shot operation and injected into the preamplifier (PA) unit [15].First, the pulse is amplified by high-gain Nd:glass rod amplifiers,and its energy is amplified from microjoules to 10 mJ. Here,the traveling wave amplification method without cavity modeis used. It has sufficient gain bandwidth, and no spectral com-ponents are lost. Then, the pulse is near-field profile shaped bya liquid crystal spatial modulator (LCSM) and amplified bymultistage Nd:glass rod amplifiers delivering a output up to 35 Jin ns-level pulse duration with a bandwidth of 15 nm. The totalgain of PA reaches up to 3.5× 107. In such high gain amplifiers,the spectral narrowing is significant. Four double-pass birefrin-gent filters (BFs) are employed to ensure that the low-coherencepulses are amplified with a broadband flat-top spectrum gain.

And their transmission spectrum profiles are designed to matchthe inverse of low-coherence pulse gain curves. Compensatingthe spatial nonuniform gain by the LCSM, the spatial profile ofthe beam is close to a 12th-order super-Gaussian with a size of42 mm× 42 mm.

The main amplifier (MA) system adopts the “six-stage five-pass” configuration pumped by xenon flash-lamps, and thesize of each Nd:glass chip is 800 mm× 500 mm× 40 mm.Through the MA, low-coherent broadband pulse is amplifiedup to 1 kJ/3 ns with a spectral bandwidth of 13 nm (the injectedenergy by PA is 26 J). The output flux is 5 J/cm2, and the inten-sity is 1.7 GW/cm2. It has good near-field uniformity with asize of 160 mm× 160 mm (flux contrast is 0.09) and far-fieldfocusing characteristics [95% energy in 8.9 times diffractionlimit (DL)]. Due to the relatively long thermal recovery timeof large-aperture optical elements and the strict requirementsof the output beam quality, our laser facility is signal shot, andthe interval between two shots is about 1.5 h. In hundreds ofoperations, there is no beam quality degradation caused bysmall-scale self-focusing and spectral and temporal degradation.The spectral shape, pulse time waveform, and near-field distri-bution can be precisely controlled by the PFs, AWG, and SLCMin the FE and RA stage.

The frequency conversion process is one of the most impor-tant capabilities of the low coherence laser driver. Using alarge-aperture (200 mm× 200 mm) type-I cutting 15%DKDP crystal, which provides a theoretical acceptance band-width of 15 nm (at 95% peak) at the retracing point (1053 nm),a 63% conversion efficiency SHG has achieved. The thickness

Letter Vol. 45, No. 24 / 15 December 2020 /Optics Letters 6841

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Fig. 3. Output of the high-power low-coherence laser: A1, near-field and A2, far-field of SH; B1, spectra and B2, temporal waveforms of the FEoutput, FW and SH; C1, the time-resolved spectrum recorded by combining a streak camera with a spectrometer (the temporal resolution of streakcamera is 10 ps); C2, the calculated coherent time of the FE output, FW, and SH.

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Fig. 4. Experimental results of beam smoothing profile of A1, traditional coherent Nd:glass laser pulse and A2, low-coherence laser pulse; B1–C2are simulation results, and B1 and B2 are the corresponding speckles evolution characteristics over time. C1 and C2 are the corresponding specklesevolution along the longitudinal direction at a certain moment.

of the nonlinear crystal is 23 mm. Figures 3A1–3A2 show theSHG results of a 1000-J pulse. To the best of our knowledge,this is the first high-efficiency, large-bandwidth SHG of low-coherence pulse based on a large-aperture low-deuterium-dopedKDP crystal. In experiments, it was found that the crystal hasspatial nonuniformity, which affects the further improvementof SHG efficiency. Experiments by small-aperture crystal [16]show that this scheme has the potential to achieve an 80%conversion efficiency. The spectrum of SH has a triangularprofile, and the full width at half-maximum (FWHM) is 3.2 nm

(Fig. 3B1). For low-coherence pulse, the SH spectrum intensitydistribution is proportional to the self-convolution of the fun-damental wave (FW) spectrum, which is the result of randomphase averages. It is expressed as I2()∝ I1(ω)⊗ I1(ω), whereI2() and I1(ω) are the spectrum distributions of SH and FW.For coherent broadband light (transform-limited pulse), theconvolution relationship is between the electric field of SH andFW. The relationship in frequency conversion of coherent lightis about the electric field but appears as a convolution of inten-sity in that of the low-coherence light. The temporal waveform

6842 Vol. 45, No. 24 / 15 December 2020 /Optics Letters Letter

of SH has the same profile with that of FW, corresponding toa pulse duration of 3 ns, as shown in Fig. 3B2. Using a streakcamera combined with a spectrometer, the time-resolved spec-trum of SH was recorded (Fig. 3C1), which demonstrate aninstantaneous broadband characteristics. At any time within thepulse duration, the spectrum contains all the frequency compo-nents, as shown as the white curves in Fig. 3C1. The coherencetime of SH is 272 fs, which is basically equivalent to that of theFW, as shown in Fig. 3C2. Here, the coherence time of SHGwas calculated by the Fourier transformation of SH spectrum,according to the equation0(τ)= |F−1{I2()}|, where0(τ) isthe contrast function. Theoretically, the coherence time of thelight pulse (first-order coherence) only depends on its spectrumwidth and distribution.

Considering the low-temporal coherence characteristics ofthe laser pulse, a scheme combining a CPP with the inducedspatial incoherence (ISI) method is introduced to obtain asmoothing target spot with a diameter of 200 µm. The designand processing target of CPP is a flat top focal spot with a diam-eter of 200µm. As a transmissive echelon plate, ISI has a divisionnumber of 8× 8 and a step depth of 250µm. The experimentalspatial distributions of the target spots of traditional coherentNd:glass laser pulse and low-coherence laser pulse are shownin Fig. 4, which demonstrates a efficient smoothing effect forthe low-coherence pulse. The rms nonuniformity is reducedfrom 40.7% to 14.7%. As the speckle patterns of the targetspot will randomly reconstruct after every coherent time, asshown in Fig. 4B2, this smoothing scheme has a fast responsetime. For coherent light, the longitudinal scale of speckles isproportional to the square of the f-number, which is the ratioof the focal length to the beam aperture (Fig. 4C1). However,for low-coherence pulse, it is determined by the f-numberand coherence length (Fig. 4C2). Thus, the smoothed focalspot of low-coherence pulse has a shorter longitudinal specklelength, and it is expected to have a positive impact on LPIsuppression.

In conclusion, the generation of 1 kJ, ns-level shaping pulseswith a bandwidth of 13 nm by the high-power low-coherenceNd:glass laser driver facility is demonstrated. The whole systemof our laser facility has delivered hundreds of high-power pulseswithout damage, showing a good beam controllability andsystem stability. To the best of our knowledge, this is the world’sfirst demonstration of a kJ-level broadband low-coherenceNd:glass laser facility. Furthermore, the low-coherence SHGusing a 200-mm-aperture 15% DKDP crystal with an efficiencyup to 63% has been first realized. Based on the improvement ofcrystal quality, this method has the potential to achieve an 80%conversion efficiency. With the smoothing scheme combiningCPP with ISI method, the target spot has a good uniformity anda fast smoothing time. It provides a new experimental platformfor laser-plasma interaction and high-energy density physicsresearch.

Funding. Science Challenge Project (TZ2016005);National Natural Science Foundation of China (11804321).

Acknowledgment. The authors thank Xiuguang Huang,Wei Wang, Zhiheng Fang, Zhiyu He, Guo Jia, and ChenWang for their help in measurement, and Academician WeiyanZhang, Xiaomin Zhang, Guixue Huang of CAEP, WanguoZheng, Yaping Dai, Xiaofeng Wei, Dong Yang of Laser FusionResearch Center CAEP, Shaoping Zhu, Yongkun Ding, Ke Lan,Shiyang Zhou, Hongbo Cai, Liang Hao of Institute of AppliedPhysics and Computational Mathematics CAEP, BaoqiangZhu, Xinqiang Lu of Shanghai Institute of Optics and FineMechanics CAS, and Wei Zou of Institute of Automation CASfor fruitful discussions.

Disclosures. The authors declare no conflicts of interest.

†These authors contributed equally to this Letter.

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