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目前,许多研究小组通过OPCPA技术实现了中心波长位于2~4 μm,脉冲能量高达毫焦量级的高能量、少周期激光源。表1总结了部分2~4 μm少周期OPCPA系统的主要相关参数并且选择了其中一些典型的工作进行详细的阐述。
表 1 2~4 μm OPCPA系统相关参数
Table 1. Relevant parameters of 2-4 μm OPCPA system
Wavelength/μm Energy/mJ Repetition rate/kHz Average power/W Duration/fs Optical cycle Reference 2.1 1.2 3 3.6 10.5 1.5 [7] 2.2 0.25 100 25 16.5 2.2 [8] 3 0.3 10 3 21 2.1 [9] 3 2.4 10 24 50 5 [10] 3.1 0.125 100 12.5 73 7 [11] 3.2 0.152 100 15.2 38 3.6 [12] 3.25 0.06 160 9.6 14.5 1.35 [13] 3.9 8 0.02 0.16 83 6.4 [14] 4 2.6 0.1 0.26 21.5 1.6 [15] 3.3 31 1 31 66 6 [16] 2.8 0.52 1 0.52 27 2.89 [17] 3.2 5.8 1 5.8 20 2 [18] 2.1 2.7 10 27 30 4.3 [19] 3.3 13.3 1 13.3 111 10 [20] 在早期工作中,F.Krausz的研究小组同时使用宽带的钛宝石激光(振荡器和放大器)作为信号和泵浦光[7]。如图1所示,将从钛宝石振荡器输出的宽带光谱中提取的1030 nm光谱成分入射到Yb:YAG薄片放大器中,从而获得与信号光束同步的泵浦光。采用啁啾周期极化铌酸锂晶体(PPLN),通过对钛宝石放大器的光谱展宽脉冲进行差频混频,产生放大阶段所需的超宽光谱种子。然后,分别在两块PPLN晶体和一块LiNbO3晶体中利用非共线光学参量放大技术(NOPAs)进行放大。最后,产生了重复频率为3 kHz,中心波长为2.1 μm的中红外输出,其脉冲能量为1.2 mJ,脉冲宽度为10.5 fs (1.5周期)。
图 1 (a) 2.1 μm少周期OPCPA系统的示意图;(b)测量(蓝色)和恢复(红色)的光谱强度和相位(黑色虚线),以及(c)测量的时间强度和相位。插图:第三阶段后测量的空间强度剖面[7]
Figure 1. (a) Schematic of 2.1 μm few-cycle OPCPA system; (b) Measured (blue) and retrieved (red) spectral intensity and phase (dashed black), and (c) measured temporal intensity and phase. Inset: measured spatial intensity profile after the third stage[7]
2020年,U. Keller的研究小组提出了一个中心波长为2.2 μm的OPCPA系统,在100 kHz的重复频率下,产生了脉宽为16.5 fs (2.2周期)的脉冲,平均功率25 W[8]。如图2所示,利用BBO晶体将钛宝石振荡器输出的种子光进行参量放大。随后,在另一个BBO晶体中通过DFG机制产生闲频光。基于PPLN晶体的连续三级NOPA系统将闲频光放大到300 μJ。最后,通过压缩器将脉冲压缩到16.5 fs,此时脉冲能量为250 μJ。同时利用该中红外OPCPA系统,演示了光谱延伸至0.6 keV的软X射线发射。
图 2 (a) 2.2 μm OPCPA系统的示意图。右上角的插图显示了系统的长期输出稳定性和圆柱形整形望远镜后的光束轮廓。(b)恢复的放大器输出脉冲形状。(c)蓝线,测量的光谱;蓝虚线,恢复的光谱;橙线,恢复的相位[8]
Figure 2. (a) 2.2 μm OPCPA layout. The inset on the top right shows the long-term output stability of the system and beam profile after cylindrical reshaping telescopes. (b) The retrieved pulse shape of the amplifier output. (c) Blue line, measured spectrum; blue-dashed line, retrieved spectrum; orange line, retrieved phase[8]
3~4 µm波段最经典的MIR-OPCPA可能来自A.Baltuska领导的研究团队。他们获得了中心波长为3.9 μm,脉冲能量为8~20 mJ,脉冲宽度为~90 fs,重复频率为20 Hz的MIR激光输出[14]。如图3所示,采用Yb:KGW克尔透镜锁模振荡器作为种子源。然后,利用KTP晶体连续三级参量放大,获得了能量为65 μJ、中心波长为1460 nm的信号光。另一方面,提取振荡器输出光谱的1064 nm分量通过Nd:YAG CPA系统放大到250 mJ的脉冲能量,作为后续OPCPA系统的泵浦源。然后构造了两级OPCPA,获得了波长在1.46 μm的信号光和3.9 μm的闲频光,其能量分别为22 mJ和13 mJ。通过压缩闲频光,获得了8 mJ,80 fs的输出脉冲。在随后的工作中,系统的能量进一步放大到了~20 mJ。
在3~4 μm波段中,J.Biegert的团队展示了一种高平均功率的MIR-OPCPA系统,获得了在3.25 μm的中心波长处输出功率为21 W,重复频率为160 kHz的中红外脉冲[13]。如图4所示,波长为3.25 μm的MIR种子是由双色光纤前端与DFG级结合产生的。然后,将MIR脉冲展宽后,在前置放大器和两个增强放大器中连续放大。采用Nd:YVO4基主振荡功率放大器(MOPA)作为泵浦源,提供了波长为1064 nm,重复频率160 kHz下1.1 mJ,9 ps的脉冲。经过三个前置放大器和四个增强放大器的放大,得到了平均功率21 W、能量为131 μJ的MIR脉冲。在惰性气体填充的反共振光子晶体光纤中,通过孤子自压缩将MIR激光输出压缩到1.35个周期,产生在3.3 μm处脉宽为14.5 fs的脉冲,其平均功率为9.6 W。
图 4 (a)高能量MIR OPCPA系统装置图。种子由双色光纤前端和DFG级组合产生。然后,拉伸后在前置放大器和两个增强放大器中对MIR脉冲进行连续放大。最大的转换效率是通过多次使用泵浦光和单独定制的种子到泵浦光脉冲持续时间来实现的。MIR输出脉冲在一个大容量拉伸器中被压缩。(b)使用Ar填充的ARR-PCF将脉冲最终压缩到单个光学周期。MIR-OPCPA系统的输出特性。MIR输出脉冲的SHG-FROG恢复。(c)光谱的振幅和相位以及(d)时间振幅和瞬时频率。(e)在30 min内测量的脉冲间功率稳定性。插图显示输出光束轮廓[13]
Figure 4. (a) Setup of the high-power, MIR OPCPA system. The seed is generated by a two-color fiber front-end in combination with a DFG stage. Afterward, the MIR pulses are stretched and consecutively amplified in a preamplifier and two booster amplifiers. Maximum conversion efficiencies are achieved by multiple use of the pump beam and by individually tailored seed-to-pump pulse durations. The MIR output is compressed in a bulk stretcher and (b) the final compression to a single optical cycle is performed using an Ar-filled ARR-PCF. Output characteristics of the MIR OPCPA system. SHG-FROG retrieval of the MIR output pulses, showing (c) the spectral amplitude and phase, and (d) the temporal amplitude and instantaneous frequency. (e) The pulse-to-pulse power stability measured over 30 min. The inset shows the output beam profile[13]
Y.Leng的研究小组报道了一种产生4 μm脉冲的OPCPA系统,该脉冲具有2.6 mJ的脉冲能量和1.6个周期的脉冲宽度[15]。如图5所示,由商用钛宝石飞秒激光器泵浦的自制OPA产生CEP稳定的波长在4 μm的种子,其能量约为120 μJ。用于MIR-OPCPA的泵浦激光来自皮秒Nd:YAG激光器,该激光器可以以100 Hz的重复频率输出能量高达300 mJ、脉宽为50 ps的1064 nm脉冲。经过两级放大后,利用光栅压缩器将放大后的4 μm啁啾脉冲压缩到105 fs,能量为11.8 mJ。为了获得4 μm的近单周期的脉冲,采用了一种内径为1 mm、长度为3 m的惰性气体填充中空纤维对中红外脉冲进行压缩。结合CaF2材料,进一步将脉冲能量为2.6 mJ的MIR脉冲压缩到21.5 fs,对应于4 μm中心波长的1.6个光学周期。
2021年,H.K.Liang的团队展示了一种由平顶光束泵浦的高能高功率3 µm OPCPA系统[10]。利用商用衍射相位板和聚焦透镜的组合,将1 µm高斯泵浦转换成平顶轮廓,光束整形率大于95%,如图6(a)所示。采用平顶光束作为MIR-OPCPA系统的泵浦光,其转换效率高达13.5%,效率提高2倍。如图6(b)~(d)所示,产生2.7 mJ、27 W、125 fs、3 µm脉冲,其重复频率为10 kHz。放大后的平顶状MIR脉冲在薄YAG晶体中进行非线性压缩,压缩到50 fs,对应5个光学周期,压缩效率约为90%。在OPCPA上安装衍射相位板作为光束整形器是一种简单、可靠、经济的方法。它原则上也可以应用于其他波长范围的其他参量转换。
图 6 (a)高能高平均功率3 µm OPCPA平顶光束整形示意图。通过周期性极化铌酸锂(PPLN)和KTA晶体,从三级OPCPA前置放大器产生中心在3 μm的MIR脉冲并放大到300 μJ。第四级OPCPA通过平顶光束整形来提高MIR输出,提高参量效率。第四级OPCPA的高斯泵浦光束入射到由相位板和聚焦透镜组成的平顶光束整形器,在透镜的成像平面上形成平顶泵浦光束。利用平顶泵浦来放大前3级OPCPA系统产生的高斯闲频光束,使得产生3 µm的高能高平均功率平顶输出。在KTA晶体上测量了带平顶光束整形器和不带平顶光束整形器的泵浦光剖面(b)和(c)。(d)测量了利用平顶(红色)和高斯(黑色)光束剖面泵浦OPCPA系统产生3 μm闲频光脉冲能量。利用平顶光束和高斯光束进行泵浦获得了2.7 mJ和1.45 mJ的MIR脉冲能量,相当于第4级OPCPA系统的的泵浦-闲频效率分别为7%和13.5%[10]
Figure 6. (a) Schematic of flat-top beam shaping of the high-energy and high-average-power 3 µm OPCPA. The MIR pulses centered at 3 µm are generated and amplified to 300 µJ from 3-stage OPCPA preamplifiers via periodically poled lithium niobate (PPLN) and KTA crystals. The 4th OPCPA stage is designed to boost up the MIR output and enhance the parametric efficiency through the flat-top beam shaping. The Gaussian pump beam of the 4th-stage OPCPA is sent to a flat-top beam shaper consisting of a phase plate and a focus lens, and the flat-top pump beam is formed at the imaging plane of the lens. The Gaussian idler beam generated from the first-3 OPCPA stages is amplified with a flat-top pump, producing a high-energy and high-average-power flat-top-like 3 µm output. The measured pump beam profiles (b) with and (c) without the flat-top beam shaper, on the KTA crystal. The cross section beam profiles on the x and y axes are included too. (d) The pulse energy measurements of the 3 µm idler pulse from the OPCPA with flat-top (red) and Gaussian (black) pump beam profiles. 2.7 mJ and 1.45 mJ MIR pulse energy are obtained from the flat-top and Gaussian pump, corresponding to 7% and 13.5% pump-to-idler efficiency for the 4th-OPCPA stage, respectively[10]
2018年,Yuxi Fu的研究小组利用双啁啾光学参量放大(DC-OPA)技术通过中心波长为0.8 μm的宽带钛宝石激光器系统产生了能量31 mJ,波长接近3.3 μm的中红外脉冲[16]。如图7所示,将钛宝石放大器产生重频为10 Hz的激光在压缩之前分成两束,其中能量为1.4 mJ、脉宽为150 ps的这部分激光依次通过多通放大器和压缩器,产生700 mJ的脉冲作为后续两级DC-OPA系统的泵浦。将另一部分激光压缩至25 fs后通过OPA和DFG机制产生中心波长接近3.3 μm 的中红外脉冲。然后使用声光可编程分散式滤波器(AOPDF)和Si将其展宽至5 ps后,通过基于MgO:LiNbO3晶体两级NOPA系统放大至31 mJ。最后利用CaF2压缩至70 fs。
Huijun He的研究团队报道了一种由钛宝石激光系统泵浦产生520 μJ、1 kHz的中红外飞秒OPCPA系统,其中心波长大约在2.8 μm[17]。如图8所示,将大约2 mJ的泵浦能量聚焦到3 mm的YAG上产生白光作为信号光。部分泵浦通过BBO晶体发生倍频后在另一块BBO内利用OPA技术将特定的部分白光 (1115 nm)放大至1.3 μJ。接下来,采用非共线结构,利用KTA晶体连续三次参量放大实现宽带输出。最后获得了31.8 mJ的信号光和520 μJ的闲频光。为了最大限度的提高闲频光的带宽,他们还提出了一种给出最优非共线角和估计转换效率以及输出谱的理论方法。
2016年,Yanchun Yin的研究小组提出了利用宽带泵浦的DC-OPA系统产生中心波长位于3.2 μm的高能量、双周期中红外脉冲的方案[18]。如图9所示,nJ量级的振荡器输出脉冲经CPA系统放大到30 mJ后分成两部分。将第一部分(28 mJ)压缩至1.79 ps,作为三级DC-OPA系统的泵浦。将剩余的2 mJ部分压缩至20 fs后,通过充满N2的空芯光纤(HCF)产生白光。然后利用啁啾镜将白光压缩至<7 fs后入射到KTA晶体中,利用DFG产生宽带中红外种子光(2.3~4.5 μm)。使用AOPDF将种子光展宽至1 ps 后入射到基于MgO:LiNbO3晶体的三级DC-OPA系统。最后经过压缩可以获得波长为3.2 μm、能量为5.8 mJ的双周期脉冲,其转换效率为17%。
Tianli Feng的团队首次演示了平均功率高达27 W,重复频率为10 kHz的2.1 μm的OPCPA系统,其中泵浦和信号来自同一个500 W的Yb:YAG的薄片激光器[19]。如图10所示,利用0.5 mJ的泵浦脉冲通过两个级联的非线性过程(SHG和DFG)产生信号脉冲。然后采用两级基于PPLN晶体的OPA机制放大信号,在两级OPA之间,使用AOPDF来控制色散,将信号压缩至19 fs。泵浦功率最大部分用于后续的两级OPA系统(分别基于BiBO晶体和BBO晶体),将信号放大至2.9 mJ。最后使用一个4 cm长的无涂层玻璃块进行脉冲压缩,获得了脉宽30 fs的高功率2 μm脉冲。
2013年,Kun Zhao的研究团队展示了一种非共线光参量变换可调谐脉冲放大系统,产生了波长在3.3~3.95 μm 内可调谐的高峰值功率中红外脉冲[20]。如图11所示,该系统采用了两个放大系统作为泵浦源,分别是飞秒钛宝石激光再生放大器和Nd:YVO4激光再生放大系统。通过二向色镜将钛宝石激光器产生的激光和20%的Nd:YVO4激光输出结合入射到2块12 mm的LiNbO3晶体,通过OPA技术产生了波长在3.3~3.95 μm范围内可调谐的闲频光,作为后续OPCPA系统的信号光。然后将其展宽至430 ps后通过基于LiNbO3晶体的非共线单级OPCPA系统放大至29.5 mJ。最后,经过压缩获得了能量为13.3 mJ,脉宽111 fs的宽调谐中红外脉冲。值得一提的是,基于~1 µm泵浦的 ~3 µm 参量转换远离简并波长,且相较长波长中红外色散变化较陡峭,因此相位匹配带宽受限,一般难以直接实现周期量级的脉冲放大,需辅助后续的非线性压缩。
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近年来,出现了许多激光波长超过5 μm的长波MIR-OPCPA系统的报道。表2总结了中心波长为5 μm、7 μm和9 μm的长波MIR-OPCPA系统。
表 2 长波中红外OPCPA系统的参数
Table 2. Parameters of long wave MIR-OPCPA system
2017年,报告了一个5 μm OPCPA系统,该系统以1 kHz的重复率提供了数千兆瓦的飞秒脉冲[21]。如图12所示,掺铒飞秒激光器用作系统的种子源。将1.5 μm种子的一部分注入非线性光纤,以提供1 μm光谱分量。然后通过DFG产生3.4 μm MIR种子,作为OPCPA系统的信号光。1.5 μm种子的另一部分也被注入非线性光纤中以产生2 μm光谱成分,该光谱成分通过CPA系统被放大后用作后续OPCPA系统的泵浦光。于是,通过在ZGP晶体中的三级OPA系统,获得了脉冲能量为0.65 mJ、脉宽为75 fs(亚五周期)的5 μm的MIR闲频光输出。
图 12 (a) 2 μm泵浦的中红外OPCPA光源的示意图。主要部件有种子源、2 μmho:YLF CPA放大器、DFG、SLM和基于ZGP晶体的三个OPA级。Regen. amp.,再生放大器;Booster,功率放大器;CVBG,啁啾体布拉格光栅;SC,超连续谱;HNLF,高非线性光纤;TFP,薄膜偏振片。(b) DFG谱(灰色)、第一级(蓝色)、第二级OPA后的信号谱(绿色);(c)第三个OPA阶段后的闲频光谱测量值(黑色)和计算值(紫色)。TFL,傅里叶变换极限[21]
Figure 12. (a) Setup of the mid-IR OPCPA source pumped at 2 μm. The main parts are the seed source, the 2 μm Ho:YLF CPA amplifiers, DFG, the SLM, and the three OPA stages based on ZGP crystals. Regen. amp., regenerative amplifier; Booster, power amplifier; CVBG, chirped volume Bragg grating; SC, supercontinuum; HNLF, highly nonlinear fiber; TFP, thin-film polarizer. (b) DFG spectrum (gray), signal spectrum after the first (blue) and second OPA stage (green); (c) Idler spectrum after the third OPA stage measured (black) and calculated (purple). TFL, Fourier-transform-limited[21]
2016年,J.Biegert的团队展示了一种高能量、少周期的7 μm OPCPA系统[22]。如图13所示,系统从Er:Tm:Ho:fiber激光器开始,该激光器通过CSP晶体中的DFG产生7 µm种子。然后在由低温冷却Ho:YLF-CPA系统泵浦的基于ZGP的OPCPA链中放大7 μm种子脉冲,该泵浦光的波长在2052 nm处,脉冲宽度16 ps、能量为260 mJ。采用两级前置放大器和增强放大器,最后通过压缩器,获得了脉冲能量为0.75 mJ、脉宽为188 fs的7 μm脉冲。
图 13 (a) 7 μm OPCPA系统示意图。MIR种子是通过DFG从一个三色光纤前端通过两个宽带飞秒输出产生的。然后,在介质体中将MIR脉冲展宽,并在用啁啾反转级分离的前置放大器和增强放大器中连续放大。通过在前置放大器和增强放大器中调整种子到泵浦脉冲的持续时间来实现OPCPA的最大效率。利用BaF2介质体棒对宽带高能中红外脉冲进行了再压缩。(b)恢复的脉冲包络具有188 fs的半高宽持续时间,(c)测量(填充灰色)和恢复的光谱(红线)和相位(绿线)[22]
Figure 13. (a) Layout of the 7 μm OPCPA. The MIR seed is generated using the two broadband femtosecond outputs from a three-color fiber frontend via DFG. Afterward, the MIR pulses are stretched in a dielectric bulk and consecutively amplified in a pre-amplifier and a booster amplifier separated with a chirp inversion stage. Maximum efficiency of the OPCPA is achieved by tailoring the seed-to-pump pulse durations in the pre-amplifier and booster amplifier. The broadband high-energy mid-IR pulses are recompressed using a dielectric bulk rod of BaF2. (b) The retrieved pulse envelope with 188 fs FWHM duration, and (c) measured (filled gray) and retrieved spectrum (red line) and phase (green line)[22]
最近,H.K.Liang的研究小组报道了一种基于LiGaS2晶体的9 μm、少周期MIR OPCPA系统,该系统是由1 μm Yb:YAG激光器以10 kHz的重复频率进行泵浦的[23]。这是第一个以1 μm波长泵浦的长波长MIR-OPCPA。如图14所示,将从Yb:YAG中分离出来的小部分泵浦注入YAG晶体中,产生中心波长为1.16 μm的白光连续谱。在两个放大级中连续放大被拉伸的信号脉冲。最后,在10 kHz重复频率下,获得了中心波长在9 μm处、脉冲能量为14 μJ、持续时间为142 fs (4.7个光周期)的长波长MIR闲频脉冲。9 μm脉冲通过KrS-5材料的进行非线性压缩,将脉冲脉冲进一步压缩到45 fs,对应于1.5个光学周期[24]。
图 14 (a) 9 μm OPCPA系统示意图. YAG,钇铝石榴石;ZnSe,硒化锌窗口;HR,高反射镜;TFP,薄膜偏振器;BS,分束器;LGS,LiGaS2晶体;Ge,锗窗口。对于TFP,S偏振的泵浦光的反射率和P偏振的信号光的透射率分别为>99%和91%。(b)SC产生后的信号脉冲光谱(蓝点)、预放大阶段(红色)和主放大阶段(黑色虚线);(c)输出闲频光的测量(黑色)和模拟(红色虚线)光谱[23]
Figure 14. (a) The schematic of the 9 μm OPCPA. YAG, Yttrium aluminum garnet; ZnSe, Zinc selenide window; HR, High reflective mirror; TFP, Thin film polarizer; BS, Beam splitter; LGS, LiGaS2 crystal; Ge, Germanium window. For TFP, the reflectance of the S-polarized pump and the transmittance of the P-polarized signal are measured as > 99% and 91% respectively. (b) The spectra of signal pulses after SC generation (blue dotted), the pre-amplification stage (red) and the main-amplification stage (black dashed); (c) The measured (black) and simulated (red dashed) spectra of the output idler pulse[23]
Development and application of mid-infrared high-energy, high-power, few-cycle optical parametric chirped pulse amplifier (Invited)
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摘要: 近十年来,超强超短脉冲是激光光学发展的一个重要趋势。尤其是在中红外(MIR)波段,由于中红外波长具有更大的有质动力并且其光谱范围几乎包含了所有分子“指纹”共振峰,这使得中红外激光的研究在强场物理、中红外光谱学、材料加工以及生物医学研究等领域中至关重要。目前已经有许多比较成熟的激光技术可以对脉冲进行整形、放大,例如差频(DFG)、啁啾脉冲放大(CPA)、光学参量放大技术(OPA)以及光学参量啁啾脉冲放大(OPCPA)等。利用OPCPA技术具有的高放大增益、高信噪比、宽增益带宽的优点在高非线性系数的非线性晶体中进行脉冲放大已经成为当前获取超强超短中红外脉冲的主要手段之一。文中总结了利用OPCPA技术在2~20 μm波长范围内产生和放大MIR少周期脉冲的研究进展,并对其在强场物理、分子频谱探测以及生物医学方面的应用进行了简要的阐述。
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关键词:
- 中红外脉冲 /
- 超短脉冲 /
- 光学参量啁啾脉冲放大
Abstract: In recent decades, ultra-intense ultrashort pulse is an important trend in the development of laser optics. Especially in the mid-infrared (MIR) band, because the mid-infrared wavelength has greater ponderomotive force and its spectral range contains almost all the molecular "fingerprint" resonance peaks, the research of mid-infrared laser is very important in the fields of strong-field physics, mid-infrared spectroscopy, material processing and biomedical research. At present, there are many mature techniques for pulse shaping and amplification, such as different frequency generation (DFG), chirped pulse amplification (CPA), optical parametric amplification (OPA) and optical parametric chirped pulse amplification (OPCPA). Using optical parametric chirped pulse amplification technology with its advantages of high amplification gain, high signal-to-noise ratio and wide gain bandwidth to amplify the pulse in nonlinear crystals with high nonlinear coefficient has become one of the main means to obtain ultra-short and ultra-intense mid-infrared pulse.This paper summarizes the research progress of generating and amplifying MIR few-cycle pulse in 2-20 μm based on OPCPA , and its applications in strong-field physics, molecular spectrum detection and biomedicine are briefly described. -
图 1 (a) 2.1 μm少周期OPCPA系统的示意图;(b)测量(蓝色)和恢复(红色)的光谱强度和相位(黑色虚线),以及(c)测量的时间强度和相位。插图:第三阶段后测量的空间强度剖面[7]
Figure 1. (a) Schematic of 2.1 μm few-cycle OPCPA system; (b) Measured (blue) and retrieved (red) spectral intensity and phase (dashed black), and (c) measured temporal intensity and phase. Inset: measured spatial intensity profile after the third stage[7]
图 2 (a) 2.2 μm OPCPA系统的示意图。右上角的插图显示了系统的长期输出稳定性和圆柱形整形望远镜后的光束轮廓。(b)恢复的放大器输出脉冲形状。(c)蓝线,测量的光谱;蓝虚线,恢复的光谱;橙线,恢复的相位[8]
Figure 2. (a) 2.2 μm OPCPA layout. The inset on the top right shows the long-term output stability of the system and beam profile after cylindrical reshaping telescopes. (b) The retrieved pulse shape of the amplifier output. (c) Blue line, measured spectrum; blue-dashed line, retrieved spectrum; orange line, retrieved phase[8]
图 4 (a)高能量MIR OPCPA系统装置图。种子由双色光纤前端和DFG级组合产生。然后,拉伸后在前置放大器和两个增强放大器中对MIR脉冲进行连续放大。最大的转换效率是通过多次使用泵浦光和单独定制的种子到泵浦光脉冲持续时间来实现的。MIR输出脉冲在一个大容量拉伸器中被压缩。(b)使用Ar填充的ARR-PCF将脉冲最终压缩到单个光学周期。MIR-OPCPA系统的输出特性。MIR输出脉冲的SHG-FROG恢复。(c)光谱的振幅和相位以及(d)时间振幅和瞬时频率。(e)在30 min内测量的脉冲间功率稳定性。插图显示输出光束轮廓[13]
Figure 4. (a) Setup of the high-power, MIR OPCPA system. The seed is generated by a two-color fiber front-end in combination with a DFG stage. Afterward, the MIR pulses are stretched and consecutively amplified in a preamplifier and two booster amplifiers. Maximum conversion efficiencies are achieved by multiple use of the pump beam and by individually tailored seed-to-pump pulse durations. The MIR output is compressed in a bulk stretcher and (b) the final compression to a single optical cycle is performed using an Ar-filled ARR-PCF. Output characteristics of the MIR OPCPA system. SHG-FROG retrieval of the MIR output pulses, showing (c) the spectral amplitude and phase, and (d) the temporal amplitude and instantaneous frequency. (e) The pulse-to-pulse power stability measured over 30 min. The inset shows the output beam profile[13]
图 6 (a)高能高平均功率3 µm OPCPA平顶光束整形示意图。通过周期性极化铌酸锂(PPLN)和KTA晶体,从三级OPCPA前置放大器产生中心在3 μm的MIR脉冲并放大到300 μJ。第四级OPCPA通过平顶光束整形来提高MIR输出,提高参量效率。第四级OPCPA的高斯泵浦光束入射到由相位板和聚焦透镜组成的平顶光束整形器,在透镜的成像平面上形成平顶泵浦光束。利用平顶泵浦来放大前3级OPCPA系统产生的高斯闲频光束,使得产生3 µm的高能高平均功率平顶输出。在KTA晶体上测量了带平顶光束整形器和不带平顶光束整形器的泵浦光剖面(b)和(c)。(d)测量了利用平顶(红色)和高斯(黑色)光束剖面泵浦OPCPA系统产生3 μm闲频光脉冲能量。利用平顶光束和高斯光束进行泵浦获得了2.7 mJ和1.45 mJ的MIR脉冲能量,相当于第4级OPCPA系统的的泵浦-闲频效率分别为7%和13.5%[10]
Figure 6. (a) Schematic of flat-top beam shaping of the high-energy and high-average-power 3 µm OPCPA. The MIR pulses centered at 3 µm are generated and amplified to 300 µJ from 3-stage OPCPA preamplifiers via periodically poled lithium niobate (PPLN) and KTA crystals. The 4th OPCPA stage is designed to boost up the MIR output and enhance the parametric efficiency through the flat-top beam shaping. The Gaussian pump beam of the 4th-stage OPCPA is sent to a flat-top beam shaper consisting of a phase plate and a focus lens, and the flat-top pump beam is formed at the imaging plane of the lens. The Gaussian idler beam generated from the first-3 OPCPA stages is amplified with a flat-top pump, producing a high-energy and high-average-power flat-top-like 3 µm output. The measured pump beam profiles (b) with and (c) without the flat-top beam shaper, on the KTA crystal. The cross section beam profiles on the x and y axes are included too. (d) The pulse energy measurements of the 3 µm idler pulse from the OPCPA with flat-top (red) and Gaussian (black) pump beam profiles. 2.7 mJ and 1.45 mJ MIR pulse energy are obtained from the flat-top and Gaussian pump, corresponding to 7% and 13.5% pump-to-idler efficiency for the 4th-OPCPA stage, respectively[10]
图 12 (a) 2 μm泵浦的中红外OPCPA光源的示意图。主要部件有种子源、2 μmho:YLF CPA放大器、DFG、SLM和基于ZGP晶体的三个OPA级。Regen. amp.,再生放大器;Booster,功率放大器;CVBG,啁啾体布拉格光栅;SC,超连续谱;HNLF,高非线性光纤;TFP,薄膜偏振片。(b) DFG谱(灰色)、第一级(蓝色)、第二级OPA后的信号谱(绿色);(c)第三个OPA阶段后的闲频光谱测量值(黑色)和计算值(紫色)。TFL,傅里叶变换极限[21]
Figure 12. (a) Setup of the mid-IR OPCPA source pumped at 2 μm. The main parts are the seed source, the 2 μm Ho:YLF CPA amplifiers, DFG, the SLM, and the three OPA stages based on ZGP crystals. Regen. amp., regenerative amplifier; Booster, power amplifier; CVBG, chirped volume Bragg grating; SC, supercontinuum; HNLF, highly nonlinear fiber; TFP, thin-film polarizer. (b) DFG spectrum (gray), signal spectrum after the first (blue) and second OPA stage (green); (c) Idler spectrum after the third OPA stage measured (black) and calculated (purple). TFL, Fourier-transform-limited[21]
图 13 (a) 7 μm OPCPA系统示意图。MIR种子是通过DFG从一个三色光纤前端通过两个宽带飞秒输出产生的。然后,在介质体中将MIR脉冲展宽,并在用啁啾反转级分离的前置放大器和增强放大器中连续放大。通过在前置放大器和增强放大器中调整种子到泵浦脉冲的持续时间来实现OPCPA的最大效率。利用BaF2介质体棒对宽带高能中红外脉冲进行了再压缩。(b)恢复的脉冲包络具有188 fs的半高宽持续时间,(c)测量(填充灰色)和恢复的光谱(红线)和相位(绿线)[22]
Figure 13. (a) Layout of the 7 μm OPCPA. The MIR seed is generated using the two broadband femtosecond outputs from a three-color fiber frontend via DFG. Afterward, the MIR pulses are stretched in a dielectric bulk and consecutively amplified in a pre-amplifier and a booster amplifier separated with a chirp inversion stage. Maximum efficiency of the OPCPA is achieved by tailoring the seed-to-pump pulse durations in the pre-amplifier and booster amplifier. The broadband high-energy mid-IR pulses are recompressed using a dielectric bulk rod of BaF2. (b) The retrieved pulse envelope with 188 fs FWHM duration, and (c) measured (filled gray) and retrieved spectrum (red line) and phase (green line)[22]
图 14 (a) 9 μm OPCPA系统示意图. YAG,钇铝石榴石;ZnSe,硒化锌窗口;HR,高反射镜;TFP,薄膜偏振器;BS,分束器;LGS,LiGaS2晶体;Ge,锗窗口。对于TFP,S偏振的泵浦光的反射率和P偏振的信号光的透射率分别为>99%和91%。(b)SC产生后的信号脉冲光谱(蓝点)、预放大阶段(红色)和主放大阶段(黑色虚线);(c)输出闲频光的测量(黑色)和模拟(红色虚线)光谱[23]
Figure 14. (a) The schematic of the 9 μm OPCPA. YAG, Yttrium aluminum garnet; ZnSe, Zinc selenide window; HR, High reflective mirror; TFP, Thin film polarizer; BS, Beam splitter; LGS, LiGaS2 crystal; Ge, Germanium window. For TFP, the reflectance of the S-polarized pump and the transmittance of the P-polarized signal are measured as > 99% and 91% respectively. (b) The spectra of signal pulses after SC generation (blue dotted), the pre-amplification stage (red) and the main-amplification stage (black dashed); (c) The measured (black) and simulated (red dashed) spectra of the output idler pulse[23]
表 1 2~4 μm OPCPA系统相关参数
Table 1. Relevant parameters of 2-4 μm OPCPA system
Wavelength/μm Energy/mJ Repetition rate/kHz Average power/W Duration/fs Optical cycle Reference 2.1 1.2 3 3.6 10.5 1.5 [7] 2.2 0.25 100 25 16.5 2.2 [8] 3 0.3 10 3 21 2.1 [9] 3 2.4 10 24 50 5 [10] 3.1 0.125 100 12.5 73 7 [11] 3.2 0.152 100 15.2 38 3.6 [12] 3.25 0.06 160 9.6 14.5 1.35 [13] 3.9 8 0.02 0.16 83 6.4 [14] 4 2.6 0.1 0.26 21.5 1.6 [15] 3.3 31 1 31 66 6 [16] 2.8 0.52 1 0.52 27 2.89 [17] 3.2 5.8 1 5.8 20 2 [18] 2.1 2.7 10 27 30 4.3 [19] 3.3 13.3 1 13.3 111 10 [20] -
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