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定时是指某件事情发生的特定时间点。通常电磁波脉冲的定时信息可以通过脉冲的时间重心(center of gravity, COG)来定义,其表达式为[13]:
$$ {{{T}}_{{\rm{COG}}}} = \mathop \int \nolimits_{ - \infty }^{ + \infty } {{t}}{\left| {{{E}}\left( {{t}} \right)} \right|^2}{{\rm{{d}}}{t}}/\mathop \int \nolimits_{ - \infty }^{ + \infty } {\left| {{{E}}\left( {{t}} \right)} \right|^2}{{{\rm{d}}t}} $$ 式中:
${{E}}\left({{t}}\right)$ 为脉冲的电场;${{t}}$ 为脉冲变化的时间。对于周期为${{T}}$ 的电磁波脉冲序列,均方根误差$\left\{{{{T}}}_{{n}}-{{n}}{{T}}|{{n}}= {1,2},3\cdots \right\}$ 称为定时抖动(Timing Jitter),用来描述脉冲的各个有效瞬时位置对其当时的理想位置的短期性随机偏离。在锁模激光器中,定时抖动定义为输出光脉冲包络与理想位置的脉冲包络线之间的时间偏差。这种时域上的抖动,在频域对应着脉冲重复频率的噪声。对于自由运转的锁模激光器来说,脉冲时序会无限制地发散并经历随机游走,如图1(a)所示。
图 1 锁模激光器输出脉冲的定时抖动 (a)时域 (b)频域
Figure 1. Timing jitter of mode-locked laser output (a) in time domain and (b) in frequency domain
在飞秒激光器的低抖动特性被广泛认同前,时钟信号源大都基于微波器件,通过射频波段的高频率载波实现授时。在微波信号源中,由于热噪声及谐振腔损耗的存在,其输出的载波信号的零点位置相对于理想周期性位置会有一定程度偏差,此偏差即为微波信号源的定时抖动。微波信号源的定时抖动,通常用衡量时间偏移量变化速率的扩散系数来表征:扩散系数越大,说明在一定的积分时间内,时间偏移量越大,定时抖动的水平也就越高。一般地,微波信号源定时抖动的扩散系数为[14]:
$$\frac{{\rm{d}}}{{{\rm{d}}}{t}}\left\langle {\Delta t_{RF}^2} \right\rangle = \frac{1}{{{{\left( {2\pi } \right)}^2}}}\frac{{T_0^2}}{{{E_{mode}}}} \cdot \frac{{kT}}{{{\tau _{cav,RF}}}}$$ 式中:
$ {T}_{0} $ 为载波信号的周期;$ k $ 为玻耳兹曼常量;$ T $ 为绝对温度,$ {E}_{mode} $ 为腔内振荡模式的能量;$ {\tau }_{cav,RF} $ 为腔内能量的衰减时间常数。与微波信号源相对应的,飞秒激光器中以放大自发辐射噪声(ASE)直接耦合至双曲正割形脉冲的定时抖动的扩散系数可以写成如下形式[14]:$$\frac{{\rm{d}}}{{{\rm{d}}t}}\left\langle {\Delta t_{ML}^2} \right\rangle = \frac{{{\pi ^2}}}{6}\frac{{\tau _P^2}}{{{E_P}}} \cdot \frac{{hv}}{{{\tau _{cav,ML}}}}$$ 式中:
$ {\tau }_{p} $ 为脉冲的半极大全宽;$ {E}_{p} $ 为单脉冲能量;$ hv $ 为单光子能量;${\tau }_{cav, \; ML}=\dfrac{{T}_{rt}}{2{g}_{s}}$ 为腔内损耗时间常数;$ {g}_{s} $ 为饱和增益。对于工作在室温下、载波频率为10 GHz的微波信号源来说,$ {T}_{0} $ 为100$\rm{ps}$ ,$kT\approx 0.025\;\rm{eV}$ ;对于工作在室温下、中心波长为1040 nm的飞秒激光器,$hv\approx 1.2\;\rm{eV}$ ,$ {\tau }_{p} $ 约为100$\rm{fs}$ 。若假设二者的腔内模式能量与脉冲能量和衰减时间常数乘积位于同一数量级,则微波信号源的扩散系数约为飞秒激光器的$ {10}^{5} $ 倍。不难看出,飞秒激光器的低定时抖动的优势主要来源于其极窄的脉冲宽度。这种现象可以解释为:在飞秒激光器腔内,脉冲能量集中在飞秒量级的超短脉冲内,单位时间内的光子密度得到了极大的提升;对于一定的自发辐射(ASE)噪声水平,脉冲光子密度的提高可以降低ASE噪声引起的脉冲时域分布的相对变化量,进而降低其对于脉冲时域位置的短期稳定性的干扰,使得脉冲的定时抖动保持在极低水平。 -
随着飞秒激光器定时抖动理论的不断完善和测量精度的逐渐提升,关于低抖动的飞秒激光源的实验研究也逐渐展开。
固体飞秒激光器由于输出脉冲具有极窄的脉冲宽度和极高的峰值功率,ASE直接耦合的定时抖动水平较低。目前为止,飞秒激光器低定时抖动的纪录是麻省理工大学的A. J. Benedick等人通过BOC方法在脉冲宽度为10 fs的钛宝石飞秒激光器内获得的13 as的残余定时抖动[22]。另外,在基于SESAM锁模的Cr:LiSAF激光器和Er:Yb-glass激光器中,也分别获得了30 as[31]和83 as[32]的定时抖动(积分带宽为[10 kHz-50 MHz])。然而,由于固体激光器往往对腔镜的空间耦合精度要求极高,且使用和维护成本较高,因此很难在实验室以外的环境大范围地推广应用。
与固体激光器相比,光纤激光器的光-光转化效率更高,热效应不明显,结构紧凑,价格低廉,更具有实用化优势。近年来,光纤飞秒激光器的定时抖动性能被不断优化,已经可以接近固体飞秒激光器的参数,进一步提高了其实用价值。2007年,韩国KAIST的J. Kim等人首次利用BOC技术实现了对掺铒光纤激光器的高频抖动的测量,在[10 kHz,10 MHz]积分带宽内获得了5 fs的定时抖动[33]。之后,宋有建等人在实验中探索了腔内净色散与脉冲序列定时抖动的耦合机制,通过优化激光器的色散参数将掺镱和掺铒光纤激光的高频定时抖动分别降低至175 as[20]和76 as[26]。2014年,天津大学的秦鹏等人通过在腔内加入窄带滤波器,使得掺镱激光器在很宽的腔色散范围内均可获得较低的的定时抖动[23],降低了低抖动飞秒激光源设计的难度。由于上述激光器均为基于非线性偏振旋转(Nonlinear Polarization Evolution, NPE)锁模的飞秒激光器,因此对于环境扰动和光纤应力较为敏感。2016年,美国IMRA公司N. Kuse等人设计了的一种基于非线性放大环形镜(Nonlinear Amplifying Loop Mirror, NALM)锁模的全保偏9字腔激光器,可获得40 as的定时抖动[34]。其研究提高了激光器长期稳定性和可重复性,为低定时抖动光纤飞秒激光器的实用化奠定了基础。
尽管上述工作在降低锁模固体飞秒激光器和光纤飞秒激光器的定时抖动方面取得了巨大进步,但是这些阿秒级定时抖动性能的激光器重复频率大多被限制在100 MHz。为了探究高重频飞秒激光器的定时抖动特性,麻省理工大学的J. Chen等人通过在200 MHz激光振荡器外加入自由空间Fabry-Perot腔作为重频倍增器,获得了重复频率为2 GHz,定时抖动为27 fs的脉冲[35]。韩国KAIST的H. Yang等人搭建了重复频率为1.13 GHz的Yb:KYW固体激光器,并测得其在[17.5 kHz,10 MHz]带宽内的定时抖动为0.7 fs[36]。北京大学的王燕等人测量了基于NPE锁模的掺Yb光纤激光器在重复频率为882 MHz时的定时抖动,在[30 kHz,5 MHz]的积分带宽内最低为10 fs[37], 并提出高重频的光纤飞秒激光器的定时抖动可以通过减小相对强度噪声与定时抖动的耦合来抑制[38]。西安光机所的张建国等人报道了一种2.68 GHz全光纤SESAM锁模光纤激光器,在[300 Hz,30 MHz]的积分带宽内测得的定时抖动为82.5 fs[39]。此外,半导体锁模激光器和微环谐振腔也是可以产生数十GHz高重频超短脉冲的可靠光源。但是,由于半导体锁模激光器的脉冲宽度通常在皮秒量级,很难实现飞秒量级以下的极低抖动[40-42]。2020年,韩国KAIST课题组的D. Jeong等人测量了一台22 GHz的硅基微环光疏的定时抖动,在[10 kHz, 3 MHz]积分带宽内获得了2.6 fs的定时抖动积分值[43],进一步证明了微环谐振腔作为高重频低抖动飞秒激光光源的应用潜力。
除单脉冲运转状态之外,飞秒激光器还能工作在多脉冲的束缚态。德国马普所的庞盟等发现,利用光机械力可以束缚住激光器内多个孤子,并编程调控其间隔,孤子之间的抖动小于100 fs[44],这种稳定的多孤子状态能够在激光器内一直稳定运转,为全光存储提供了一种新的思路。天津大学的师浩森等人发现,激光器内,光孤子尾部相互作用产生的紧束缚态孤子对的定时抖动<1 fs[45],这种稳定的双孤子结构有可能为全光信息处理提供一种新型多级字母表编码单元(multi-alphabet coding unit)。
此后,基于具有极限分辨能力的测量手段与激光腔参数的优化,不同结构的低定时抖动的飞秒激光器层出不穷,为其在前沿领域的广泛应用奠定了坚实的基础[46]。表1列举了近年来低抖动飞秒激光器的相关研究进展。表2展示了目前商用飞秒激光器的定时抖动水平。
表 1 低抖动飞秒激光器的相关研究进展
Table 1. Representative progress of low timing jitter femtosecond laser
Laser source Integrated
timing jitterIntegrated Fourier
frequency rangeMeasurement
methodResearch
instituteClassification Laser cavity parameter Er-doped
fiber laser194-MHz NPR,soliton mode-
locking regime (2007)[47]18 fs [1 kHz–10 MHz] PD MIT 3-GHz SESAM,soliton mode-
locking regime (2009)[48]19 fs [10 kHz–40 MHz] PD MIT 200-MHz NPR with spectral filter,stretched-pulse mode-locking regime (2010)[49] 17.4 fs [1 kHz–10 MHz] PD Shanghai Jiao Tong University 1-GHz SESAM,soliton mode-
locking regime (2010)[50]22 fs [1 kHz–10 MHz] PD MIT 80-MHz NPR,cavity dispersion
$-0.002\;\rm{ps}^{2}$ (2011)[26]0.07 fs* [10 kHz–40 MHz] BOC KAIST 80-MHz CNT-SA,cavity dispersion
$-0.02\;\rm{ps}^{2}$ (2013)[51]0.5 fs [10 kHz–40 MHz] BOC KAIST 463-MHz MoS2-SA,soliton mode-
locking regime (2015)[52]33 fs [1 kHz–1 MHz] PD Shanghai Jiao Tong University 80-MHz NPR,cavity dispersion
$+0.002\;\rm{ps}^{2}$ (2015)[53]0.7 fs [10 kHz–10 MHz] BOC KAIST 129-MHz NPR with spectral filter,cavity dispersion$+0.008\;\rm{ps}^{2}$ (2015)[54] 3.46 fs [10 kHz–10 MHz] BOC KAIST 75-MHz NALM,cavity dispersion
$-0.003\;\rm{ps}^{2}$,soliton molecule (2018)[45]0.83 fs [10 Hz–2 MHz] BOC Tianjin University 36.56 MHz NALM,soliton mode-
locking regime (2019)[55]7.3 fs [10 kHz–1 MHz] FDL KAIST 2.68 GHz SESAM,cavity dispersion
$\sim 50\;\rm{fs}^{2}$ (2019)[39]82.5 fs [300 Hz-30 MHz] PD Xi'an Institute of Optics and Precision Mechanics of CAS Yb-doped
fiber Laser80-MHz NPR,stretched-pulse mode-
locking regime (2011)[20]0.18 fs* [10 kHz–40 MHz] BOC KAIST 80-MHz NPR,soliton laser (2011)[20] 1.8 fs [10 kHz–40 MHz] BOC KAIST 80-MHz NPR,amplifier-similariton (2011)[20] 2.9 fs [10 kHz–40 MHz] BOC KAIST 80-MHz, NPR with spectral filter,cavity dispersion $+0.008\;\rm{ps}^{2}$ (2014)[23] 0.57 fs [10 kHz–10 MHz] BOC Tianjin University 10-MHz SESAM,all-PM,cavity dispersion $+0.46\;\rm{ps}^{2}$ (2016)[24] 5.9 fs [10 kHz–5 MHz] BOC Tianjin University 880 MHz NPR,stretched-pulse mode-
locking regime (2018)[37]10 fs [30 kHz–5 MHz] BOC Peking University Tm-doped
fiber laser690-MHz harmonic mode-locking NPR,soliton mode-locking regime (2015)[56] 6 ps [100 Hz–1 MHz] PD Tsinghua University 253 MHz SBR Tm/Ho fiber laser (2016)[57] 20 fs [100 Hz–2 MHz] PD Boston University 1.5 GHz SESAM (2018)[58] 940 fs [10 Hz–1 MHz] PD South China University of Technology Solid
laser80-MHz KLM Ti:sapphire laser (2012)[22] 0.013 fs* [100 Hz–40 MHz] BOC MIT 100-MHz SESAM Cr:LiSAF laser (2012)[31] 0.03 fs [10 kHz–50 MHz] BOC MIT 100-MHz, SESAM, Er:Yb-glass laser (2013)[32] 0.083 fs [10 kHz–50 MHz] BOC CSEM 500-MHz SESAM Er:Yb-glass laser (2015)[25] 0.016 fs [10 kHz–250 MHz] OH University of Colorado Semiconductor
laser10 GHz SCOWA-based harmonically mode-locked nested cavity laser (2018)[59] 3.4 fs [10 Hz,100 MHz] PD University of Central Florida 21 GHz InAs/InP quantum dash laser (2018)[41] 400 fs [10 kHz,100 MHz] PD University College Cork 19 GHz integrated heterogeneous silicon/III-V colliding pulse mode-locked laser (2018)[42] 1.2 ps [100 kHz,100 MHz] PD University of California Optical
Microresonator22 GHz silica microcomb (2020)[43] 2.6 fs* [10 kHz–3 MHz] FDL KAIST 注:*表示该类激光器测得的最低定时抖动值
* indicates the measured lowest timing jitter of lasers in this category表 2 商用飞秒激光器的定时抖动
Table 2. Timing jitter of commercial femtosecond laser
Laser source Integrated timing jitter Integrated Fourier frequency range Company Figure 9 fiber laser <2 fs [10 kHz–10 MHz] Menlo System Figure 9 Er- fiber laser[34] 0.04 fs [10 kHz–10 MHz] IMRA America MENHIR-1550 ≤30 fs [1 kHz–10 MHz] MenHir Photonics ORIGAMI 5-40 <30 fs [1 kHz–10 MHz] NKT Photonics
Ultra-low timing jitter femtosecond laser technology (Invited)
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摘要: 飞秒激光器的时间抖动(或定时抖动)是指其输出脉冲的时域位置相对于理想周期信号的短期随机偏差。在毫秒量级的时间尺度上,飞秒激光器的脉冲序列具有严格的一致性,其定时抖动甚至低至阿秒量级。飞秒激光器的这种独特性质及其支持的前沿应用构成了“阿秒时间精度的超快光子学”这一全新的超快研究分支。文中回顾了近年来飞秒激光器定时抖动研究进展、高时间分辨率的定时抖动测量技术、以及不同类型的飞秒激光源能够达到的最低抖动水平。最后介绍了低抖动飞秒激光器在大科学装置同步、高速模数转换、绝对测距、相干脉冲合成等领域的应用。Abstract: The time jitter of a femtosecond laser is the short-term deviation of the optical pulse position relative to its ideal equally spaced pulse position. Femtosecond lasers emit uniformly spaced ultrashort pulse train. The quantum-noise-limited timing jitter can be as low as few tens of attoseconds in millisecond time scale. This unique property and its advanced applications constitute a new branch of ultrafast research, "Attosecond precision ultrafast photonics". In this paper, the recent advances in femtosecond laser timing jitter research, high-precision timing jitter characterization methods, and the ultralow timing jitter that can be achieved by different kinds of femtosecond laser sources were reviewed. Finally, the application of low-jitter femtosecond lasers in the fields of synchronization of large-scale scientific instruments, high-speed analog-to-digital conversion, absolute ranging technology and coherent beam combination are introduced.
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Key words:
- laser optics /
- femtosecond laser /
- timing jitter /
- mode-locked laser
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表 1 低抖动飞秒激光器的相关研究进展
Table 1. Representative progress of low timing jitter femtosecond laser
Laser source Integrated
timing jitterIntegrated Fourier
frequency rangeMeasurement
methodResearch
instituteClassification Laser cavity parameter Er-doped
fiber laser194-MHz NPR,soliton mode-
locking regime (2007)[47]18 fs [1 kHz–10 MHz] PD MIT 3-GHz SESAM,soliton mode-
locking regime (2009)[48]19 fs [10 kHz–40 MHz] PD MIT 200-MHz NPR with spectral filter,stretched-pulse mode-locking regime (2010)[49] 17.4 fs [1 kHz–10 MHz] PD Shanghai Jiao Tong University 1-GHz SESAM,soliton mode-
locking regime (2010)[50]22 fs [1 kHz–10 MHz] PD MIT 80-MHz NPR,cavity dispersion
$-0.002\;\rm{ps}^{2}$ (2011)[26]0.07 fs* [10 kHz–40 MHz] BOC KAIST 80-MHz CNT-SA,cavity dispersion $-0.02\;\rm{ps}^{2}$ (2013)[51]0.5 fs [10 kHz–40 MHz] BOC KAIST 463-MHz MoS2-SA,soliton mode-
locking regime (2015)[52]33 fs [1 kHz–1 MHz] PD Shanghai Jiao Tong University 80-MHz NPR,cavity dispersion $+0.002\;\rm{ps}^{2}$ (2015)[53]0.7 fs [10 kHz–10 MHz] BOC KAIST 129-MHz NPR with spectral filter,cavity dispersion $+0.008\;\rm{ps}^{2}$ (2015)[54]3.46 fs [10 kHz–10 MHz] BOC KAIST 75-MHz NALM,cavity dispersion
$-0.003\;\rm{ps}^{2}$ ,soliton molecule (2018)[45]0.83 fs [10 Hz–2 MHz] BOC Tianjin University 36.56 MHz NALM,soliton mode-
locking regime (2019)[55]7.3 fs [10 kHz–1 MHz] FDL KAIST 2.68 GHz SESAM,cavity dispersion
$\sim 50\;\rm{fs}^{2}$ (2019)[39]82.5 fs [300 Hz-30 MHz] PD Xi'an Institute of Optics and Precision Mechanics of CAS Yb-doped
fiber Laser80-MHz NPR,stretched-pulse mode-
locking regime (2011)[20]0.18 fs* [10 kHz–40 MHz] BOC KAIST 80-MHz NPR,soliton laser (2011)[20] 1.8 fs [10 kHz–40 MHz] BOC KAIST 80-MHz NPR,amplifier-similariton (2011)[20] 2.9 fs [10 kHz–40 MHz] BOC KAIST 80-MHz, NPR with spectral filter,cavity dispersion $+0.008\;\rm{ps}^{2}$ (2014)[23]0.57 fs [10 kHz–10 MHz] BOC Tianjin University 10-MHz SESAM,all-PM,cavity dispersion $+0.46\;\rm{ps}^{2}$ (2016)[24]5.9 fs [10 kHz–5 MHz] BOC Tianjin University 880 MHz NPR,stretched-pulse mode-
locking regime (2018)[37]10 fs [30 kHz–5 MHz] BOC Peking University Tm-doped
fiber laser690-MHz harmonic mode-locking NPR,soliton mode-locking regime (2015)[56] 6 ps [100 Hz–1 MHz] PD Tsinghua University 253 MHz SBR Tm/Ho fiber laser (2016)[57] 20 fs [100 Hz–2 MHz] PD Boston University 1.5 GHz SESAM (2018)[58] 940 fs [10 Hz–1 MHz] PD South China University of Technology Solid
laser80-MHz KLM Ti:sapphire laser (2012)[22] 0.013 fs* [100 Hz–40 MHz] BOC MIT 100-MHz SESAM Cr:LiSAF laser (2012)[31] 0.03 fs [10 kHz–50 MHz] BOC MIT 100-MHz, SESAM, Er:Yb-glass laser (2013)[32] 0.083 fs [10 kHz–50 MHz] BOC CSEM 500-MHz SESAM Er:Yb-glass laser (2015)[25] 0.016 fs [10 kHz–250 MHz] OH University of Colorado Semiconductor
laser10 GHz SCOWA-based harmonically mode-locked nested cavity laser (2018)[59] 3.4 fs [10 Hz,100 MHz] PD University of Central Florida 21 GHz InAs/InP quantum dash laser (2018)[41] 400 fs [10 kHz,100 MHz] PD University College Cork 19 GHz integrated heterogeneous silicon/III-V colliding pulse mode-locked laser (2018)[42] 1.2 ps [100 kHz,100 MHz] PD University of California Optical
Microresonator22 GHz silica microcomb (2020)[43] 2.6 fs* [10 kHz–3 MHz] FDL KAIST 注:*表示该类激光器测得的最低定时抖动值
* indicates the measured lowest timing jitter of lasers in this category表 2 商用飞秒激光器的定时抖动
Table 2. Timing jitter of commercial femtosecond laser
Laser source Integrated timing jitter Integrated Fourier frequency range Company Figure 9 fiber laser <2 fs [10 kHz–10 MHz] Menlo System Figure 9 Er- fiber laser[34] 0.04 fs [10 kHz–10 MHz] IMRA America MENHIR-1550 ≤30 fs [1 kHz–10 MHz] MenHir Photonics ORIGAMI 5-40 <30 fs [1 kHz–10 MHz] NKT Photonics -
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