西安交通大学学术报告
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西安交通大学学术报告. 固体飞秒激光的阿秒同步及相干控制. 魏志义. 中国科学院物理研究所. 2005 年4月26日. 目 录. 引言 飞秒激光的被动同步机理 飞秒钛宝石与镁橄榄石激光的被动同步 不同飞秒钛宝石激光的被动同步 同步飞秒激光的载波相位测量与控制 总结. 一.引言. l 1. l2. 测量装置. 常规飞秒激光脉冲的局限性. 在常规情况下,飞秒激光所输出的飞秒脉冲仅工作在某一特定波长。. 而在泵浦探测、飞秒激光的差频、相干合成、光频标、量子纠缠通讯、快点火激光聚变等领域中,需要两个不同的同步超短激光脉冲,.

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固体飞秒激光的阿秒同步及相干控制

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西安交通大学学术报告

固体飞秒激光的阿秒同步及相干控制

魏志义

中国科学院物理研究所

2005年4月26日


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目 录

  • 引言

  • 飞秒激光的被动同步机理

  • 飞秒钛宝石与镁橄榄石激光的被动同步

  • 不同飞秒钛宝石激光的被动同步

  • 同步飞秒激光的载波相位测量与控制

  • 总结


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一.引言


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l1

l2

测量装置

常规飞秒激光脉冲的局限性

在常规情况下,飞秒激光所输出的飞秒脉冲仅工作在某一特定波长。

而在泵浦探测、飞秒激光的差频、相干合成、光频标、量子纠缠通讯、快点火激光聚变等领域中,需要两个不同的同步超短激光脉冲,


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飞秒激光的传统被动同步技术 I

共享增益

D.R.Dykaar et al, Opt Lett, Vol. 18, 634(1993)


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飞秒激光的传统被动同步技术II

两激光共用同一增益介质

A.Leitensdorfer et al, Opt. Lett. Vol. 20, 916(1995)


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被动同步激光的腔长允许失配量

允许失配量~1mm

A.Leitensdorfer et al, IEEE.STQE. Vol. 2, 743(1996)


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飞秒激光的主动同步技术

SynchroLock,

Coherent Inc

LoktoClock,

Spectra-Physics Inc.

主动同步的同步精度(Timing Jitter):200 fs - ps

S.A.Crooker et al; Rev.Sci.Instr, Vol.67, 2068(1996)

J.Y.Sohn et al; Appl.Opt, Vol.38, 5899(1999)


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主、被动同步飞秒技术的比较


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不同同步飞秒激光的特性

  • 主动同步

    复杂的电子学设备,同步精度一般100fs-ps

  • OPO 和OPA

    产生双波长飞秒脉冲,但稳定性差,输出功率低

  • 被动同步技术

    可以获得高同步精度的双波长飞秒脉冲。

    但增益竞争阻碍了被动同步的发展.

    通常同步精度约几fs,腔长允许失配量在1~3 m

问题?

在传统被动同步中,钛宝石晶体同时充当两束激光的增益介质及产生互相位调制效应的耦合介质


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用于频标研究的主动同步飞秒激光

近年来低于1fs时间抖动的主动同步飞秒激光已有报道

R.K.Shelton, et al; SPIE 4269, 105 (2001)

R.K.Shelton et al;Opt Lett., Vol.27,312 (2002)

T.R.Schibli et al; Opt Lett., Vol.28, 947(2003)


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原有同步飞秒激光技术的主要特点

  • 仅用一块增益介质,结构简单、成本低

  • 增益竞争效应,导致稳定性差

  • 同步精度有限,最好仅2fs

  • 同一块增益介质,仅能用一台泵浦源,导致输出功率低

  • 由于两激光共用同一增益介质,输出波长相近,应用稍强于单波长激光

  • 腔长允许失配量小(~1 m),致使同步不能长期运行,这也是迄今国际上无此类飞秒激光产品的原因

问题:能否将两增益介质分开?


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I(t)

重复频率 f2

重复频率 f1

激光1

激光2

t

dI(t)

dt

t

二、飞秒激光的被动同步机理

同步前:

激光1与激光2以F=f1-f2的频率在晶体内交叉

频谱扩展(SPM&XPM,假定I2>I1)

接近同步时:

若激光1落后于激光2, 在激光2下降沿相互作用

dI2/dt <0 w1 (with -GVD )  trip time t 

若激光2落后于激光1,在激光2上升沿相互作用

dI2/dt >0 w1  (with -GVD )  trip time t 

同步


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三、飞秒钛宝石与镁橄榄石激光的被动同步


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实验方案设计

掺钛蓝宝石激光

泵浦: 4.5W (532nm)

锁模功率:600mW

掺铬镁橄榄石激光

泵浦: 9.5W (1064nm)

锁模功率: 200mW

增益介质I:Ti:Al2O3; 增益介质II:Cr:Mg2SiO4

耦合介质:Ti:Al2O3; (用于提供XPM)

无增益竞争效应

M1~M7: CVI 反射镜,曲率10cm; M8:微型镜片; P1,P2:熔石英棱镜; P3,P4: SF6 棱镜; PZT: 压电陶瓷; OS:示波器; PD:快速光电二极管; M1,M2,M8:中心波长850nm高反镜; M3~M7:中心波长1230nm高反镜.


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同步的实现

(a)

(b)

Ti:Al2O3激光与Cr:Mg2SiO4激光的锁模踪迹,示波器由Ti:Al2O3激光触发。上图为Ti:Al2O3激光脉冲波形,下图为Cr:Mg2SiO4脉冲波形。(a)为临界同步时的波形,(b)为同步后的波形。


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同步的腔长失配特性

(a)

(b)

钛蓝宝石与镁橄榄石激光重复频率随腔长失配量的变化特性。腔长允许失配量~5mm

Z.Wei al; Opt Lett, Vol. 26, 1806(2001)


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钛宝石与镁橄榄石激光自相关曲线

(a)

(b)

干涉自相关曲线。在高斯脉冲假设下,对应脉冲分别为:

(a)54 2fs (b)65 1fs

Z.Wei al; Appl Phys B, Vol. 74, S171(2002)


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PZT Driver

PZT

Oscalliscope

1250nm/65fs

Cr:forsterite laser

BBO

Grating

B.S

820nm/54fs

PMT

Ti:sapphire laser

互相关曲线测量装置示意图

B.S: 金属分束片. 为方便观察,互相关及自相关信号经光栅展开


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从互相关曲线估计的时间抖动

FWHM:

74 2fs

右图为钛宝石激光倍频光(蓝)、镁橄榄石激光倍频光(红)及和频光 (绿)图片 。左图为对应的互相关曲线,其半高宽度为:742fs,时间抖动曲线由下式计算:

计算得时间抖动为0


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从互相关的半峰值抖动估算时间抖动

I(t)

Timing

jitter

V=41.424( average)

DV = 0.494438 (standard deviation )

t

DKD V/V

D K

DKD V/V

t

互相关信号半峰值处的强度起伏正比于两束激光的时间抖动。在1KHz带宽下, 在5秒时间间隔内我们得到0.88fs的时间抖动


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进一步优化的可能

尽管增益介质是独立的,但是一个增益介质仍然同时承担了提供增益和互相位调制的作用,这在同步的过程是否有副作用?


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四、掺钛蓝宝石飞秒激光的被动同步


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两独立掺钛蓝宝石飞秒激光的同步

Pump Lasers

PZT

增益介质I:Ti:Al2O3I

增益介质II: Ti:Al2O3II

耦合介质:Ti:Al2O3III (用于提供XPM)

增益介质与耦合介质完全独立

系统占用空间: 2× 0.8m


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同步的实现

泵浦功率: 4W @ 532nm.

输出功率: 400mW.

脉冲宽度: ~ 30 fs

同步失调量:>10mm, 手细调端镜启动同步


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典型的同步的光谱及自相关曲线

波长:730 nm~850 nm可独立调谐

脉冲宽度:30~70 fs可调


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Laser1

Laser2

Slit

Lens

M4

M3

BBO

M2

PMT

M1

M5

M6

Translation Stage

Oscilloscope

Stepping Motor

Computer

互相关测量装置

Laser 1 and laser 2: 飞秒掺钛蓝宝石激光

BBO: 0.1mm ; Lens: 熔融石英, f=10cm


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互相关信号半峰值的起伏估算同步精度

时间抖动:400as

Sub-femtosecond passively synchronized Ti:sapphire lasers,

物理学报, Vol. 54, No.1, 129~133(2005)


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同步飞秒掺钛蓝宝石激光的实用化设计

尺寸: 0.8 m  0.4 m = 0.32 m2


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同步状态的观察

同步启动: 用手细调端镜

可以推断:存在超长的腔长允许失配量

同步持续时间:>8小时


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同步的相关技术指标


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被动同步掺钛蓝宝石激光的样机设计

该激光器中国科技大学近代物理系已取得的应用研究成果

  • 首次实现纠缠光子对之间的同步

  • 首次实现不同来源纠缠光子对的纠缠交换

  • 量子力学的非定域性检验


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五、同步飞秒激光的载波相位测量与控制


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飞秒激光的载波相移

carrier-envelope phase offset (CEP, CEO)

Df

L.Xu et al., Opt Lett, 21, 2008 (1996)

Prof. Dr. Theodor W. Hänsch


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载波相位的控制飞秒激光频梳

f =nfrep+ d

2nfrep+d

控制腔镜以稳定频率漂移d

控制腔长以稳定重复频率frep

Df=2(nfrep+d)-(2nfrep+d)= d

nfrep+d

2(nfrep+d)


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超宽带飞秒钛宝石激光器

f=10 cm

M1

M2

Ti:sa

5W 532nm

Millennia

OC T=10%

M3

Rainbow,Femtolasers Inc

我们最新的结果,支持5fs的脉宽

1st Asian Workshop on HHG and attosecond laser. Korea, Feb 23, 2005


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fDFG= (m-n)frep

f2=d+nfrep

f1=d+mfrep

Consideration on the DFG and amplification

DFG: 627-1000nm1680nm(w)

Free CE phase, f1= nfrep

SHG: 1680nm 840nm(2w)

CEP locked, f2= 2nfrep

I (f)

d

frep

frep

f

CE phased locked sub-harmonic waves can be obtained based on the frequency conversion of 1680nm and 840nm

1680nm(w), 840nm(w+w), 560nm(w+2w),420nm(2w+2w),336nm(2w+3w )

Method 1: directly double and sum frequency 1680nm and 840nm.

Method 2: Amplify 840nm to ~100nJ to mJ by CPA,

low repetition rate, higher energy for 840nm

Method 3: Amplify 1680nm by OPCPA,

CEP locked ~100nJ 840nm as pump


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同步亚谐波激光的相位控制原理

Df1

I (f)

fCF=d2+nfrep

fTS=d1+mfrep

frep

frep

f

Df2

d2

d1

Dfi =2pdi/frep

fTS=d1+ mfrepfCF=d2+ nfrep

frep: the repetition rate of both lasers

frep


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0

frep

-20

-40

frep+f

frep -f

f

Intensity (dBm)

-60

-80

0

20

40

60

80

100

Frequency(MHz)

同步飞秒钛宝石与镁橄榄石激光相位漂移测量方案——直接测量

C:F Laser, 1250nm/52fs

830nm/43fs

T:S laser

B.S

PDF

STAGE

40X

Filter

PIN

Diode

Spetrum

Analyzer

Heterodyning the Cr:forsterite laser with the PCF-generated continuum from the Ti:sapphire laser. The beat signal f=(d2 -d1)directly reveals the carrier-envelope phase relation between two lasers. PDF: 25cm/1.8mm photonic crystal fiber.


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M1

C:F laser

F

1

F3

F4

C3

P.H

M6

PMT

C2

B.S

C1

M3

F

2

F.L

M2

M4

DM

M5

Ti:S laser

M7

PC

Oscilloscope

Spectrum

Analyzer

同步飞秒钛宝石与镁橄榄石激光相位漂移测量方案II——间接测量

镁橄榄石激光三次谐波与钛宝石激光二次谐波外差装置

拍频 f= (3d2 -2d1) 反映两束激光的载波包络相位移动

信噪比: 30dB,带宽: <5MHz

Z.Wei et al; Opt Lett, Vol. 27, 2121(2002)


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拍频信号与对应的光谱及其动力学特性

The drifts of the beat frequency (left) and spectra (right) as tuning the Ti:sapphire laser cavity, which reveal the shifting period is about 0.19mm and is reasonable with the calculation based on Dl /lT:S =1/4.


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高动态范围及优化结果

信噪比40dB


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相位锁定与相干控制的实现

Locked beat signal obtained by spectrum analyzer. The spectrum width is 3 kHz (RBW).

Opt Lett, Vol.28, (2003), IEEE J Topics of Quantum Electron, 2003


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同步钛宝石激光的CEP测量

重复频率frep:79MHz

拍频信号:~28MHz

信噪比: 41dB

f1=d1+nfrep

f2=d2+nfrep

f1- f2= d1- d2


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High stably 1.3W synchronized femtosecond Ti:sapphire lasers with attosecond timing jitter and ultralong tolerance

-----CLEO 2005, USA. Accepted

Independently tunable 1.3W femtosecond Ti:sapphire lasers passively synchronized with attosecond timing jitter and ultrahigh robustness

-----Optics Letters, 2005, Accepted

I am pleased to accept your manuscript for publication in Optics Letters based on the review(s) appended below. I encourage you to take advantage of this opportunity to finalize your paper on the basis of the reviewers’ comments.

The authors show passive synchronization between two-modelocked Ti:sapphire laser by novel synchronization scheme. Similar passively-synchronized two lasers have been reported, however, this scheme is new and can be applied to various laser medium. I would like to recommend the manuscript for publication after minor revision.


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I have carefully read and examined manuscript 60807 by J. Tian et al. on "Synchronized two independent 1.3W femtosecond Ti:sapphire lasers with attosecond timing jitter and ultralong tolerance". The paper reports on a new and rigorous design of a self-synchronized two-color laser system with very attractive operating performance. Since this setup might proof useful for a row of applications in ultrafast spectroscopy, the paper is clearly suited for publication in Optics Letters. Especially, the split design using separate media for gain and cross mode locking should make the laser easier to align as compared with previous proposals. Also, the measured sub-fs timing jitter, the tolerance of cavity length mismatch in the 10 micron range and output powers of 1.3 W for each branch are very impressive. The only point I would like to criticize is the moderate level of correctness of English grammar in this manuscript. Sometimes, the text is a bit difficult to understand for the non-specialist reader. As an example, the title might be changed to something like "Independently tunable 1.3W femtosecond Ti:sapphire lasers passively synchronized with attosecond timing jitter and ultrahigh robustness"... For correction of the rest of the manuscript, the authors might want to contact e.g. a native speaker of English for assistance.


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同步飞秒激光的应用

激光脉冲的相干合成


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飞秒激光相干控制在强场中的应用

在极短脉冲的产生、超快非线形光学、 相干合成等领域有重要的意义


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六、总 结

  • 提出了将增益介质与XPM耦合介质完全独立的被动同步技术方案,给出了同步机理的物理模型,基于该原理研制成功了多台高精度同步飞秒激光器

  • 在同步精度、同步的维持运行时间、允许腔长失配量、锁模输出功率、锁模稳定性等方面得到了远高于原国际先进水平的领先结果。

  • 研制成功紧凑实用的产品化样机,填补了国际上无此类产品的空白。

  • 已在量子光学研究领域取得成功应用,可望为超快泵浦探测、飞秒差频光的产生、超宽超连续光谱的获得、光学频率测量等研究提供新型的前沿研究手段。


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谢谢各位


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