optical frequency comb ofc ( 光頻梳 ) 施宙聰. outline what is an ofc? how to setup an ofc? ofc...
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Optical Frequency CombOFC ( 光頻梳 )
施宙聰
Outline
• What is an OFC?• How to setup an OFC?• OFC in NTHU and Taiwan• Some applications
Comb 梳子
Ruler
Optical frequency comb is a frequency ruler in optical region.
Optical Frequency CombA novel and powerful tool for
optical frequency measurement
Why frequency measurement?
Frequency standard: Atomic clock 1 × 10-15
Cs | 6s 2S1/2, F >
Cs | 6p 2P3/2, F >
F = J + I
9.192631770 GHz (exact)
Frequency can be measured with by far the highest precision of all physical quantities. In the radio frequency (RF) domain (say up to 100 GHz), frequency counters have existed for a long time.
☆Why Optics ?
Mechanics~1000 HzElectronics~100 GHzOptics~1000 THz or higher
=500 nm, c=3 × 108 m/sec, = 6 × 1014 Hz
Why precision frequency measurement
•Hänsch sounded an important precept by quoting Townes' former colleague, Arthur Schawlow: "Never measure anything but frequency," meaning that signal frequency can essentially be measured with higher precision than any other physical quantity.
•quoted from Physics News Update 749
(10GHz) 倍頻 ((10GHz) 倍頻 ) 倍頻 (((10GHz) 倍頻 ) 倍頻 ) 倍頻 ((((10GHz) 倍頻 ) 倍頻 ) 倍頻 )倍頻
(((((10GHz) 倍頻 ) 倍頻 ) 倍頻 ) 倍頻 ) 倍頻… ..
2007/12/07
Transfer-limited pulses of light
• The uncertainty principle says that the product of the temporaland spectral pulse widths is greater than ~1.
Long pulse
Short pulse
What is a transfer-limited broadband source?
Answer: “Mode-locked” Laser
frequency difference between modes, or repetition rate
Delta f = (n+1)c/2L – nc/2L= c/2L
Standing wave criteria in laser cavity
L=n/2 , L=cavity length, n= integer
•Locking the phases of the laser modes yields an ultrashort pulse.
W1T-W2T=2n, T=round trip time
Numerical simulation of mode-locking
Ultrafast lasers often have thousands of modes.
Fourier Transform of Comb Function
Optical Frequency Comb Optical spectra consisting equidistant lines
Optical (frequency) ruler
The output of any mode-locked laser is an OFC.
phase velocity ≠ group velocity
Mode-locked laser is not a good frequency ruler.
一把左右晃動的尺
Frequency Comb
Ti:Saphire Kerr-lensmode locked laser
r=1/T, T: repetition timeControlled by cavity length
pulse-to-pulse carrierenvelope phase shift Controlled by pulsed power
f-2f self-referencing technique
Frequency Comb
= 1× 10-15 上限 = 1× 10-18
Phase-controlled OFC
Both r and CE are locked.
n = nr + CE
fn = nfr + fCE
Optical frequency ruler
Octave generation by photonic crystal fiber
Zero dispersion
Smaller core
Larger nonlinear effect
Supercontinuum over an octave
25
Supercontinuum !!!
The spectrum of supercontinuum
Our OFC
f-2f inter-ferometer
Our OFCOur OFC
1. KLM Ti:sapphire laser Repetition Rate ~ 1 GHz average power ~ 650 mW pulse duration ~ 30 fs.2. Wavelength coverage: 500 ~ 1100 nm3. Frequency accuracy : ~ 10-12 level
“frequency ruler” to measure any unknown frequency at optical region
Frequency measurement
Our ongoing projects
I2 Hyperfine structures 531 nm NIR (760 - 780 nm)
6,7Li: 2S-4S two photon transition Thallium: 6P1/2-7S1/2
3,4He: 21S - 21P
2007/12/07 32
Our preliminary results
TransitionTransition Value (MHz)Value (MHz)
II22 0-14 R78 a10 384,222,471.442(10)
77LiLi F=2-2 407, 808, 975.87(13)
77LiLi F=1-1 407, 809, 282.12(16)
OFC at ITRI Ti:S laser comb generator
Fiber laser comb generator
Cost 5000 k$bulkyfine alignment
Cost 400 k$compactportablerobust
工研院量測中心 彭錦龍博士
Other OFC Researches
• Attosecond pulse formation 孔慶昌 ( 中研院原分所 ) 、潘犀靈 ( 清華
大學 )• Direct OFC spectroscopy 鄭王曜 ( 中研院原分所 )• Precision spectroscopy using OFC 崔祥辰、蔡錦俊 ( 成功大學 )
Applications
• Optical clock• Astro-comb• Direct frequency comb spectroscopy• Distance measurement
Optical Clock
• Stabilized laserLinewidth < 1 HzStability better than 1@10-18
How to link between optical and microwave frequency?
Optical clock
2007/12/07 38
Astro-comb ( 太空光頻梳 )
• Astro-comb: search of the Earth-analogous extra-solar planets
• A laser frequency comb that enables radial velocity measurements with a precision of
1 cm/s• “Astro-comb: a novel tool for the exoplanet
search and the direct observation of our universe expansion”
李志豪
Hansch
Cavity-enhanced OFC spectroscopyDetect the Breath of Disease
Cavity-enhanced frequency comb Fourier transform spectroscopy
Comb 1
Comb 2
Absolute measurement of a longdistance to less than an optical fringe
2007/12/07 45
“For their contributions to the development of laser-based precision
spectroscopy, including the optical frequency
comb technique”
2007/12/07 46
John (Jan) L. Hall雷射穩頻第一高手
Theodor (Ted) W. Hänsch氫原子光譜第一高手They are Good friends!
2007/12/07 47
J. L. Hall, K. T. Udem, R. Drever and T. W.Hansch
Summer 2005
J. L. Hall, K. T. Udem, R. Drever and T. W.Hansch
Summer 2005
2007/12/07 48
量測所使用的工具決定了物理的說法 -- J. S. Schwinger
人類永無止境的好奇由精密量測完成
Ruler of Light Cundiff S, Ye J, Hall J, SCIENTIFIC AMERICAN ( 科學人 ) Volume: 298
Issue: 4 Pages: 74-81 , APR 2008 2005 年諾貝爾物理獎得主 --- 漢希 (T.W.
Hansch) 與霍爾 (J.L. Hall)http://psroc.phys.ntu.edu.tw/bimonth/v28/15.pdf