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