Optical Frequency Comb OFC (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 Comb A novel and powerful tool for optical frequency measurement

Why frequency measurement?

Frequency standard: Atomic clock 1 × 10-15 Cs | 6p 2P3/2, F > F = J + I 9.192631770 GHz (exact) Cs | 6s 2S1/2, F >

l=500 nm, c=3 × 108 m/sec, n= 6 × 1014 Hz Mechanics~1000 Hz Electronics~100 GHz Optics~1000 THz or higher ☆Why Optics ? l=500 nm, c=3 × 108 m/sec, n= 6 × 1014 Hz 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 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)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 temporal and spectral pulse widths is greater than ~1. Long pulse Short pulse

What is a transfer-limited broadband source? Answer: “Mode-locked” Laser

Standing wave criteria in laser cavity L=nl/2 , L=cavity length, n= integer frequency difference between modes, or repetition rate Delta f = (n+1)c/2L – nc/2L= c/2L

Locking the phases of the laser modes yields an ultrashort pulse. W1T-W2T=2np, 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-lens mode locked laser wr=1/T, T: repetition time Controlled by cavity length D pulse-to-pulse carrier envelope phase shift Controlled by pulsed power

f-2f self-referencing technique

Frequency Comb Dn/n = 1× 10-15 上限 Dn/n = 1× 10-18

Phase-controlled OFC Both wr and wCE are locked. wn = nwr + wCE fn = nfr + fCE Optical frequency ruler

Octave generation by photonic crystal fiber Zero dispersion Smaller core Larger nonlinear effect Supercontinuum over an octave

Supercontinuum !!!

The spectrum of supercontinuum

Our OFC f-2f inter- ferometer

2. Wavelength coverage: 500 ~ 1100 nm Our OFC “frequency ruler” to measure any unknown frequency at optical region 1. KLM Ti:sapphire laser Repetition Rate ~ 1 GHz average power ~ 650 mW pulse duration ~ 30 fs. 2. Wavelength coverage: 500 ~ 1100 nm 3. Frequency accuracy : ~ 10-12 level

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

Our preliminary results Transition Value (MHz) I2 0-14 R78 a10 384,222,471.442(10) 7Li F=2-2 407, 808, 975.87(13) F=1-1 407, 809, 282.12(16) 2007/12/07

OFC at ITRI Ti:S laser comb generator Cost 5000 k$ bulky fine alignment Fiber laser comb generator Cost 400 k$ compact portable robust 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 laser Linewidth < 1 Hz Stability better than 1@10-18 How to link between optical and microwave frequency?

Optical clock

2007/12/07

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 spectroscopy Detect the Breath of Disease

Cavity-enhanced frequency comb Fourier transform spectroscopy

Comb 1 Comb 2

Absolute measurement of a long distance to less than an optical fringe

“For their contributions to the development of laser-based precision spectroscopy, including the optical frequency comb technique” 2007/12/07

John (Jan) L. Hall 雷射穩頻第一高手 Theodor (Ted) W 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

J. L. Hall, K. T. Udem, R. Drever and T. W.Hansch Summer 2005 2007/12/07

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 2007/12/07

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