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.

Slide 1 Optical Frequency Comb OFC ( 光頻梳 ) 施宙聰 Slide 2 Outline What is an OFC? How to setup an OFC? OFC in NTHU and Taiwan Some applications Slide 3 Comb 梳子 Slide 4 Ruler Optical frequency comb is a frequency ruler in optical region. Slide 5 Optical Frequency Comb A novel and powerful tool for optical frequency measurement Slide 6 Why frequency measurement? Slide 7 Frequency standard: Atomic clock 1 × 10 -15 Cs | 6s 2 S 1/2, F > Cs | 6p 2 P 3/2, F > F = J + I 9.192631770 GHz (exact) Slide 8 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 Hz Electronics~100 GHz Optics~1000 THz or higher =500 nm, c=3 × 10 8 m/sec, = 6 × 10 14 Hz Slide 9 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 Slide 10 (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 ….. Slide 11 2007/12/07 Slide 12 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 Slide 13 What is a transfer-limited broadband source? Answer:“Mode-locked” Laser Slide 14 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 Slide 15 Locking the phases of the laser modes yields an ultrashort pulse. W 1 T-W 2 T=2n , T=round trip time Slide 16 Numerical simulation of mode-locking Ultrafast lasers often have thousands of modes. Slide 17 Fourier Transform of Comb Function Slide 18 Optical Frequency Comb Optical spectra consisting equidistant lines Optical (frequency) ruler  The output of any mode-locked laser is an OFC. Slide 19 phase velocity ≠ group velocity Mode-locked laser is not a good frequency ruler. 一把左右晃動的尺 Slide 20 Frequency Comb Ti:Saphire Kerr-lens mode locked laser  r =1/T, T: repetition time Controlled by cavity length  pulse-to-pulse carrier envelope phase shift Controlled by pulsed power Slide 21 f-2f self-referencing technique Slide 22 Frequency Comb  = 1× 10 -15 上限  = 1× 10 -18 Slide 23 Phase-controlled OFC Both  r and  CE are locked.  n = n  r +  CE f n = nf r + f CE Optical frequency ruler Slide 24 Octave generation by photonic crystal fiber Zero dispersion Smaller core Larger nonlinear effect Supercontinuum over an octave Slide 25 25 Supercontinuum !!! Slide 26 The spectrum of supercontinuum Slide 27 Our OFC f-2f inter- ferometer Slide 28 Slide 29 Our OFC 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 ruler” to measure any unknown frequency at optical region Slide 30 Frequency measurement Slide 31 Our ongoing projects  I 2 Hyperfine structures  531 nm  NIR (760 - 780 nm)  6,7 Li: 2S-4S two photon transition  Thallium: 6P 1/2 -7S 1/2  3,4 He: 2 1 S - 2 1 P Slide 32 2007/12/0732 Our preliminary results Transition Value (MHz) I2I2I2I2 0-14 R78 a10384,222,471.442(10) 7 Li F=2-2407, 808, 975.87(13) 7 Li F=1-1407, 809, 282.12(16) Slide 33 OFC at ITRI Ti:S laser comb generator Fiber laser comb generator Cost 5000 k$ bulky fine alignment Cost 400 k$ compact portable robust 工研院量測中心 彭錦龍博士 Slide 34 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 ) Slide 35 Applications Optical clock Astro-comb Direct frequency comb spectroscopy Distance measurement Slide 36 Optical Clock Stabilized laser Linewidth < 1 Hz Stability better than 1@10 -18 How to link between optical and microwave frequency? Slide 37 Optical clock Slide 38 2007/12/0738 Slide 39 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 Slide 40 Hansch Slide 41 Cavity-enhanced OFC spectroscopy Detect the Breath of Disease Slide 42 Cavity-enhanced frequency comb Fourier transform spectroscopy Slide 43 Comb 1 Comb 2 Slide 44 Absolute measurement of a long distance to less than an optical fringe Slide 45 2007/12/0745 “For their contributions to the development of laser-based precision spectroscopy, including the optical frequency comb technique” Slide 46 2007/12/0746 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! Slide 47 2007/12/0747 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 Slide 48 2007/12/0748 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 Slide 49 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 http://psroc.phys.ntu.edu.tw/bimonth/v28/15. pdf