1. Optical Frequency Comb OFC (光頻梳)
施宙聰

2. Outline What is an OFC? How to setup an OFC? OFC in NTHU and Taiwan
Some applications

3. Comb 梳子

4. Ruler
Optical frequency comb is a frequency ruler in optical region.

5. Optical Frequency Comb
A novel and powerful tool for
optical frequency measurement

6. Why frequency measurement?

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

8. 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.

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

10. (((((10GHz)倍頻)倍頻)倍頻)倍頻)倍頻…..

11. 2007/12/07

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

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

14. 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

15. Locking the phases of the laser modes yields an ultrashort pulse.
W1T-W2T=2np, T=round trip time

16. Numerical simulation of mode-locking
Ultrafast lasers often have thousands of modes.

17. Fourier Transform of Comb Function

18. Optical Frequency Comb
Optical spectra consisting equidistant lines
Optical (frequency) ruler
 The output of any mode-locked laser is an OFC.

19. phase velocity ≠ group velocity Mode-locked laser is not a good frequency ruler.
一把左右晃動的尺

20. 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

21. f-2f self-referencing technique

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

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

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

25. Supercontinuum !!!

26. The spectrum of supercontinuum

27. Our OFC
f-2f inter-
ferometer

29. 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 : ~ level

30. Frequency measurement

31. Our ongoing projects I2 Hyperfine structures
531 nm
NIR ( nm)
6,7Li: 2S-4S two photon transition
Thallium: 6P1/2-7S1/2
3,4He: 21S - 21P

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

33. 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

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)

35. Applications Optical clock Astro-comb
Direct frequency comb spectroscopy
Distance measurement

36. Optical Clock Stabilized laser Linewidth < 1 Hz
Stability better than
How to link between optical and microwave frequency?

37. Optical clock

38. 2007/12/07

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

40. Hansch

41. Cavity-enhanced OFC spectroscopy Detect the Breath of Disease

42. Cavity-enhanced frequency comb Fourier transform spectroscopy

43. Comb 1
Comb 2

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

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
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
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
2007/12/07

49. Ruler of Light Cundiff S, Ye J, Hall J, SCIENTIFIC AMERICAN (科學人) Volume: 298 Issue: 4 Pages: , APR 年諾貝爾物理獎得主---漢希(T.W. Hansch)與霍爾(J.L. Hall)