Optical Frequency Combs

Transcription

1 Optical Frequency Combs Ronald Holzwarth Max-Planck-Institute for Quantum Optics Garching, Germany And Menlo Systems GmbH Martinsried, Germany Engelberg, March 6th. 2007

2 The Team Team MPQ MenloSystems Ted Hänsch Th. Udem Ch. Gohle M. Hermann S. Knünz A. Ozawa N. Kolachevsky E. Peters J. Alnis K. Predehl T. Wilken J. Alnis M. Mei M. Fischer P. Adel P. Fendel

3 Frequency comb generation f M = N f rep + f 0

4 Pulsed Lasers Tasks: Create short pulses Sustain short pulses in the cavity

5 Kerr effect for modelocking intensity dependent index of refraction n(i) = n0 + I(t) n2 Ti:Sapphire Or: saturable absorbers

6 Group Velocity Dispersion blue blue substrate red red dielectric coating

7

8 Is the mode spacing equal? mode locked Ti:Sapph. laser 0 single mode fiber -20 I 580,000 modes 44 THz log Intensity Wavelength [nm] 851nm 972nm Yes! 907nm MPQ 1998 Average: 0.054± mhz ( 3 x )

9 First frequency comb measurement x2 x2 485 nm f 4f 7f MPQ nm x2 3.5 f : 4 f f I 4f 3.5f 44 THz gap between 848 nm and 972nm 3.39 µm λ Cs atomic clock 10 MHz 70 fs Ti:sapphire mode locked laser quarz fiber

10 Octave Spanning Spectra Rainbow Fiber (Lucent Technologies, 1999) Photonic Crystal Fiber J.C. Knight, W.J. Wadsworth, P. St.J. Russel University of Bath, UK

11 Spectral Broadening in PCF

12 Comb detail

13 Self referenced frequency comb ω optical = N ω rep + ω offset JILA, MPQ 2000

14 Historic Overview: Harmonic Frequency chain ONE Optical reference frequency

15 Frequncy comb system

16 In real life

17 Comb System Schematic I Laser II Stabilization III Application Optical layer Optical frequency comb generator (OFCG) Optical amplifier (OFCG dependent) Nonlinear spectral broadening (OFCG dependent) Nonlinear Interferometer A B C CEO det. D Beat detection unit (BDU) E Continuouswave Laser Pump and control electronics Pump and control electronics Control electronics (optional) Control electronics Detection electronics Electronic layer Control and measurement electronics (optional) Radio-frequency reference Optical frequency measurement result

18 Other comb sources Cr:LiSAF in collaboration with Fraunhofer Institut für Lasertechnik, Aachen

19 Fiber laser Add free space part to adjust Repetition frequency Tamura, Ippen et. al. 1994

20 Laser Specs 0 8 interferometric AC intensity AC intensity [db] intensity Wavelength [nm] time [fs] 3 db bandwidth: 70nm, autocorrelation length: approx. 100fs Average power: approx 200 mw / 100 mw Repetition frequency: 100 MHz / 250 MHz

21 Offset beat signal 0-20 RF level (dbm) Frequency (MHz)

22 Beat signal free running -40 Rf level [db] khz (10 db) Frequency [MHz]

23 Beat signal locked 0 bandwidth 1 Hz spectral density [dbm] frequency offset from setpoint [Hz]

24 Ti:Sa Phase Noise 100 mrad

25 Commercial fiber based frequency comb Founded in 2001 as a spin of from MPQ to commercialize Frequency comb technology

26 Difference Frequency comparision -21 weighted average: µhz ± 1.27 µhz ( 2.0x10 )

27 Comparison of two fiber based frequncy combs Stabilized 1.5 µm laser 2 x PTB comb MPQ comb frequency deviation [Hz] deviation = (-0,0052 ± 0,0381) Hz ,0 time [sec] In collaboration with Harald Schnatz Gesine Grosche 100 MHz reference frequency [khz] 4462,5 4462,0 <PTB> = ,769 ± 0,403 Hz (sd= 73,97 Hz) <MPQ> = ,772 ± 0,463 Hz (sd= 84,90 Hz) 4461, time [sec]

28 Test of neighboring comb lines NIST, Science, 303, 1843 (2004)

29 Frequency comb I 0 Hz 450 THz 750 THz Frequency "Oscillations per Second" The Nobel Prize in Physics 2005 John L. Hall and Theodor W. Hänsch for their contributions to the development of laserbased precision spectroscopy, including the optical frequency comb technique

30 What is it good for? Time domain: CEO phase Distance measurement Infrared spectroscopy Direct comb spectroscopy Fiber laser combs Precision spectroscopy Optical Frequency Synthesizer 450 THz 750 THz Frequency "Oscillations per Second" Difference frequency combs Harmonic combs Dissemination of time and frequncy

31 Optical Frequencies 532 nm or 560 THz oscillations per second

32 Femtosecond Laser Frequency Combs Applications An Enabling Technology ultra-stable lasers at once Revolutionary optical wave meter Clockwork for optical atomic clocks Ultra-stable microwave source Tool for fundamental measurements Optical waveform synthesizer Source of phase-stabilized femtosecond pulses Key to attosecond physics

33 Measuring the Frequency of Hydrogen with a Laser Comb Nobel Poster Nobel Prize in Physics 2005, Nobel Poster

34 Hydrogen 1S 2S Transition

35 Resolving the line

36 Hydrogen Spectroscopy max. drift 200 Hz in 3.68 yrs f(1s2s)/f(cs) < 5 x yr -1

37 Hydrogen at MPQ since 1986

38 History

39 Why Hydrogen? Rydberg constant Lamb Shift QED Test Hydrogen vs. Antihydrogen H-D Isotope shift Are fundamental constants constant?

40 Are the fundamental constants really constant?

41 Comparison of optical transitions mercury+ hydrogen ytterbium+ Hydrogen, , MPQ Mercury+, , NIST Ytterbium+, , PTB

42 All area

43 Astro quasar spectra + G UT Keck/HIRES area ESO VLT

44 Optical clocks A clock consists of a oscillator, and a counter that counts these uniform oscillations. The finer the partition of time, the more accurate the clock can be From 3500 BC Sun dial: One oscillation per day Pendulum clock: One oscillation per second Quartzuhr: oscillations Per second Cesium atomic clock: Hz Optical atomic clock: Hz

45 Optical clock some candidates Laser-cooled trapped ions Hg +, In +, Yb +, Sr +, Ca +,... Paul trap Cold neutral atoms: H, Ca, Sr, Yb, Ag,... Molecules: I 2, C 2 H 2,... Optical lattice Atomic fountain Atom chip

46 Shelving Scheme cooling transition (broad, dipole allowed) Clock transition (narrow, forbidden) Shelving scheme (Hans Dehmelt): - Shine in clock laser - Shine in cool laser to see whether the ion is still In the ground state

47 Optical vs. Microwave Clocks 1.0E E-10 Essen's Cs clock iodine-stabilised HeNe Fractional uncertainty 1.0E E E E E E-16 Cs redefinition of the second Microwave atomic clocks Optical frequency standards femtosecond combs Ca H Hg +,Yb +,Ca Yb + Sr Hg + Cs fountain clocks Year H H +

48 GPS as reference

49 Two frequency combs with slightly different repetition rates Ti:S laser 800nm GaSe 30 THz 125 MHz Ti:S laser 125 MHz + GaSe detector sample In collaboration with F. Keilmann, MPI Biochemistry Opt. Lett. 29, 1542 (2004)

50 IR Spectrum mapped to rf

51 Single shot performance f r = MHz =29.93 Hz τ= 70 µs 2cm -1 NH 3 cell classical FTIR

52 Load fs pulses into enhancement cavity 112 MHz 6 nj 400 nj injected: 22 fs pulse duration, 0.65 W average, 200 kw peak circulating: 27 fs pulse duration, 45 W average, 15 MW peak Enhancement factor ~ 66 MPQ: Nature 436, (2005). JILA: Phys. Rev. Lett. 94, (2005).

53 Dispersion causes mismatch laser frequency comb passive cavity

54 Pulse characterization laser oscillator pulsein build-up cavity

55 High Harmonic Generation

56 Setup

57 Making use of the harmonics Wavelength/nm

58 UV Frequency combs Christoph Gohle et al., Nature 436, (2005).

59 Ultrafast meets Ultrastable I(f) ϕ f f 0 f r f c Frequency Comb Phase sensitivity

60 Nonlinear interactions with few cycle pulses High-harmonic generation A. Baltuska et al., Nature 421, 611 (2003) Above-threshold ionization G.G. Paulus et al.,, PRL 91, (2004) Photocathode emission A. Apolonski et al., PRL 92, (2004) Photocurrents in semiconductors T. Fortier et al., PRL 92, (2004)...

61 An oscilloscope for light waves Light wave oscilloscope Photoelectron kinetic energy [ev] Vector potential, A L (t) [fs MV/cm] Delay t [fs] E. Goulielmakis et al., Science 305, 1267 (2004)

62 Above Threshold Ionization (ATI) Stereo ATI G.G. Paulus et al.,, PRL 91, (2004)

63 ART SCHALOW Never measure anything but frequency! Arthur L. Schawlow

64 End