Optical Frequency Combs Ronald Holzwarth Max-Planck-Institute for Quantum Optics Garching, Germany And Menlo Systems GmbH Martinsried, Germany Engelberg, March 6th. 2007
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
Frequency comb generation f M = N f rep + f 0
Pulsed Lasers Tasks: Create short pulses Sustain short pulses in the cavity
Kerr effect for modelocking intensity dependent index of refraction n(i) = n0 + I(t) n2 Ti:Sapphire Or: saturable absorbers
Group Velocity Dispersion blue blue substrate red red dielectric coating
1978 1978...
Is the mode spacing equal? mode locked Ti:Sapph. laser 0 single mode fiber -20 I 580,000 modes 44 THz log Intensity -40-60 -80 750 800 850 900 950 1000 1050 Wavelength [nm] 851nm 972nm Yes! 907nm MPQ 1998 Average: 0.054± 0.111 mhz ( 3 x 10-18 )
First frequency comb measurement x2 x2 485 nm f 4f 7f MPQ 1999 848 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
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
Spectral Broadening in PCF
Comb detail
Self referenced frequency comb ω optical = N ω rep + ω offset JILA, MPQ 2000
Historic Overview: Harmonic Frequency chain ONE Optical reference frequency
Frequncy comb system
In real life
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
Other comb sources Cr:LiSAF in collaboration with Fraunhofer Institut für Lasertechnik, Aachen
Fiber laser Add free space part to adjust Repetition frequency Tamura, Ippen et. al. 1994
Laser Specs 0 8 interferometric AC intensity AC -10 6 intensity [db] -20-30 intensity 4-40 2-50 0 1450 1500 1550 1600 1650 Wavelength [nm] -200-100 0 100 200 time [fs] 3 db bandwidth: 70nm, autocorrelation length: approx. 100fs Average power: approx 200 mw / 100 mw Repetition frequency: 100 MHz / 250 MHz
Offset beat signal 0-20 RF level (dbm) -40-60 -80 0 50 100 150 200 250 300 Frequency (MHz)
Beat signal free running -40 Rf level [db] -50-60 -70-80 110 khz (10 db) -90 19 20 21 Frequency [MHz]
Beat signal locked 0 bandwidth 1 Hz spectral density [dbm] -20-40 -60-80 -100-40 -20 0 20 40 frequency offset from setpoint [Hz]
Ti:Sa Phase Noise 100 mrad
Commercial fiber based frequency comb Founded in 2001 as a spin of from MPQ to commercialize Frequency comb technology
Difference Frequency comparision -21 weighted average: -0.99 µhz ± 1.27 µhz ( 2.0x10 )
Comparison of two fiber based frequncy combs Stabilized 1.5 µm laser 2 x 10-16 PTB comb MPQ comb frequency deviation [Hz] 300 200 100 0-100 deviation = (-0,0052 ± 0,0381) Hz -200-300 0 10000 20000 30000 40000 4463,0 time [sec] In collaboration with Harald Schnatz Gesine Grosche 100 MHz reference frequency [khz] 4462,5 4462,0 <PTB> = 4462194,769 ± 0,403 Hz (sd= 73,97 Hz) <MPQ> = 4462194,772 ± 0,463 Hz (sd= 84,90 Hz) 4461,5 0 10000 20000 30000 40000 time [sec]
Test of neighboring comb lines NIST, Science, 303, 1843 (2004)
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
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
Optical Frequencies 532 nm or 560 THz 560 000 000 000 000 oscillations per second
Femtosecond Laser Frequency Combs Applications An Enabling Technology 100 000 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
Measuring the Frequency of Hydrogen with a Laser Comb Nobel Poster Nobel Prize in Physics 2005, Nobel Poster
Hydrogen 1S 2S Transition
Resolving the line
Hydrogen Spectroscopy max. drift 200 Hz in 3.68 yrs f(1s2s)/f(cs) < 5 x 10-14 yr -1
Hydrogen at MPQ since 1986
History
Why Hydrogen? Rydberg constant Lamb Shift QED Test Hydrogen vs. Antihydrogen H-D Isotope shift Are fundamental constants constant?
Are the fundamental constants really constant?
Comparison of optical transitions mercury+ hydrogen ytterbium+ Hydrogen, 1999-2003, MPQ Mercury+, 2000-2002, NIST Ytterbium+, 2000-2003, PTB
All area
Astro quasar spectra + G UT Keck/HIRES area ESO VLT
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 1656 1918 1955 2002 Sun dial: One oscillation per day Pendulum clock: One oscillation per second Quartzuhr: 32 768 oscillations Per second Cesium atomic clock: 9 192 631770 Hz Optical atomic clock: 1 267 402 452 899 920 Hz
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
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
Optical vs. Microwave Clocks 1.0E-09 1.0E-10 Essen's Cs clock iodine-stabilised HeNe Fractional uncertainty 1.0E-11 1.0E-12 1.0E-13 1.0E-14 1.0E-15 1.0E-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 1950 1960 1970 1980 1990 2000 2010 Year H H +
GPS as reference
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)
IR Spectrum mapped to rf
Single shot performance f r =125.130 MHz =29.93 Hz τ= 70 µs 2cm -1 NH 3 cell classical FTIR
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, 234 237 (2005). JILA: Phys. Rev. Lett. 94, 193201 (2005).
Dispersion causes mismatch laser frequency comb passive cavity
Pulse characterization laser oscillator pulsein build-up cavity
High Harmonic Generation
Setup
Making use of the harmonics 0.18 0.16 0.14 0.12 0.10 0.08 0.06 0.04 0.02 0.00-0.02 20 40 60 80 100 120 140 160 Wavelength/nm
UV Frequency combs Christoph Gohle et al., Nature 436, 234 237 (2005).
Ultrafast meets Ultrastable I(f) ϕ f f 0 f r f c Frequency Comb Phase sensitivity
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, 253004 (2004) Photocathode emission A. Apolonski et al., PRL 92, 073902 (2004) Photocurrents in semiconductors T. Fortier et al., PRL 92, 147403 (2004)...
An oscilloscope for light waves Light wave oscilloscope Photoelectron kinetic energy [ev] 90 80 70 60 20 10 0-10 -20 Vector potential, A L (t) [fs MV/cm] 50 0 2 4 6 8 10 12 14 16 18 20 22 Delay t [fs] E. Goulielmakis et al., Science 305, 1267 (2004)
Above Threshold Ionization (ATI) Stereo ATI G.G. Paulus et al.,, PRL 91, 253004 (2004)
ART SCHALOW Never measure anything but frequency! Arthur L. Schawlow
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