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Optical Frequency Combs
Ronald Holzwarth
Max-Planck-Institute for Quantum OpticsGarching, Germany
And
Menlo Systems GmbHMartinsried, Germany
Engelberg, March 6th. 2007
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Team
The Team
Ted HänschTh. UdemCh. GohleM. HermannS. KnünzA. OzawaN. KolachevskyE. PetersJ. AlnisK. PredehlT. WilkenJ. Alnis
MPQ
MenloSystems M. MeiM. FischerP. AdelP. Fendel
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Frequency comb generation
fM = N frep + f0
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Pulsed Lasers
Tasks:
• Create short pulses
• Sustain short pulses in the cavity
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Kerr effect for modelocking
Ti:Sapphire
intensity dependent index of refraction
n(I) = n0 + I(t) n2
Or: saturable absorbers
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Group Velocity Dispersion
dielectriccoating
substrate
blue
red
red
blue
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1978
1978....
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Is the mode spacing equal?
I
907nm
972nm851nm
580,000 modes
44 THz
mode lockedTi:Sapph. laser
single modefiber
750 800 850 900 950 1000 1050
-80
-60
-40
-20
0
Wavelength [nm]
Average: 0.054± 0.111 mHz( 3 x 10 -18 )
log In
tens
ityYes!
MPQ 1998
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First frequency comb measurement
4ff 7f
λ
x2
70 fs Ti:sapphiremode locked laser
quarz fiber
Csatomic clock
10 MHz
4f 3.5f
x2
x2
f
3.39 µm
848 nm
485 nm
I
MPQ 19993.5 f : 4 f
44 THz gap between848 nm and 972nm
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Octave Spanning Spectra
“Photonic Crystal Fiber”
J.C. Knight, W.J. Wadsworth, P. St.J. RusselUniversity of Bath, UK
„Rainbow Fiber“
(Lucent Technologies, 1999)
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Spectral Broadening in PCF
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Comb detail
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Self referenced frequency comb
ωoptical= N ωrep + ωoffset
JILA, MPQ 2000
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HistoricOverview:
Harmonic Frequency chain
ONEOptical reference frequency
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Frequncy comb system
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In real life
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Comb System Schematic
Optical frequency comb generator
(OFCG)
Pump and control
electronics
Optical amplifier(OFCG
dependent)
Pump and control
electronics
Nonlinear spectral
broadening(OFCG
dependent)
Control electronics(optional)
NonlinearInterferometer
CEO det.
Control electronics
Beat detectionunit (BDU)
Detectionelectronics
Control and measurement electronics
Radio-frequencyreference
Continuous-waveLaser
EA B C D
(optional)
Opt
ical
laye
rEl
ectro
nic
laye
r
Optical frequencymeasurement
result
I Laser II Stabilization III Application
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Other comb sources
Cr:LiSAFin collaboration with Fraunhofer Institut für Lasertechnik, Aachen
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Fiber laser
Add free space part to adjust Repetition frequency
Tamura, Ippen et. al. 1994
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Laser Specs
1450 1500 1550 1600 1650
-50
-40
-30
-20
-10
0
inte
nsity
[dB]
Wavelength [nm]
-200 -100 0 100 200
0
2
4
6
8
inte
nsity
time [fs]
interferometric AC intensity AC
3 dB bandwidth: 70nm,
autocorrelation length: approx. 100fs
Average power: approx 200 mW / 100 mW
Repetition frequency: 100 MHz / 250 MHz
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Offset beat signal
-80
-60
-40
-20
0
0 50 100 150 200 250 300
Frequency (MHz)
RF
leve
l (db
m)
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Beat signal free running
-90
-80
-70
-60
-50
-40
19 20 21
Frequency [MHz]
Rf l
evel
[dB]
110 kHz(10 dB)
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Beat signal locked
-40 -20 0 20 40
-100
-80
-60
-40
-20
0bandwidth 1 Hz
sp
ectra
l den
sity
[dBm
]
frequency offset from setpoint [Hz]
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Ti:Sa Phase Noise 100 mrad
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Commercial fiber based frequency comb
Founded in 2001 as a spin of from MPQ to commercialize Frequency comb technology
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Difference Frequency comparision
weighted average: -0.99 µHz ± 1.27 µHz ( 2.0x10 )-21
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Comparison of two fiber based frequncy combs
Stabilized 1.5 µm laser 2 x 10 -16
0 10000 20000 30000 400004461,5
4462,0
4462,5
4463,0
<PTB> = 4462194,769 ± 0,403 Hz (sd= 73,97 Hz) <MPQ> = 4462194,772 ± 0,463 Hz (sd= 84,90 Hz) fre
quen
cy [k
Hz]
time [sec]
0 10000 20000 30000 40000-300
-200
-100
0
100
200
300
deviation = (-0,0052 ± 0,0381) Hz
frequ
ency
dev
iatio
n [H
z]
time [sec]PTB comb MPQ comb
In collaboration withHarald SchnatzGesine Grosche
100 MHz reference
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Test of neighboring comb lines
NIST, Science, 303, 1843 (2004)
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Frequency comb
I
Frequency"Oscillations per Second"
450 THz 750 THz0 Hz
The Nobel Prize in Physics 2005
John L. Hall and Theodor W. Hänsch
for their contributions to the development of laser-based precision spectroscopy, including the optical frequency comb technique
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What is it good for?
Infraredspectroscopy Direct comb
spectroscopyTime domain:
CEO phase
Frequency"Oscillations per Second"
450 THz 750 THz
Fiber laser combsDistance
measurement
Harmonic combsPrecision
spectroscopyDissemination
of time and frequncyOptical Frequency Synthesizer
Differencefrequency combs
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Optical Frequencies
532 nm or 560 THz
560 000 000 000 000 oscillations per second
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Applications
100 000 ultra-stable lasers at onceRevolutionary optical wave meterClockwork for optical atomic clocksUltra-stable microwave source
Tool for fundamental measurementsOptical waveform synthesizer
Source of phase-stabilized femtosecond pulsesKey to attosecond physics
Femtosecond Laser Frequency Combs
An Enabling Technology
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Nobel Poster
Measuring the Frequency of Hydrogen with a Laser Comb
Nobel Prize in Physics 2005, Nobel Poster
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Hydrogen 1S 2S Transition
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Resolving the line
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Hydrogen Spectroscopy
max. drift 200 Hz in 3.68 yrs ⇒ f(1s2s)/f(Cs) < 5 x 10-14 yr-1
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Hydrogen at MPQ since 1986
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History
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Why Hydrogen?
Lamb ShiftQED Test
Rydbergconstant
Are fundamental constantsconstant?
Hydrogenvs.
Antihydrogen
H-D Isotopeshift
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Are the fundamental constants really constant?
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Comparison of optical transitions
ytterbium+
hydrogen
mercury+
Mercury+, 2000-2002, NISTHydrogen, 1999-2003, MPQ
Ytterbium+, 2000-2003, PTB
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All
area
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Astro
area
quasar spectra+ GUT
Keck/HIRES
ESO VLT
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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 20021955
Optical atomic clock: 1 267 402 452 899 920 Hz
Sun dial:One oscillationper day
Pendulum clock:One oscillation per second
Quartzuhr:32 768 oscillations Per second
Cesium atomic clock: 9 192 631770 Hz
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Optical clock – some candidates
Laser-cooled trapped ions
Hg+, In+, Yb+, Sr+, Ca+, ... Paul trap
Cold neutral atoms:
H, Ca, Sr, Yb, Ag, ...Optical lattice
Molecules:Atomic fountain
I2, C2H2, ...
Atom chip
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Shelving Scheme
Clock transition(narrow, forbidden)
cooling transition(broad, dipole allowed)
„Shelving scheme“ (Hans Dehmelt):- Shine in clock laser- Shine in cool laser to see whether the ion is still
In the ground state
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Optical vs. Microwave Clocks
femtosecond combs
Yb+
1.0E-16
1.0E-15
1.0E-14
1.0E-13
1.0E-12
1.0E-11
1.0E-10
1.0E-09
1950 1960 1970 1980 1990 2000 2010Year
Frac
tiona
l unc
erta
inty
Microwave atomic clocksOptical frequency standards
Essen's Cs clock
Cs redefinition of the second
Cs fountain clocks
Sr+
Hg+,Yb+,Ca
iodine-stabilised HeNe
H
H
H
Ca
Hg+
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GPS as reference
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Two frequency combs with slightly different repetition rates
Ti:S laser
Ti:S laser
GaSe
GaSe
125 MHz
125 MHz + ∆
sample
800nm 30 THz
detector
In collaboration with F. Keilmann, MPI Biochemistry Opt. Lett. 29, 1542 (2004)
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IR Spectrum mapped to rf
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Single shot performance
NH3 cell
classical FTIR
fr=125.130 MHz∆=29.93 Hzτ= 70 µs2cm-1
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Load fs pulses into enhancement cavity
6 nJ 400 nJ
112 MHz
Enhancement factor ~ 66
injected: 22 fs pulse duration, 0.65 W average, 200 kW peak
circulating: 27 fs pulse duration, 45 W average, 15 MW peak
MPQ: Nature 436, 234–237 (2005).JILA: Phys. Rev. Lett. 94, 193201 (2005).
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Dispersion causes mismatch
pas
sive
cavi
tyla
ser
freq
uen
cy
com
b
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Pulse characterization
laser oscillator pulse in build-up cavity
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High Harmonic Generation
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Setup
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Making use of the harmonics
20 40 60 80 100 120 140 160-0.02
0.00
0.02
0.04
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0.08
0.10
0.12
0.14
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0.18
Wavelength/nm
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UV Frequency combs
Christoph Gohle et al., Nature 436, 234–237 (2005).
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Ultrafast meets Ultrastable
∆ϕ
f0
I(f)
f
fcfr
Phase sensitivityFrequency Comb
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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).......
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Light wave oscilloscope
-20
-10
0
10
20
Delay ⊗t [fs]
Phot
oele
ctro
n ki
netic
ene
rgy
[eV]
Vect
or p
oten
tial,
A (t)
[fs
MV/
cm]
L
2 4 8 10 14 18 200 6 12 16 22
50
60
70
80
90
E. Goulielmakis et al., Science 305, 1267 (2004)
An “oscilloscope” for light waves
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Above Threshold Ionization (ATI)
“Stereo” ATI
G.G. Paulus et al., , PRL 91, 253004 (2004)
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ART SCHALOW
“Never measure anything but frequency!”
Arthur L. Schawlow
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End