femtosecond lasers: the gears of optical atomic clocks · femtosecond lasers: the gears of optical...
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Femtosecond lasers: the gears of optical atomic clocks
Scott A. DiddamsTime & Frequency Division
National Institute of Standards and TechnologyBoulder, Colorado 80305
Guest Researchers:Albrecht Bartels (U. Aachen)Eugene Ivanov (U. West. Aust.) Long-Sheng Ma (U. Colorado & BIPM)Lennart Robertsson (BIPM)Utako Tanaka (CRL, Japan)Carol Tanner (Notre Dame U.)Thomas Udem (MPQ)Karl Weber (U. Melbourne)Tim Birks (U. of Bath)Robert Windeler (OFS)
NIST, Time & Frequency Division:
• Optical Frequency MeasurementsLeo HollbergAnne Curtis (grad student)Chris OatesTanya Ramond (post-doc)Isabell Thomann (grad student)Kristan Corwin (post-doc)Nate Newbury
• Ion StorageJim Bergquist Sebastian Bize (post-doc)Bob DrullingerWayne ItanoWindell Oskay (post-doc)Dave Wineland
• Atomic StandardsSteve JeffertsTom HeavnerTom Parker
What Makes a Clock?
Earth Rotation SundialPendulum Clock Gears/HandsQuartz Crystal Electronic Counter
Oscillator + Counting Mechanism
ATOMIC CLOCKSMicrowave Transition + Oscillator Electronic CounterOptical Transition + Laser Frequency Chain
Optical ClocksWhy are they interesting?
• Optical standards have superior stability:e.g. Ca optical standard<2x10-16 at 1 s
• Optical standards have the potential for greatly improved accuracy: e.g. approaching 1x10-18 for single trapped ions
ττσ 1~)(
Nff
o
∆
10-17
10-16
10-15
10-14
10-13
Alla
n D
evia
tion
-- In
stab
ility
10-2 100 102 104 106
Averaging Time (s)
H-maser
Cs
Hg+
Ca
1 day 1 monthCa
ττσ 1~)(
yInstabilit Limited Quantum
Nff
o
∆
Oscillator Stabilityσ(
τ)
Single Hg+ Ion Optical Standard
Tprobe = 20 ms
Tprobe = 120 ms
~ 6.5 Hz
2S1/2
2D5/2
2P1/2
Observefluorescence(λ = 194 nm)
“clock” transition @fo ≈ 1.06x1015 Hz
199Hg+
F = 1
F = 0
F=2F=3
“Clock”Transition(λ=282 nm)
F=0F=1
Q=1.6×1014 !!
J. Bergquist, et al. (NIST)
Femtosecond-Laser-BasedOptical Synthesizer
• What is it? A device that phase-coherently connects optical and RF/microwave domains.
Optical Synthesizer
nm×
µ-wave in
optical in
µ-wave out
optical out
Femtosecond-Laser-Based SynthesizerPump Power Control of fo
Ti:SapphireGain
Output650 mW
frep= 1 GHz
PZT Control of frep
AOM5-8 W of532 nm
-60
-50
-40
-30
-20
Rela
tive
Powe
r (dB
)
12001000800600Wavelength (nm)
input output
MicrostructureFiber
S.A. Diddams, et al.Proc. SPIE vol. 4269 (200)
+1
0
Optical Clock with aFemtosecond Synthesizer
fm
PLL 2 fbOptical Standard (fHg )
frClock Output
fr = fHg ÷ m(m~106)
Femtosecond Laser +Microstructure Fiber
I(f)
f
fox2 f2n=fo+2nfrfn=fo+nfr
PLL 1
fr÷100
fr÷100
S. Diddams, et al. Science 293, 825 (2001)
Comparison of Hg+ Optical Clock to a H-maser
949,700
949,600
949,500
949,400
f Hg/
2 - 5
32 3
60 8
04 0
00 0
00 (H
z)
300025002000150010005000Time (s)
5 s gate timeScatter: 37 Hz
10-15
2
4
6810-14
2
4
68
Alla
n D
evia
tion
1 10 100 1000Averaging Time (s)
τ−1/2
Instability limitedby H-maser
Comparison of Hg+ (optical) to Cs (microwave)
-30
-20
-10
0
10
20
30f H
g-1
064
721
609
899
143.
4 (H
z)
Aug 00 Feb 01 Aug 01 Feb 02 Aug 02Measurement Date
Weighted Average of all Data:...899 143.4 (1.0) Hz Original Measurement: ...899 142.6(2.5) PRL 86, 4996 (2001)
Hg+ -- Cs comparison limits possible variation of α
constant assumed are and if yr101.1
or
yr107 to
of s variationpossible constrains dataPresent
1-15
1-15
03.6
×≤
×≤
∝
−
−
p
eCs
Hg
Cs
p
eCs
Hg
Cs
mmg
mmg
αα
νν
ανν
&
Dzuba, Flambaum, WebbPRA 59, 230 (1999)
S. Bize, et al. (submitted to Phys. Rev. Letters)
Hg-Ca Optical Comparison
fm
PLL 2 fbOptical Standard (fHg )
Femtosecond Laser +Microstructure Fiber
I(f)
f
fo
Self-Referencing
PLL 1
fCa
1.2
1.0
0.8
0.6
0.4
0.2Beat
am
plitu
de (a
.u.)
-1,000 -500 0 500 1,000Frequency (Hz)
1.0
0.8
0.6
0.4
0.2
0.0
Beat
Am
plitu
de (a
.u.)
-10,000 -5,000 0 5,000 10,000Frequency (Hz)
40,0000-40,000Frequency (Hz)
a)
b)
“Beat” between Hg+ and Ca across 76 THzMillions of Narrow Linewidth Oscillators
Femtosecondlaser
Hg+Standard
CaStandard
180 mfiber
10 mfiber
10 m fiber noise
180 m fiber noise
Hg-Ca beat
Testing the Femtosecond Synthesizer
Diode Laser456 THz
fs Comb #1
fs Comb #2
fr1 fo1
fr2 fo2
Optical Heterodyne(tests comb teeth)
Stability: <6ä10-16 τ-1
Accuracy: <4ä10-17
PMT
X-Correlation(tests envelope)Jitter: 400 as (1-100 Hz)Stability: <2ä10-15 τ-1
RF Mixing(tests microwave output)
Stability: ~2ä10-14 τ-1
Stability of Microwave and Optical Signals
10-17
10-16
10-15
10-14
10-13
Alla
n D
evia
tion
0.1 1 10 100Averaging Time (s)
Nonlinear X-Correlation Photodection +
Microwave RF Mixing
2x10-14 τ−1
2x10-15 τ−1
(Measurement Limited)
-200
-180
-160
-140
-120
-100
-80
100 101 102 103 104 105
Frequency [Hz]
L(f)
[dBc
/Hz]
a
b
cde
f
g
Comparison of Various Oscillators/Synthesizers
Phase noise for 1 GHz carrier
a. Premium quartz oscillatorb. Low noise synthesizerc. Sapphire oscillatord. Ca optical (projected)e. Hg+ optical cavityf. fs synthesizer: optical pulse traing. fs synthesizer: microwave output
Potential Limitations to RF Stability
•Shot Noise:rfr
shoty P
feiR6nf21)( ∆
τπτσ =
kHz 150 10dBm, mA, 4 for 101)( -115 =−==×= − ∆fPi rfshoty ττσ
•Excess noise from Laser or Microstructure Fiber
→ AM-PM conversion exists. For example, timing jitter of 1-10 ps/mW in various photodetectors.
→ Saturation with high peak power??
•Excess Phase Noise in Photodetection
Amplification of Fundamental Noise in Microstructured Fibers
-60
-40
-20 Power (dB
)140012001000800600
Wavlength (nm)
-140
-120
RIN
(dB
c/H
z)
(a)
(b) λL λR
-120
-100
RIN
(d
Bc/
Hz)
1.00.80.60.40.20.0Pulse Energy (nJ)
600
400
Wid
th (n
m)
(a)
(b)
K. Corwin, N. Newbury, J. Dudley,S. Coen, K. Weber, S. Diddams,R. Windeler (to appear in PRL)
ExperimentTheory
• High repetition rate—800 mW total power• Compact, 5-element ring design
532 nmpump
L M1 M2
M3 OC
Ti:Sa
A New, Simpler Tool:1 GHz Ti:sapphire Octave-Spanning Oscillator
A. Bartels and H. Kurz, Opt. Lett. 27, 1839 (2002)
KEY ELEMENT: 1000mm ROC convex mirror (M3) increased self-amplitude modulation shorter pulses enhanced
self-phase modulation
10-6
10-5
10-4
10-3
10-2
10-1
100
101
102
Pow
er P
er 1
GH
z M
ode
(µW
)
140012001000800600
Wavelength (nm)
Frequency triple 960nm and double 640nm to obtain 320nm heterodyne (fo)
3fn – 2fm = 3(nfr + fo) – 2(mfr + fo) = fo
Output spectrum of laser
Detection of fo without microstructure fiber
No critical alignment of nonlinear elementsCan be phase-locked nearly indefinitely….
Ramond et al. Optics Letters 27, 1842
Ti:Salaser
BBO640 nm
LiIO3
480 nm960 nm
960 nm
BBO
320 nm
Single ModeFiber
PMT320 nm
2f
3ffo
U. Morgner, et al. Phys. Rev. Lett. 86, 5462 (2001).T. Ramond, et al. Opt. Lett. 27, 1842 (2002).
Long-term Phase Locking of Broadband Laser
-5
0
5
c)
b)
a)
fLD Drift (kH
z)
f R -
998
,092
,449
.54
Hz
(mH
z)
f b - 6
00 M
Hz
(mH
z)
f 0 - 1
00 M
Hz
(mH
z)
0
50
100
0 2 4 6 8 10 12 14 16 18 20-0.20.00.20.40.6
Time (h)
-100
0
100
200
300
Offset Frequency
Beat with StabilizedLaser Diode
RepetitionRate
Control of femtosecond laser: <6ä10-18 @ 10 sïcan count >1019 optical cycles without missing a single one!
Summary + Outlook•Femtosecond lasers combined with cold atom standards will be the basis of future atomic clocks
(stability ~1µ10-16 @1s, accuracy < 1µ10-17 )
•Emerging applications and uses:--secure communications--ultra low noise microwaves (RADAR)--length metrology--time/frequency transfer over fiber networks--remote sensing--extreme nonlinear optics
•Smaller, more compact, more robust--novel solid state femtosecond lasers--broader spectra, different wavelength regimes