Conférence Universitaire de Suisse OccidentaleProgramme doctoral en physique, Printemps 2014 1Vapour cell atomic frequency standards
Lecture 4, Gaetano Mileti, 13.03.2014
ATOMIC CLOCKS: BASIC PRINCIPLES AND APPLICATIONSLecture 4 Vapour cell atomic frequency standards
Gaetano Mileti, C. Affolderbach, Laboratoire Temps – Fréquence (LTF), Université de Neuchâtel
CUSO – Conférence Universitaire de Suisse OccidentaleProgramme doctoral de Physique – Printemps 2014
13.03.2014
Conférence Universitaire de Suisse OccidentaleProgramme doctoral en physique, Printemps 2014 2Vapour cell atomic frequency standards
Lecture 4, Gaetano Mileti, 13.03.2014
PROGRAM OF CUSO LECTURES 2014 (3RD EDITION)
Thursday February 20, lecture # 1G. Mileti, Laboratoire Temps‐Fréquence (LTF), Université de NeuchâtelIntroduction to the lectures and to atomic clocks, Cs thermal beam standards
Thursday February 27, lecture # 2L.‐G. Bernier, Laboratoire de Photonique, Temps et Fréquence, Institut fédéral de métrologie (METAS)Atomic time scale, Allan deviation, time transfer, Hydrogen Masers & its applications
Thursday March 6, lecture # 3S. Schilt and R. Matthey, Laboratoire Temps‐Fréquence (LTF), Université de NeuchâtelFundamentals in laser spectroscopy and laser frequency stabilisations. Examples of applications
Thursday March 13, lecture # 4G. Mileti and C. Affolderbach, Laboratoire Temps‐Fréquence (LTF), Université de NeuchâtelVapour cell standards, chip‐scale atomic clocks, applications in telecommunications and navigation
Thursday March 20, lecture # 5J. Guéna, LNE‐SYRTE (Laboratoire National de Métrologie et d'Essais, SYRTE), Observatoire de ParisAtomic fountains, primary frequency standards
Thursday March 27, lecture # 6T. Südmeyer, LTF‐UniNe and T. Kippenberg, Laboratoire de Photonique et Mesures Quantiques, EPFLIntroduction to optical combs and applications. Examples of recent developments.
Thursday April 3, lecture # 7C. Salomon, Laboratoire Kastler Brossel, Département de Physique Ecole Normale Supérieure, ParisLaser cooling and trapping of atoms. Bose‐Einstein Condensation. The ACES experiment on the ISS
Thursday April 10, lecture # 8S. Bize, LNE‐SYRTE (Laboratoire National de Métrologie et d'Essais, SYRTE), Observatoire de ParisOptical frequency standards and applications
Conférence Universitaire de Suisse OccidentaleProgramme doctoral en physique, Printemps 2014 3Vapour cell atomic frequency standards
Lecture 4, Gaetano Mileti, 13.03.2014
Rubidium clocks:
• Chapter 8 of book J. Vanier and C. Audoin, Adam-Hilger, Bristol (1989)
• Article of J. Camparo, Physics Today, November (2007)
From the lecturer (Rb clocks):
• PhD of G. Mileti thesis, Université de Neuchâtel (1995)
• ESA bulletin, vol. 122 (May), p. 53, (2005)
• Proc. SPIE, vol. 5830, p. 159, (2005)
• Comptes-rendus du Congrès Intern. de Chronométrie, p. 91 (2007)
• Journal and Web site of the Swiss Physical Society, July, (2008)
• Tutorial on Rb clocks joint IFCS EFTF conference, Prague, July, (2013)
ESSENTIAL BIBLIOGRAPHY ON RB CLOCKS
Conférence Universitaire de Suisse OccidentaleProgramme doctoral en physique, Printemps 2014 4Vapour cell atomic frequency standards
Lecture 4, Gaetano Mileti, 13.03.2014
1. Vapour cell standards basic principle
2. Laser-pumped Rubidium clock
3. Coherent Population Trapping
4. Chip-scale atomic clocks
CONTENTS OF LECTURE 4
Conférence Universitaire de Suisse OccidentaleProgramme doctoral en physique, Printemps 2014 5Vapour cell atomic frequency standards
Lecture 4, Gaetano Mileti, 13.03.2014
1. Vapour cell standards basic principle
2. Laser-pumped Rubidium clock
3. Coherent Population Trapping
4. Chip-scale atomic clocks
CONTENTS OF LECTURE 4
Conférence Universitaire de Suisse OccidentaleProgramme doctoral en physique, Printemps 2014 6Vapour cell atomic frequency standards
Lecture 4, Gaetano Mileti, 13.03.2014
Heart of the clock: a Rubidium vapour cell
5S1/2
F=1
F=2mF = 0mF = -1mF= -2
mF = 1mF = 2
mF = 0mF = -1
mF = 1
6.8346 GHz
87Rb
Rb partial pressure: 10-5 torr(1011-1012 atoms)
RUBIDIUM VAPOUR CELL
We apply to the atoms in the vapourphase one or more
resonant electromagnetic
fields.
Conférence Universitaire de Suisse OccidentaleProgramme doctoral en physique, Printemps 2014 7Vapour cell atomic frequency standards
Lecture 4, Gaetano Mileti, 13.03.2014
Absorption spectrum of natural rubidiumD2 line (780 nm)with 30 mb of nitrogen
Rb 85 - F= 2
Rb 87 - F= 2
Rb 85 - F= 3
Rb 87 - F= 1
Optical frequency detuning [GHz]0 2 4 6 8
S
P
RUBIDIUM ISOTOPES (REMINDER FROM LECTURE 1)
Conférence Universitaire de Suisse OccidentaleProgramme doctoral en physique, Printemps 2014 8Vapour cell atomic frequency standards
Lecture 4, Gaetano Mileti, 13.03.2014
Each atom in the vapor undergoes the following interactions:
• Collisions: buffer gas, other atoms, walls
• Static magnetic fieldUsually collinear with laser propagation
• Resonant interaction with the optical beam
• Resonant interaction with a microwave field
In lecture # 1, we have shown how these interactions may be described
S
P
light
‐wave
5S1/2
F=1
F=2mF = 0mF = -1mF= -2
mF = 1mF = 2
mF = 0mF = -1
mF = 1
6.8346 GHz
87Rb
RFBH
ˆ
EdH
ˆˆ
BASIC INTERACTIONS OF THE ATOMS IN THE CELL
Conférence Universitaire de Suisse OccidentaleProgramme doctoral en physique, Printemps 2014 9Vapour cell atomic frequency standards
Lecture 4, Gaetano Mileti, 13.03.2014
S
P
Thermal equilibrium
S
P
Complete optical pumping
S
P
Partial optical pumping
Lamp Rb87 filter Rb85 cell Rb87
HYPERFINE OPTICAL PUMPING (WITH LAMP AND FILTER)
Absorption spectrum of natural rubidiumD2 line (780 nm)with 30 mb of nitrogen
Rb 85 - F= 2
Rb 87 - F= 2
Rb 85 - F= 3
Rb 87 - F= 1
Optical frequency detuning [GHz]0 2 4 6 8
Conférence Universitaire de Suisse OccidentaleProgramme doctoral en physique, Printemps 2014 10Vapour cell atomic frequency standards
Lecture 4, Gaetano Mileti, 13.03.2014
Rb87 LampRb85 filtre
Rb87 resonance cell
détector
Microwave cavity
5.304x106 5.306x106 5.308x106 5.310x106 5.312x1060.108
0.112
0.116
0.120
0.124
0.128
Tra
nsm
itted
ligh
t [V
on
10k
]
6.84 GHz - Synthesiser frequency [Hz]
S
P
Double resonance
light
‐wave
DOUBLE RESONANCE (WITH A DISCHARGE LAMP)
Conférence Universitaire de Suisse OccidentaleProgramme doctoral en physique, Printemps 2014 11Vapour cell atomic frequency standards
Lecture 4, Gaetano Mileti, 13.03.2014
I
III
1II
III
tiIIIIIIRb
IIIIII
tiIIII
III
IIIIII
tiIIII
III
IIII
eii
e
e
)()(
)(Im)(
)(Im)(
12
121
121
2
22
22
2
2
1
1
Bloch equations approach
« Fictitious » spinU: dipole component in phaseV: dipole component out of phaseV: difference of population
)-(
0//1
1
vwwvuv
vuu
RATE EQUATIONS WITH A 3-LEVELS MODEL
Conférence Universitaire de Suisse OccidentaleProgramme doctoral en physique, Printemps 2014 12Vapour cell atomic frequency standards
Lecture 4, Gaetano Mileti, 13.03.2014
I II
tiIII e
0 2 ( )I II ll
( ) ( )
( )
Im
ll 0 1
1
4 I IIi t
I II Rb I IIi t
e
i i e
2
2
0 22)(1
Rb2
124 / ll
i i Spin Exchange i Wall Collisions i Buffer GasCollisions / / / i 1,2
)(22 21// III
Broadening (relaxations):
Rate equations (equivalent to Bloch equations):
Stationary solution:
Note: here the interaction with light is described as a relaxation process
RELAXATION PROCESSES AND STATIONARY SOLUTIONS
Conférence Universitaire de Suisse OccidentaleProgramme doctoral en physique, Printemps 2014 13Vapour cell atomic frequency standards
Lecture 4, Gaetano Mileti, 13.03.2014
variation du flux d'énergie
lumineuse
=
nombre d'atomes rencontrés par
les photons
x
nombre de photons absorbés par atome
et par seconde
x énergie
d'un photon
[W / cm2] [1 / cm2] [1 / s] [J]
I n l hn l h
I I Rb
II II Rb
222
222
)(442
)()(
Rb
IIIIIIIIIRbhlnI
llll
Linearabsorption
Opticalpumping
Micro-wave
Clock signal
THEORETICAL “CLOCK” DOUBLE RESONANCE SIGNAL
Conférence Universitaire de Suisse OccidentaleProgramme doctoral en physique, Printemps 2014 14Vapour cell atomic frequency standards
Lecture 4, Gaetano Mileti, 13.03.2014
5.304x106 5.306x106 5.308x106 5.310x106 5.312x1060.108
0.112
0.116
0.120
0.124
0.128
Tra
nsm
itted
ligh
t [V
on
10k
]
6.84 GHz - Synthesiser frequency [Hz]
Resonant photons (typical values):
– 2·1012 / (s ·mm2) incident
– 6·1011 / (s ·mm2) transmitted
– 1.5·1011 / (s ·mm2) Double Resonance
MEASURED “CLOCK” DOUBLE RESONANCE SIGNAL
Conférence Universitaire de Suisse OccidentaleProgramme doctoral en physique, Printemps 2014 15Vapour cell atomic frequency standards
Lecture 4, Gaetano Mileti, 13.03.2014
0 20 40 60 80 1000
100
200
300
400
500
600
1° h
arm
onic
sig
nal w
idth
[Hz]
Nitrogen pressure [mbar]
Exemple expérimentalAvec une celluleCylindrique de 1-2 cm3
Parois
Gaz + Spin-Exchange
EFFECT OF COLLISIONS
Experimental example with a 1 x 2 cm cylindrical cell
Alternative: use of inner wall coating (like in H Masers)
Conférence Universitaire de Suisse OccidentaleProgramme doctoral en physique, Printemps 2014 16Vapour cell atomic frequency standards
Lecture 4, Gaetano Mileti, 13.03.2014
EXAMPLES OF RB CLOCKS (OBSERVATOIRE NE 1985-1995)
Conférence Universitaire de Suisse OccidentaleProgramme doctoral en physique, Printemps 2014 17Vapour cell atomic frequency standards
Lecture 4, Gaetano Mileti, 13.03.2014
EXAMPLE OF COMMERCIAL PRODUCTION (SPECTRATIME NE)
Conférence Universitaire de Suisse OccidentaleProgramme doctoral en physique, Printemps 2014 18Vapour cell atomic frequency standards
Lecture 4, Gaetano Mileti, 13.03.2014
GPS (USA) GLONASS (RU) GALILEO (EU)
Other similar developments:Rb clock for Cassini-Huygens mission, China (Wuhan, etc.) and Japan (Anritsu, etc.)
EXAMPLES OF SPACE RB CLOCKS
Conférence Universitaire de Suisse OccidentaleProgramme doctoral en physique, Printemps 2014 19Vapour cell atomic frequency standards
Lecture 4, Gaetano Mileti, 13.03.2014
Communications:
SDH (Synchronous Digital Hierarchy) CCITT G811,G812 Mobile communications base stations reference clock Spread Spectrum secure radio communication systems Digital Radio&Video Broadcast systems (DRB , DVB)
Instrumentation:
Telecom SDH synch. Test sets , Cellular base stations test sets … Synthesizers, Counters , Laboratory , Metrology GPS time receivers.
OTHER APPLICATIONS OF RB CLOCKS
Conférence Universitaire de Suisse OccidentaleProgramme doctoral en physique, Printemps 2014 20Vapour cell atomic frequency standards
Lecture 4, Gaetano Mileti, 13.03.2014
Conférence Universitaire de Suisse OccidentaleProgramme doctoral en physique, Printemps 2014 21Vapour cell atomic frequency standards
Lecture 4, Gaetano Mileti, 13.03.2014
Conférence Universitaire de Suisse OccidentaleProgramme doctoral en physique, Printemps 2014 22Vapour cell atomic frequency standards
Lecture 4, Gaetano Mileti, 13.03.2014
+ price !
Conférence Universitaire de Suisse OccidentaleProgramme doctoral en physique, Printemps 2014 23Vapour cell atomic frequency standards
Lecture 4, Gaetano Mileti, 13.03.2014
1. Vapour cell standards basic principle
2. Laser-pumped Rubidium clock
3. Coherent Population Trapping
4. Chip-scale atomic clocks
CONTENTS OF LECTURE 4
Conférence Universitaire de Suisse OccidentaleProgramme doctoral en physique, Printemps 2014 24Vapour cell atomic frequency standards
Lecture 4, Gaetano Mileti, 13.03.2014
Examples of Laser diodes
Solitary Fabry-Perot (FP) Extended cavity lasers (ECDL) Distributed Bragg Reflectors (DBR) Distributed Feedback (DFB) FP with DBR optical fiber Vertical Cavity Surface Emitting (VCSEL) MEMS based ECDL and VCSELs Discrete mode lasers Etc.
780, 795, 852, 894nm the atom may be changedSingle mode, mode-hop free tuningTypical specs: 5-10 mW, LW < 5 MHzLow intensity and frequency noise
1.50um
ECDL
DFB
DBR
VCSEL
FP (RWL)
TUNABLE AND FREQUENCY-CONTROLLED LASER DIODES
Conférence Universitaire de Suisse OccidentaleProgramme doctoral en physique, Printemps 2014 25Vapour cell atomic frequency standards
Lecture 4, Gaetano Mileti, 13.03.2014
-4 -2 0 2 4 6 80
5
10
15
20
25
30
D2 lines of Rb87
F = 1F = 2
Phot
ocur
rent
[ mA]
Laser diode frequency [GHz]
-4 -2 0 2 4 6 80
50
100
150
200
250
300
350
400
450
Maximal slope : 600 Hz / GHz
Maximal slope : 420 Hz / GHz
Rubi
dium
clo
ck fr
eque
ncy
[Hz]
Laser diode frequency [GHz]-0.3
-0.2
-0.1
0.0
0.1
0.2
0.3
F=2
Total(F=1)+(F=2)
F=1
Ligh
t shi
ft (1
0 -7
)
Laser wave length
0 2 4 6 8 103500
3505
3510
3515
3520
3525
(0,1 mW/ mm2)
(S = 90 mm2)
Fit : 3500,4 + 2,31 x I
Fit : 3500,3 + 2,45 x I
Laser tuned to F=1
Laser tuned to F=2
"Clo
ck" F
requ
ency
[Hz]
Photocurrent [m A]
dfclock/dIlaser
dfclock/dflaser
LIGHT-SHIFT EFFECT (AC STARK SHIFT)
Conférence Universitaire de Suisse OccidentaleProgramme doctoral en physique, Printemps 2014 26Vapour cell atomic frequency standards
Lecture 4, Gaetano Mileti, 13.03.2014
-4 -2 0 2 4 6 80
5
10
15
20
25
30
D2 lines of Rb87
F = 1F = 2
Phot
ocur
rent
[ mA]
Laser diode frequency [GHz]
-4 -2 0 2 4 6 80
50
100
150
200
250
300
350
400
450
Maximal slope : 600 Hz / GHz
Maximal slope : 420 Hz / GHz
Rubi
dium
clo
ck fr
eque
ncy
[Hz]
Laser diode frequency [GHz]
-1000 -800 -600 -400 -200 0 200 400 600 8000.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
1.1
1.2
10-8
Refere
nce a
bsor
ption
line
"10
MH
z" C
lock
freq
uenc
y (-
9'99
9'99
6) [H
z]
Laser frequency detuning [MHz]
-200 -180 -160 -140 -120 -100 -80 -60 -40 -20 0 200.64
0.68
0.72
0.76
0.80
0.84
0.88
"zer
o lig
ht-s
hift"
lase
r fre
quen
cy
Rb8
7 C
O 2
1-23
Rb8
7 C
O 2
2-23
2·10-9
Reference saturated absorption"1
0 M
Hz"
clo
ck fr
eque
ncy
(-9'
999'
996)
[Hz]
Laser frequency detuning [MHz]
LIGHT-SHIFT EFFECT (AC STARK SHIFT)
This effect exists also with lamps but its control is different when using a laser source.
Conférence Universitaire de Suisse OccidentaleProgramme doctoral en physique, Printemps 2014 27Vapour cell atomic frequency standards
Lecture 4, Gaetano Mileti, 13.03.2014
1 2 3 4 5 6 7 8 9 10
10-13
10-12
4.10-14
9.10-14
2,5.10-13
RIN = 1.10-12
4.10-12
9.10-12
5 KHz/Hz0,5
10 KHz/Hz0,5
20 KHz/Hz0,5
50 KHz/Hz0,5Slaser = 100 KHz/Hz0,5
Pred
icte
d A
llan
devi
atio
n y
-
1/2
DC Photocurrent [A]
21
).(2.0
NSQ
Iy
EFFECT OF THE LASER AM AND FM NOISE (SHORT-TERM)
Noise subtraction is possible!
Fundamental limit: shot noise.
Conférence Universitaire de Suisse OccidentaleProgramme doctoral en physique, Printemps 2014 28Vapour cell atomic frequency standards
Lecture 4, Gaetano Mileti, 13.03.2014
RUBIDIUM VAPOR CELL – CELL FILLING SYSTEM
LTF cell filling facility Cell temperature coefficient
• Typical TC ≈ 1x10-11/K2
• Clock instability ≤ 1x10-15
132
130
128
126
124
122
120
118
116
114
reso
nanc
e fre
quen
cy s
hift
[Hz]
656055504540
Cell temperature [°C]
data quadratic fit
TINV
Light-shift
x109
300
290
280
270
260
250
240
706050403020100-10Laser Power (uW)
Fe3 Fe23 Fe13
Sig
nal
Laser frequency
Fe3Fe23
Fe13
Freq
uenc
y sh
ift a
t 6.8
GH
z (H
z)
Laser power (uW)
Cellno.
Batch no.
Quadratic behaviour
inversion point TINV (± 2°C)
3120 1 Yes 56°C
3125 2 Yes 58°C
3131 3 Yes 56°C
3134 4 Yes 53°C
Goodreproducibility
C. Affolderbach, F. Droz, G. Mileti, IEEETrans. on Instrumentation & Measur., Vol.55, No. 2, pp. 429-435, (2006).
Conférence Universitaire de Suisse OccidentaleProgramme doctoral en physique, Printemps 2014 29Vapour cell atomic frequency standards
Lecture 4, Gaetano Mileti, 13.03.2014
STATE-OF-THE-ART LASER-PUMPED RB CLOCK
Laserhead
PhysicsPackage
Clock assembly
25 c
m
23 cm 25 mm glass cell Volume (LH+PP) < 1.7 liters Mass (LH+PP) < 2 kg
T. Bandi, C. Affolderbach, C. E. Calosso, and G. Mileti, High-Performance Laser-Pumped Rubidium Frequency Standard for Satellite Navigation, Electronics Letters, Vol. 47, No. 12, p. 698–699, (2011) + talk and poster at this conference.
Conférence Universitaire de Suisse OccidentaleProgramme doctoral en physique, Printemps 2014 30Vapour cell atomic frequency standards
Lecture 4, Gaetano Mileti, 13.03.2014
EXAMPLE OF PROJECT: MCLOCKS (EMRP JRP-IND55)
Compact and high-performing microwave clocks for industrial applications
Objective:
Realization of clocks joining features of commercial Rb clocks with performances of Hmaser: frequency stability 10-13 at 1 s and in the 10-15 range for day.
Tasks:
1) Pulsed optical pumping (POP) clock implementation of a Rb cell clock optimizing thesize, the reliability and suitability to operate in industry-like environment.
2) Isotropic cooling use of Rb atoms to overcome the limits of the previousimplementation made with Cs. It is a clock based on cold atom and can provide anaccuracy specification.
3) CPT technique implementation of a particularly compact clock.
Slide courtesy of Salvatore Micalizio, INRIM
Conférence Universitaire de Suisse OccidentaleProgramme doctoral en physique, Printemps 2014 31Vapour cell atomic frequency standards
Lecture 4, Gaetano Mileti, 13.03.2014
MCLOCKS (EMRP JRP-IND55)
The consortium
INRIM (Coordinator) Pulsed Rb clock
OBSPARIS (SYRTE) Rubiclock
UFC (Femto-St) CPT clock
Tubitak CPT clock and industrial tests
Muquans Rubiclock
LTF-UniNe Support on cells, lasers, wave, cavity and physics
Slide courtesy of Salvatore Micalizio, INRIM
Conférence Universitaire de Suisse OccidentaleProgramme doctoral en physique, Printemps 2014 32Vapour cell atomic frequency standards
Lecture 4, Gaetano Mileti, 13.03.2014
POP RB CLOCK: OBJECTIVES IN THE JRP
Slide courtesy of Salvatore Micalizio, INRIM
AOMLaser
zLMicrowaveSynthesizer
RF Oscillator
Optical Signal
= Switch
OCXO
Servotp
t1 T
td
Magnetic ShieldsW Cavity
C-field coil
Cell
• Lab prototype compact prototype• New Physics Package (INRIM+UniNe);• Transportable device: clock in a “box”
Improved performances:
• Reducing laser noise transfer(INRIM-OBSPARIS);
• Reducing Dick effect (low-noise electronicsINRIM-UFC);
• Increasing S/N ratio (multi-pumping technique,INRIM);
• Improved thermal design;• Test in an industry-like environment (UME)
Conférence Universitaire de Suisse OccidentaleProgramme doctoral en physique, Printemps 2014 33Vapour cell atomic frequency standards
Lecture 4, Gaetano Mileti, 13.03.2014
RUBICLOCK: A COMPACT CLOCK BASED ON COLD ATOMS
Slide courtesy of Luigi De Sarlo, SYRTE
T = 35 K, Nat ~ 106
- Simple laser cooling
- Ramsey interrogation
Based on more than 10 years of R&D at LNE-SYRTE
Operation on Cs demonstrated,
Short-term relative stability ~ 2.3 x 10-13
Long-term relative stability ~ 3 x 10-15
Accuracy (estimated) ~ 10-15
Rb clock under development :
- bring cold-atoms tech to the industry
- provide a replacement to GPS+Cs Clockensemble
Conférence Universitaire de Suisse OccidentaleProgramme doctoral en physique, Printemps 2014 34Vapour cell atomic frequency standards
Lecture 4, Gaetano Mileti, 13.03.2014
10‐9
10‐10
10‐11
10‐12
10‐13
10‐141 s 10 s 100 s 1000 s 104 s 105 s
Compact atomic clocks
Telecoms(s)
Navigation(ns)
2‐5 L
0.05‐0.5 L
< 0.02 L
STABILITY OF RB CLOCKS
Power grids
Conférence Universitaire de Suisse OccidentaleProgramme doctoral en physique, Printemps 2014 35Vapour cell atomic frequency standards
Lecture 4, Gaetano Mileti, 13.03.2014
10 -16
10 -15
10 -14
10 -13
10 -12
10 -11
10 -10
1 10 100 1000 10 4 10 5 10 6 10 7
Cs beam, magneticCs-beam, laser H-maser, activeH-maser, passiveRb cell, lampRb or Cs cell, laser CS cold
Time interval (s)
Alla
n de
v.COMPARISON OF RB, CS & H FREQUENCY STANDARDS
Conférence Universitaire de Suisse OccidentaleProgramme doctoral en physique, Printemps 2014 36Vapour cell atomic frequency standards
Lecture 4, Gaetano Mileti, 13.03.2014
GENERAL PROPERTIES OF RUBIDIUM CLOCKS
• Passive, microwave, secondary frequency standard
Main frequency shift due to collisions (several kHz, 10-6)
Stability @ 1s: 10-12-10-10, @10’000 s: 10-14-10-11
• Produced commercially
Probably several 10’000s per year, price: 1k – 5k $
• They have ground and space applications
Instrumentation, telecommunication, power grids, navigation, etc.
• Current R&D includes
Miniaturization, cost reduction, stability improvement
Conférence Universitaire de Suisse OccidentaleProgramme doctoral en physique, Printemps 2014 37Vapour cell atomic frequency standards
Lecture 4, Gaetano Mileti, 13.03.2014
Prof. Gaetano MiletiLaboratoire Temps – Fréquence (LTF)Faculté des Sciences, Université de NeuchâtelAvenue de Bellevaux 51CH-2000 Neuchâtel, Switzerland
www.unine.ch/ltf
THANK YOU FOR YOUR ATTENTION !
38Conférence Universitaire de Suisse OccidentaleProgramme doctoral en physique, Printemps 2014
Lecture 4 : Vapour cell atomic frequency standardsCPT and miniature atomic clocks, C. Affolderbach, 13.03.2014
ATOMIC CLOCKS: BASIC PRINCIPLES AND APPLICATIONSLECTURE 4: VAPOUR CELL ATOMIC FREQUENCY STANDARDSPART 2: COHERENT POPULATION TRAPPING (CPT) AND MINIATURE ATOMIC CLOCKS
13.03.2014
Christoph Affolderbach and Gaetano Mileti
([email protected], [email protected], )
CUSO – Conférence Universitaire de Suisse OccidentaleProgramme doctoral de Physique – Printemps 2014
39Conférence Universitaire de Suisse OccidentaleProgramme doctoral en physique, Printemps 2014
Lecture 4 : Vapour cell atomic frequency standardsCPT and miniature atomic clocks, C. Affolderbach, 13.03.2014
Motivation
3. Coherent Population Trapping (CPT)A. Basic CPT phenomenonB. CPT-based atomic clocks
4. Miniature & chip-scale atomic clocksA. Motivation and applicationsB. Generic technologiesC. Examples of realizationD. Recent trends
OUTLINE
40Conférence Universitaire de Suisse OccidentaleProgramme doctoral en physique, Printemps 2014
Lecture 4 : Vapour cell atomic frequency standardsCPT and miniature atomic clocks, C. Affolderbach, 13.03.2014
BASIC CELL CLOCK SCHEMES
Double-resonance
1 light field (optical pumping), and 1 microwave field (interrogation)
2cm
Coherent Population Trapping (CPT)
2 light fields, Simultaneous preparation and interrogation
Laser source
Rbcell
• Employs coherent laser effects potential for new & improved clocks
(first proposal: Cyr et al., IEEE TIM, 1993)
• No microwave cavity needed more radical clock miniaturization
Cavity size scales with the clock transition wavelength.e.g. 87Rb: 6.8GHz = 4.4 cm
(one can go slightly smaller: dielectric loading, special structures)
Laser source
Rbcell
Det.
fMicrowave = fHFS = fclock
Light 2 light fields:f1 – f2 = fHFS = fclock
u-wavesource
cavity
Servoloop
u-wavesource
Servoloop
Det.
P
S
P
S
fuwave
trans
mis
sion few %
fuwave
trans
mis
sion ≤ 1 %
41Conférence Universitaire de Suisse OccidentaleProgramme doctoral en physique, Printemps 2014
Lecture 4 : Vapour cell atomic frequency standardsCPT and miniature atomic clocks, C. Affolderbach, 13.03.2014
PART 1
COHERENT POPULATION TRAPPING
42Conférence Universitaire de Suisse OccidentaleProgramme doctoral en physique, Printemps 2014
Lecture 4 : Vapour cell atomic frequency standardsCPT and miniature atomic clocks, C. Affolderbach, 13.03.2014
First observation:
Dark lines in Na fluorescence excited by a pulsed laser
G. Alzetta et al., Il Nuovo Cimento 1976 & 1979
COHERENT POPULATION TRAPPING
“The resonant character of these lines is pointed out by the change of their position when thelaser mode spacing is varied. In fact, they occur where the magnetic‐field value matches [i.e.tunes into resonance] a ground‐state hyperfine transition with a frequency difference betweentwo laser modes.”
• multi‐optical‐frequency resonance
• very narrow lines (á optical linewidth)
• absence of fluorescence (i.e. absorption) on resonance
H
Laser beam
43Conférence Universitaire de Suisse OccidentaleProgramme doctoral en physique, Printemps 2014
Lecture 4 : Vapour cell atomic frequency standardsCPT and miniature atomic clocks, C. Affolderbach, 13.03.2014
Simple 3-level systeme.g. 87Rb
BASIC CPT EFFECT
|3
|2
|1
12
L
R
HFS
Apply & scan only 1:
1(E3- E1) / h(L =0)
trans
mis
sion
5S1/2, F=1, mF=0
5S1/2, F=2, mF=0
5P1/2, or 5P3/2(unresolved)
Keep 1 fixed, scan only 2:
2R =0tra
nsm
issi
on
R = 1 – 2 – hfs =0CPT resonance condition:
“dark line”
Laser source
Rbcell
Det. Abs
orpt
ion,
flu
ores
cenc
e
Fluorescence:~ absorption,anisotropic
44Conférence Universitaire de Suisse OccidentaleProgramme doctoral en physique, Printemps 2014
Lecture 4 : Vapour cell atomic frequency standardsCPT and miniature atomic clocks, C. Affolderbach, 13.03.2014
3-level “-system”
CPT: 3-LEVEL SCHEME
2 1~C 21 ie
2 1~NC 12 ie
0 ~C3 222
21int iti ReH
01~NC3 2int iti ReH for R=0
|3 |3
|2|NC
|1|C
12
L
R
1, 2
HFS
New basis
R = 1 – 2 – hfs
Properties of |NC- no excitation by the 2 light fields- Eigenstate of Hint: stationary- depends on relative phase of laser fields- requires phase-stable fields !
Coupling to |3 with fields 1 & 2
Destructive interference of excitation amplitudes
45Conférence Universitaire de Suisse OccidentaleProgramme doctoral en physique, Printemps 2014
Lecture 4 : Vapour cell atomic frequency standardsCPT and miniature atomic clocks, C. Affolderbach, 13.03.2014
-system with relaxations:
DENSITY MATRIX DESCRIPTION
|3
|2
|1
12
L
R
HFS
R = 1 – 2 – hfs
• symmetric system:Relaxation opt = 32 = 31
Rabi frequency g = g1 = g2
• neglected one-photon background• Lorentzian lineshape with:
32
31
12
Solution of density-matrix
Absorption
Dispersion
46Conférence Universitaire de Suisse OccidentaleProgramme doctoral en physique, Printemps 2014
Lecture 4 : Vapour cell atomic frequency standardsCPT and miniature atomic clocks, C. Affolderbach, 13.03.2014
Kramers-Kronig relation:Absorption resonance is always accompanied by dispersion in the refractive index
NUMERICAL RESULTSab
sorp
tion
abso
rptio
n
Ref
ract
ive
inde
x (
rel.
units
)
Light absorption for 3-level system
• vanishing absorption at R = 0• narrow linewidth, determined by 12
• steep dispersion at R = 0,combined with vanishing absorption ! “slow-light”, optically controlled all-optical light switching
47Conférence Universitaire de Suisse OccidentaleProgramme doctoral en physique, Printemps 2014
Lecture 4 : Vapour cell atomic frequency standardsCPT and miniature atomic clocks, C. Affolderbach, 13.03.2014
3‐level system solution :CPT LINESHAPES: LASER INTENSITY
SII2 FWHM 12
IIIA
SCPT
2
~
FWHM slope CPTA
~ I1
~ I1
~ const.
~ I2
~ const.
~ I2
212 W/cm1 GI optS
CPT saturation intensity:IS
CPT on Cs D2 line, buffer‐gas cell:
1000‐times lower than optical saturation intensity: slow ground‐state relaxation !
very low light intensities are sufficient
48Conférence Universitaire de Suisse OccidentaleProgramme doctoral en physique, Printemps 2014
Lecture 4 : Vapour cell atomic frequency standardsCPT and miniature atomic clocks, C. Affolderbach, 13.03.2014
Typical situation:(buffer-gas cell)
CPT SIGNAL DETECTION
For improved signal-to-noise (S/N) ratio in the detection: Use FM modulation of one of the laser fields (1) and lock-in detection.
( see course of 06.03.2014, by S. Schilt & R. Matthey)
• linewidth: 100Hz – 10 kHz range• contrast: ≤ 1 % (max. few %)
Laser source
Rbcell
Det.
tMtiEE m101 sin exp t
Lock-in amplifier
mon 1 ...2 M M 01110 JJS
... M M 11-10 JJC
direct access to absorption & dispersion can exploit the steep dispersion at low light absorption dispersive line-shapes for locking in atomic clocks
Lowest-order signals• in-phase: phase-shift (dispersion) information
• quadrature: absorption information
jj iT j-exp transmission:
49Conférence Universitaire de Suisse OccidentaleProgramme doctoral en physique, Printemps 2014
Lecture 4 : Vapour cell atomic frequency standardsCPT and miniature atomic clocks, C. Affolderbach, 13.03.2014
A REAL MULTI-LEVEL SYSTEM
87Rb D1-lineZeeman sub-structure (in magnetic field):
“trap” state
• + light polarization (circular)• optical transitions selection rule m = +1
Laser source
Dc magnetic field
cell4-plate
• pumping of atoms to the F=2, mF=2 state
• reduced atomic population in mF=0 states
not all -systems are simultaneously “dark”
reduced useful clock signal
Standard scheme & setup:
50Conférence Universitaire de Suisse OccidentaleProgramme doctoral en physique, Printemps 2014
Lecture 4 : Vapour cell atomic frequency standardsCPT and miniature atomic clocks, C. Affolderbach, 13.03.2014
Example: 87RbExcited-state hyperfine-structure:
A REAL MULTI-LEVEL SYSTEM (2)
D1 line 795nm
D2 line 780nm
Optical selection rules: F= 0, ±1
D1-line:
• stronger signal(less atoms lost)
• narrower linewidth(less coherence lost)
D2
D2
D1
D1
D1
D2
M. Stähler et al., Opt. Lett. 2002
one-photon transitions to F’= 1 or 4 loss channels on the D2-line
51Conférence Universitaire de Suisse OccidentaleProgramme doctoral en physique, Printemps 2014
Lecture 4 : Vapour cell atomic frequency standardsCPT and miniature atomic clocks, C. Affolderbach, 13.03.2014
PHASE-STABLE LIGHT-FIELD GENERATION
Laser diode 2
Phase-lockedloop
Laser diode 1
2 Phase-locked lasers
fmod
fopt
External modulator(Electro-optical modulator)
Laser diode EOM
fmod
foptfopt
Direct modulation(laser current)
Laser diode
fmod
fmod = fhfs
+ ideal 2-color light field+ full freedom of parameters– needs 2 lasers– fast PLL electronics
+ only one laser + easy implementation– bulky modulator– multi-frequency light field
+ only one laser + easy & direct– limited modulation bandwidth– strong amplitude modulation– multi-frequency light field
fmod = fhfs or fmod = fhfs /2,typ. 1.5 to 9 GHz
52Conférence Universitaire de Suisse OccidentaleProgramme doctoral en physique, Printemps 2014
Lecture 4 : Vapour cell atomic frequency standardsCPT and miniature atomic clocks, C. Affolderbach, 13.03.2014
OPTICAL EXCITATION SCHEMES
fopt
2 phase-locked lasers:+ all light power contributes to CPT
resonances
fopt
fopt
fmod = fhfs
fhfs
fmod = fhfs /2
carrier
(carrier)
-1
-1 +1
Laserspectrum
Atomic absorption
Modulation at fmod = fhfs:– off-resonant sidebands only add
background and noise to the signal, not CPT signal
– P(nth SB) ~ Jn (M)
Modulation at fmod = fhfs/2 :– off-resonant sidebands add background
and noise– residual carrier can deteriorate CPT– Pcarrier = 0 for M ≈ 2.4
+12
53Conférence Universitaire de Suisse OccidentaleProgramme doctoral en physique, Printemps 2014
Lecture 4 : Vapour cell atomic frequency standardsCPT and miniature atomic clocks, C. Affolderbach, 13.03.2014
CPT ATOMIC CLOCKS
A SELECTION
54Conférence Universitaire de Suisse OccidentaleProgramme doctoral en physique, Printemps 2014
Lecture 4 : Vapour cell atomic frequency standardsCPT and miniature atomic clocks, C. Affolderbach, 13.03.2014
FIRST CPT ATOMIC CLOCK PROPOSAL
1993 !
87Rb
Edge-emitting diode laser at 780nm
fm = 6.83 GHz / 6 = 1.14GHz
Clock transition linewidth = 6 kHz
(at 6.8GHz, with buffer-gas)
55Conférence Universitaire de Suisse OccidentaleProgramme doctoral en physique, Printemps 2014
Lecture 4 : Vapour cell atomic frequency standardsCPT and miniature atomic clocks, C. Affolderbach, 13.03.2014
CPT ATOMIC CLOCK: HELSINKI UNIVERSITY OF TECHNOLOGY
M. Merimaa et al., JOSAB 2003.
Clock loop
Laser loop Laser spectrum
3.0 GHz
56Conférence Universitaire de Suisse OccidentaleProgramme doctoral en physique, Printemps 2014
Lecture 4 : Vapour cell atomic frequency standardsCPT and miniature atomic clocks, C. Affolderbach, 13.03.2014
CPT clock using the Cs D2 line.
One of the first studies on VCSEL-based CPT atomic clocks.
Optical and microwave loops use the same signal, at different modulation frequencies.
CPT ATOMIC CLOCKS AT NIST
S. Knappe et al., JOSAB 2001.J. Kitching et al., El. Lett. 2001.
Pure-gas cell
Temperature-compensated cell
Compact physics package
Optical laser lock loop
CPT clock loop
57Conférence Universitaire de Suisse OccidentaleProgramme doctoral en physique, Printemps 2014
Lecture 4 : Vapour cell atomic frequency standardsCPT and miniature atomic clocks, C. Affolderbach, 13.03.2014
Light shift:Variation of the “clock” transition frequency due to variations in light frequency or intensity.
Increasing modulation index M (mod. power) Favours more negative intensity light shift
Also affects frequency light shift “quadratic” only
Control button for light shifts
LIGHT SHIFT IN CPT CLOCKS
M. Zhu et al., Proc. PTTI 2000.
F. Levi et al., Trans. UFFC 2000.
fopt
Laserspectrum
Atomic absorption
carrier
Ligh
t shi
ft
58Conférence Universitaire de Suisse OccidentaleProgramme doctoral en physique, Printemps 2014
Lecture 4 : Vapour cell atomic frequency standardsCPT and miniature atomic clocks, C. Affolderbach, 13.03.2014
• optical CPT excitation
• detect the microwave field emitted by the ground-state coherence
• maser approach: high-Q cavity (Q=8000)
THE CPT MASER
EOM drive ~ 3.4 GHzM ≈ 2.4
F. Levi et al., Proc. EFTF 2004.
59Conférence Universitaire de Suisse OccidentaleProgramme doctoral en physique, Printemps 2014
Lecture 4 : Vapour cell atomic frequency standardsCPT and miniature atomic clocks, C. Affolderbach, 13.03.2014
Main limitations:
• low microwave power levels
• detection noise
THE CPT MASER
good clock stability
optical detection still preferable
F. Levi et al., Proc. EFTF 2004.
60Conférence Universitaire de Suisse OccidentaleProgramme doctoral en physique, Printemps 2014
Lecture 4 : Vapour cell atomic frequency standardsCPT and miniature atomic clocks, C. Affolderbach, 13.03.2014
• goal: increased signal contrast• one frequency-modulated laser to excite CPT (beam-1, fmod=3.4 GHz)• one additional, single-frequency laser• generates an optical frequency to satisfy the 4-beam Raman condition
4-WAVE MIXING WITH CPT
V. Shah et al., Opt. Lett. 2007.
demonstrates the possibilities with non-linear techniques
somewhat complicated setup…
100 nW
61Conférence Universitaire de Suisse OccidentaleProgramme doctoral en physique, Printemps 2014
Lecture 4 : Vapour cell atomic frequency standardsCPT and miniature atomic clocks, C. Affolderbach, 13.03.2014
• 87Rb CPT scheme using a VCSEL
• conventional cell and physics package
• smaller than most OCXO, < 50 cm3
• applications: UMTS, holdover, etc.
• power consumption: 5W at 25°C
COMMERCIAL PRODUCT
J. Deng et al., Proc. EFTF&IFCS 2008.
62Conférence Universitaire de Suisse OccidentaleProgramme doctoral en physique, Printemps 2014
Lecture 4 : Vapour cell atomic frequency standardsCPT and miniature atomic clocks, C. Affolderbach, 13.03.2014
PART 2
MINIATURE ATOMIC CLOCKS
63Conférence Universitaire de Suisse OccidentaleProgramme doctoral en physique, Printemps 2014
Lecture 4 : Vapour cell atomic frequency standardsCPT and miniature atomic clocks, C. Affolderbach, 13.03.2014
Bring atomic timing precision to the size and power range previously covered by quartz oscillators
MINIATURE ATOMIC CLOCKS: MOTIVATION
PrimaryStandard
CommercialBeam Clock
CompactAtomic Clock
WristwatchQuartz
Accuracy: 10‐15 10‐13 10‐11 10‐7 10‐5Timing error: 10ns/yr 1s/yr 0.1s/day 100s/day 1s/daySize: 107 cm3 104 cm3 100 cm3 1‐10 cm3 10 mm3
Power: kW 100’s W 1 W 100 mW 10 WCost: >$1 M $50 k $2,000 $100 $1
PrecisionQuartz
Decreasing performance and size/power/cost
MiniatureAtomic Clock
New clocks !
10‐101s/day 10 cm3
120mW$300
64Conférence Universitaire de Suisse OccidentaleProgramme doctoral en physique, Printemps 2014
Lecture 4 : Vapour cell atomic frequency standardsCPT and miniature atomic clocks, C. Affolderbach, 13.03.2014
Main goals
• small size portable & mobile instrumentation• low power cons. battery-powered devices• 10-11 stability improvement compared to quartz (1s timing)• low unit price parallel micro-fabrication techniques• improved aging & shock behavior, compared to quartz
MINIATURE ATOMIC CLOCKS
Applications
• network synchronization• improved hold-over capabilities • secure communication• improved GNSS / positioning receivers• mobile end-user equipment • …
Ultimate development goals (e.g. US DARPA CSAC project)
Stability: ≤ 6x10‐10 ‐1/210‐11 at > 3’600s
Size: 1 cm3
Power: 30 mWTiming error: 1 s/day
runs ≈ 7 days from a smartphone battery
Power: 30 mW
65Conférence Universitaire de Suisse OccidentaleProgramme doctoral en physique, Printemps 2014
Lecture 4 : Vapour cell atomic frequency standardsCPT and miniature atomic clocks, C. Affolderbach, 13.03.2014
subtitle
text
MINIATURE ATOMIC CLOCK APPLICATIONS
66Conférence Universitaire de Suisse OccidentaleProgramme doctoral en physique, Printemps 2014
Lecture 4 : Vapour cell atomic frequency standardsCPT and miniature atomic clocks, C. Affolderbach, 13.03.2014
J. Kitching et al., Appl. Phys. Lett 2002.
TOWARDS CHIP-SCALE ATOMIC CLOCKS
Proposal for a chip-scale atomic clock:
Use of micro‐fabrication technology & CPT Wafer-level assembly of all components:
dicing
Miniature clock physics package
Compo
nents wafers
67Conférence Universitaire de Suisse OccidentaleProgramme doctoral en physique, Printemps 2014
Lecture 4 : Vapour cell atomic frequency standardsCPT and miniature atomic clocks, C. Affolderbach, 13.03.2014
COMPONENTS:
MICRO-FABRICATED ALKALI CELLS
VCSEL DIODE LASERS
68Conférence Universitaire de Suisse OccidentaleProgramme doctoral en physique, Printemps 2014
Lecture 4 : Vapour cell atomic frequency standardsCPT and miniature atomic clocks, C. Affolderbach, 13.03.2014
Buffer-gas: avoids ground-state relaxation by collisions with the cell walls.
Diffusion:
Collisions:
Total:(Cs in Ne)
LIMITATIONS IN BUFFER-GAS CELLS
gas
Rb
S. Brandt, Phys. Rev. A 1997.
CPT line,Cs + Ne
1/ p
p
CPT linewidth depends on cell size and gas pressure
to be optimized for a miniature cell
69Conférence Universitaire de Suisse OccidentaleProgramme doctoral en physique, Printemps 2014
Lecture 4 : Vapour cell atomic frequency standardsCPT and miniature atomic clocks, C. Affolderbach, 13.03.2014
CPT-line Q-factor (f0/), 1mm cell size
SHORT-TERM STABILITY AND BUFFER-GAS CELL SIZE
Coated cell
Buffer-gas cell
≈ 5x10-11
J. Kitching et al., Appl. Phys. Lett 2002.
Predicted short‐term stability:(at optimized temperature, etc.)
2/1
0 //2.0
fNSy
nalkali (T),Ilaser
Ilaser Pgas
Optical absorption: 20 kPa - cell
≈ 500 Hz
70Conférence Universitaire de Suisse OccidentaleProgramme doctoral en physique, Printemps 2014
Lecture 4 : Vapour cell atomic frequency standardsCPT and miniature atomic clocks, C. Affolderbach, 13.03.2014
Main requirements and challenges
• Reactive alkalis
Hermetic cell sealing, under vacuum compatible cell materials
(Si + glass ok, metals? )
• sealing at elevated temperatures
control of alkali quantity control of buffer-gas pressure incompatible with anti-relaxation wall coatings
(known paraffines have low melting points)
• reliability of cells !
MICRO-FABRICATED VAPOR CELLS
Technologies overview
• cell sealing
anodic bonding low-temperature sealing
• different cell geometries one or 2 chambers spherical
• alkali filling
metallic: beam dispensing, pipetting post-activation: chemical, electrolytic
71Conférence Universitaire de Suisse OccidentaleProgramme doctoral en physique, Printemps 2014
Lecture 4 : Vapour cell atomic frequency standardsCPT and miniature atomic clocks, C. Affolderbach, 13.03.2014
MICRO-FABRIATED CELLS WITH ANODIC BONDING
J. Kitching et al., Metrologia 2005, S. Knappe et al., Opt. Lett. 2005.
Glass‐Si‐glass anodic bonding (NIST process): Refined alkali filling:
72Conférence Universitaire de Suisse OccidentaleProgramme doctoral en physique, Printemps 2014
Lecture 4 : Vapour cell atomic frequency standardsCPT and miniature atomic clocks, C. Affolderbach, 13.03.2014
Dia=5 mm
L =10x10 mm 200 - 500 um
500-2000 um
Silicon wafer
Photolithography and cavity etching by DRIE
Wafer-level anodic bonding of Si with glass
Dicing
Cell closing / Anodic bonding of glass lid
200 - 500 um
Rb deposition
Dia=5 mm
L =10x10 mm 200 - 500 um
500-2000 um
Silicon wafer
Photolithography and cavity etching by DRIE
Wafer-level anodic bonding of Si with glass
Dicing
Cell closing / Anodic bonding of glass lid
200 - 500 um
Rb deposition
Y. Pétremand, et al., Proc. EFTF 2010
SWISS ANODIC BONDING TECHNOLOGY
IMT‐SAMLAB
73Conférence Universitaire de Suisse OccidentaleProgramme doctoral en physique, Printemps 2014
Lecture 4 : Vapour cell atomic frequency standardsCPT and miniature atomic clocks, C. Affolderbach, 13.03.2014
POST-ACTIVATED VAPOUR CELLS
L. Nieradko et al., FEMTO-ST (Besançon) & Wroclaw UT
74Conférence Universitaire de Suisse OccidentaleProgramme doctoral en physique, Printemps 2014
Lecture 4 : Vapour cell atomic frequency standardsCPT and miniature atomic clocks, C. Affolderbach, 13.03.2014
POST-ACTIVATED VAPOUR CELLS (C’TD)
A-L. Liew et al., Appl. Phys. Lett. 2007 F. Gong et al., Rev. Sci. Instrum. 2006
700 V540 °C
Cs-enriched glass
Cs liberation from CsN3
• liberates Cs and N2 buffer-gas in sealed cells• activation by UV light• explosive !
Electrolytic release of Cs in sealed cellsLiberate Cs from glass by electrolytic exchange with Na+ ions
75Conférence Universitaire de Suisse OccidentaleProgramme doctoral en physique, Printemps 2014
Lecture 4 : Vapour cell atomic frequency standardsCPT and miniature atomic clocks, C. Affolderbach, 13.03.2014
MICRO-FABRICATED VAPOUR CELLS
J. Eklund et al., MEMS 2007.K. Tsujimoto et al., MEMS 2011.
Sacrificial glass-frit channel sealingAt 470°C, 250 kPa
Micro glass-blown cells
76Conférence Universitaire de Suisse OccidentaleProgramme doctoral en physique, Printemps 2014
Lecture 4 : Vapour cell atomic frequency standardsCPT and miniature atomic clocks, C. Affolderbach, 13.03.2014
SWISS SPECIAL MINIATURE CELLS
R. Straessle et al., MEMS 2012.Y. Pétremand et al., JMM 2012.
IMT‐SAMLAB
4-mm size Rb cellsMicro-fabrication technology for precise control of cell geometryMulti-stack anodic bonding: Thick glass core wafer 2 Si layers + 2 glass windows 4 steps of anodic bonding
Indium cell sealingLow-temperature sealing (≤ 140°C) alkali control & wall coatings
Working alkali cells
5mm
77Conférence Universitaire de Suisse OccidentaleProgramme doctoral en physique, Printemps 2014
Lecture 4 : Vapour cell atomic frequency standardsCPT and miniature atomic clocks, C. Affolderbach, 13.03.2014
High modulation bandwidth CPT !
LIGHT SOURCES: VCSEL
www.ulm-photonics.de www.oclaro.com
Low‐power light sources
Higher relaxation frequency R for a VCSEL: mainly due to the higher photon density p0 in the resonator
pR
pA 0
A: laser gainp: photon lifetime in resonator
0.45 mW@ 2mA≈ 12%
Typical linewidths≈ 20 – 100 MHz
78Conférence Universitaire de Suisse OccidentaleProgramme doctoral en physique, Printemps 2014
Lecture 4 : Vapour cell atomic frequency standardsCPT and miniature atomic clocks, C. Affolderbach, 13.03.2014
Fabrication process:Chip size:Phase noise: Power consumption:Frequency tuning step: Output RF power:
Phase noise @ 4.6 GHzCircuit architecture
CMOS 130 nm1.7 x 1.2 mm = 2 mm2
-86 dBc/Hz @ 5 kHz15 mW (vdd = 1.2 V)0.5 mHz (10-13)0 dBm
Chip photography
Obtained results
Y. Zhao, S. Tanner, P-A. Farine, ESPLAB-EPFL
This low power, highly integrated and high performance frequency synthesizer is suited for realizing chip-scale atomic clocks.
Stability with Cs micro-cell and ASIC
Exp. Evaluation by LTF-UniNe: L. Schneller, F. Gruet, C. Affolderbach
SINGLE-CHIP 4.6 GHZ SYNTHESIZER FOR CS CPT CSAC
Y. Zhao et al., IFCS-EFTF 2013
79Conférence Universitaire de Suisse OccidentaleProgramme doctoral en physique, Printemps 2014
Lecture 4 : Vapour cell atomic frequency standardsCPT and miniature atomic clocks, C. Affolderbach, 13.03.2014
MINIATURE ATOMIC CLOCK REALIZATIONS
AN OVERVIEW
MINIATURE ATOMIC CLOCKS
80Conférence Universitaire de Suisse OccidentaleProgramme doctoral en physique, Printemps 2014
Lecture 4 : Vapour cell atomic frequency standardsCPT and miniature atomic clocks, C. Affolderbach, 13.03.2014
• Fully micro-fabricated physics package
• 75mW power dissipation
FIRST PIONEERING PROTOTYPES AT NIST
S. Knappe et al., Appl. Phys. Lett. 2004
1mm
with drift
drift removed
81Conférence Universitaire de Suisse OccidentaleProgramme doctoral en physique, Printemps 2014
Lecture 4 : Vapour cell atomic frequency standardsCPT and miniature atomic clocks, C. Affolderbach, 13.03.2014
SYMMETRICOM CSAC PROTOTYPE
R. Lutwak et al., Proc. PTTI 2007.
• CPT on Cs D1-line (894nm)• VCSEL operating at 85°C• Temperature-compensated cell• 1cm3 PP
82Conférence Universitaire de Suisse OccidentaleProgramme doctoral en physique, Printemps 2014
Lecture 4 : Vapour cell atomic frequency standardsCPT and miniature atomic clocks, C. Affolderbach, 13.03.2014
SYMMETRICOM CSAC PROTOTYPE
R. Lutwak et al., Proc. PTTI 2007.
Pcb with PP
Power budget
• 1 cm3 PP volume • 15 cm3 total volume• 125 mW consumption• 10 MHz output
• 0.35 cm3 PP volume • ≈1 cm3 total volume• 30 mW consumption• 4.6 GHz output (uncal.)• No shields, etc.
Limit from the 1cm3 PP:Impact of electronics !
0.35 cm3 PP clock
83Conférence Universitaire de Suisse OccidentaleProgramme doctoral en physique, Printemps 2014
Lecture 4 : Vapour cell atomic frequency standardsCPT and miniature atomic clocks, C. Affolderbach, 13.03.2014
SYMMETRICOM CSAC PRODUCT
www.symmetricom.com
84Conférence Universitaire de Suisse OccidentaleProgramme doctoral en physique, Printemps 2014
Lecture 4 : Vapour cell atomic frequency standardsCPT and miniature atomic clocks, C. Affolderbach, 13.03.2014
SOME OTHER DARPA-CSAC PROTOTYPES
J. F. DeNatale et al., Proc. IEEE 2008.
Teledyne scientific Corp.87Rb CPT on D1-line, folded beam geometry 1 cm3 volume 30 mW power consumption (with electron.)
Honeywell Aerospace Research87Rb CPT on D1-line, 1.7 cm3 volume 57 mW power consumption (with electron.)
Yougner et al., Proc. Transducers 2007.
5¢
85Conférence Universitaire de Suisse OccidentaleProgramme doctoral en physique, Printemps 2014
Lecture 4 : Vapour cell atomic frequency standardsCPT and miniature atomic clocks, C. Affolderbach, 13.03.2014
ESA-PROJECT: MUSO
Miniature Ultra-Stable Oscillator
For secure satellite communication
• low-power VCSEL laser (795nm)
• miniature Rb cells
100% Swiss-Made !
C. Schori et al., Proc. EFTF 2010.
86Conférence Universitaire de Suisse OccidentaleProgramme doctoral en physique, Printemps 2014
Lecture 4 : Vapour cell atomic frequency standardsCPT and miniature atomic clocks, C. Affolderbach, 13.03.2014
EU-FP7: MAC-TFC
Miniature Atomic Clock for Timing, Frequency Control and Communication
For ground‐based applications
• Low‐power VCSEL on Cs D1‐line (Ulm)
• Miniature Cs cells (Besançon, Neuchâtel, Wroclaw)
• Full miniature electronics (Neuchâtel)
Cs-Cells VCSEL-Laser
ASIC microwave electronics
All key components are developed within the project consortium !
Test setup
www.mac‐tfc.eu
Packaging
Clockdemonstrator
87Conférence Universitaire de Suisse OccidentaleProgramme doctoral en physique, Printemps 2014
Lecture 4 : Vapour cell atomic frequency standardsCPT and miniature atomic clocks, C. Affolderbach, 13.03.2014
SWISS MINIATURE ATOMIC CLOCK (SWISS–MAC )
Swiss–MAC
Slide courtesy of Jacques Haesler and Steve Lecomte, CSEM
Assembled prototype
88Conférence Universitaire de Suisse OccidentaleProgramme doctoral en physique, Printemps 2014
Lecture 4 : Vapour cell atomic frequency standardsCPT and miniature atomic clocks, C. Affolderbach, 13.03.2014
DOUBLE-RESONANCE MINIATURE ATOMIC CLOCKS
A. Braun et al., Proc. PTTI 2007
“End-state clock” (W. Happer, 2003)
• Strong end-state double-resonance• Requires magnetic field lock• microwave coupling loops
89Conférence Universitaire de Suisse OccidentaleProgramme doctoral en physique, Printemps 2014
Lecture 4 : Vapour cell atomic frequency standardsCPT and miniature atomic clocks, C. Affolderbach, 13.03.2014
RECENT TRENDS IN
MINIATURE ATOMIC CLOCKS
MINIATURE ATOMIC CLOCKS
90Conférence Universitaire de Suisse OccidentaleProgramme doctoral en physique, Printemps 2014
Lecture 4 : Vapour cell atomic frequency standardsCPT and miniature atomic clocks, C. Affolderbach, 13.03.2014
new components for a miniature double-resonance clock
demonstrate clock operation
SINERGIA PROJECT: MINIATURE ATOMIC CLOCKS AND SENSORS
UniNe: LTF
EPFL:SAMLAB, LEMA,LMTS, LPM2
5mm
Microfabricated Rb-cells: new geometries and low-temperature bonding
Miniature Rb lamp
LTCC packaging
Miniature micro-wave resonator
Miniature DR clock demonstrator & stability:5.5x10-12 -1/2 and < 10-12 at > 100 s
http://macqs.epfl.ch/
10-13
2
4
68
10-12
2
4
68
10-11
Ove
rlapp
ing
Alla
n D
evia
tion
y(
)
1 10 100 1000Integration Time, , [s]
S/N limit: 5.5 x 10-12 -1/2
Shot Noise Limit: 2.5 x 10-12 -1/2
91Conférence Universitaire de Suisse OccidentaleProgramme doctoral en physique, Printemps 2014
Lecture 4 : Vapour cell atomic frequency standardsCPT and miniature atomic clocks, C. Affolderbach, 13.03.2014
MINIATURE MICROWAVE CAVITY
Volume: < 0.9 cm3
Loaded Q = 26 Injected power a few W Power loss 50 nW Fine tuning range > 100 MHz Patent submitted
M. Violetti et al., European Microwave Week 2012
6 mm
92Conférence Universitaire de Suisse OccidentaleProgramme doctoral en physique, Printemps 2014
Lecture 4 : Vapour cell atomic frequency standardsCPT and miniature atomic clocks, C. Affolderbach, 13.03.2014
MINIATURE DISCHARGE LAMPS FOR MINIATURE DR CLOCK
1 cm
V. Venkatraman et al., Trans. UFFC 2012 & Appl. Phys. Lett. 2014
Dielectric Barrier Discharge (DBD)• Micro-fabricated cell with Al-electrodes
500 µm Pyrex
500 µm Pyrex2 mm Si
Al Electrodes
Cavity filled with few µl of Rb + 70 mbar of Argon
5 mm
• 2 to 500 MHz RF drive• ≈10 mW RF power coupled to the cell 100W light output
LMTS+SAMLAB
Optical pumping and Zeeman DR in a micro-cell
93Conférence Universitaire de Suisse OccidentaleProgramme doctoral en physique, Printemps 2014
Lecture 4 : Vapour cell atomic frequency standardsCPT and miniature atomic clocks, C. Affolderbach, 13.03.2014
MEMS TECHNOLOGY FOR TIME & FREQUENCY
• Need of low-consumption, low-weight and low-cost T&F devices
Example: GNSS receivers able to lock on PRN signals
• Key MEMS-based building blocks:
– Resonators
– Filters
– Oscillators
– (Chip-scale atomic clocks)
e.g. Q = 15’248at f = 1.46 GHz( = 95 kHz)
94Conférence Universitaire de Suisse OccidentaleProgramme doctoral en physique, Printemps 2014
Lecture 4 : Vapour cell atomic frequency standardsCPT and miniature atomic clocks, C. Affolderbach, 13.03.2014
SUMMARY
• CPT-based clocks exploit coherence effects from 2-colour laser systems:purely optical interrogation, but stringent laser requirements
• Miniature atomic clocks bring atomic timing precision from a quartz outlinedevice:
o Typically s timing stabiltiy at 1 day: holdover capabilities,for communication networks and smart grid
o Low power consumption: mobile applications or remote networks
• Miniature atomic clocks based on alkali-cells are well-established, a product is available in the USA
• Interesting research on alternative approaches is ongoing :
• Miniature cell clocks using DR scheme
• Non-cell miniature clock schemes for improved long-term stability(cold atoms, ions, …)
95Conférence Universitaire de Suisse OccidentaleProgramme doctoral en physique, Printemps 2014
Lecture 4 : Vapour cell atomic frequency standardsCPT and miniature atomic clocks, C. Affolderbach, 13.03.2014
CPT
G. Alzetta et al., Il Nuovo Cimento B 36, 5 (1976) & Il Nuovo Cimento B 52, 204 (1979)E. Arimondo, Progress in Optics vol. 35, 257 (1996)C. Affolderbach, PhD Thesis, Bonn 2002.M. Stähler et al., Opt. Lett. 27, 1472 (2002)Related reading: S. E. Harris, Physics Today 50, 36 (1997), L. V. Hau et al., Nature 397, 594 (1999)
CPT clocks
S. Knappe et al., JOSAB 18, 1545 (2001)J. Vanier, Appl. Phys. B 81, 421 (2005)
Miniature atomic clocks
S. Knappe, Comprehensive Microsystems vol.3, 571 (2007)
Time&Frequency conference proceedingswww.eftf.org (free)www.pptimeeting.org (free)www.ieee-uffc.org/main/publications/fcs/index.asp (on subscription)
REFERENCE LITERATURE
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