Meet the world’s best commercial cesium beam clock. Dr. Patrick Berthoud’s ITSF 2015 presentation has the details

1. High performance optically-
pumped cesium beam clock
Dr. Patrick Berthoud, Chief Scientist Time & Frequency
ITSF 2015, Edinburgh, UK, 2nd – 5th November

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Outline
• Motivation and applications
• Cs clock: magnetic vs. optical
• Cs clock prototype development
• Conclusion

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Identified markets
• Telecommunication network reference
• Telecom operators, railways, utilities, …
• Science
• Astronomy, nuclear and quantum physics, …
• Metrology
• Time scale, fund. units measurement
• Professional mobile radio
• Emergency, fire, police
• Defense
• Secured telecom, inertial navigation
• Space (on-board and ground segments)
• Satellite mission tracking, GNSS systems

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Available Cs clock commercial products
• Long life magnetic Cs clock
• Stability : 2.7E-11 t-1/2, floor = 5E-14
• Lifetime : 10 years
• Availability : commercial product
• High performance magnetic Cs clock
• Stability : 8.5E-12 t-1/2 , floor = 5E-15
• Lifetime : 5 years
• vailability : commercial product
• High perfermance and long life optical Cs clock
• Stability : 3.0E-12 t-1/2 , floor = 5E-15
• Lifetime : 10 years
• Availability : under development

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Timing error prediction
• T0 depends on timing reference, meas. resol. and noise
• Accuracy depends on reference accuracy, meas. resol. and noise, and
flicker frequency noise floor of the DUT
• Environmental sensitivities are usually periodic variation of frequency,
zero on average
• Frequency drift common in quartz and cell stds (Rb, maser), negl. for Cs
• Intrinsic noise sources of the DUT (white and flicker FM)
Timing error
Initial timing
calibration
Freq accuracy +
Env. changes
Freq linear drift
Noise timing error
sx(t)  sy(t) * t

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Frequency stability (ADEV)
1E-15
1E-14
1E-13
1E-12
1E-11
1E-10
1.E+00 1.E+01 1.E+02 1.E+03 1.E+04 1.E+05 1.E+06 1.E+07 1.E+08
Allanstdfrequencydeviation[Hz/Hz]
Averaging time [s]
Std. Perf.Magnetic
High Perf. Magnetic
High Perf. Optical

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Timing error prediction
1.E-12
1.E-11
1.E-10
1.E-09
1.E-08
1.E-07
1.E-06
1.E-05
1.E-04
1.E+00 1.E+01 1.E+02 1.E+03 1.E+04 1.E+05 1.E+06 1.E+07 1.E+08
Timingerror[s]
Averaging time [s]
Std. Perf.Magnetic
High Perf. Magnetic
High Perf. Optical
30 ns
6days
60days

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Motivation for an Optical Cs clock
• Improved performance (short and long-term stability) for:
• ePRTC applications (extended holdover period)
• Metrology and time scales
• Science (long-term stability of fundamental constants)
• Inertial navigation (sub-marine, GNSS)
• No compromise between lifetime and performance
• Same Cs reservoir capacity
• Same Cs oven temperature
• Same vacuum pumping capacity
• Large improvement of Cs beam efficiency by laser optical pumping

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Outline
• Motivation and applications
• Cs clock: magnetic vs. optical
• Cs clock prototype development
• Conclusion

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Magnetic Cesium clock operation
• Cs beam generated
in the Cs oven
(vacuum operation)
• Cs atoms state
selection by magnets
• Cs clock frequency
probing (9.192 GHz)
in the Ramsey
cavity
• Atoms detection and
amplification by
electron multiplier
(vacuum)
• RF source servo loop
using atomic signal
Ramsey
cavity
Magnetic
selectors
Magnetic
shield + coil
FM
User
10 MHz
Cs
Oven
Vacuum
enclosure
Electron
multiplier
RF
source
Sync
Detect
N
S
N
S
Cs
beam

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Optical Cesium clock operation
• Cs beam generated
in the Cs oven
(vacuum operation)
• Cs atoms state
selection by laser
• Cs clock frequency
probing (9.192 GHz)
in the Ramsey
cavity
• Atoms detection and
amplification by
photodetector (air)
• Laser and RF sources
servo loops using
atomic signals
Ramsey
cavity
Light
Collectors
Magnetic
shield + coil
FM
User
10 MHz
Laser
Cs
Oven
Vacuum
enclosure
Photo-
detectors
RF
source
Sync
Detect
FM
Laser
source
Sync
Detect
Cs
beam

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133Cs atomic energy levels
• Stable ground states
(F=3 and F=4)
• Switching between
ground states F by RF
interaction 9.192 GHz
• Unstable excited
states (F’=2,3,4,5)
• Switching between
ground states F and
excited states F’ by
laser interaction 852
nm (or 351 THz)
6S1/2 nhf = 9.192 GHz
F=4
F=3
6P3/2
F’=4
F’=3
F’=2
F’=5

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Cesium clock: Magnetic vs. Optical
• Weak flux
• Strong velocity selection (bent)
• Magnetic deflection (atoms kicked
off)
• Typical performances:
• 2.7E-11 t-1/2
• 10 years
• Stringent alignment (bent beam)
• Critical component under vacuum
(electron multiplier)
F=3,4
• High flux (x100)
• No velocity selection (straight)
• Optical pumping (atoms reused)
• Typical performances:
• 2.7E-12 t-1/2
• 10 years
• Relaxed alignment (straight beam)
• Critical component outside vacuum
(laser)
N
S
N
S
F=3,4

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Outline
• Motivation and applications
• Cs clock: magnetic vs. optical
• Cs clock prototype development
• Conclusion

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Clock functional bloc diagram
• Cs tube
• Generate Cs atomic beam in ultra
high vacuum enclosure
• Electrical and optical feedthoughs for
atomic signal generation
• Optics
• Generate 2 optical beams from 2
lasers modules (cold redundancy)
• Electronics
• Cs core for driving the Optics and
the Cs tube
• External modules for power
supplies, management, signals
outputs
Cesium tube
Magnetic field and shields
Cs
Oven
Collect CollectRamsey cavity
Optics
Laser Splitter Mirror
Clock electronics
RF
Source
Clock Ctrl Power
Supply
Photo
Detect
Photo
Detect
4xSyncout(1PPS)
Expansion electronics
Serial(RS232)
Syncin(1PPS)
Display
10MHzsine
10MHzsine
10or5MHzsine(option)
10or100MHzsine(option)
Metrology
Manage
ment
Remote(TCP/IP)
PPS DC/DC AC/DC Battery
ExternalDCsupply
ExternalACsupply

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Cs clock design
• 19’’, 3U, 460 mm
rack
• Cs core is not
customizable
• Cs clock expansions
are customizable:
• Sine waves outputs
• 1PPS sync In/Out
• Local & Remote
management
• Display
• DC/AC power
supplies
• Internal battery

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Cs tube design
Photo-detector inserts
Pinch-off tubeRF feedth. Laser viewports
Electrical feedth.
Ion pump
Vacuum enclosure

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Optics sub-system design
Redundant laser modulesDFB laser modules
Off-the-shelf optical parts
Free space optics
Tunable optical mounts

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Cs clock prototype

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Laser frequency locking
• Laser current ramp
(yellow)
• Atomic fluorescence
signal (pink)
• FM demodulated
atomic signal
(green) used as
laser frequency
error signal
• Automatic line
identification
algorithm
• Automatic laser
frequency lock

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Ramsey fringe
• Atomic RF frequency
discrimination
signal
• Inverted fringe
(minimum amplitude
at resonance)
• Fringe amplitude
• 700 pA
• Signal/Noise
• 18’500 Hz1/2
• Fringe linewidth
• 740 Hz
• Atomic quality factor
• 12.4E6

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Short-term frequency stability
• Measured Allan deviation
• 2.7E-12 t-1/2
• Theoretical prediction
• 2.4E-12 t-1/2
• Proves proper clock
tuning parameters
• Performance limitations
• Short-term:
Spurious light
• Long-term:
Single servo loop in
operation (OCXO)
2.70E-12E t-1/2

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Outline
• Motivation and applications
• Cs clock: magnetic vs. optical
• Cs clock prototype development
• Conclusion

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Conclusion
• Development of the world best commercial Cs beam clock
• Laser optical pumping technology inside
• 10x better frequency stability (<3E-12 t-1/2)
• Long lifetime (10 years), no compromise with performance
• Standard 19” rack, 3U high, 460 mm deep
• Management: serial, remote, display
• Signals: 5, 10, 100 MHz, 1 PPS
• Acknowledgment: this work is partially financed by the European Space
Agency (contract number 21603/08/D/JR and 4000111645/14/NL/CVG)

25. Thank You
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