High Performance Optically Pumped Cesium Beam Clock

High performance optically-pumped cesium beam clock

Dr. Patrick Berthoud, Chief Scientist Time & Frequency

ITSF 2015, Edinburgh, UK, 2nd – 5th November

© 2015 ADVA Optical Networking. All rights reserved. Confidential.2

Outline

• Motivation and applications

• Cs clock: magnetic vs. optical

• Cs clock prototype development

• Conclusion


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


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


• vailability : commercial product

• High perfermance and long life optical Cs clock• Stability : 3.0E-12 t-1/2 , floor = 5E-15


• Availability : under development


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 timingcalibration

Freq accuracy +Env. changes

Freq linear drift

Noise timing error

sx(t) sy(t) * t


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

Alla

n s

td f

req

uen

cy d

evia

tio

n [

Hz/

Hz]

Averaging time [s]

Std. Perf.Magnetic

High Perf. Magnetic

High Perf. Optical


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

Tim

ing

erro

r [s

]

Averaging time [s]

Std. Perf.Magnetic

High Perf. Magnetic

High Perf. Optical

30 ns

6 d

ays

60 d

ays


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


Outline




• Conclusion


Magnetic Cesium clock operation

• Cs beam generatedin the Cs oven(vacuum operation)

• Cs atoms state selection by magnets

• Cs clock frequencyprobing (9.192 GHz) in the Ramseycavity

• Atoms detection and amplification by electron multiplier (vacuum)

• RF source servo loopusing 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


Optical Cesium clock operation

• Cs beam generatedin the Cs oven(vacuum operation)

• Cs atoms state selection by laser

• Cs clock frequencyprobing (9.192 GHz) in the Ramseycavity

• Atoms detection and amplification by photodetector (air)

• Laser and RF sources servo loops usingatomic signals

Ramsey

cavityLight

Collectors

Magnetic

shield + coil

FM

User

10 MHz

Laser

Cs

OvenVacuum

enclosure

Photo-

detectors

RF

source

Sync

Detect

FM

Laser

source

Sync

Detect

Cs

beam


133Cs atomic energy levels

• Stable ground states (F=3 and F=4)

• Switching betweenground states F by RF interaction 9.192 GHz

• Unstable excitedstates (F’=2,3,4,5)

• Switching betweenground states F and excited states F’ by laser interaction 852 nm (or 351 THz)

6S1/2 nhf = 9.192 GHz

F=4

F=3

6P3/2F’=4

F’=3

F’=2

F’=5


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


Outline




• Conclusion


Clock functional bloc diagram• Cs tube

• Generate Cs atomic beam in ultrahigh vacuum enclosure

• Electrical and optical feedthoughs foratomic 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, signalsoutputs

Cesium tube

Magnetic field and shields

Cs Oven

Collect CollectRamsey cavity

Optics

Laser Splitter Mirror

Clock electronicsRF

Source

Clock Ctrl Power Supply

Photo Detect

Photo Detect

4x S

ync

ou

t (1

PPS

)

Expansion electronics

Seri

al (

RS2

32)

Sync

in (

1PPS

)

Dis

pla

y

10 M

Hz

sin

e10

MH

z si

ne

10 o

r 5

MH

z si

ne

(opt

ion

)10

or

100

MH

z si

ne

(opt

ion

)

MetrologyManage

ment

Rem

ote

(TC

P/IP

)

PPS DC/DC AC/DC Battery

Exte

rnal

DC

sup

ply

Exte

rnal

AC

su

ppl

y


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 & Remotemanagement

• Display• DC/AC power

supplies

• Internal battery


Cs tube design

Photo-detector inserts

Pinch-off tubeRF feedth. Laser viewports

Electrical feedth.

Ion pump

Vacuum enclosure


Optics sub-system design

Redundant laser modulesDFB laser modules

Off-the-shelf optical parts

Free space optics

Tunable optical mounts


Cs clock prototype


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


Ramsey fringe

• Atomic RF frequencydiscrimination 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


Short-term frequency stability

• Measured Allan deviation• 2.7E-12 t-1/2

• Theoretical prediction• 2.4E-12 t-1/2

• Proves proper clocktuning parameters

• Performance limitations• Short-term:

Spurious light

• Long-term:Single servo loop in operation (OCXO)

2.70E-12E t-1/2


Outline




• Conclusion


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)

Thank You

IMPORTANT NOTICE

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Copyright © for the entire content of this presentation: ADVA Optical Networking.

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Date post:	09-Feb-2017
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High Performance Optically Pumped Cesium Beam Clock

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