At PTTI 2023, our team, led by Patrick Berthoud, discussed our industry-first high-performance optical cesium atomic clock technology. This cutting-edge solution offers unparalleled levels of precise synchronization and stability.

1. Industrial optically pumped cesium beam clock
Precise Time and Time Interval Meeting, January 23-26, 2023, Long Beach, CA, USA
P. Berthoud, M. Haldimann, F. Kroll, V. Dolgovskiy

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Outline
Motivation
Design
Performance
Conclusion
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Improve performance while keeping lifetime and compactness
Telecommunication network reference
• Telecom operators, railways, transport, power, …
Enterprise / Data-centers (ICPs) / Finance
• Synchronization of distributed databases,
• Speed trading
PNT market
• Multi verticals GNSS backup
Defense
• Secured telecom, inertial navigation
Space
• Satellite mission tracking, GNSS systems
Science
• Astronomy, nuclear and quantum physics, …
Metrology
• Time scale, fundamental unit measurement
Markets for cesium clocks

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Outline
Motivation
Design
Performance
Conclusion
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Cs tube
• Produce a cesium atomic beam
• Provide mechanical means to
measure Cs resonance frequency
Optical bench
• Provide laser beams to the Cs
tube for the optical pumping and
optical detection processes
Electronics
• Control the laser
• Control the Cs tube
• Provide an output signal to the
user based on the Cs atom
reference frequency
Optically-pumped cesium clock architecture
Collector
Cs Oven
Collector
Ramsey cavity
C-field coil + magnetic shields
Vacuum enclosure
Laser
FM
sinus
ADC
A A B
Demod
Laser
source
DAC
ADC
FM square +
time sequencer
Demod
RF synth.
DAC
RF pwr
OCXO
DAC
RF freq
DAC
C-field
C-field
source
Output
frequency

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Cesium tube
Collector
Cs Oven
Collector
Ramsey cavity
C-field coil + magnetic shields
Vacuum enclosure
Laser
A A B
Cesium oven
• Temperature = 100°C
• Compatible with 10 yr lifetime
Ramsey cavity
• Dark fringe (p pulse between arms)
• Loaded quality factor = 500
Atomic fluorescence collectors
• Efficiency >50%
• Laser light trap (50 dB)
Magnetic C-field and shields
• Static and uniform
• Amplitude = 100 mG

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Optical bench
Collector
Cs Oven
Collector
Ramsey cavity
C-field coil + magnetic shields
Vacuum enclosure
Laser
A A B
Simplest optical setup
• Single laser frequency (D2:3→4’) for
atomic preparation and detection
• DFB laser module
• Free space optics
• No acousto optic modulator
• No optical isolator
Beams (3)
• A: laser frequency lock
(50 mW)
• A’: optical pumping completion
(1.5 mW)
• B: clock transition
detection (50 mW)

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Laser lock loops
• Sine wave FM (20 kHz)
• Optical frequency by current and temp.
• Optical power by current and temp.
Clock lock loops
• Square wave FM (10 Hz)
• Time sequencer for multiple loops
• Quartz oscillator frequency
• RF field power
• Magnetic C-field amplitude
• Light shift compensation
• All error signals come from atomic signals
and are differential
Processing
• Real time
• Fully digital (FPGA)
Electronics
Collector
Cs Oven
Collector
Ramsey cavity
C-field coil + magnetic shields
Vacuum enclosure
Laser
FM
sinus
ADC
A A B
Demod
Laser
source
DAC
ADC
FM square +
time sequencer
Demod
RF synth.
DAC
RF pwr
OCXO
DAC
RF freq
DAC
C-field
C-field
source
Output
frequency

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Industrial product OSA-3300 HP
19’’ rack, 3U (450 x 133 x 460 mm3)
35 W
23 kg (battery included)
10 years lifetime

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Outline
Motivation
Design
Performance
Conclusion
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2
3
4

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Symmetric spectrum
Dark fringe behavior
• Minimum signal at resonance
Contrast: 50%
Linewidth: 740 Hz (FWHM)
Signal-to-noise ratio: 15’000 Hz1/2
Fringes per sub-level: 1
• Large destructive fringe interference by the
wide atomic velocity distribution
Open loop performances (Ramsey fringes)

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Test setup:
• Phase comparision of cesium clock vs. active
hydrogen maser
• Active maser frequency monitored versus GPS
• Carrier frequency: 10 MHz
• Measurement time: 50 days
• Sampling period: 10 s
• Standard lab conditions
Frequency stability
• Overlapping Allan deviation sy(t)
• White FM noise (t-1/2 slope)
• sy(t) = 4E-12 t-1/2 (0.004 ppb @1s)
• Not limited by any flicker FM noise @4E-15
Frequency stability (Allan deviation)

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Test setup
• Identical as before (same data)
Cesium clock vs. active hydrogen maser
Time interval error
• ± 6 ns over 50 days in free running mode
Time interval error (TIE)
Good candidate to hold time during long GNSS outages (PNT)

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3300 SHP experimental batch
Test setup
• Cesium clocks vs. active hydrogen maser
Repeatability
• Statistics over 10 clocks
• About ±25% stability dispersion between
clocks
• All clocks comply with HP standard, with a
10 years lifetime basis

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Outline
Motivation
Design
Performance
Conclusion
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2
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• Design description of an industrial and
compact optically-pumped thermal
cesium beam clock (19’’, 3U rack)
• High performance frequency stability
demonstration using optical pumping
technology (long lifetime)
• Good industrial ground-based back-up
solution for PNT architecture (in case of
GNSS outages)
Conclusions

17. Thank you
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