International audienceIn the field of Time and Frequency metrology, the most stable frequency source is based on a microwave whispering gallery mode sapphire resonator cooled near 6 K. Provided the resonator environment is sufficiently free of vibration and temperature fluctuation, the Cryogenic Sapphire Oscillator (CSO) presents a short term fractional frequency stability of better than 1 x 10-15. The recent demonstration of a low maintenance CSO based on a pulse-tube cryocooler paves the way for its deployment in real field applications. The main drawback which limits the deployment of the CSO technology is the large electrical consumption (three-phase 8 kW peak / 6 kW stable operation) of the current system. In this paper, we describe an optimized cryostat designed to operate with a low consumption cryocooler requiring only 3 kW single phase of input power to cool down to 4 K a sapphire resonator.We demonstrate that the proposed design is compatible with reaching a state-of-the-art frequency stabilit

Dubois, Benoît

FLUHR, Christophe

Giordano, Vincent

Grop, Serge

Le Tetu, Guillaume

Paris, Julien

Soumann, Valérie

English

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HAL Id: hal-02392625
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A Low Power Cryogenic Sapphire Oscillator with better
than 10-15 short term frequency stability
Christophe Fluhr, Benoît Dubois, Valérie Soumann, Guillaume Le Tetu,
Vincent Giordano, Serge Grop, Julien Paris
To cite this version:
Christophe Fluhr, Benoît Dubois, Valérie Soumann, Guillaume Le Tetu, Vincent Giordano, et al..
A Low Power Cryogenic Sapphire Oscillator with better than 10-15 short term frequency stability.
International Frequency Control Symposium and European Frequency and Time Forum, Apr 2019,
Orlando, United States. ￿hal-02392625￿
A Low Power Cryogenic Sapphire Oscillator
with better than 10−15
short term frequency stability
C. Fluhr∗, B. Dubois∗, V. Soumann†, G. Le Tetuˆ†, V. Giordano† and S. Grop‡ J. Paris§
—–
∗FEMTO Engineering
15B avenue des Montboucons, 25030 Besanc¸on Cedex, FRANCE
—–
†FEMTO-ST Institute, Time & Frequency Dpt.
26 Chemin de l’Epitaphe, 25030 Besanc¸on Cedex, FRANCE
—–
‡Spectratime
Vauseyon 29, 2000 Neuchaˆtel, SWITZERLAND
—–
§ My Cryo Firm SARL
104 Avenue de la Re´sistance, 93100 Montreuil, FRANCE
Abstract—In the field of Time and Frequency metrology, the
most stable frequency source is based on a microwave whispering
gallery mode sapphire resonator cooled near 6 K. Provided the
resonator environment is sufficiently free of vibration and tem-
perature fluctuation, the Cryogenic Sapphire Oscillator (CSO)
presents a short term fractional frequency stability of better than
1 × 10−15. The recent demonstration of a low maintenance
CSO based on a pulse-tube cryocooler paves the way for its
deployment in real field applications. The main drawback which
limits the deployment of the CSO technology is the large electrical
consumption (three-phase 8 kW peak / 6 kW stable operation)
of the current system.
In this paper, we describe an optimized cryostat designed to
operate with a low consumption cryocooler requiring only 3 kW
single phase of input power to cool down to 4 K a sapphire
resonator. We demonstrate that the proposed design is compatible
with reaching a state-of-the-art frequency stability.
I. INTRODUCTION
The Cryogenic Sapphire Oscillator (CSO) is the microwave
oscillator which features the highest short-term frequency sta-
bility [1]. With the advent of low maintenance CSO based on a
closed cycle cryocooler, 1×10−15 fractional frequency stabil-
ity is now avaible for real field applications. The autonomous
CSO enables the operation of laser-cooled microwave clocks
at the quantum limit [2], [3], the improvement of the resolution
in Very Long Baseline Interferometry (VLBI) and in the
deep-space netwoks for space-vehicle ranging and Doppler
tracking [4]–[9]. It can enhance the calibration capability of
Metrological Institutes, and help for the qualification of high
performances clocks and oscillators [10], [11]. Nevertheless,
the large electrical consumption (three-phase 8 kW peak/6 kW
stable operation) of the current system is the main obstacle to
the deployment of the CSO technology.
This paper describes an optimized cryostat designed to
operate with a low consumption cryocooler requiring only
3 kW single phase of input power to cool down to 4 K a
sapphire resonator. The proposed design is compatible with
reaching a state-of-the-art frequency stability.
II. HIGH PERFORMANCES CSOS OF THE
OSCILLATOR-IMP PROJECT
The Oscillator IMP project targets at being a facility ded-
icated to the measurement of noise and short-term stability
of oscillators and devices in the whole radio spectrum (from
MHz to THz), including microwave photonics. A set of three
CSOs constitutes one of the reference of the Oscillator IMP
metrological platform [1]. The figure 1 shows these three
instruments that have been successively assembled since 2012.
Fig. 1. The three CSOs of the OSCILLATOR-IMP platform.
They are in principle equivalent, but differ from several de-
tails. Two of them (CSO-1 and CSO-2) are equiped with a PT-
405 cryocooler from Cryomech requiring a 6 kW compressor.
The last one incorpores a PT-410 and a 8 kW compressor.
The three-cornered-hat method has been used to extract the
individual fractional frequency stability of each CSO. The
three oscillators present a stability better than 1 × 10−15
between 1 s and 10,000 s (see Fig. 2) [12].
−1610
10 −15
CSO−2
CSO−3
CSO−1
 1  10000  100000
Integration Time (s)
In
di
vi
du
al
 A
D
EV
 
 100 10  1000
Fig. 2. The three individual ADEV obtained from the 3-cornered-hat method.
III. THE ULISS PROJECT
One of these CSOs, codenamed ULISS, can be transported
with a small truck. In two years ULISS visited several Eu-
ropean sites, traveling more than 10,000 km [11]. Measuring
ULISS before and after traveling, we can validate stability and
spectral purity at destination. It has been used for example to
check on the stability of a laser stabilized on a ULE cavity,
and to run with a cold atom fountain at SYRTE (see Fig. 3).
Fig. 3. ULISS in SYRTE, where it was used as flywheel oscillator to run the
fountain atomic clock.
The ULISS Odissey was the opportunity to demonstrate
the potentiality and the reliability of our technology. It was
also pointed out that the electrical consumption and the heat
released by the compressor could turn out to hinder the
deployment of this technology. The project ULISS-2G was
launched to solve this technological difficulty with the aim to
divide by two the electrical consumption.
IV. NEW CRYOSTAT DESIGN OVERVIEW
Our design is based on a 3 kW PT-403 cryocooler from Cry-
omech delivering typically 0.25 W at 4 K. For this cryocooler
the frequency of the thermodynamical cycle is about 1.4 Hz.
The levels of mechanical displacement of the 2nd stage cold
plate induced by the gas flow inside the Pulse-Tube are 25 µm
peak-to-peak in the vertical direction and 12 µm peak-to-peak
in the horizontal one [13]. The temperature variation on the
PT 2nd stage is typically 100 mK rms.
Our main objective is to develop a cryostat as simple as
possible avoiding a high cost and a complex optimisation.
Thus we choose a simple passive filtering inspired from
Tomaru et al. [14]. Fig. 4 shows an overview of the CSO
cryostat.
sensors
Temperature
Cold head
flexible tube
Bellows
Springs
Rotary
Valve
Thermal shields
Microwave cable
Flexible
link
thermal
Vacuum can
300 K
1st Stage
Resonator
Thermal ballast
2nd Stage
Heater
Fig. 4. CSO cryostat: design overview (left), picture of the prototype (right).
The rotary valve is located remotely from the cryostat.
The microwave resonator is rigidly attached to the top flange
of the cryostat whose external surrounding walls are made
as rigid as possible keeping reasonable mass and space. The
rotary valve unit, which could generate large vibrations, is
separated from the cold-head. The cold-head itself still remains
an important source of vibrations. Thus it is supported by a
set of 8 springs providing the vibration insulation. A bellows
makes the connection with the vacuum can. The remaining
vibrations come from the pressure oscillation in the pulse tube
and regenerator transmitted to the cold-stage through thermal
links. The vibration reduction is obtained by using flexible
metal heat links (copper braids). The temperature variations
are passively filtered by the thermal ballast constituted by
the stainless steel top flange of the 2nd stage thermal shield.
Associated with the thermal resistance of the flexible thermal
link it is equivalent to a first order filter. The cavity is
attached to the thermal ballast. A thermal sensor and a 25 Ω
resistive heater are anchored in the cavity foot. They allow
the control of the resonator temperature. Four microwave UT-
085 semirigid Be/Cu cables link the resonator assembly to the
external circuits. These cables are thermalized on each stage.
To optimize this design with regard to the CSO performance,
trade-offs have been made between the vibration attenuation,
the thermal conductance of the links and the ballast thermal
mass [15].
The figure 5 shows a 3D-view of the cryostats internal
part and a picture of the 2nd stage suspension. The V-shaped
suspension rods enable to stiffen the suspension in the
transverse direction. To limit the thermal conduction these
suspensions are made in Mylar and simply realized using a
3D-printer.
Fig. 5. Internal 3D view of the cryostat and picture of the 2nd stage
suspension.
V. EXPERIMENTAL VALIDATION
A resonator was assembled and mounted inside the cryostat.
The sapphire resonator is made from a Kyropoulos monocrys-
tal [16]. The temperatures evolution during the cool down is
shown in Fig. 6:
3.4 K
80K
4.1 K
 3 K
 0
 50
 100
 150
 200
 250
 300
 350
 0  10  20  30  40  50  60
Te
m
pe
ra
tu
re
 (K
)
Time (h)
 4 K
Fig. 6. Temperatures evolution during cool-down. Resonator temperature
(bold line), 2nd-stage shield temperature (dotted line), 1st stage shield tem-
perature (dashed line).
Fig. 7 shows the modulus of the resonator response around
the whispering gallery mode at ν0 ∼ 10 GHz measured at the
minimum achievable resonator temperature, i.e. 4.1 K. The
bandwidth is 7.2 Hz equivalent to a loaded Q-factor QL ∼
1.4× 109.
The resonator temperature is stabilized using a commercial
temperature controller. By changing the temperature set-point
step-by-step we obtain the resonator frequency evolution (see
Fig. 8). The resonator presents a turnover temperature equals
to 5.24 K. Stabilized at this point, its frequency is no longer
ν−ν (Hz)0
7.2 Hz
−40
−20
−40 −20  0  20  40
|S2
1| (
dB
)
−30
Fig. 7. Transmissiom resonator response around the resonance frequency
ν0 ≈ 10 GHz.
 0
 4  4.5  5  5.5  6  6.5
Temperature (K)
Fr
eq
ue
nc
y 
sh
ift
 (H
z)
 
5.
24
 K
−100
−80
−60
−40
−20
Fig. 8. Resonator frequency evolution around the turnover temperature.
sensitive to the temperature fluctuations and a high frequency
stability can be obtained.
This resonator has been inserted in an oscillator loop
complemented with a Pound servo and a power stabilization.
The oscillator delivers 10 dBm at ν0 = 10.02 GHz.
In a first step, the close-to-carrier phase noise of the
prototype has been evaluated. Its 10 GHz output signal is
mixed with those delivered by one of the ultra-stable 6 kW
CSO. The obtained 26 MHz beatnote is sent to a Symmetricom
5125A phase noise test set. The observed phase noise is shown
in the Fig. 9. The upper curve was recorded when the springs
supporting the PT cold-head were intentionally blocked: the
bolt getting through every spring was tightened. In this case
the vibration of the cold-head are almost integrally transferred
to the cryostat. The spectrum shows a large line at 1.47 Hz
and its harmonics, which are the signature of the cold-head
vibration. The other smaller line at 1.41 Hz comes from the
6 kW CSO. When the springs are released (see Fig. 9 lower
curve), the 1.47 Hz line almost completely disappears and
its harmonics are also strongly attenuated. From this second
spectrum, the resonator displacement is estimated to be less
than 0.1 µm.
To evaluate the frequency performance of the prototype, it
has been simultaneously compared to two high stability 6 kW
CSOs. The three-cornered-hat method [12], [17] has been
used to extract the prototype fractional frequency stability.
φFourier frequency (Hz)
Fourier frequency (Hz)
2
φS 
  (f
)  (
dB
rad
  /H
z)
2
S 
  (f
)  (
dB
rad
  /H
z)
−70
−60
−120
−110
−100
−90
−80
−70
−60
−120
 1
 10
1.47 Hz
1.41 Hz
"bellows springs blocked"
"bellows springs released"
−110
 1
−100
−90
−80
 10
Fig. 9. Upper curve: Phase noise when the bellows springs are blocked.
Lower curve: bellows springs realesed.
The result is presented in the Fig. 10. The new instrument
ULISS−2G
10−16
10
10−17
−14
10−15
 10000 100  1000 10
Fr
ac
tio
na
l F
re
qu
en
cy
 S
ta
bi
lit
y
Integration Time (s)
 1
Fig. 10. Fractional frequency stability of the 3 kW prototype evaluated
using the three-cornered-hat-method with two ultra-stable 6 kW CSOs used
as references [12].
presents a fractional short term frequency stability below
1 × 10−15. At long integration time the fractional frequency
stability is degraded proportionnaly to τ . The observed drift
(approximately 3 × 10−14/day) could result from the large
temperature variations inside the laboratory. The three ref-
erence CSOs have recently been moved in a temperature
stabilized room at 22◦C ±0.5◦C. The new prototype stays in
another experimental room equipped only with the standard
air-conditioning system of the flat. Depending on the sunlight
the temperature near the cryostat can vary of several degrees
during the day.
VI. CONCLUSION
A new cryostat has been designed to cool-down a sapphire
microwave resonator intented to serve as the frequency refer-
ence of an ultra-stable oscillator. This new cryostat is equipped
with a 3 kW PT cryocooler sufficient to reach a temperature
of 4.1 K. A specially designed suspension permits to filter
the 1.4 Hz vibration generated by the pulse-tube. A prelim-
inary performance evaluation shows a short term fractional
frequency stability below 1× 10−15 for 1 s≤ τ ≤ 2000 s.
ACKNOWLEDGMENT
This work has been realized in the frame of the ANR
projects: Equipex Oscillator-Imp and Emergence ULISS-2G.
The authors would like to thank the Council of the Re´gion de
Franche-Comte´ for its support to the Projets d’Investissements
d’Avenir and the FEDER (Fonds Europe´en de De´veloppement
Economique et Re´gional) for funding one CSO.
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A Low Power Cryogenic Sapphire Oscillator with better than 10-15 short term frequency stability

Fluhr, Christophe

HAL - Université de Franche-Comté

https://hal.archives-ouvertes.fr/hal-02392625/document

A Low Power Cryogenic Sapphire Oscillator with better than 10-15 short term frequency stability

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