1 Reliability and Reproducibility of the Cryogenic Sapphire Oscillator Technology.

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Reliability and Reproducibility of the Cryogenic

Sapphire Oscillator Technology.

Christophe Fluhr∇, Benoˆ

ıt Dubois∇, Guillaume Le Tetu], Valerie Soumann],

Julien Paris[, Enrico Rubiola]⊗and Vincent Giordano]

Abstract—The cryogenic sapphire oscillator (CSO) is a highly

specialized machine, which delivers a reference signal exhibiting

the lowest frequency ﬂuctuations. For the best units, the Allan

deviation (ADEV) is σy(τ)<10−15 for integration time

between 1and 104s, with a drift <10−14 in one day. The

oscillator is based on a sapphire monocrystal resonating at 10

GHz in a whispering-gallery mode, cooled at 6K for highest

Q-factor and zero thermal coefﬁcient. We report on the progress

accomplished implementing eleven CSOs in about 10 years since

the ﬁrst sample delivered to the ESA station in Argentina. Short-

term stability is improved by a factor of 3-10, depending on

τ, and the refrigerator’s electric power is reduced to 3 kW.

Frequency stability and overall performances are reproducible,

with unattended operation between scheduled maintenance every

two years. The CSO is suitable to scientiﬁc applications requiring

extreme frequency stability with reliable long-term operation.

For example, the ﬂywheel for primary frequency standards,

the ground segment of GNSS, astrometry, VLBI, and radio

astronomy stations.

Index Terms—Time and frequency metrology. ultra-stable

oscillators, frequency stability.

I. INTRODUCTION

SHORT term fractional frequency stability in the 10−15

range has been demonstrated more than 20 years ago

with the use of high Q-factor microwave dielectric sapphire

resonator cooled near the liquid helium temperature [1], [2].

In the early 2000s, Cryogenic Sapphire Oscillator (CSO)

breakthrough performances and early uses in the ﬁeld of

Time and Frequency Metrology have been demonstrated with

prototypes still operating within a liquid He bath [3], [4], [5],

[6], [7]. In 2010, at the FEMTO-ST Institute, we demonstrated

for the ﬁrst time the possibility to use a cryocooler while

maintaining a state-of-the-art frequency stability [8]. Since,

we undertaken large engineering efforts to rationalize the CSO

design and its development, reduce its electrical consumption

and improve its immunity to environmental perturbations [9],

[10], [11], [12].

Currently, there is no competing technology. The closest

are the laser stabilized to a cryogenic Fabry-Perot cavity

Manuscript created October 2022

∇France Comt´

e Innov, 25000 Besanc¸on, France.

]FEMTO-ST Institute, Dept. of Time and Frequency, Universit´

e de Bour-

gogne and Franche-Compt´

e (UBFC), and Centre National de la Recherche

Scientiﬁque(CNRS), E-mail: giordano@femto-st.fr, Address: ENSMM, 26

Rue de l’Epitaphe, 25000 Besanc¸on, France.

[My Cryo Firm, 94120 Fontenay-sous-Bois, France.

⊗Physics Metrology Division, Istituto Nazionale di Ricerca Metrologica

INRiM, Torino, Italy.

and the Hydrogen maser, but they do not show the best

stability in same region of the σy(τ)plot, and have quite

different “personalities”. The short term frequency stability

achieved by the CSO is today only surpassed by some high-

class laser prototypes stabilized on a cryogenic ultra-stable

Fabry-Perot cavity [13], [14]. Commercial versions based

on a room temperature Ultra-Low Expansion (ULE) optical

cavity approach the short-term CSO frequency stability

but still present a large long term frequency drift, i.e.

∼10−11 −10−12/day [15], [16]. Moreover, those optical

sources operate generally in the near IR, and then require a

metrological femto-second laser to derive useful signals in

the microwave or the VHF bands.

Our upgraded CSO technology, code named ULISS-2G,

is today sufﬁciently mature to be offered as a commercial

product consuming only 3 kW and able to run continuously

for year. A simple maintenance operation, which can be

performed by the user, is only required every two years.

In this paper, we present the performance as well as the

operating history of all the CSOs that we have built and

validated since 2009. The objective here is to demonstrate

the reliability and the reproducibility of our CSO technology.

II. FEMTO-ST CSO DESIGN

A block diagram of the CSO is represented in the ﬁgure 1.

Monitoring System

Compressor

Temp.

Control PULSE

TUBE

10 GHz

100 MHz

10 MHz

Data

100 MHz

cooling water

Pound

Control Sustaining stage

Cryostat

Phase comparator

Synthesizer

Power

Control

Resonator

Fig. 1. CSO block diagram: we distinguish 3 mains subsets: the ultra-stable

oscillator itself, the frequency synthesizer and the monitoring system.

arXiv:2210.05545v1 [physics.ins-det] 11 Oct 2022

The high Q sapphire resonator is maintained near 6K

in a cryostat cooled with an autonomous Pulse-Tube

(PT) cryocooler. The CSO is a Pound-Galani oscillator:

the resonator is used in transmission mode in a regular

oscillator loop, and in reﬂection mode as the discriminator

of the classical Pound servo. The sustaining stage and the

control electronics are placed at room temperature. The

CSO output signal at the resonator frequency ν0drives

the frequency synthesizer, which eventually delivers several

output frequencies: 10 GHz, 100 MHz and 10 MHz in the

typical implementation. Eventually the synthesizer outputs

can be disciplined at long term on an external 100 MHz

signal coming from an Hydrogen Maser for example. A

monitoring system scrutinizes all the key parameters to

follow the CSO status. The technological choices relating to

the various subsets have already been described in previous

publications to which we refer the reader [17], [18], [19],

[20]. Here we will simply recall the key features, which make

the originality of our design.

A. Sapphire resonator

Fig. 2. Cryogenic sapphire resonator operating on the WGH15,0,0mode at

9.99 GHz.

The cylindrical resonator 54-mm-diameter and 30-mm-height

is made in a high purity sapphire monocrystal (see Fig. 2). It

operates on the quasi-transverse magnetic whispering-gallery

mode WGH15,0,0at ν0= 9.99 GHz ±5MHz. This choice

greatly simpliﬁes the design of the frequency synthesis

(see below). The sapphire resonator frequency shows near

6 K a turnover temperature T0for which the resonator

sensitivity to temperature variations nulls at ﬁrst order. The

appearance of this turning point results from the presence in

Al2O3of paramagnetic impurities as Cr3+ or Mo3+, whose

concentration are typically well below 1ppm in weight. The

exacte value of T0is speciﬁc to each resonator and for high

quality sapphire crystal is typically found between 5and

9K [21]. At T0, the unloaded Q factor can achieve two

billion depending on the sapphire crystal quality and on the

resonator adjustment and cleaning.

The sapphire resonator includes an integral spindle (see

Fig. 2), such that the sapphire piece can be mounted with no

signiﬁcant ﬁxing stress in the cylinder’s critical circumferential

region, where the WG mode’s ﬁeld is concentrated. Since

2009, we have ordered more than 25 sapphire resonators from

several high-quality crystal manufacturers that we selected

after preliminary tests on samples. Of these resonators, 17

were actually tested at low temperature. The following ﬁgures

show the reproducibility of resonator main characteristics.

1) ν0, resonator frequency:

CA B

9.99

9.98

9.985

Manufacturer

9.995

WGH15 mode frequency (GHz)

Fig. 3. WGH15,0,0mode frequency of the tested resonators.

We had set the tolerances on resonator dimensions to

achieve a frequency of 9.99 GHz ±5MHz which simpliﬁes

the design of frequency synthesis (see Sect. II-C ). Only

one among the tested resonators presents a frequency

notably shifted from the expected value at about 9.998 GHz.

Nevertheless, it could be integrated into a CSO by making

some minor modiﬁcations to the synthesis.

2) T0, resonator turnover temperature:

ABC

Manufacturer

WGH15 mode turnover temperature (K)

Fig. 4. WGH15,0,0mode turnover temperature of the tested resonators. The

grey zone: 4.6K≤T0≤8K corresponds to our speciﬁcation.

The lowest value of the acceptable temperature range

is 4.6K, which is the lowest achievable temperature in

ULISS-2G increased by 0.5 K. It represents the lowest

allowable T0for an effective temperature stabilization. On

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1ReliabilityandReproducibilityoftheCryogenicSapphireOscillatorTechnology.ChristopheFluhrr,BenotDuboisr,GuillaumeLeTetu],ValerieSoumann],JulienParis[,EnricoRubiola]andVincentGiordano]AbstractThecryogenicsapphireoscillator(CSO)isahighlyspecializedmachine,whichdeliversareferencesignalexhibitingthelo...

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