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High resolution frequency standard dissemination via
optical ber metropolitan network
F Narbonneau, M Lours, S Bize, A Clairon, G Santarelli, O Lopez, Ch
Daussy, Anne Amy-Klein, Ch Chardonnet
To cite this version:
F Narbonneau, M Lours, S Bize, A Clairon, G Santarelli, et al.. High resolution frequency standard
dissemination via optical ber metropolitan network. Review of Scientic Instruments, American
Institute of Physics, 2006, �10.1063/1.2205155��. �hal-03151495�
arXiv:physics/0603125 v1 15 Mar 2006
High Resolution Frequeny Standard Dissemination via Optial Fibre
Metrop olitan Network
F. Narb onneau, M. Lours, S. Bize, A. Clairon, and G. Santarelli
LNE-SYRTE, Observatoire de Paris, 61 Avenue de l'Observatoire, 75014 Paris, Frane
O. Lop ez, Ch. Daussy, A. Amy-Klein, and Ch. Chardonnet
Laboratoire de Physique des Lasers, Université Paris XIII, Vil letaneuse, Frane
We present in this paper results on a new dissemination system of ultra-stable referene signal
at 100 MHz on a standard bre network. The 100 MHz signal is simply transferred by amplitude
modulation of an optial arrier. Two dierent approahes for ompensating the noise introdued
by the link have b een implemented. The limits of the two systems are analyzed and several solution
suggested in order to improve the frequeny stability and to further extend the distribution dis-
tane. Nevertheless, our system is a go od tool for the best old atom fountains omparison between
laboratories, up to 100 km, with a relative frequeny resolution of 10
−14
at one seond integration
time and 10
−17
for one day of measurement. The distribution system may b e upgraded to fulll the
stringent distribution requirements for the future optial lo ks.
I. INTRODUCTION
Ultra-stable frequeny and time soures play an im-
portant role in many modern Time and Frequeny
metrology and fundamental physis appliations (lok
evaluation, relativity tests, fundamental onstants test
...)(e.g. [1℄, [2℄, [3℄, [4℄). In the eld of partiles
physis, mo dern large linear aelerators require RF
distribution system with minimal phase drifts and er-
rors for the neutrons and p ositrons generation [5℄. In
radio-astronomy, e.g. in the ase of the ALMA (At-
aama Large Millimetri Array) pro jet or for VLBI
(Very Long Baseline Interferometry), the ombination
of high frequeny and long baselines of the interfer-
ometer needs the distribution of a loal osillator with
low phase noise and low phase drift through the ar-
ray [6℄, [7℄. For the Deep Spae Network (DSN), the
Jet Propulsion Lab oratory (JPL) has developed a bre
link to distribute referene signals from an H-Maser to
synhronize eah antenna of the DSN [8℄, [9℄.
Modern old atoms frequeny standards in the mi-
rowave domain have already demonstrated an au-
ray in the 10
−15
range with the p otential to reah
the 10
−16
level or b etter. Frequeny stabilities, de-
ned by the Allan standard Deviation (ADEV), are
ommonly of 10
−13
τ
−
1
2
for suh standards and a few
10
−14
τ
−
1
2
have b een demonstrated using more ad-
vaned tehniques [10℄. Cold atom optial loks have
the p otential to reah the 10
−17
auray level [11℄,
[12℄, [13℄, [14℄. The emergene of mo dern mirowave-
to-optial synthesizers based on mode-lo ked femtose-
ond lasers allows high resolution omparisons b etween
mirowave and optial lo ks [15℄, [16℄, [17℄. Cloks
omparisons are urrently performed by satellite, as for
example GPS or TWSTFT (Two-Way Satellite Time
and Frequeny Transfer. Measurements are limited by
the transmission system to about 10
−15
at one day av-
eraging time [18℄. Theses metho ds are thus insuient
for measuring the ultimate p erformane of a mirowave
or an optial standard (Fig. 1).
Upgrades of the orbital equipments are expetable
to improve the urrent p erformane, but are quite
10
0
10
1
10
2
10
3
10
4
10
5
10
-17
10
-16
10
-15
10
-14
10
-13
H-MASER
Best cold caesium atoms fountain
(FO2 / LNE-SYRTE)
Future optical
frequency standards
Allan standard deviation
y
(
)
Integration time (s)
Cryogenic Sapphire
Oscillator
Figure 1: Allan deviation of some frequeny standards
omplex and exp ensive. Moreover, the two previous
systems deliver only a synhronization signal not al-
lowing diret short-term stability omparisons. Then
for muh of appliations a referene signal is needed.
Hene, the opp ortunity to ompare mirowave and op-
tial loks by the development of a new type of a
ground frequeny dissemination by optial bre seems
appropriate, even when the lab oratories are separated
by 100 km [19℄, [20℄, [21℄. One an indeed take advan-
tage of b oth the low attenuation and low disp ersion
in the bre, whih allow reahing long distane fre-
queny transfer by maintaining a go od signal-to-noise
ratio (SNR).
Moreover the aess to an ultra-stable frequeny refer-
ene for a large number of lab oratories op en the way
to p erform new exp eriments in fundamental physis.
The development and op eration of a state-of-the-art
frequeny standard remain a strong limitation and an
be overome by a bre distribution system onneting
Time and Frequeny Metrology lab oratories to users.
The simplest way to develop a bre distribution is to
use the redundany of the teleom network. In this pa-
2
per, we present the transfer of high frequeny stability
signal at 100 MHz, by using the existing teleommuni-
ation bre network, over a few tens kilometers, with
ompensation of the phase noise introdued by the link.
II. PRINCIPLE AND OBJECTIVE
The goal of the dissemination is the distribution of a
referene signal at a frequeny of 100 MHz, synthesized
from a frequeny standard, by amplitude mo dulation
of an optial arrier, without degradation of the phase
noise of the distributed signal. The referene signal
modulates the bias urrent of a DFB laser dio de, at
1.55
µ
m, whih is transmitted through a bre optial
link to users. At the link extremity, a photodio de
detets the amplitude modulation and onverts the
optial signal to a radio-frequeny signal osillating at
the referene frequeny and phase oherent with the
mirowave referene soure.
The high stability and low phase noise of the trans-
ferred signal are degraded by the residual phase noise
of the optial link and by the attenuation in the
bre. We op erate in urban environment by using
the existing teleom network. Thus, bre layout and
installation asp ets are not ideal and the stability
of the optial link an b e aeted by environmental
eets. Optial length of the bre is mo died by
mehanial stresses and temp erature utuations. The
rst one aets phase noise and short-term frequeny
stability p erformanes of the transmitted signal. The
seond eet, is a slowly hanging phenomenon and
has an impat on the long-term stability.
These instabilities have b een studied on two optial
links using the dense Frane Teleom network and
onneting LNE-SYRTE to Laboratoire de Physique
des Lasers (LPL) (about 43 km), and LNE-SYRTE
with Lab oratoire Kastler Brossel (LKB - University
Paris VI) (ab out 3 km).
10
0
10
1
10
2
10
3
10
4
10
5
10
6
10
-16
10
-15
10
-14
10
-13
Allan Standard Deviation
y
(
)
Integration time [s]
LNE-SYRTE / LPL (44 km) [May 03]
LNE-SYRTE / LPL (44 km) [July 01]
LNE-SYRTE / LKB Jussieu (3 km) [Dec 03]
LNE-SYRTE / LKB Jussieu (3 km) [Feb 04]
Figure 2: Frequeny stability measurements of the LNE-
SYRTE/LPL and LNE-SYRTE/LKB optial links
Measurements, realized at dierent perio ds, are pre-
sented in gure 2 and show non-stationary eets de-
pending on the ativities around the link. Periodi
eets as daily temp erature variations app ears as a
bump at the half perio d, on the ADEV. The frequeny
instabilities related to a sinusoidal temp erature p ertur-
bations an b e alulated from the equation (1):
σ
y
(τ) = ∆T ×
T CD × n × L
c
×
sin
2
(πτν
0
)
τ
(1)
with
∆T
the amplitude of the temperature utuation
[
◦
C℄,
T CD
the thermal oeient of delay [ppm/
◦
C℄
of the optial bre (typially 7 ppm/
◦
C for standard
teleom SMF28 bre),
n
the bre ore index,
L
the
optial link length [km℄,
c
the light veloity in vauum
[3
×
10
8
m/s℄,
ν
0
the p erturbation frequeny [Hz℄, and
τ
the averaging time [s℄. For example, if we onsider a
sinusoidal p erturbation of 0.2
◦
C with a p eriod of 1000s
due to air onditioning and ating on a setion of 50
meters of the optial link, the ADEV of the link ould
be limited to ab out 7x10
−16
at 500 s integration time.
In the same way, a daily 0.5
◦
C temp erature variation
on 43 kilometers of optial bre is onverted into an
instability of the order of 1.3x10
−14
at 43200 s averag-
ing time.
Consequently, the distribution system needs an ative
ontrol lo op to omp ensate for these phase variations
indued on the signal transmitted through the link re-
lated to the environment (mehanial vibrations, tem-
perature utuations ...).
The ob jetive of the dissemination b eing lo k om-
parisons or delivery of a referene signal oming from
an H-Maser or a Cryogeni Sapphire Osillator (CSO),
the omp ensation set-up must introdue a phase noise
lower than the referene signal. In this persp etive we
have to develop a system whih delivers a referene sig-
nal at 100 MHz, showing a relative frequeny stability
σ
y
(τ) ≤ 2.10
−14
[
τ
= 1s℄ (
< 10
−16
1d), that implies
a residual iker phase noise of -120 dBrad
2
/Hz at 1
Hz and a white phase noise o or with a level of -140
dBrad
2
/Hz.
II I. ACTIVE PHASE FLUCTUATIONS
COMPENSATION SYSTEM
A. Presentation
The priniple of the phase utuations omp ensa-
tion, is displayed in gure 3. At the link extremity,
the deteted signal an not be diretly ompared to
the referene signal and thus the orretion of the
phase p erturbations an b e only arried out at the
link emission. A two-way distribution, using the same
optial bre link, allows determination of the phase
perturbation aumulated along a full round trip with
the hypothesis that the forward and the bakward
signals are orrupted by the same p erturbation. The
ompensation rests then on the measurement of the
phase of the signal after one round trip to apply a
orretion on the emitted signal.
3
R e f e r e n c e s i g n a l
R F p r o c e s s
P h a s e c o r r e c t i o n
r e f
f
0
=+
)()(
tt
pc
ff
pc
ff
-=
P h a s e p e r t u r b a t i o n
f
p
(
t
)
)()(
tt
pcr e f
fff
++
U s e r - e n d
P h a s e f l u c t u a t i o n s c o m p e n s a t o r
)( t
cr e f
ff
+
O p t i c a l f i b r e l i n k
Figure 3: Shemati of the phase utuations omp ensation
The referene signal at the frequeny f
ref
=
ω
ref
/2π
is used for modulating a laser diode. The amplitude
modulated signal is then orreted by a phase term
φ
c
. This orretion term is provided either by phase
shifting the RF modulating signal or by mo difying the
propagation delay in the bre. At the user-end, the
signal orrupted by the environmental perturbations
is deteted:
V
RF deteted
(t) ∝ sin(ω
ref
t + φ
ref
+ φ
c
+ φ
p
)
(2)
This signal is split in two signals: one part for the
user appliations and the other to b e re-injeted via
an optial irulator in the same optial bre. After
one round-trip, the signal, twie orrupted by the term
φ
p
is deteted. A RF proess allows generation of an
error signal, applied to the phase orretor. Two dier-
ent laser soures, op erating at slightly dierent wave-
lengths, are used for generating the forward and the
bakward optial signals and optial add/drop fun-
tions are realized with optial irulators.
Dierent approahes of phase ompensation have b een
studied and are desrib ed here.
B. Eletroni phase utuations ompensator
In the ase of an eletroni phase utuations om-
pensator (f g. 4), the orretion is p erformed by
ating on the phase of the injeted signal in the optial
link, that we all
φ
input
.
E l e c t r o n i c
p h a s e s h i f t e r
-
+
L o o p
f i l t e r
O p t i c a l
t r a n s c e i v e r
1
O p t i c a l
t r a n s c e i v e r
2
O p t i c a l l i n k
R e f e r e n c e s i g n a l
r e f r e f
,
fw
p
f
r e f o u t p u t r e f
,
ffw
=
i n p u t
f
U s e f u l s i g n a l
r
f
Figure 4: Simplied shemati of the phase onjugator
We dene by
φ
r
the phase of the round-trip signal,
and
φ
output
the phase of the deteted signal at the user-
end, equal to:
φ
output
(t) = φ
input
(t − τ) +
Z
t
t−τ
φ
p
(ξ)dξ
(3)
where
τ
is the propagation delay in the optial
bre link and
φ
p
(ξ)
is the distributed phase p er-
turbation along the bre. The main eet of the
delay
τ
is to limit the lo op bandwidth. In the fol-
lowing disussion, we neglet the inuene of the delay.
The output signal must b e phase oherent with the
referene soure of frequeny
ω
ref
and on average of
phase
φ
ref
, and thus the orretion applied to the
emitted signal must b e equal to the opp osite of the
phase perturbation
φ
p
. Consequently, on average (or
for time muh longer than
τ
) the phase of the input
signal,
φ
input
is:
φ
input
= φ
ref
− φ
p
(4)
Then, the phase of the round-trip signal b eomes:
φ
r
= φ
input
+ 2 × φ
p
= φ
ref
+ φ
p
The phase oherene of the output signal is hene im-
posed by maintaining a onjugation relationship b e-
tween the input and the round trip signal of the optial
link:
(φ
input
− φ
ref
) = −(φ
r
− φ
ref
)
(5)
A simplied sheme of the phase onjugator is shown
in gure 4. The orretion is p erformed with a phase
shifter in series with the referene signal, whih is
used as the input signal. The referene signal is p ower
divided to drive two phase detetors. Phase detetion
between the referene signal, the input orreted
signal and the round-trip signal, allow generation of
two baseband signals, onneted to the inputs of a
low noise dierential amplier. The output of this
amplier is used for driving a lo op lter, ontrolling
the eletroni phase shifter until the phase onjuga-
tion, and thus a zero level at the amplier output is
reahed. Although the simpliity of operation, this
system suers from various drawbaks. First, the
phase orretion is limited by the dynami of the
phase shifter. Eletroni phase shifters have a typial
dynami of 180 degrees with a non linear resp onse,
induing variable insertion losses. Moreover the phase
shifter an present a phase noise exess, ompared
to the other omp onents of the phase onjugator.
Seondly, phase detetors are quite sensitive to the
driving levels and it is diult to ensure the same
sensitivity for the two detetors of gure 4.
The pratial realization leads to a very p oor eetive
system of the phase p erturbations anellation. A
new sheme, regarding the previous onsiderations
and introdued by the JPL [22℄ is shown in gure 5.
Two symmetrial signals are pro dued by frequeny
shift (f
shift
) of the referene signal (f
±
=f
ref
±
f
shift
).
4
L a s e r D i o d e
L a s e r D i o d e
P L L
1 1 0 M H z
P L L
9 0 M H z
1
1 0
L o o p
f i l t e r
U s e r e n d @ 1 0 0 M H z
R e f e r e n c e s i g n a l
@ 1 0 0 M H z
O p t i c a l f i b r e l i n k
V C X O @
1 0 0 M H z
1 0 M H z
1 0 M H z
Figure 5: Blo k diagram of the full eletroni omp ensation
system
This sheme allows replaement of the double phase
measurements (Fig. 4) by a muh more aurate dou-
ble frequeny mixing and a single phase measurement.
The dynami and the linearity of the phase orretion
is improved by using a voltage ontrolled quartz osil-
lator (VCXO), as a phase shifter, delivering a signal at
the referene frequeny with a stable amplitude. The
VCXO presents thus the advantage to orret all phase
perturbation in the orretion bandwidth of the phase
ompensator, whih is limited by the round-trip prop-
agation delay in the optial link (ab out 0.3 ms in the
ase of the 43-km LNE-SYRTE to LPL optial link).
The 100 MHz output signal of the VCXO modulates
the bias urrent of the DFB laser dio de. The optial
signal is launhed in the optial bre link to the user.
At the user end, a simple system allows detetion and
regeneration of the bakward signal. The deteted sig-
nal after a one-way distribution is prop ortional to:
V
User end
(t) ∝ sin(ω
os
× t + φ
os
+ φ
p
)
(6)
The bakward optial signal is submitted to the same
phase perturbation and after one omplete round-trip,
the deteted signal has the following form:
V
round trip
(t) ∝ sin(ω
os
× t + φ
os
+ 2 × φ
p
)
(7)
The servo lo op fores the VCXO at 100 MHz b oth to be
phase oherent with the referene soure and to om-
pensate for the phase p erturbation. For obtaining the
phase onjugation, two signals separated by 10 MHz
around the referene frequeny (one at 90 MHz and the
other at 110 MHz) are pro dued by frequeny mixing
between the referene signal and itself frequeny di-
vided by ten. Two dierent systems, based on PLL
(Phase Lo k Lo op) are used for ltering eah signal
issue from the previous frequeny mixing. The signal,
from the "down onversion", at 90 MHz, is mixed with
the mo dulating signal, delivered by the VCXO, to ob-
tain a signal at 10 MHz:
V
1
(t) ∝ sin((ω
os
− 2 π ×
90 MHz
) × t + φ
os
−
9
10
φ
ref
)
(8)
In parallel, the signal at 110 MHz is mixed with the
round-trip signal, pro duing another signal at 10 MHz:
V
2
(t) ∝ sin((2 π×
110 MHz
−ω
os
)×t+
11
10
φ
ref
−φ
os
−2φ
p
)
(9)
The phase omparison at 10 MHz allows generation of
a base-band signal, ontaining the three phase terms:
V
error
(t) ∝ φ
os
+ φ
p
− φ
ref
(10)
whih is anelled in normal op eration. The phase of
the VCXO is then:
φ
os
= φ
ref
− φ
p
(11)
By this pro ess, the stability and the auray of the
referene soure is transmitted to the user end in the
system bandwidth.
The apaity of the phase ompensator to rejet phase
perturbations in the ontrol bandwidth is dened by
the rejetion fator, equal to the ratio b etween the
phase variations in op en and in losed lo op. The p er-
formane of the distribution system dep end b oth of the
intrinsi system phase noise and of the rejetion fator.
Figure 6: Blok diagram of the omp ensation system test
benh
Figure 6 displays the set-up used for the harater-
ization of the phase onjugator. Simulation of phase
perturbations are realized by perio dially heating a
2.5-km bre sp ool with an amplitude of 4
◦
C and a
perio d of ab out 4000 s. This p erturbation indues a
phase mo dulation of the order of 200 mrad on the
100-MHz transmitted signal. In op eration, when
the phase onjugator is ativated, the residual phase
modulation measured at the link output is redued to
0.4 mrad (f. gure 7), that implies a rejetion fator
of the phase p erturbations along the link of ab out 500
(53 dB).
0 2000 4000 6000 8000 10000 12000 14000
-0.5
-0.4
-0.3
-0.2
-0.1
0.0
0.1
0.2
0.3
0.4
Time [s]
Pha se var iation s in cl osed l oop [mr ad]
-100
-80
-60
-40
-20
0
20
40
60
80
100
Pha se var iation s in ope n loop [mrad]
Figure 7: Phase shift indued by temp erature modulation
of the transmitted signal, in op en and losed lo op at 100
MHz
