High resolution frequency standard dissemination via optical fiber metropolitan network

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High resolution frequency standard dissemination via optical fiber 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 fiber metropolitan network. Review of Scientific Instruments, American

Institute of Physics, 2006, �10.1063/1.2205155��. �hal-03151495�

arXiv:physics/0603125 v1 15 Mar 2006

Metropolitan Network

F. Narbonneau, M. Lours, S. Bize, A. Clairon, and G. Santarelli

LNE-SYRTE, Observatoire de Paris, 61 Avenue de l'Observatoire, 75014 Paris, Frane

O. Lopez, Ch. Daussy, A. Amy-Klein, and Ch. Chardonnet

Laboratoire de Physique des Lasers, Université Paris XIII, Villetaneuse, Frane

Wepresent inthis paperresults onanewdissemination system ofultra-stable referene signal

at100MHzonastandardbrenetwork. The100MHz signalis simplytransferredbyamplitude

modulationofanoptialarrier. Twodierentapproahes for ompensatingthe noiseintrodued

bythelinkhavebeenimplemented. Thelimitsofthetwosystemsareanalyzedandseveralsolution

suggestedin orderto improve the frequeny stability and to further extendthe distribution dis-

tane. Nevertheless,oursystemisagoodtoolforthebestoldatomfountainsomparisonbetween

laboratories, upto100km,witharelativefrequenyresolutionof10

−14

atoneseondintegration

timeand10

−17

foronedayofmeasurement. Thedistributionsystemmaybeupgradedtofulllthe

stringentdistributionrequirementsforthefutureoptialloks.

I. INTRODUCTION

Ultra-stablefrequenyandtimesouresplayanim-

portant role in many modern Time and Frequeny

metrologyandfundamentalphysisappliations(lok

evaluation,relativitytests,fundamentalonstantstest

...)(e.g. [1℄, [2℄, [3℄, [4℄). In the eld of partiles

physis, modern large linear aelerators require RF

distributionsystemwithminimalphasedriftsand er-

rors fortheneutrons andpositronsgeneration [5℄. In

radio-astronomy, e.g. in the ase of the ALMA (At-

aama Large Millimetri Array) projet or for VLBI

(VeryLongBaselineInterferometry),theombination

of high frequeny and long baselines of the interfer-

ometerneedsthedistributionofaloalosillatorwith

low phase noise and low phase drift through the ar-

ray [6℄, [7℄. Forthe DeepSpae Network (DSN), the

JetPropulsionLaboratory(JPL)hasdevelopedabre

linktodistributereferenesignalsfromanH-Maserto

synhronizeeahantennaoftheDSN[8℄, [9℄.

Modern old atoms frequeny standards in the mi-

rowavedomain have already demonstrated an au-

ray in the 10

−15

range with the potential to reah

the 10

−16

level or better. Frequeny stabilities, de-

ned by the Allan standard Deviation (ADEV), are

ommonlyof10

−13τ^{−12 forsuh standardsandafew}

10

−14 τ^{−12 have been} demonstrated using more ad- vanedtehniques[10℄. Coldatom optiallokshave

the potential to reah the 10

−17

auray level [11℄,

[12℄, [13℄, [14℄. Theemergeneof modernmirowave-

to-optialsynthesizersbasedonmode-lokedfemtose-

ondlasersallowshighresolutionomparisonsbetween

mirowave and optial loks [15℄, [16℄, [17℄. Cloks

omparisonsareurrentlyperformedbysatellite,asfor

exampleGPS orTWSTFT (Two-Way Satellite Time

andFrequenyTransfer. Measurementsarelimitedby

thetransmissionsystemtoabout10

−15

atonedayav-

eragingtime[18℄. Thesesmethodsarethusinsuient

formeasuringtheultimateperformaneofamirowave

oranoptialstandard(Fig. 1).

Upgrades of the orbital equipments are expetable

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

Figure1: Allandeviationofsomefrequenystandards

omplex and expensive. Moreover, the two previous

systems deliver only a synhronization signal not al-

lowingdiret short-term stability omparisons. Then

for muh of appliations a referene signalis needed.

Hene,theopportunitytoomparemirowaveandop-

tial loks by the development of a new type of a

groundfrequenydisseminationbyoptialbreseems

appropriate,evenwhenthelaboratoriesareseparated

by100km[19℄,[20℄,[21℄. Oneanindeed takeadvan-

tage of both the low attenuation and low dispersion

in the bre, whih allow reahing long distane fre-

quenytransferbymaintainingagoodsignal-to-noise

ratio(SNR).

Moreovertheaesstoanultra-stablefrequenyrefer-

ene for alarge numberof laboratories open theway

to perform new experiments in fundamental physis.

The development and operation of a state-of-the-art

frequenystandardremainastronglimitationandan

beoveromebyabredistributionsystemonneting

TimeandFrequenyMetrologylaboratoriesto users.

The simplest way todevelopabredistribution isto

per,wepresentthetransferofhighfrequenystability

signalat100MHz,byusingtheexistingteleommuni-

ationbre network,overafew tenskilometers, with

ompensationofthephasenoiseintroduedbythelink.

II. PRINCIPLE ANDOBJECTIVE

Thegoalofthedisseminationisthedistributionofa

referenesignalatafrequenyof100MHz,synthesized

from a frequenystandard, byamplitude modulation

ofanoptialarrier,withoutdegradationofthephase

noise of the distributed signal. The referene signal

modulates the bias urrent of a DFB laser diode, at

1.55µ^{m,whih is}transmittedthroughabre optial link to users. At the link extremity, a photodiode

detets the amplitude modulation and onverts the

optialsignaltoaradio-frequenysignalosillatingat

the referene frequeny and phase oherent with the

mirowavereferenesoure.

The high stability and low phase noise of the trans-

ferredsignalaredegradedbytheresidual phasenoise

of the optial link and by the attenuation in the

bre. We operate in urban environment by using

the existing teleom network. Thus, bre layout and

installation aspets are not ideal and the stability

of the optial link an be aeted by environmental

eets. Optial length of the bre is modied by

mehanialstressesandtemperatureutuations. The

rstoneaets phasenoiseand short-termfrequeny

stability performanes of thetransmittedsignal. The

seond eet, is a slowly hanging phenomenon and

hasanimpatonthelong-termstability.

These instabilities have been 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 Laboratoire Kastler Brossel (LKB - University

ParisVI) (about3km).

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/LPLandLNE-SYRTE/LKBoptiallinks

sentedin gure2andshownon-stationaryeetsde-

pending on the ativities around the link. Periodi

eets as daily temperature variations appears as a

bumpatthehalfperiod,ontheADEV.Thefrequeny

instabilitiesrelatedtoasinusoidaltemperaturepertur-

bationsanbealulatedfromtheequation(1):

σy(τ) = ∆T ×T CD×n×L

c ×sin²(πτ ν0)

τ ⁽¹⁾

with∆T ^{theamplitudeofthe}temperatureutuation [

◦

C℄, T CD ^{the thermal oeient of delay [ppm/◦C℄}

of theoptial bre (typially7 ppm/

◦

C for standard

teleom SMF28 bre), n ^{the bre ore index,} L ^the

optiallinklength[km℄,c^{thelightveloityinvauum}

[3×^108m/s℄,ν0^theperturbationfrequeny[Hz℄,and

τ ^{theaveragingtime[s℄. Forexample,ifweonsidera}

sinusoidalperturbationof0.2

◦

Cwithaperiodof1000s

due to aironditioning and ating ona setionof 50

metersoftheoptiallink,theADEVofthelinkould

belimitedtoabout7x10

−16

at 500sintegrationtime.

In thesameway, adaily 0.5

◦

Ctemperaturevariation

on 43 kilometers of optial bre is onverted into an

instabilityoftheorderof1.3x10

−14

at43200saverag-

ingtime.

Consequently,thedistribution systemneedsanative

ontrolloop to ompensatefor these phasevariations

induedonthesignaltransmittedthroughthelinkre-

lated totheenvironment(mehanialvibrations,tem-

peratureutuations...).

The objetive of the dissemination being lok om-

parisonsordeliveryof areferenesignalomingfrom

anH-MaseroraCryogeniSapphireOsillator(CSO),

theompensationset-upmustintrodueaphasenoise

lowerthanthereferenesignal. Inthisperspetivewe

havetodevelopasystemwhihdeliversareferenesig-

nalat100MHz,showingarelativefrequenystability

σy(τ)≤2.10^−14[τ ^=1s℄(<10^{−161d),that implies}

a residual ikerphase noise of -120dBrad

2

/Hz at 1

Hz and awhite phasenoise oorwith alevelof -140

dBrad

2

/Hz.

III. ACTIVE PHASEFLUCTUATIONS

COMPENSATION SYSTEM

A. Presentation

The priniple of the phase utuations ompensa-

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 perturbations an be only arried out at the

link emission. Atwo-waydistribution,usingthesame

optial bre link, allows determination of the phase

perturbationaumulatedalongafullroundtripwith

the hypothesis that the forward and the bakward

signals are orrupted by the sameperturbation. The

ompensation rests then on the measurement of the

phase of the signal after one round trip to apply a

orretionontheemittedsignal.

3

Reference signal

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

)(tc

r e f f

f +

O p t i c a l f i b r e l i n k

Figure3:Shematiofthephaseutuationsompensation

Thereferenesignalatthefrequenyf

ref

=ω^ref/2π

is used for modulating a laser diode. The amplitude

modulated signal is then orreted by a phase term

φc^{. This orretion term is provided either by phase}

shiftingtheRFmodulatingsignalorbymodifyingthe

propagation delay in the bre. At the user-end, the

signal orrupted by the environmental perturbations

isdeteted:

V^{RF deteted}(t)∝sin(ω^ref t+φ^ref+φc+φp) ⁽²⁾

This signal is split in two signals: one part for the

user appliations and the other to be re-injeted via

an optial irulator in the same optial bre. After

oneround-trip,thesignal,twieorruptedbytheterm

φp ^{is deteted. A RFproess allowsgeneration ofan}

errorsignal,appliedtothephaseorretor. Twodier-

entlasersoures, operatingat slightlydierentwave-

lengths, are used for generating the forward and the

bakward optial signals and optial add/drop fun-

tionsarerealizedwithoptialirulators.

Dierentapproahesofphaseompensationhavebeen

studiedandaredesribedhere.

B. Eletroniphase utuationsompensator

Intheaseofaneletroni phaseutuationsom-

pensator (f g. 4), the orretion is performed by

atingonthephaseoftheinjetedsignalintheoptial

link,thatweallφinput .

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 , f

w p f

r e f o u t p u t

w r e f ,f = f

i n p u t

f

U s e f u l s i g n a l

r f

Figure4: Simpliedshematiofthephaseonjugator

Wedene byφr ^{thephaseofthe round-tripsignal,}

andφoutput

thephaseofthedetetedsignalattheuser-

end, equalto:

φ^output(t) =φ^input(t−τ) + Z t

t−τ

φp(ξ)dξ ⁽³⁾

where τ ^{is the} propagation delay in the optial bre link and φp(ξ) ^{is the} distributed phase per- turbation along the bre. The main eet of the

delay τ ^{is to limit the loop bandwidth. In the fol-}

lowingdisussion,weneglettheinueneofthedelay.

Theoutput signalmustbephaseoherentwith the

referene soure of frequeny ω^{ref and on average of}

phase φref^{, and thus the orretion applied to the}

emitted signal must be equal to the opposite of the

phase perturbation φp^. Consequently, on average (or for time muh longer than τ^{) the phase of the input}

signal,φ^inputis:

φinput=φref−φp ⁽⁴⁾

Then, thephaseoftheround-tripsignalbeomes:

φr = φinput+ 2×φp

= φ^ref+φp

Thephaseohereneoftheoutputsignalisheneim-

posed by maintaining a onjugation relationship be-

tweentheinputandtheroundtripsignaloftheoptial

link:

(φ^input−φ^ref) =−(φr−φ^ref) ⁽⁵⁾

A simplied sheme ofthe phaseonjugatoris shown

in gure4. Theorretion isperformedwith aphase

shifter in series with the referene signal, whih is

usedastheinputsignal. Thereferenesignalispower

dividedto drivetwophasedetetors. Phasedetetion

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 loop 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

phaseshifter. Eletroniphaseshifters haveatypial

dynami of 180 degrees with a non linear response,

induingvariableinsertionlosses. Moreoverthephase

shifter an present a phase noise exess, ompared

to the other omponents of the phase onjugator.

Seondly, phase detetors are quite sensitive to the

driving levels and it is diult to ensure the same

sensitivityforthetwodetetorsof gure4.

Thepratialrealizationleadstoaverypooreetive

system of the phase perturbations anellation. A

new sheme, regarding the previous onsiderations

and introduedbytheJPL[22℄isshownin gure5.

Twosymmetrialsignalsareproduedbyfrequeny

shift (f

shift

) of the referene signal (f

±

=f

ref ± ^fshift ).

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

11 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

Figure5: Blokdiagramofthefulleletroniompensation

system

This sheme allows replaement of the double phase

measurements(Fig. 4)byamuh moreauratedou-

blefrequenymixingandasinglephasemeasurement.

Thedynamiandthelinearityofthephaseorretion

isimprovedbyusingavoltageontrolledquartzosil-

lator(VCXO),asaphaseshifter,deliveringasignalat

thereferene frequenywithastable amplitude. The

VCXOpresentsthustheadvantagetoorretallphase

perturbationin theorretionbandwidthofthephase

ompensator,whihislimitedbytheround-tripprop-

agation delayin theoptial link (about0.3ms in the

aseofthe43-kmLNE-SYRTEto LPLoptiallink).

The 100MHz output signalof the VCXO modulates

the biasurrentof theDFB laser diode. The optial

signalislaunhedintheoptial brelink totheuser.

Attheuserend,asimplesystemallowsdetetionand

regenerationofthebakwardsignal. Thedetetedsig-

nalafteraone-waydistributionisproportionalto:

VUserend(t)∝sin(ωos×t+φos+φp) ⁽⁶⁾

Thebakwardoptialsignalissubmittedtothesame

phaseperturbationandafteroneompleteround-trip,

thedetetedsignalhasthefollowingform:

V^roundtrip(t)∝sin(ω^os×t+φ^os+ 2×φp) ⁽⁷⁾

TheservoloopforestheVCXOat100MHzbothtobe

phaseoherentwiththe referenesoureand toom-

pensateforthephaseperturbation. For obtainingthe

phase onjugation, twosignals separated by 10 MHz

aroundthereferenefrequeny(oneat90MHzandthe

otherat 110MHz) areproduedbyfrequeny mixing

between the referene signal and itself frequeny di-

vided by ten. Two dierent systems, based on PLL

(Phase Lok Loop) are used for ltering eah signal

issuefromthepreviousfrequenymixing. Thesignal,

fromthe"downonversion",at90MHz,ismixedwith

themodulatingsignal,deliveredbytheVCXO,toob-

tainasignalat10MHz:

V1(t)∝sin((ωos−2π×^90MHz)×t+φos− 9 10φref)

(8)

In parallel, the signalat 110MHz is mixed with the

round-tripsignal,produinganothersignalat10MHz:

V2(t)∝sin((2π×^110MHz−ω^os)×t+11

10φ^ref−φ^os−2φp)

(9)

Thephaseomparisonat10MHzallowsgenerationof

abase-bandsignal,ontainingthethreephaseterms:

Verror(t)∝φos+φp−φref

(10)

whih isanelled in normaloperation. Thephaseof

theVCXOisthen:

φ^os=φ^ref−φp ⁽¹¹⁾

Bythis proess, the stabilityand theauray of the

referene soureis transmitted to theuser endin the

systembandwidth.

Theapaityofthephaseompensatortorejetphase

perturbations in the ontrol bandwidth is dened by

the rejetion fator, equal to the ratio between the

phasevariationsin openand inlosed loop. Theper-

formaneofthedistributionsystemdependbothofthe

intrinsisystemphasenoiseandoftherejetionfator.

Figure 6: Blok diagramofthe ompensation systemtest

benh

Figure 6displaystheset-upused fortheharater-

ization of the phaseonjugator. Simulationof phase

perturbations are realized by periodially heating a

2.5-km bre spool with an amplitude of 4

◦

C and a

period of about 4000 s. This perturbation indues a

phase modulation of the order of 200 mrad on the

100-MHz transmitted signal. In operation, when

the phaseonjugator is ativated, the residual phase

modulationmeasuredat thelinkoutputisreduedto

0.4 mrad(f. gure7), thatimpliesarejetion fator

ofthephaseperturbationsalongthelink ofabout500

(53dB).

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]

Phasevariationsinclosedloop[mrad]

-100 -80 -60 -40 -20 0 20 40 60 80 100

Phasevariationsinopenloop[mrad]

Figure7: Phase shiftinduedby temperaturemodulation

of the transmittedsignal, inopenand losed loop at 100

MHz