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F. Acernese, P. Amico, M. Al-Shourbagy, S. Aoudia, S. Avino, D. Babusci, G.
Ballardin, R. Barillé, F. Barone, L. Barsotti, et al.
To cite this version:
F. Acernese, P. Amico, M. Al-Shourbagy, S. Aoudia, S. Avino, et al.. The Virgo interferometric grav- itational antenna. Optical Diagnostics and Monitoring (OPTIDIMON), Mar 2004, Bacoli (Napoli), Italy. pp.1-16. �in2p3-00024243�
F.A cernese,P.A m icox,M .A l-Shourbagyxi,S.A oudia∗∗,S.Avino, D .Babusci§,G .Ballardin†,R .Barillé†,F.Barone,L.Barsottixi, M .Barsuglia††,F.Beauville∗,M .A .Bizouard††,C .Boccara‡‡, F.Bondu∗∗,L.Bosix,C .Bradaschiaxi,S.Braccinixi,A .Brillet∗∗, V.Brisson††,L.Broccoxii,D .Buskulic∗,E.C alloni,E.C am pagna‡, F.C avalier††,R .C avalieri†,G .C ellaxi,E.C hassande-M ottin∗∗,C . C ordaxi,A .-C .C lapson††,F.C leva∗∗,J.-P.C oulon∗∗,E.C uoco†, V.D attilo†,M .D avier††,R .D e R osa,L.D iFiore,A .D iV irgilioxi, B.D ujardin∗∗,A .Eleuteri,D .Enard†,I.Ferrantexi,F.Fidecaroxi, I.Fiorixi,R .Flam inio∗,J.-D .Fournier∗∗,S.Frascaxii,F.Frasconi†, A .Freise†,L.G am m aitonix,A .G ennaixi,A .G iazottoxi,G .G iordano§, L.G iordano,R .G ouaty∗,D .G rosjean∗,G .G uidi‡,S.H ebri†,
H .H eitm ann∗∗,P.H ello††,L.H ollow ay†,S.K reckelbergh††, P.La Penna†,V.Loriette‡‡,M .Loupias†,G .Losurdo‡,
J.-M .M ackow ski¶,E.M ajoranaxii,C .N .M an∗∗,M .M antovanixi, F.M archesonix,F.M arion∗,J.M arque†,F.M artelli‡,A .M asserot∗, M .M azzoni‡,L.M ilano,C .M oins†,J.M oreau‡‡,N .M orgado¶, B.M ours∗,A .Paixii,C .Palom baxii,F.Paoletti†,S.Pardi,
A .Pasqualetti†,R .Passaquietixi,D .Passuelloxi,B.Perniola‡,
F.Piergiovanni‡L.Pinard¶,R .Poggianixi,M .Punturox,P.Puppoxii, K .Q ipiani,P.R apagnanixii,V.R eita‡‡,A .R em illieux¶,F.R iccixii, I.R icciardi,P.R uggi†,G .R usso,S.Solim eno,A .Spallicci∗∗, R .Stanga‡,R .Taddei†,D .Tom bolato∗,M .Tonellixi,A .Toncellixi, E.Tournefier∗,F.Travassox,G .Vajentexi,D .Verkindt∗,F.Vetrano‡, A .V iceré‡,J.-Y.V inet∗∗,H .Voccax,M .Y vert∗ and Z.Zhang†
∗Laboratoire d’Annecy-le-Vieux de Physique desParticules,Annecy-le-Vieux, France;
†European G ravitationalO bservatory (EG O ),Cascina (Pi),Italia;
‡INFN,Sezione diFirenze/U rbino,Sesto Fiorentino,and/orU niversità di Firenze,and/orU niversità diU rbino,Italia;
§INFN,LaboratoriNazionalidiFrascati,Frascati(Rm ),Italia;
¶LM A,Villeurbanne,Lyon,France;
INFN,sezione diNapoliand/orU niversità diNapoli"Federico II"
Com plesso U niversitario diM onte S.Angelo,and/orU niversità diSalerno, Fisciano (Sa),Italia;
∗∗D epartem entArtem is– O bservatoire de la Côte d’Azur,BP 42209 06304
Nice,Cedex 4,France;
††Laboratoire de l’AccélérateurLinéaire (LAL),IN2P3/CNRS-U niv.de Paris-Sud,O rsay,France;
‡‡ESPCI,Paris,France;
xINFN,Sezione diPerugia and/orU niversità diPerugia,Perugia,Italia;
xiINFN,Sezione diPisa and/orU niversità diPisa,Pisa,Italia;
xiiINFN,Sezione diRom a and/orU niversità "La Sapienza",Rom a,Italia.
E-m ail:Corresponding author: P. La Penna <paolo.lapenna@ego-gw.it>
A bstract. The interferom etric gravitationalw ave detectorsrepresentthe ultim ate evolution of the classicalM ichelson interferom eter. In order to m easure the signalproduced by the passage of a gravitational w ave, they aim to reach unprecedent sensitivities in m easuring the relative displacem ents of the m irrors. O ne of them ,the 3-km -long Virgo gravitational w ave antenna,w hich w illbe particularly sensitive in the low frequency range (10-100 H z),is presently in itscom m issioning phase.In thispaperthe varioustechniquesdeveloped in order to reach itstargetextrem e perform ance are outlined.
Subm itted to:Class.Q uantum G rav.
PACS num bers:04.80.N n,95.55.Y m
1.Introduction
The present ground-based interferom etrical gravitational w ave detectors Virgo [2], LIG O [3], TA M A 300 [4], G EO 600 [5]) aim to reach a spectral strain sensitivity of h about 10−23/√
H z−10−22/√
H z in a frequency range around 100 H z. This m eans, in term s of relative displacem ents of the test m asses, to m easure length varitions of less than 10−19m/√
H z (see figure 1). In order to reach this extrem e sensitivity, special optical configurationshave been developed.
The passage ofa gravitationalw ave can be detected in the outputinterferom etric signalasa relative displacem entofa setofquasi-free falling m asses(suspended m irrors).Virgo,m ainly consisting ofa3-km -long M ichelson interferom eter,w ith Fabry-Perotcavitiesin thearm sand pow errecycling,sharesw ith otherexperim ents,such asLIG O [3],TA M A [4]and G EO [5], substantially the sam e opticaldetection principle,and itaim s to detectgravitationalw aves em itted by astrophysicalsources[2]in a frequency range betw een a few H z and a few kH z.
In particular,Virgo,thanks to its peculiarattenuating system providing the highestpassive isolation perform ance,w illbe m ore sensitive than the otherdetectors atlow frequency (10- 100 H zrange),aim ing atadisplacem entsensitivity of10−17m/√
H zat10 H zand atthelevel 10−19m/√
H zatabout100 H z.In term soffringe sensitivity,the progressw ith respectto the M ichelson and M orley experim entw ould befrom their1/100 [1]to thepresent1/1012ofthe fringe (butthe frequency reange atw hich the m easurem entisperform ed isdifferent).
10 100 1000 10000 10-20
10-18 10-16 10-14 10-12 10-10
VIRGO LIGO
m/sqrt(Hz)
Hz
Figure 1.Com parison ofthe design displacem entsensitivity ofthe tw o interferom etersVirgo and LIG O ,expressed in m/√
H z,asa function ofthe frequency.
2.Basic principlesofoperation ofan interferom etric antenna
The first idea of the interferom etric detection of gravitational w aves rem ounts up to the 60’s[6]. The passage ofa gravitationalw ave,com ing from a direction perpendicularto the line connecting tw o m irrors,suspended as quasi-free testm asses ata certain distance L one from the other,w illbe seen by a laserlightbeam travelling along thisline asa displacem ent ofthe sam e testm asses. The resultw illbe a change in the interference condition betw een the beam s com ing from the tw o opposite directions. Since the action of the gravitational w ave on a length L isessentially described by itsadim ensionalstrain h=∆L/L,the quantity
∆L to be m easured is enhanced ifthe length L is larger. G iven the expected values foran adim ensionalstrain ofastrophysicalsource (about10−23/√
H z−10−22/√
H z in the 10-100 H z frequency range),the∆L to be m easured is extrem ely sm all. Forthis reason the ground based gravitationalw ave interferom eters are as long as possible,ofthe orderofseveralkm (Virgo is3 km long,LIG O is4 km long).
2.1.O pticalcavities
In orderto increase the opticalpath,the firstpossibility w ould be to use delay lines:the light could be m ade travelm any tim esinside tw o m irrorsspaced severalkm apart.Thisdevice is too difficultto control,thus the use ofopticalcavities (Fabry-Perot)has been preferred. In a Fabry-Perotcavity having finesse F the dephasing ofthe reflected lightis enhanced by a factor2/π×F .Therefore,w ith a finesse F=50,a cavity 3-km -long can be seen asoptically equivalentto an about120-km -long one.
2.2.M ichelson configuration
In principle a single cavity could be used. H ow ever,the signalcom ing from a single cavity w ould bedom inated by thelaserfrequency noise∆ν(typically oftheorderof104H z/(f√
H z). Fora cavity having length L ofthe orderof1 km ,and a laserfrequencyν=2.8×1014,the equivalentdisplacem entnoise w ould be∆L=∆ν/ν×L∼=3×10−10H z/√
H zat100 H z.In orderto overcom e thislim itthe interferom eterconfiguration isused:in an idealM ichelson, having no arm sasym m etry,there isno frequency noise injection in the interference betw een thebeam scom ing from thetw o arm s.Thisisthereason becauseallthegravitationalantennas have the M ichelson interferom eter configuration. In a gravitational w ave interferom etric antenna high-quality optics are suspended to actas quasi-free testm asses atthe end ofthe M ichelson interferom eterarm s,the arm sbeing,in Virgo,LIG O and TA M A ,opticalcavities.
2.3.D ark fringe
U sing laser beam s, the first fundam ental noise source is the shot noise connected to the corpuscularnature ofthe light.In orderto im prove the shotnoise signal-to-noise ratio,itcan be show n thatthe shotnoise signalto noise ration ism axim ized ifthe interferom eteriskept on the dark fringe,thatis the tw o beam s com ing from the tw o arm s are m ade destructively interfering.A llthe lightistherefore reflected back tow ardsthe lasersource.In thisw ay,the w hole interferom eterbehaveslike a m irror,reflecting back allthe incom ing light. Since the shotnoise signal-to-noise ratio im provesw ith the square rootofthe pow er,itisconvenientto increase asm uch aspossible the lightstored inside the interferom eter.
2.4.Powerrecycling
O nce the lim itin the increase ofthe incom ing laserpow erisreached,a furtherim provem ent can be obtained by reflecting again,tow ardsthe interferom eter,the lightcom ing back to the laser.Forthisreason,betw een the laserand the interferom eterbeam splitter,anotherm irror, called the recycling m irror,is placed,w hich,togetherw ith the interferom eteritself,form s a furtheropticalcavity in w hich the lightisstored.The lightpow erinside the recycling cavity, i.e. the lightim pinging onto the interferom eterbeam splitter,is enhanced by a "recycling"
factor.
2.5.H eterodyne detection
A further lim it to the sensitivity are the pow er fluctuations of the laser light. The presently available noise source are notshotnoise dom inated in the frequency of interest forgravitationalw avedetection (10 H z-10 kH z).O netrick to overcom ethepow erfluctuation problem isto shiftthe detection frequency in a frequency range w here the laserbeam isshot noisedom inated.Forthisreason thelaserlightism odulated in phaseatafrequency ofseveral M H z,w here the laseris shotnoise dom inated,before entering the interferom eter,and then dem odulated by the sam e reference,in the standard Pound-D rever-H all(PD H )schem e[7][8].
A t the dark fringe output of the interferom eter the beating betw een the carrier and the
sidebands is detected (heterodyne detection),w hen the destructive interference condition is m odified by the passage of a gravitationalw ave. In order to have an available signal,the sidebands have to be partially transm itted to the dark port: this is obtained by introducing a slightasym m etry in the arm s length,so thatw hen the carrieris on the perfectdestructive interference the sidebandsare partially transm itted (Schnupp’stechnique[9]).
The arm s asym m etry im posed by the Schnupp’s technique, together w ith other arm s asym m etries (like cavity finesse asym m etries), have as a consequence that part of the frequency noise ofthe laseris reintroduced in the interferom eter. This im poses to stabilize the laserfrequency,up to a levelw here itseffectislow erthan the shotnoise (the frequency stabilization schem e adopted in Virgo w illbe described in 4.3.
2.6.Autom atic alignm ent
In addition to the interferom eterislongitudinallocking,an angularcontrolsystem isneeded to m aintain the m irrors in an aligned position w ith respectto one anotherand the incom ing beam .The alignm entsystem isnotreferred to the ground,itratherkeepsthe interferom eter autom atically aligned on theincom ing beam .Severaltechniques,know n asw avefrontsensing techniques[10] are used in the gravitationalw ave interferom eters: they alltake advantage, analogously to the Pound-D rever-H alltechnique,of a high frequency (M H z) m odulation- dem odulation technique. The autom atic alignm entschem e designed forVirgo [12]uses the Anderson technique[11].Them odulation frequency ischosen so thatthefirstordertransverse m odes(T E M01)ofthe sidebandsare resonantin the arm cavities.O nce allthe cavitiesofthe interferom eterare locked attheirresonance,the transm itted lightis detected by differential w ave-frontsensors,producing photo currents w hich are dem odulated and then opportunely m ixed to achieve signalsproportionalto the m isalignm entsbetw een the opticalcom ponents.
Thesesignalsarethen filtered and sentby feedback to them irrorsatthelevelofthem arionette w ith a controlbandw idth ofa few H z,so thatnoise isnotreintroduced in the detection band.
2.7.Locking
In orderto attain thedesired sensitivity,theinterferom eterhasto beplaced and keptin aspec- ified w orking point,i.e.w ith the arm satthe opticalresonance ofthe cavities,the M ichelson on theoutputdark fringeand therecycling cavity atitsresonance.Thism eansthatthevarious lengthsand positionsofthe m irrorshave to be actively controlled,thisoperation being called longitudinallocking ofthe interferom eter.
3.The V irgo interferom eter 3.1.Virgo generallayout
TheopticallayoutofVirgo isshow n in figure2:alaserbeam (20W @ 1064nm )isproduced by aN d :YV O4high pow erlaserinjection,locked to a1 W N d :YAG m asterlaser.Thelaserlight
ism odulated in phase ata frequency of6.26 M H z before entering the vacuum system atthe injection bench (IB),and isthen pre-stabilized by the 144 m long inputm ode-cleaner(IM C), to a few tens of kH z using a standard Pound-D rever-H all(PD H ) schem e[7][8]. The low frequency pre-stabilization isperform ed by actively controlling the length ofthe IM C to lock the laserfrequency to the length ofa 30 cm m onolithic triangularcavity (RFC)suspended in vacuum .A 10 W pow erbeam em itted from theinjection system entersinto theinterferom eter (ITF)through the pow er-recycling m irror(PR).The beam is splitatthe levelofthe beam - splitterm irror(BS),and entersthe tw o 3km long Fabry-Perotcavities(north cavity and west cavity).
Together w ith the PR m irror, the M ichelson ITF form s a Febry-Perot cavity, the power recycling cavity,w ith an opticalrecycling gain of50 w hen the ITF is atits w orking point.
In thisstate,the expected pow erupon the BS is500 W .The 3-km Fabry-Perotcavitieshave a finesse of50 (opticalgain about32):the finalpow ercirculating inside the ITF istherefore estim ated to bearound 8 kW .W ith theITF attheoperating point,thegravitationalw avesignal isextracted on the dark portbeam ,w hich passesthrough the outputm ode-cleaner(O M C),to reach a setof16 InG aA S photodiodes (B1),by w hich the dark portsignalis reconstructed.
O thersignalsare extracted from the ITF,essentially forcontrolpurposes:figure 2 show sthe benchesdetecting the beam stransm itted by the long Fabry-Perotcavities(N B and W B),the beam reflected by the ITF (D T),and the beam reflected by the second face ofthe BS (D B).
In figure 3the noise sourceslim iting the Virgo sensitivity are show n:the m ain lim iting noise contributionsw illbe seism ic disturbancesbelow 4 H z,therm alnoise up to 100 H z and shot noise athigherfrequencies.
3.2.O ptics
The Virgo otpicaldesign im posesvery large (diam eter350 m m )and heavy optics(20 kg)for the interferom etersuspended m irrorsand beam splitter.The totallossesat1064 nm ofeach m irror (including absorption,scattering and large-scale w avefrontdeform ation) should not exceed 100 parts per m illion (ppm ). There are specific constraints on the absorption (< 5 ppm )due to the therm allensing and on the scattering level(< 5 ppm )to m inim ize the noise on the interferom eteroutput,due to the scattered light. Forthese reasons m irrors and beam splittersaresuper-polished piecesm adeofanew typesofsilica(Suprasil311 SV,Suprasil312 SV ),w ith very low absorption and scattering,m anufactured by theG erm an com pany H eraeus, developed in collaboration w ith ESPCI(Paris). The O H contentis very low («50 ppm ),the refractive index ishom ogeneousin alldirectionsand the birefringence isvery low (< 5.10-4 rad/cm ). The bulk absorption ofthe silica substrates crossed by the V IRG O laserbeam has been m easured[13]as being less than 0.7 ppm /cm . The flatness ofthese large com ponents is 8 nm RM S on 150 m m . The lim iting factor of the w avefrontflatness is the substrate:
since the polishers can notguarantee every tim e betterw avefronts,the substrate surface is corrected before deposition by using a Corrective Coating technique[14]. The m irrors,after finalcoating w ith reflective quarterw avelength layers ofSiO 2 and Ta2O 5,exhibita global RM S flatness ofthe orderof3 nm over150 m m ,an average absorption ofless than 1 ppm
Figure 2.Virgo opticallayout
Figure 3.Virgo design sensitivity w ith the lim iting noise sources:seism ic noise up to 4 H z, therm alnoise up to 100 H z and shotnoise athigherfrequencies.
and a scattering ofthe orderof5 ppm ,thusbeing the m ostperform ing existing large optics.
3.3.Suspensions
In allthe presentground based interferom eterthe testm asses are m irrors,isolated from the ground by suspending them to pendula.TheVirgo seism icisolation,being m orecom plex than thoseofthedetectors,providesthehighestpassiveisolation perform ance.Thetestm assesare located in an ultra-high vacuum system (from 10−9m barforH 2 up to 10−14forhydrocarbon) and suspended from a sophisticated seism ic isolation system the Superattenuator (SA ).The SA isam ultistage,10-m -tall,m ultipendularsuspension,w hich iseffectivein isolating thetest m assesfrom the seism ic noise forfrequency higherthan a few H z (the pendulum resonance frequency are allconfined below a couple ofH z). The seism ic attenuation system is either passive and active.The passive filtering isprovided by the SA ,a chain ofm echanicalfilters.
The firststage ofthe SA [15]is an inverted pendulum (IP)preisolating stage [16]. A chain offive m echanicalfiltersissuspended from the top ofthe IP.From the laststage ofthe chain (the so called "filter7" [17])an anvilshaped steelstage,the so called "m arionette" [18],is suspended by a steelw ire.The payload,suspended from the m arionette,isform ed by the test m assand by an alum inum reference m ass(RM ),independently suspended behind the m irror.
The passive attenuation ofthe w hole chain isbetterthan 10−14at10 H z,corresponding to an expected residualm irrorm otion of10−18m/√
H zatthe sam e frequency.D ue to the residual low frequency m otion oftensofm icronsatthe resonancesofthe SA ,the frequenciesofthe w holechain norm alm odesranging betw een 40 m H zand 2 H zw ith quality factorsup to 103, the SA isdesigned to allow an active controlofthe m irrorposition overa very large dynam ic range. In orderto allow lock acquisition,i.e. to confine the residualm irrorm otion below 1µm ,controlforcesareexerted on threeactuation points:attheIP top stagelevel,perform ing an inertialactive dam ping ofthe resonantm otionsofthe SA [19];using the m arionette coils to steerthe suspended m irror[18]w ith respectto the laststage ofthe chain;and directly on the m irror,through coils supported by the RM w hich can acton fourm agnets m ounted on the holderofthe m irror. A localcontrolsystem referred to the ground,is active in the bot- tom partofeach SA in orderto keep the longitudinaldisplacem entofthe m irrorsbelow 1µm rm s. Ituses as a signaltw o laserbeam s em itted by lasers leaning on ground outside each tow er.These beam senterinto the tow erthrough an opticalw indow,one isreflected one by a m irroron the m arionetta and the otherone by the suspended m irroritself,and then im pinge on respectivePSD detectors,afterpassing through dedicated opticalsystem s,thusbeing used as opticallevers forreconstructing the m irrorm ovem ents. A correction feedback using this signalsallow sto dam p and controlthe localangularm otion ofthe m irrorsbelow 1µrad RM S and m akestheacquisition ofthelongitudinallock oftheinterferom eterpossiblew ith alim ited actuation force,thuspreventing noise reintroduction in the detection band.Since thissystem is referred to the ground,and therefore lim ited by the seism ic noise,once the longitudinal locking ofthe interferom eterisacquired the localcontrolsystem issw itched offand replaced by the autom atic alignm entsystem .
Figure 4. Schem e ofthe Virgo Superattenuatorchain: in the CITF the feedback is exerted atthree stages: inertialdam ping is perform ed atthe inverted pendulum stage,localcontrol through them arionette,and theinterferom eterlocking keeping forceactson them irrorthrough the reference m ass.
4.The com m issioning ofthe 3 km long V IR G O
In orderto testthestability and robustnessofallofthesub-system sinvolved in theoperations and to getsom eexperiencein designing thevariouscontrolsystem s,theVirgo com m issioning activity hasbeen organized in stepsofincreasing com plexity:the separate com m issioning of the N orth and W estFabry-Perotcavities,follow ed by the com m issioning ofthe recom bined M ichelson Fabry-PerotITF,and eventually the com m issioning of the recycled M ichelson Fabry-PerotITF.Shortperiodsofcontinuousdata-taking (the so-called com m issioning runs) have taken place every tw o to three m onths since N ovem ber 2003,in order to check the evolution of the detector and the consequent progress in the level of sensitivity. Five com m issioning runshave been perform ed so far:
• C1 -N orth cavity longitudinally controlled (14-17 N ovem ber2003);
• C2 -N orth cavity longitudinally controlled,plus autom atic alignm ent(20-23 February 2004);
• C3 -Tw o configurations: N orth cavity as in C2 plus the frequency stabilisation servo (23-26 A pril2004);firstdata-taking w ith the ITF locked in recom bined m ode (26-27 A pril2004);
• C4 -ITF longitudinally controlled in recom bined m ode,w ith suspension tidalcontrol, autom atic alignm enton both the arm s,frequency stabilization servo (24-29 June 2004);
• C5 -Tw o configurations:ITF in recom bined m ode asin C4,plusend suspensionsw ith fullhierarchicalcontrol(2-6 D ecem ber2004);firstdata-taking w ith the ITF locked in recycled m ode (6-7 D ecem ber2004);
The com m issioning of a single arm w as concluded w ith C3,w ith an autom atic alignm ent and a frequency servo running forthisconfiguration.The evolution ofthe detectorsw orking in recom bined and in recycled m ode w illbe described in sections 4.2 and 4.5,w here the attention w illfocusupon the tw o m ostrecentdata-taking during C4 and C5.
4.1.The longitudinalcontrol
The nom inalsensitivity ofan interferom etric detectorsuch asVirgo isachieved by selecting an appropriate w orking point,w ith laserlightresonantin the opticalcavities,and the output porttuned on the dark fringe.These conditionstranslate into fixed relationshipsbetw een the laserlightw avelength and fourindependentlengthsofthe ITF [20]:
• the length ofthe recycling cavity (PRCL),
• the differentiallength ofthe shortM ichelson arm s(M ICH ),l1−l2;
• the com m on (CARM )and the differential(D ARM )length ofthe tw o long arm s,L1+L2 and L1−L2.
W hile the expected sensitivity is ofthe orderof10−18m/√
H z,the allow ed deviation from thew orking pointis10−12m rm s.A feedbackcontrolsystem isneeded to keep theITF locked on therequired interferenceconditions.Relativedisplacem entofthem irrorsisdetected using a carrierbeam phase m odulated at6 M H z. U sing a standard PD H technique allthe lengths involved can be reconstructed by m ixing the signals produced by the photodiodes,w hich are placed atdifferentoutputports ofthe ITF.These errorsignals are digitized and sentto the Virgo globalcontrolsystem (G lobalControl[20]),w hich com putesthe correctionsto be applied to the m irrorsby the actuatorsatthe levelofthe reference m ass.
A localcontrolsystem ,referred to theground,isactivein thebottom partofeach SA in order to keep thelongitudinaldisplacem entofthem irrorsbelow 1µm rm s.Thisalso keepsthelocal angularm otion ofthe m irrorsbelow 1µrad rm sand allow sthe acquisition ofthe longitudinal lock oftheinterferom eterusing alim ited actuation force,thuspreventing noisereintroduction in the detection band.
4.2.The recom bined interferom eter
A san interm ediate step tow ardsthe fullconfiguration,the interferom eterw ascom m issioned in recom bined m ode fora large partof2004. In this m ode the opticalschem e differs from
the finalconfiguration in thatthe PR m irror is significantly m isaligned,so thatonly three lengthsinstead offourhaveto belongitudinally controlled:CARM ,D ARM and M ICH .U sing the end transm itted signalsthe tw o long arm scan be controlled independently,acting on the corresponding end m irrors. A s soon as the cavities are locked,M ICH is controlled w ith the outputportdem odulated signal(oralternatively thereflected dem odulated signal)filtered and sentto the BS.By applying thisstrategy the lock isusually acquired in a few seconds.
Because ofthe low pow erupon the end photodiodes,the transm itted signals are electronic noise-lim ited:once the lock isacquired,they have to be replaced by anothersetoflessnoisy signals.In asteady stateCARM isalso attained using by thein-phasedem odulated com ponent ofthe lightreflected by the ITF.Instead,D ARM isattained by the in-phase outputportlight, w hich isslightly contam inated by M ICH ,m ainly provided by the otherquadrature ofthe ITF reflected light.Linearcom binationsoftheerrorsignalsarecom puted to providethecorrection forcesto the m irrors.Thisphase iscalled linearlocking.
4.3.The Com m issioning Run C4
D uring C4, the interferom eter w as operated for five days in recom bined m ode. The longitudinal degrees of freedom w ere locked according to the linear locking schem e previously described,w ith theautom aticalignm entrunning on both arm sand suspension tidal controlon the end m irrors.The laserfrequency w asactively stabilized on CARM ,w hich w as locked on thereferencecavity in linew ith thefrequency servo strategy developed in Virgo,the so-called Second StageofFrequencystabilization.TheO M C w aslocked on thedark fringe,so thatD ARM could becontrolled by thefiltered outputdem odulated signal.Theinterferom etric schem e in the data-taking m ode isdescribed in figure 5.The longestcontinuouslock during C4 w asabout28 hours.A ll9 lock lossesthatoccurred w ere analyzed and understood.A tthe beginning ofthe run som e acoustic noise injection w asperform ed in the laserand detection laboratory,to study the possible couplings w ith the dark fringe signal[21]. A softw are and hardw are injection ofinspiralevents w as also perform ed [23]during the data-taking,to test som eoftheelem entsoftheanalysischain and to characterizethedetectorstability during the run.
A t the beginning of the run som e calibration noise injection w as perform ed in order to produce the sensitivity curve. The resultis plotted in figure 6,togetherw ith the m ain noise contributions.The sensitivity isstilllim ited by controlnoise atlow frequency,by the m irror actuatornoisein theinterm ediatefrequency rangeand by laserfrequency noise,starting from som e hundredsofH z.
4.4.Reduction ofactuation noise
Longitudinal control during C4 w as acquired and m aintained by acting on the m irror at the levelof the reference m ass. The noise injected by the recoilm ass actuators into the interferom eterisa severe lim itto the sensitivity ofVirgo:at20 H z itism ore than 1000 tim es largerthan the design sensitivity. Itis m ainly contributed to by the 16 bitDAC noise (300 nV /sqrt(H z))and thecoildrivernoise(70 nV /sqrt(H z))and isconverted into equivalentm irror
Figure 5.C4 configuration:the data-taking m ode consisted ofthe ITF locked in recom bined m ode according to the linear locking schem e, w ith the autom atic alignm ent running on both arm s. In this state, the frequency stabilization controlsystem is engaged. W ith the laser frequency pre-stabilized on the IM C,the CARM locking loop is sw itched off and the corresponding PD H errorsignalis added into the errorpointofthe IM C-loop (bandw idth 1 kH z). The sam e signalis applied to the length ofthe IM C,w ith a bandw idth ofabout200 H z:in thisw ay the laserfrequency and the length ofthe IM C are stabilized on CARM ,w hich providesa betterfrequency stability atfrequencieshigherthan the internalresonancesofthe SA .The low frequency stabilization isachieved locking CARM on the RFC length.
displacem entby a reasonably large coupling factor: 130 m icrons/V.Such a large coupling factorhas been adopted in orderto ease the lock acquisition. O nce the lock is acquired,the residualforce to be exerted is largely in the low frequency region (D C-5 H z),w here tidal drifts and resonantm otion have to be com pensated,and very sm allelsew here. Therefore, the gain ofthe coildriver(and the corresponding noise)cannotbe reduced,unless a large fraction of the low frequency force is reallocated to the upper stages. This is in factthe Virgo suspension hierarchicalcontrolstrategy: once the lock is acquired,the locking force is splitover three actuation stages in a hierarchicalw ay. The correction in the range D C- 0.01 H z,thatcom pensatesforearth tides,isreallocated upon the softinverted pendulum ;the force in the range 0.01-8 H z,w here allthe suspension resonances fall,is reallocated to the m arionette.Consequently,the residualforce on the reference m assisstrongly reduced,and a strong reduction ofthe coildrivergain becom espossible.
W ith respectto the reference m ass-m irror system ,the upper actuation stage reveals m ore com plex dynam ics.There isan intrinsic and non negligible coupling betw een the horizontal actuators pushing on the m arionette and the pitch m otion induced on the m irror. Therefore, thelocking oftheITF from them arionette,requirestheuseofallfouractuatorsavailablew ith a properfrequency dependentdiagonalization.
Figure 6. Sensitivity curve in C4 expressed in m/√
H z,w ith the m ain noise contributions:
controlnoise atlow frequency,m irror actuator noise (essentially 16 bitDAC noise) in the interm ediate frequency range and laserfrequency noise starting from som e hundredsofH z.
O nce the lock is acquired using the recoilm ass only,the recom bined ITF should be fully locked before reallocating the force to the upperstage: itis necessary thatsecond stage of frequency stabilization is engaged,otherw ise the frequency noise w ould cause saturation of the m arionette actuators. A fter reallocation,the reference m ass residualcorrection alw ays rem ains below 10 m V,allow ing a reduction of the coildriver am plification by a factor of 1000.
The ITF w asrunning in recom bined m ode during the firstpartofC5,w hen a fullhierarchical controlofthe end suspensionsw assuccessfully tested.
4.5.The lock acquisition ofthe recycled ITF
A sVirgo and LIG O have sim ilaropticalset-ups,the lock acquisition strategy developed [24]
and adopted in the LIG O interferom eter [25] w as taken as a starting point for the lock acquisition schem e of the Virgo recycled interferom eter. This baseline technique consists in sequentially controlling the four degrees of freedom of the ITF,dynam ically changing the opticalsensing m atrix to com pensate the variation of the fields in the course of lock acquisition. Som e interm ediate stable states w ere locked applying the LIG O strategy,and som e fulllock acquisition trials w ere perform ed. A tthe sam e tim e an alternative technique w asdeveloped:thefirsttestsrapidly provided prom ising results,and experim entalactivity on the baseline technique w assubsequently interrupted.
4.6.Lock acquisition
The basic idea of the new lock acquisition technique is thatthe ITF is locked outside the w orking pointforthe dark fringe. In this w ay a good fraction oflightescapes through the outputportand thepow erbuild-up in therecycling cavity islow.Then theITF isadiabatically broughton to thedark fringe.Thistechniqueisreferred to asvariablefinesse[22],becausethe finesse ofthe recycling cavity changesduring the lock acquisition path.The procedure starts w ith the PR m irrorinitially m isaligned by som e µrads. The sim ple M ichelson is controlled on the half fringe,using the outputportD C signal,w hile the tw o arm s are independently locked using the end photodiodes,as in the recom bined configuration. The pow errecycling cavity length iscontrolled using the reflected 3f-dem odulated signal.In thisw ay allthe four longitudinaldegreesoffreedom oftheITF arelocked in astablew ay from thebeginning ofthe lock acquisition procedure,preventing excitation ofthe m irrors.From thisstarting condition the PR isrealigned,w hile alw aysm aintaining the M ichelson on the halffringe,giving a very low recycling gain.
In orderto increase the recycling gain the M ichelson hasto be broughton to the dark fringe:
this is done adiabatically,decreasing the offsetin the M ichelson errorsignal. A tthe sam e tim e,the controlschem e changes. The end photodiodes can only be used to independently controlthecavitiesw hen theITF isfarfrom thedark fringe:w hen nearing thedark fringethey begin to couplestrongly and acom m on and differentialcontrolhasto beactivated to keep the lock. Then a frequency stabilization servo is engaged,controlling CARM w ith a very high bandw idth:consequently,thecontam ination by thisdegreeoffreedom on allthephotodiodes is cancelled. D ARM is keptin a locked state by one of the end photodiode signals. The finalstep consistsofsw itching from theD C to adem odulated signalto controltheM ichelson length.Eventually the offsetin the M ichelson errorsignalisrem oved,the ITF goeson to the dark fringe and the recycling cavity gain increasesup to the m axim um value.
A pplying this technique the lock acquisition ofthe fullVirgo ITF w as reached forthe first tim e on 26th O ctober 2004,and tested in the latter partof C5. A typicallock acquisition sequence takes few m inutes and itprovides a determ inistic and repeatable lock. The final recycling cavity gain w asm easured to be around 25.
4.7.Sensitivity progress
The 3 km Virgo detectorhas been in com m issioning foraboutone and halfyears. The first lock ofa single Fabry-Perotarm w as realized in O ctober2003: afterexactly one year,the lock ofthe recycled ITF w asperform ed (see figure 7).In betw een,the com m issioning ofthe recom bined ITF w asalso realized,w ith thecontinousim provem entofthevarioussub-system s and controls involved in the operations: longitudinallock,autom atic alignm ent,frequency stabilization servo,fullhierarchicalsuspension control.
D uring this one-year period the displacem entsensitivity of the detector has evolved from 10−11m/√
H zto lessthan 10−16m/√ H z.
Figure 7.Progressin the Virgo sensitivity in approxim ately one yearofcom m issioning.The factthatduring the sum m er of 2004 a laser pow er attenuator w as installed in the injection system also need to be considered: in C5 the laserlightentering the ITF w as around 0.7 W , instead of7 W asin the previousruns.
5.C onclusion
The gravitationalw ave interferom eters are the extrem e evolution ofthe classicalM ichelson interferom eter.Even iftheirgoalisthe m easurem ent,atthe typicalfrequency of100 H z,of spectraldensitiesofrelative displacem ents,ratherthan static lengths,the orderofm agnitude ofthetargetsensitivity (m uch lessthan thediam eterofan atom icnucleus)soundsastounding.
The progress in the sensitivity ofthe Virgo interferom etertow ards the targetsensitivity is a good dem onstration ofthe w ay in w hich this m easurem ents can be attained. A fterone year ofupgrades,the displacem entsensitivity ofthe detectorhas evolved from 10−11m/√
H z to lessthan 10−16m/√
H z.Presently,the com m issioning ofthe recycled ITF ison-going,w ith the goalto im prove the robustnessofthe longitudinallock,atthe sam e tim e putting into op- eration otherm ain controlsystem successfully im plem ented in the recom bined configuration and reducing externalnoise sources. The characterization ofthe noise contributions to the sensitivity constitutes anotherfundam entaltask,w ith the prospectofa science run,close to the targetsensititivy,before the end of2005.
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