The Virgo Experiment

The Virgo ExperimentMichele PunturoINFN Perugia3rd VESF School on Gravitational Waves

3rd VESF school - Michele Punturo - VirgoBuild up the interferometerNS/NS collapse @ Virgo clusterKilometric detectorQuadrupolar nature of the gravitational wave:A Michelson interferometer seems a very appropriate detectorE1E2EinEoutInterference term*

3rd VESF school - Michele Punturo - Virgo

3rd VESF school - Michele Punturo - Virgo* Build up the interferometerLet suppose that TGW>>2L/c:The largest high vacuum system in Europe:About 7000 m31.2 m diameter pipe @ 10-7mbar (H2 partial pressure)Reduction of light fluctuation given by air flux7 long towers (9m long) with differential vacuum:Usual 10-7 mbar vacuum in the upper part10-9 mbar in the lower part


3rd VESF school - Michele Punturo - Virgo*Why a Fabry-Perot?In a Michelson the sensitivity to an arm length difference DL=hL is given by the slope, in the gray fringe, of laser0l/2-l/2Resonant cavity:


3rd VESF school - Michele Punturo - Virgo*Why power recycled?The gray fringe working point is not the right choice:The ITF is not a Null Instrument, that is the output is not null when the input is null: large DCWe want to operate in the dark fringe: no DC if zero inputWhat to do with the light wasted in the input port?laserRecycle it!Shot noise reduced by the recycling factor, but how to extract the GW signal if we work at the dark fringe, where


3rd VESF school - Michele Punturo - Virgo*Modulation-DemodulationTo operate in the dark fringe, but converting the FP in a linear instrument we need to adopt a modulation-demodulation scheme:Pound-Drever techniqueOutcarriersidebands


*modulation-demodulationThe carrier is resonant in the cavity, but not the sidebands (p shift). Hence, the reflected beam isLet suppose that there is a GW signal that modulates the phase of the incoming field. Its effect is present only in the carrier, because it is resonant in the cavityAt the output of the interferometer, the photodiode reads the power, averaged over Wc, hence we must evaluate the square ofThe mixed product term gives:Demodulating the wmod disappears and the output is proportional to the gravitational signal:We build a linear null instrument3rd VESF school - Michele Punturo - Virgo


3rd VESF school - Michele Punturo - Virgo*E(Wlaserwgw)GWh(wgw)AntennaITF mechanical partDL(wgw)ParametrictransducerITF optical partLaserE(Wlaser)E(Wlaserwmod)R.F.OscillatorPockelcellDL(wmod)V(wmod)PhotodiodeSig.L.O.i(wmod wgw)PreampV(wmod wgw)RFMixerSig.L.O.OutV(wgw)V(wmod)GW interferometer as aDouble-Superheterodyne ReceiverSig. = SignalL.O. = Local Oscillatorwmod


*Virgo simplified Optical Scheme3km140m3rd VESF school - Michele Punturo - Virgo


*The injection system: The Laser1W master laserInjection benchMode Cleaner L = 143 m ITF 22 W slave laserReferencecavityTelescopeSlave Nd:YVO4 LaserNd:YAG l=1.064 mmDiode pumpULEmonolithic20 W, Nd:YVO4 laser, two pumping diodesInjection locked to a 0.7 W Nd:YAG laserRequired power stability: dP/P~10-8 Hz-1/2 Required frequency stability: 10-6 Hz1/2

3rd VESF school - Michele Punturo - Virgo*Gaussian beamsUntil now we considered, for the light, the plane wave approximationBut the beam, coming from a laser, shows a finite size and a approximately gaussian shapeIn effect the propagation law of an electromagnetic field in an homogeneous medium gives:Where n are the Hermite-gaussian functions:w(x) represents the beam size:MinimumbeamwaistR(x) represents the curvature radius of the beam:and is defined by:


3rd VESF school - Michele Punturo - Virgo*Transverse Modes 00 and 01


3rd VESF school - Michele Punturo - Virgo*Transverse Modes 11 and 22


3rd VESF school - Michele Punturo - Virgo*Fabry Perot as mode cleanerThe Fabry-Perot cavity is a resonator that can be tuned to select the desired resonant frequencyIn fact, the resonance condition is defined by the request that the complete round trip phase delay of the light inside the cavity is a integer multiple of 2p.Let suppose for simplicity:The resonance conditions becomes:


3rd VESF school - Michele Punturo - Virgo* FP as mode cleanerIf we want to select the gaussian mode 00, we choose the length of the cavity in such a way exists a p1 index satisfying the previous resonance condition: l00p1 resonantThe mode 00 is then transmitted by the cavity


*Input Mode CleanerMode cleaner cavity: filters laser noise, select TEM00 mode

Input mode-cleaner: dihedronInput mode-cleaner: curved mirrorMode cleaner cavity: filters laser noise, selects TEM00 mode

3rd VESF school - Michele Punturo - Virgo*Light filtering: output mode cleaner, 3.6 cm long monolithic cavityLight detection: InGaAs photodiodes, 3 mm diameter, 90% quantum efficiencySuppression of TEM01 by a factor of 10Length control via temperature (Peltier cell)Detection benchOutput Mode-CleanerOutput Optics


3rd VESF school - Michele Punturo - Virgo*OMC filtering effectbefore OMCafter OMC


*Seismic NoiseThe correct and usual way to realize an interferometer in an University Lab is to rigidly clamp the optics to the tableWe cannot adopt this solution, mainly, because of the seismic noise:The simplest seismic filter is an harmonic oscillator, for frequencies larger than the resonant one:A pendulum is an harmonic oscillator of natural frequency:A cascade of N pendulums is a multistage filter whose transfer function is:

*XYZ pendulum chainsto reduce seismic motion by a factor 1014 above 10 HzVirgo SuperattenuatorsBladesMagnetic anti-springs

*Passive Isolation performanceExpected seismic displacement of the mirror (TF measured stage by stage):

Thermal noise is dominant above 4 HzIsolation sufficient also for advanced interferometers

Residual motion too high at the resonances (tens of microns): could be a problem for the ITF operation need of damping!3rd VESF school - Michele Punturo - Virgo


*Working conditionsA FP is mainly a non-linear device. It can be used only at resonance where it is sensitive and linear:4 secondsPr_B8_DCKeep the main cavity locked to enhance the phase response3rd VESF school - Michele Punturo - Virgo


3rd VESF school - Michele Punturo - Virgo* working conditions 2The power recycling cavity must be kept locked to reduce the shot noiseKeep the ITF in the dark fringe to reduce the dependence on the power fluctuation


3rd VESF school - Michele Punturo - Virgo*Interferometer ControlTo push the ITF in the working conditions we need to know the status of the cavitiesMainly, we need to know 4 length and the angles of the mirrors respect to the beamsPhotodiodes Bx provide the error signals to control the 4 independent length of the interferometerQuadrant photodiodes provide the error signals to control the angular positions of the mirrors


3rd VESF school - Michele Punturo - Virgo*Locking a cavityerrorcorrectionLocking trialsLocked


3rd VESF school - Michele Punturo - Virgo*Locking the ITF


3rd VESF school - Michele Punturo - Virgo*How the correction is applied?Three application pointsTop of the inverted pendulum (Filter 0)MarionetteMirrorLocking requirement:dL 10-12 m Tidal strain over 3 km:dL 10-4 m Resonant motions of the mirrorsdL 10-4 m

Need to control the mirror position in a large range of frequency and displacement:Need of hierarchical control


3rd VESF school - Michele Punturo - Virgo*Mirror Actuators


*TidesTides stretch Virgo arms up to 200 mm in 6 hoursThe coils at the mirror level can support up to 10V corresponding to 100 mm displacementThe tide displacement causes a saturation of the coil voltage and a consequent delock of the ITFTide predictionTide effectCoil voltagecavity power Loss of lock saturation

3rd VESF school - Michele Punturo - Virgo*Tide compensationExample of hierarchical strategy:The mirror level coil drivers havent enough dynamical range to compensate the tidal effectThis effect is very low frequencyMoving the inverted pendulum (IP) is easy and softUse the low frequency part of the interferometer signal as error signal


*Designing the detector sensitivityBest seismic damping with low Q suspensionBut not all the stages can be highly dissipative because of the suspension thermal noise The ITF mirrors are suspended by an oscillator (the pendulum) that vibrates (Brownian motion) because of its finite temperatureThe mirror mass itself is a system of oscillators (internal modes) that oscillate because T>0How to evaluate the thermal noise contribution?You surely know the Nyquist theorem that defines the voltage noise at the end of a resistor of impedance R:Translating the electrical impedance into mechanical one we have the fluctuation-dissipation theorem:

3rd VESF school - Michele Punturo - Virgo*Fluctuation-Dissipation theoremThe previous formulation of the FD theorem is equivalent to:whereIntroducing the transfer function of a mechanical system H(w):For an harmonic oscillator:Thermal noise issues require mechanical Q as high as possible


*Special feature of a PendulumA pendulum is an harmonic oscillator where the restoring force is mainly given by the gravitation (lossless force). The dissipation is, instead, due to the elastic force of the suspension wire.Very thin (and strong) suspension wiresLow loss steel wires

3rd VESF school - Michele Punturo - Virgo*Contribution to the pendulum QThe loss angle that enters in the thermal noise formula is not only due to the intrinsic material loss, but other excess losses must be taken in account:Clamping lossesFrictional losses / eddy currentsResidual gas losses


3rd VESF school - Michele Punturo - Virgo*Mirror thermal NoiseIn the modal expansion approximation we can consider the Virgo mirrors as composed by an infinite number of oscillators.In the limit w

3rd VESF school - Michele Punturo - Virgo*Virgo MirrorsThe Virgo mirrors are the largest (and more expensive) mirrors in the current GW detectorsSince the substrate specifications are very stringent a special fused silica (Suprasil 311 SV) have been realized on purpose for Virgo:Low absorption: 0.7 ppm/cmLow OH content (

3rd VESF school - Michele Punturo - Virgo*350 mm100 mm


3rd VESF school - Michele Punturo - Virgo*Optical read-out noiseOptical read-out noise is the (incoherent) sum of the shot noise and radiation pressureIn the current ITFs only the shot noise plays a relevant role, but it is instructive to see a formalism (KLMTV) that reports the two noises in a single expression:Shot noiseRadiationpressurePBS is the power impinging the beam splitter:


3rd VESF school - Michele Punturo - Virgo*Virgo Nominal Sensitivity


3rd VESF school - Michele Punturo - Virgo*The real life!The design of a GW interferometric detector is an hard job, but the attainment of the design sensitivity is even harderIn fact, a GW detector is a complex machine that needs a deep tuning of many parametersMethods and technologies are completely new5 years of commissioning needed in LIGOSimilar time spent in Virgo


3rd VESF school - Michele Punturo - Virgo*Commissioning evolutionPhase A: Commissioning of interferometer arms Test all aspects of control systems with a simple optical configuration- locking, automatic alignment, second stage of frequency stabilization and suspension hierarchical control (tidal and marionette) First shake of the sub-systems



Phase B: Commissioning of interferometer in recombined mode Useful intermediate step towards full interferometer lock Verify functioning of BS longitudinal control Re-run all aspects of control system in a more complex configuration Start noise investigations




Phase C: Commissioning of Recycled Fabry-Perot interferometer Run full locking acquisition process Verify functioning of PR mirror longitudinal control Re-run SSFS, tidal control and marionette control Implement complete wave-front sensing control Continue noise investigations




Phase C: Commissioning of Recycled Fabry-Perot interferometer Run full locking acquisition process Verify functioning of PR mirror longitudinal control Re-run SSFS, tidal control and marionette control Implement complete wave-front sensing control Continue noise investigations

Phase D: Noise hunting


3rd VESF school - Michele Punturo - Virgo*Sensitivity Improvement


3rd VESF school - Michele Punturo - Virgo*Sensitivity of the global networkNSNS detection distance~6Mpc


*Noise BudgetUnclear excess noise sourcesShot noise dominatedResidual lightscattering

Scientific case:Thermal effects in the Virgo mirrorsThe 3km Virgo arms are a long Fabry Perot cavity:3rd VESF school - Michele Punturo - Virgo*HRAR~10-3 10-4Hence, actually, each arm is a double FP cavity:Etalon effectLALM


Etalon EffectHence, the Finesse of the cavity and all the fundamental parameters of the ITF are affected by the input mirror optical thickness variationBut, why the mirror optical thickness fluctuates?3rd VESF school - Michele Punturo - Virgo*Temperature!!Hence, knowing the mirror temperature it is possible to predict some of the ITF performancesOK, but how to measure the mirror temperature?


Resonant mode techniqueObviously the resonant frequencies of a body depend on the temperature of the bodyFor a Virgo mirror we evaluated this dependence with a ANSYS based FEM*Drum mode: 0.61 Hz/KButterfly mode(s): 0.28 Hz/K

Calibration crosscheckTo crosscheck the FEM calibration we compared the prediction with the temperature measured just outside the towers*Scientific run

Evidence of the Etalon effectThe confidence in the resonant mode is critical in this evaluation.Could we find an independent confirmation?Etalon effect in the input mirrors

*B7/B8 phdDP/PDTAR coating

VSR1 BNS horizon During the VSR1 the horizon grown from 3.7 to 4 Mpc thanks to the limited commissioning activityNow we are at about 6MpcThe fluctuation in the horizon was caused byBad weather Alignment fluctuationEtalon effect combined with control issues3rd VESF school - Michele Punturo - Virgo*


*The dark side of the experiment!The photodiode that contains the GW signal is just one.We are sensible up to few kHz:A sampling rate of 20kHz is correctly oversized20kHz 8Bytes = 160kB/s of expected data recordingInstead we write about 7-8MB/s why?Many control channels:Secondary beamsActuation and error signalsEnvironmental monitoring

3rd VESF school - Michele Punturo - Virgo*The online architectureFront-End Data CollectionFrame ProcessingFrame Building


The evolutionVirgo concluded its first long run (VSR1) in parallel with the LIGO-S5 run the 1st of October 2007During the winter we had an intense commissioning and upgrade activity that reduced the noise level mainly at low frequency and improved our sensitivity and detector understanding In the next summer a long shutdown is scheduled to install a powerful laser (reduction of the shot noise), an improved injection optical system and new control and DAQ electronics3rd VESF school - Michele Punturo - Virgo*Virgo+ project


Detection ProbabilityThe Initial LIGO and Virgo detectors are often called 1st generation ITF detectorsWhat is the detection probability with them?It depends by the typical merger rate in a galaxy times the number of galaxies visible by our detectorThe merger rate is quite uncertain (CBC group)3rd VESF school - Michele Punturo - Virgo*Hence the detection rate is not well defined (but low!):

SourceRminRreRplRupNS-NS (MWEG-1 Myr-1)0.110010002000

AbbreviationRate statementPhysical significanceUpUpper limitRates should be no higher than PlPlausible optimistic estimateRates could be as high asReRealistic estimateRates are likely to beMinExpected minimal estimateRates should be at least

SourceRminRreRplRupNS-NS initial LIGO (yr-1)1.510-50.0150.150.3


3rd VESF school - Michele Punturo - Virgo*Credit: R.PowellBasic IdeaLet suppose that the distribution of the galaxies in the universe is uniform:Increasing the detection distance by 2 (10), a factor 8 (1000) in event rate could be expectedIn effect the galaxies density is not constant:R


3rd VESF school - Michele Punturo - Virgo*LIGO - VirgoLIGO+ - Virgo+AdvLIGO - AdvVirgoCredit: R.PowellDetector Evolution programCurrent detectors:1st generation: Virgo & LIGO

Detector evolution timeline3rd VESF school - Michele Punturo - Virgo*


Key points of the advanced detectorsReduction of the shot noise:Higher laser power (160W laser)Higher cavity finesseDC detectionSignal recyclingReduction of the mirror thermal noiseImproved materials for the substratesHeavier substratesImproved coatings materials and geometriesImproved beam geometries (Virgo)Reduction of the suspension thermal noiseMonolithic fused silica suspensionsReduction of seismic noiseActive suspensions (LIGO)3rd VESF school - Michele Punturo - Virgo*


Sensitivity evolution3rd VESF school - Michele Punturo - Virgo*


3rd generation: ETAdvanced detectors are an evolution in situ of the current detectorsTo gain a further factor 10 in sensitivity the current infrastructures are insufficientWe need: Underground sitesReduction of the seismic and Newtonian noiseCryogenic infrastructuresReduction of the thermal noise and of the optics lensingGEO and Virgo collaborators started, with the European Commission support, a design study of a 3rd generation GW observatory: ET3rd VESF school - Michele Punturo - Virgo*


3rd VESF school - Michele Punturo - Virgo*


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The Virgo Experiment

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