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The VIRGO experiment The commissioning of VIRGO Towards a global network Conclusions

The status of VIRGO E. Tournefier LAPP(Annecy)-IN2P3-CNRS Journées SF2A, Strasbourg 27 juin – 1er juillet 2005. The VIRGO experiment The commissioning of VIRGO Towards a global network Conclusions. How to detect gravitational waves?. Suspended mirror. Suspended mirror. Beam splitter.

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The VIRGO experiment The commissioning of VIRGO Towards a global network Conclusions

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  1. The status of VIRGOE. TournefierLAPP(Annecy)-IN2P3-CNRSJournées SF2A, Strasbourg27 juin – 1er juillet 2005 • The VIRGO experiment • The commissioning of VIRGO • Towards a global network • Conclusions

  2. How to detect gravitational waves? Suspended mirror Suspended mirror Beam splitter Light Detection LASER () • Effect of a gravitational wave on free masses: • A Michelson interferometer is suitable: • suspend mirror with pendulum => ‘free falling masses’ • Gravitational wave => phase shift • h = L/L • L = length difference between the 2 arms • L = arm length 

  3. The shot noise and the VIRGO optical design 1/ Fabry-Perot cavities to increase the effective length: ( F = finesse ) => L’ = 100km for L=3km and F=50 2/ Recycling mirror to increase the effective power: P’ = R P (R = recycling gain) => P’ = 1kW with P=20W and R=50 Gravitational wave signal Limitation of a Michelson interferometer due to photon shot noise: the minimum measurable relative displacement is =>Can reachh ~ 3.10-23 withL=100kmandP=1kW How to achieve that?

  4. Noise sources in interferometers Seismic Noise Acoustic Noise Thermal Noise Index fluctuation Shot Noise Detection Noise Laser Noises

  5. Noise sources: seismic noise Seismic noise measured on the Virgo site m/Hz 10-12 100 Hz • Seismic noise spectrum for f few Hz: • a ~ 10-6 - 10-7 •  shot noise ! • Need a very large attenuation! Solution: suspend the mirrors to a chain of pendulums Transfert function • With 6 pendulums: attenuation of the seismic noise by more than 10 orders of magnitude above 4 Hz!

  6. Suspensions and control of the interferometer All mirrors are suspended to a cascade of 6 pendulums: • Large attenuation in the detection band ( > 10 Hz) • Large residual motion at low frequencies: < ~1mm • Need active controls to: • maintain the interferometer’s alignment • maintain the required interference conditions 1/ Local control of the suspensions: • Residual motion ~2 m/sec • Obtain interference fringes 2/ To keep the interferometer at interference conditions: • Need to control the cavity length to 10-12 m • Use the interferometer signals (photodiodes)

  7. VIRGO design sensitivity Shot noise 1 • Main sources of noise limiting the VIRGO design sensitivity Seismic noise Thermal noise Shot noise

  8. Gravitationnal waves sources and VIRGO design sensitivity(sources: see talk by N. Leroy) Distance to the Virgo cluster = 10Mpc

  9. French-italian collaboration (CNRS – INFN) Site : Cascina close to Pisa 5 french labs: Annecy (LAPP), Lyon (LMA), Nice (OCA), Paris (ESPCI), Orsay (LAL) 6 italian labs: Firenze, Frascati, Napoli, Perugia, Pisa, Roma (all INFN) VIRGO

  10. The commissioning of VIRGO • control of the north FP cavity: Oct 2003 • - control of the west FP cavity: Dec 2003 - recombined (Michelson) ITF: Feb 2004 - recycled (full VIRGO) ITF: Oct 2004 Fabry-Perot cavities input mode cleaner l=150m beam splitter laser l=6m L=3km recycling mirror output mode cleaner • Technical runs (3 to 5 days) at each step • C1(Nov 2003),…, C5(Dec 2004) • Lock stability • Sensitivity/noise studies • Data taking on ‘long’ period • Started in summer 2003 • The steps of the VIRGO commissioning: West arm North arm Gravitational wave signal

  11. Recombined interferometer Example of lock acquisition Power ‘stored’ inside the FP cavities Power at the interferometer output Lock on the dark fringe • Recombined interferometer: keep the two Fabry-Perot cavities on resonance + the Michelson on the dark fringe

  12. The lock of the full VIRGO + POWER IN THE RECYCLING CAVITY Lock acquisition sequence With power recycling Recycling gain ~ 30 Laser - Without power recycling • Lock of the recycled interferometer (full VIRGO): • Need to control 4 degrees of freedom (3 cavities + Michelson) • The lock is acquired in several steps: • Start without recycling • Slowly increase the recycling gain • 2 technical runs: • C5 (3 days, Dec 2004) • C6 (2 weeks) planned for this summer

  13. Noise studies Sensitivity measured during C4 run and identified sources of noise Noise hunting => see talk by R. Gouaty 1/ Identify the sources of noise which limit the sensitivity 2/ Perform the necessary improvements / implement new controls Attention a l’unite!

  14. Typical unforseen difficulties • Injection bench: • A small fraction (bigger than expected) of the light reflected by the interferometer is retro-diffused by the input mode cleaner mirror • spurious interferences Temporary solutions: - rotate the mode cleaner mirror - reduce the incident light (/10) • We are now working with only Pin = 0.7 Watts Final solution: install a Faraday isolator • A new input bench will be installed in september

  15. Sensitivity summary Single arm, P=7 W Recombined, P=7 W Recycled, P=0.7 W P = 10W h ~3. 10-21/Hz The VIRGO sensitivity will significantly improve with: - the implementation of the automatic alignment of the mirrors (low frequency) - the full power (high frequency)

  16. Data analysis • Calibration and reconstruction of the signal: Watts -> meters - Apply a known displacement to the mirrors • A lot of tests on simulated data including interferometer noise • Test of the data analysis on real data from the technical runs: • Test the full chain of data analysis • Learn how to put vetoes • Inject events in the real data: software and hardware injections -> measure efficiencies, false alarm rate,… • Start collaboration with LIGO: Coincident analysis will help the detection of gravitational waves =>decrease false alarm rate (rare events in non gaussian noise) Combined data analysis is necessary to extract the source parameters - Injected events

  17. Towards a worldwide network LIGO GEO VIRGO TAMA AIGO • Look for events in coincidence • Combined analysis is needed to extract information on the source

  18. Status of LIGO reaching the Virgo cluster ! LIGO Hanford Observatory (LHO) H1 : 4 km arms H2 : 2 km arms LIGO Livingston Observatory (LLO) L1 : 4 km arms • Two sites: • Hanford (Washington): 4km and 2 km interferometer • Livingston (Louisiana): 4km interferometer • Same optical configuration as Virgo • Less sophisticated suspensions • The commissioning started in 1999 • The three interferometers are operational • Long science runs have started: • S1 (Aug 2002) • S2 (March-April 2003) • S3 (Nov-Dec 2003) • S4 (Fev-March 2005) • 6 month run this year • The LIGO sensitivity is now very close to the design sensitivity !

  19. Conclusion • VIRGO commissioning is progressing • The recycled (full VIRGO) interferometer is working • Sensitivity will make big progress with • Automatic alignment of the mirrors • New input bench • First scientific run in 2006? • LIGO is very close to its design sensitivity • Long science runs will start this year • The detection with the first generation of detectors is not guaranteed • A global network is needed • A second generation of detectors is being prepared to reach h~few 10-24 /Hz => Upgraded VIRGO and LIGO ~ 2010-2013

  20. Future: how to improve the sensitivity? Shot noise The first generation of detectors might not be able to see gravitational waves • Need to push the sensitivity further down: • Seismic noise: • The VIRGO suspensions already meet the requirements for next generation interferometers • The main limit: thermal noise • Monolitic suspensions (silica) • Better mirrors (material, geometry, coating) • Shot noise • More powerful lasers • Signal recycling technique • And the technical noises • Better sensors • Better electronics • Better control systems

  21. Future: How to go to lower frequencies • Frequency range limited on the earth due to seismic noise => go to the space: the LISA project • Much lower frequencies: 10-4 – 1 Hz • It is complementary to terrestrial detectors • LISA: • Spatial interferometer (NASA-ESA) • 3 satellites, size = 5.106 km • start: 201?

  22. GEO (UK, Germany) • 600m long arms • An interferometer for the development of new techniques: • Signal recycling • Monolitic suspensions (-> reduce thermal noise) 600m arm (no FP) Power recycling Laser Signal recycling

  23. TAMA (Japon) • Located at Tokyo • Same optical configuration as VIRGO • Started the commissioning in 1997 • Reached first a sensitivity of h ~ 3.10-21 Hz –1/2 • But limited by the small arm length design 10-21

  24. Gravitational wave detectors: resonant bars • The gravitational wave excites the resonant mode of the bar • Good sensitivity for frequency = mode of the bar • First detectors in 1960 • Many improvements since then: • Cryogenic • New transducers for the detection of bar oscillation • Several detectors in operation => perform coincidence data analysis

  25. Bars events • NAUTILUS (Italie) • EXPLORER (CERN) • Coincident analysis between Explorer and Nautilus • 2001 data • Small  excess of events when the detectors are optimally oriented with respect to the galactic plane • Excess not confirmed by recent data taking

  26. The mirrors • Fused silica mirrors • Coated in a class 1 clean room at SMA-Lyon (unique in the world). • Low scattering and absorption: < few ppm • Good uniformity on large dimension: < 10-3 400 mm • Large mirrors (FP cavities): •  35 cm, 10 cm thick • 20 kg

  27. The injection and detection systems • Laser: powerful and stable • 20W • Power stability: 10-8 • Frequency stability: Hz • The input and output mode cleaners: • optical filter => improve signal to noise ratio • Signal detection: • - InGaAs photodiodes, high efficiency

  28. The commissioning of the CITF unit = meters! • Commissioning of the central interferometer: 09/2001 -> 07/2002 • CITF = Recycled Michelson interferometer (no Fabry-Perot cavities) • a lot of common points with VIRGO • The evolution: configuration and sensitivity: 4 runs of 3 days each - E0/E1: Michelson - E2: Recycled Michelson - E3: + automatic angular alignment - E4: + final injection system • Results: • Viability of the controls • Sensitivity curve understood • And gain experience for the VIRGO commissioning - Improvements triggered by the CITF experience

  29. Data analysis • Supernovae • The signal shape is not well known • several techniques are developed to detect bursts • Problem of non gaussian detector noise • Detection in coincidence with other detectors is needed • Binary coalescences • Well known signal • Use a matched filtering technique • The parameters of the sources can be extracted • Pulsars • Need to integrate on long periods • But the signal is distorted by Doppler effect due to the earth’s rotation • Huge parameter space • Limited by computational resources

  30. Fabry-Perot cavities input mode cleaner l=150m beam splitter laser l=6m L=3km recycling mirror output mode cleaner Control of the laser frequency Control of the cavity

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