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

Explore the VIRGO experiment on detecting gravitational waves with advanced technologies like Michelson interferometers and suspended mirror systems. Learn about noise sources, sensitivity measurements, and the commissioning process of VIRGO.

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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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